Anno Accademico 2005/2006 Università degli Studi di Roma “LA SAPIENZA” Facoltà di Ingegneria Dipartimento di Scienza e Tecnica dell’Informazione Corso di Laurea Specialistica in Ingegneria delle Telecomunicazioni Tesi di Laurea Modelli architetturali per soluzioni di accesso innovative basate sulla convergenza fra reti 3G/4G, DVB-H e WiMAX, in rispetto del Framework IMS Candidato Cioccari Francesca Relatore Chiar.mo Prof. Roberto Cusani Correlatore Ing. Gennaro Galdo CONSEL - Junior Consulting
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Anno Accademico 2005/2006
Università degli Studi di Roma “LA SAPIENZA”
Facoltà di Ingegneria Dipartimento di Scienza e Tecnica dell’Informazione
Corso di Laurea Specialistica in Ingegneria delle Telecomunicazioni
Tesi di Laurea Modelli architetturali per soluzioni di
Il nuovo modo di comunicare sta evolvendo verso l’integrazione, in un'unica
struttura, di tutti i servizi che generalmente sono presenti in un’organizzazione (cioè
servizi dati, telefonici e video). Il protocollo IP sembra essere ormai la struttura di
riferimento per questo nuovo sviluppo tecnologico.
I modelli architetturali delle reti attuali, però, prevedono una netta separazione
dei servizi voce, video e dati. I servizi sono amministrati generalmente in modo
autonomo a livello di cablaggio, di apparati e a livello applicativo. Il nuovo modello
di rete, quindi, dovrebbe prevedere una struttura indipendente dalla applicazione, con
apparati dotati di caratteristiche multiservizio e flessibilità, adatti a concentrare le
diverse tipologie di traffico e convogliarle su canali unici sia in ambito LAN sia
WAN. Questa nuova occorrenza sta muovendo i grandi operatori delle
telecomunicazioni, e i protagonisti come Nortel, Samsung, Alcatel insieme con quelli
del settore del networking come Cisco e 3Com, stanno già avanzando le loro
proposte parlando di convergenza.1
Finora, lo sviluppo dei servizi voce, dati e video era avvenuto prevalentemente
in modo autonomo e la loro implementazione era gestita, molto spesso, da operatori
diversi. Dal punto di vista dell’utente, l’offerta dei servizi telematici era troppo
rigida. Dal punto di vista dell’efficienza e dell’ottimizzazione del traffico, una rete
dati richiede larghezze di banda decisamente superiore rispetto a quella voce,
soprattutto a causa dello sviluppo dei servizi come videoconferenza, video streaming,
e-commerce, ecc... La nascita di offerte e soluzioni innovative scaturiscono quasi
sempre da nuove esigenze o precise richieste. Nel settore delle telecomunicazioni e
del networking si sta assistendo ad una crescita vertiginosa del traffico dati contro un
andamento della domanda praticamente costante del traffico voce, tanto che si stanno
diffondendo nel mercato soluzioni che trattano la trasmissione voce come un
particolare pacchetto dati. Da queste realtà sono nate tecnologie mirate
all’ottimizzazione delle architetture di reti con l’obiettivo di trasportare su un unico
1 “Le reti convergenti”, Matteo Santucci
Introduzione
2
supporto video, voce e dati implementando, cioè, una rete convergente. D’altro canto
è ormai largamente riconosciuto che l’IP (Internet Protocol) sarà il protocollo di
riferimento per qualunque tipo di rete. L’adozione di questo protocollo da parte di
numerosi operatori e utilizzatori per il trasporto video, voce, dati implica quasi
obbligatoriamente che una rete convergente si basi anch’essa sull’IP. Anche il
protocollo SIP ha un ruolo cardine in questa evoluzione e ampliamento dell’offerta
dei servizi. La sua flessibilità permette, infatti, rimanendo in uno stesso contesto
tecnologico, di fornire servizi con caratteristiche molto differenziate, e di integrare,
in una esperienza cliente omogenea, diverse forme di comunicazione come la
conversazione, la messaggistica, lo streaming di video.
Questa dinamicità dell'industria mondiale delle telecomunicazioni, garantita
dai numerosi exploit tecnologici e dalle nuove soluzioni proposte di continuo dai
player, inducono gli analisti a mantenere alta l'attenzione sui driver che stanno
trainando il mercato, e cioè la domanda costante di connettività a banda larga con
elevata capacità e di servizi mobili sistematicamente più sofisticati e convergenti,
con un rapporto qualità-prezzo sempre più competitivo.2 L’intero settore delle
telecomunicazioni è attualmente caratterizzato da grandi innovazioni che ne stanno
cambiando radicalmente i connotati. E’ normale che, in queste situazioni di forte
transizione tecnologica, praticamente tutto venga sconvolto e messo in discussione. Il
vecchio declina mentre il nuovo per consolidarsi richiede scelte difficili,
investimenti, nuove dimensioni imprenditoriali e manageriali, e soprattutto tempo.
Come già detto, la grande dinamicità e il ristagnare delle rendite legate ai
tradizionali servizi di telefonia fanno da preludio ad un profondo mutamento: la
rivoluzione della convergenza. La strategia che stanno perseguendo tutti gli operatori
di mercato è quella che punta a realizzare un’infrastruttura unica di trasporto, basata
su protocollo IP, lo stesso della rete internet, in grado di supportare servizi innovativi
in banda larga sia per il fisso che per il mobile. “In un futuro ormai prossimo,
diciamo cinque anni, ci sarà la convergenza su un'unica piattaforma delle tecnologie
di comunicazione. L’utilizzo dei telefoni portatili cambierà: il cellulare, grazie alla
2 Insight Research, "The Future of Telecommunications 2006 – 2011”. Maggio 2006
Introduzione
3
convergenza e alla possibilità di trasmettere e ricevere una grande quantità di dati,
diventerà un apparecchio molto più simile ad un tradizionale PC. Ciò significa che
sulla stessa rete potremo contemporaneamente comunicare a voce, navigare su
Internet, vedere televisione, fare videogiochi da remoto, discutere in
videoconferenza.”. E’ quanto sottolinea Roberto Guadagni, responsabile
infrastrutture di rete e di calcolo dell'Enea (Ente per le nuove tecnologie, l’energia e
l’ambiente).3 Tale infrastruttura rappresenta la base per lo sviluppo di una
piattaforma comune attraverso la quale offrire gli stessi prodotti e servizi su
qualunque terminale (PC, TV, Telefonino, ecc), mediante diverse tecnologie (ADSL,
HSDPA, DVB-H, UMA, ecc) e consentendo al cliente di decidere dove, come, e
quando accedere a tali servizi.
In questo contesto, l’offerta di Mobile TV, la fruizione di contenuti televisivi su
apparecchi mobili tramite l’utilizzo della tecnologia DVB-H (Digital Video
Broadcasting - Handheld), appare ricca e ben distribuita. Grazie allo standard DVB-
H sarà possibile guardare la televisione sul cellulare in maniera interattiva e dunque
rendere concreto il concetto di portabilità. Con lo sviluppo di offerte multimediali
VoD (Video on Demand) e PVR (Personal Video Recorder), combinate con il lancio
della Mobile TV, il mercato entrerà inoltre nell’era della Personal TV.
La convergenza non rende più possibile la distinzione tra telefonia fissa e
mobile. Proprio quest’ultima sta acquisendo ampiezze di banda che per molti servizi
non farà rimpiangere la potenza delle reti fisse. E manterrà quel grande vantaggio
che è la mobilità totale, requisito ormai indispensabile per gli utenti finali. Ma le reti
fisse potranno riappropriarsi almeno di una parte del traffico che hanno perso con i
nuovi sviluppi degli accessi WiFi e WiMax, che consentiranno di fare con il
telefonino, da casa, dall’ufficio e forse dalla strada comunicazioni che di fatto
transiteranno sulla rete fissa.
3 Conferenza "e-Infrastrutture per lo sviluppo", organizzata dal Consortium GARR, ideatore e gestore della rete telematica nazionale per l'Istruzione, l'Università e la Ricerca Scientifica.
Introduzione
4
Ormai la marcia verso grandezze di banda sempre più importanti è
inarrestabile. Si calcola che entro il 2015 il 20 per cento degli europei potrà contare
su una banda di 100 mega, il che consentirà di fare assolutamente tutto».4
La banda larga, verso la quale si stanno rivolgendo con ingenti investimenti le
principali compagnie che offrono servizi di telefonia fissa, non è un semplice
passaggio dalla banda stretta della telefonia vocale a quella larga che ci consente già
di vedere la televisione attraverso le reti di telecomunicazione. E’ una svolta
progressiva che apre nuovi mondi di servizi, non solo di intrattenimento, ma
soprattutto di utilità per la produttività delle imprese e per lo sviluppo di
indispensabili servizi sociali. Molti servizi innovativi di comunicazioni mobili, in
primis la navigazione su Internet, sono limitati o quasi impossibili per la carenza di
banda, e lo stesso accade nelle comunicazioni fisse. Servono servizi innovativi per la
sanità, per i trasporti, per la sicurezza, per l’insegnamento a distanza, per la
formazione professionale, per la gestione aziendale per giustificare nuovi
investimenti in banda larga, e la convinzione generale è che sarà la crescita della
banda a stimolare la nascita dei servizi nuovi.
Infine, la forte avanzata delle tecnologie ti telecomunicazioni introducono
importanti fattori: la capacità dei sistemi basati su protocollo IP di combinare e
trasportare ovunque voce, video e dati; la forte varietà di soluzioni wired e wireless
proposte a livello locale; la continua richiesta di connettività e la conseguente guerra
al ribasso delle tariffe; la crescente domanda di soluzioni per garantire la sicurezza
informatica e, di pari passo, il compito sempre più arduo di proteggere con efficacia
gli internauti dai mille pericoli della Rete.
4 Carlo Mario Guerci, Professore Ordinario di Economia Politica presso la Università degli Studi di Milano
Introduzione
5
Il Progetto Wind – “Convergent Access Network”
Questo lavoro è il risultato di un progetto commissionato dalla divisione reti di
Wind Spa. Il progetto, della durata di sei mesi, prevedeva l’analisi del mercato delle
telecomunicazioni, ed in particolare lo studio dei soggetti coinvolti e delle loro
strategie di mercato. Sono state studiate le tecnologie e le innovazioni nel campo
della comunicazione e della telefonia, sia fissa che mobile, con una focalizzazione
sulle nuove potenzialità offerte. Sono stati proposti nuovi servizi convergenti che
incontrassero queste nuove esigenze e le possibilità offerte dalle tecnologie
innovative. Infine, è stato analizzato il mercato ed i suoi trend, e sono stati
evidenziati i bisogni espressi dai consumatori e le loro nuove esigenze.
Il lavoro è scritto in Inglese, su specifica richiesta del committente, che con la
recente acquisizione da parte di Weather Investments ha assunto un carattere
internazionale. L’attività è stata sottoposta a periodiche verifiche da parte del
committente e del referente aziendale, Maria Rita Spada, mediante diversi incontri,
con lo scopo di verificare lo stato di avanzamento dei lavori e di definire gli step
successivi. Infatti, per poter raggiungere l’obiettivo del lavoro si è proceduto tramite
diverse fasi, sintetizzate nella Figura 1.
Figura 1: Fasi del Progetto "Convergent Access Network"
Introduzione
6
Come si evince, il progetto ha una doppia valenza: una di carattere prettamente
tecnologico e l’altra di carattere economico. Proprio questa interdisciplinità ha
contribuito all’accrescimento delle mie competenze trasversali e all’approfondimento
delle tematiche d’interesse di questa tesi.
Il lavoro da me svolto nell’ambito di questo progetto è rivolto alla componente
tecnologica che ha reso possibile la proposta di un servizio integrato basato su una
nuova architettura convergente.
Sviluppato nell’ambito del programma formativo Junior Consulting, presso il
Consorzio ELIS (CONSEL) di Roma, il progetto ha visto lavorare, sotto il
coordinamento del Team Leader Gennaro Galdo, quattro laureandi in diverse
discipline: Grace Augustine, laureata in B.A. Organizational Studies presso
l’università del Michigan (U.S.A.), Andrea Chiarello, laureando in Ingegneria
Elettronica (indirizzo Microelettronica), presso l’università di Bari, e Tommaso
Chiocchi, laureando in Ingegneria Gestionale, presso l’università Tor Vergata di
Roma.
Struttura della Tesi
L’obiettivo di questa tesi è quello di individuare un possibile modello
architetturale che favorisca soluzioni di convergenza fra varie tecnologie di accesso
nomadiche, mobili e di broadcasting. Le tecnologie studiate per questo scopo sono:
DVB-H, UMTS Release 6, Mobile WiMAX (802.16e), IMS, SIP e IPv6.
Come prima fase è stato necessario uno studio approfondito sulle reti di
accesso delle principali tecnologie (UMTS, WiMAX e DVB-H) che sono state scelte
per l’integrazione [Capitolo 1]. In particolare, sono state analizzate le varie
caratteristiche che contraddistinguono queste tre tecnologie standardizzate e le
rendono le più efficienti nel loro specifico ambito d’interesse. Questo studio è stato
fatto per poter capire fino a che punto potersi ‘spingere’ al fine di ottenere la
convergenza verso un’unica rete di accesso.
Introduzione
7
La seconda fase, infatti, ha come scopo l’individuazione proprio di un modello
architetturale unico per una generica rete di accesso [Capitolo 2]. Per questo è stata
necessaria dapprima un’analisi delle reti di accesso rivolta principalmente ad
individuare dove e come queste tecnologie si agganciano al “mondo IP” e quali
specifiche funzionalità ogni singolo dispositivo fisico svolge [§ 2.2 - 2.4]. A valle di
questa analisi preliminare è stato possibile eseguire un paragone a livello funzionale
tra i vari elementi che compongono le diverse reti di accesso per sottolineare cosa
hanno in comune (funzioni parallele o simili) ed individuare le diversità e peculiarità
di ogni tecnologia [§ 2.5]. Ne deriva un modello architetturale unico tra le varie
tecnologie di accesso in cui vengono individuati dei blocchi funzionali. Ognuno di
questi blocchi è composto da funzionalità, necessarie per la rete di accesso, che sono
dislocate o accorpate nei dispositivi delle reti di accesso delle tre tecnologie prese in
considerazione [§ 2.6].
Come passo successivo si è ritenuto opportuno approfondire l’analisi della
piattaforma IMS (IP Multimedia Subsystem) [Capitolo 3], la piattaforma
multimediale che permette proprio l’integrazione tra diversi servizi, basandosi sul
protocollo SIP. La rete IMS è stata individuata, quindi, come Core Network all-IP
based.
Un ulteriore passo verso una totale convergenza ha reso necessario un
approfondimento su come poter legare le reti di accesso alla Core Network unica
IMS. Questo ha portato a dover tener conto di un insieme di elementi necessari per
ottenere tale connessione. Il risultato di questo lavoro è la proposta di una Edge
Network [Capitolo 4]. L’approccio utilizzato nella definizione di questa sezione di
rete è stato quello di suddividere il lavoro in varie fasi. Come primo passo sono stati
individuati i requisiti e le funzionalità che la Edge Network deve soddisfare [§ 4.2].
Sulla base di questo sono stati definiti gli elementi di cui la Edge Network è
composta per le tre specifiche tecnologie di accesso, UMTS, WiMAX e DVB-H. Per
ognuno di questi dispositivi è stata svolta un’analisi approfondita e soprattutto uno
studio ed una definizione del loro stack protocollare [§ 4.3 - 4.5]. Successivamente si
è presentato un modello di architettura convergente, composto da Access Network,
Edge Network e Core Network, ed è chiaramente spiegato in che modo la Edge
Network permette di ottenere tale architettura [§ 4.6]. Ovviamente, la proposta finale
Introduzione
8
di questa tesi è il risultato di ampie valutazioni e considerazioni, per cui è necessaria
anche una descrizione dell’evoluzione che la Edge Network, e di conseguenza
l’architettura convergente, subisce a partire dall’esistenza di tre reti di accesso del
tutto separate al raggiungimento di una rete completamente integrata, tenendo in
considerazione anche possibili sviluppi futuri [§ 4.6.1]. Terminando, sono indicati i
vantaggi che questa soluzione apporta e le sue caratteristiche [§ 4.6.2].
Infine, viene presentato come l’architettura convergente risultante può essere
implementata [Appendice A]. Per poter fare ciò viene preso come esempio un case
pilot di possibile servizio convergente, chiamato m-Advertising game [§ A.1]. È stato
scelto questo tipo di servizio perché mette in evidenza il ruolo svolto da uno degli
elementi della Edge Network, il Content Provider server, che è appunto un
dispositivo cruciale nell’implementazione della Edge Network [§ A.2]. Questo
componente della Edge Network ha il compito di connettere il sistema di
trasmissione broadcast DVB-H alla Core Network IMS, quindi oltre le funzionalità
che tale server già possiede deve essere in grado di svolgere funzioni aggiuntive. Una
di queste è l’instaurazione di sessione, la quale è messa in evidenza nel caso preso in
considerazione. Perciò, per simulare un ambiente IMS, in cui il Content Provider
server si trova, ho usato un SIP proxy che gestisce la funzione di presence. Si mostra
in questo modo come il Content Provider server può instaurare una sessione HTTP
con gli utenti [§ A.3].
Uno studio preliminare del mercato delle Telecomunicazioni è stato necessario
per poter capire verso cosa sono rivolte le continue evoluzioni nel mondo delle
telecomunicazioni e quali sono le possibilità che le innovazioni tecnologiche offrono,
quindi è presentato un overwiev sullo scenario internazionale ed italiano [Appendice
B].
Technical Overview
9
CCHHAAPPTTEERR OONNEE
1. TECHNICAL OVERVIEW
The objective of this thesis is a convergent architecture between different access
technologies. In order to achieve this convergence the access networks considered
are UMTS (Release 6), Mobile WiMAX and DVB-H.
These three technologies were chosen because of different reasons. First, they are
standard solutions. Standardization is very important when interworking is necessary,
especially in this case. ETSI (European Telecommunication Standards Institute) has
chosen DVB-H as European standard for the broadcasting mobile TV. UMTS is
standardized by 3GPP (3rd Generation Partnership Project) which is a co-operation
between ETSI in Europe and other Associations worldwide. WiMAX standard is
IEEE 802.16. This is the 16th working group of IEEE (Institute of Electrical and
Electronic Engineers) 802, which is specialized in wireless broadband access. The
IEEE 802.16 standard specifics just the air interface (physical and MAC layer), the
last 802.16e one allows this technology to be used by mobile and nomadic clients.
Furthermore, they are the most common, especially in Europe where UMTS is
already spread, DVB-H is now coming and WiMAX will soon be realized. Moreover
there is a wide industry support for them.
In addition, they cover the whole service spectrum: DVB-H is the one that provides
the broadcasting transmission to mobile terminals, UMTS is the mobile
telecommunication system and it also provides a wide range of services, at last
WiMAX gives the broadband wireless connection, mobile, portable and fixed.
A Convergent Access Network can be done with any kind of access technology but
in this thesis DVB-H, UMTS and WiMAX are taken into account because they are
the most efficient technologies in comparison with their competitors. For example,
DVB-H provides a spectral efficiency higher then MediaFLO5.
In Figure 1 the main property of each technology is summarized. DVB-H is based on
the DVB-T transmission but it provides mobility with high data rate in addition,
5 MediaFLO is one of the main Broadcasting systems
Technical Overview
10
thanks to its new features that allow battery saving, increased general robustness and
support for seamless handover. It is also important to underline that DVB-H is IP-
Based.
UMTS is very flexible, thanks to the CDMA6 it supplies a negotiable bit rate, delay
and BER and it offers different QoS parameters. UMTS’s maximum achievable bit
rate is 2 Mbps, very improved compared to other mobile systems like GSM/GPRS.
WiMAX is the broadband wireless access technology that can accomplish very high
data rates, up to 10 Mbps. It is able to allocate different QoS flow classes and it is
very scalable.
In this chapter these standardized technologies are introduced just to present them in
general in order to understand what their features are. In the successive chapter, they
are analyzed to see how convergence can be achieved.
Figure 1: Technology Features
6 CDMA (Code Division Multiple Access) is a method of multiple access that does not divide up the channel by time (as in TDMA), or frequency (as in FDMA), but instead encodes data with a special code associated with each channel and uses the constructive interference properties of the special codes to perform the multiplexing.
Technical Overview
11
1.1 UMTS Release 6 (Universal Mobile Telecommunication System)
1.1.1 Introduction
3G Systems are intended to provide a global mobility with wide range of services
including telephony, paging, messaging, Internet and broadband data. International
Telecommunication Union (ITU) started the process of defining the standard for
third generation systems, referred to as International Mobile Telecommunications
2000 (IMT-2000). In Europe European Telecommunications Standards Institute
(ETSI) was responsible of UMTS standardization process. In 1998 Third Generation
Partnership Project (3GPP) was formed to continue the technical specification work.
The first UMTS Release is “R99” that was finalized in 2000, the subsequently
Releases were numbered Rel4, Rel5 and Rel6. Release 99 defines the Core Network
evolution starting from the GSM/GPRS system one, in the Release 4 control and
transport planes are separated in the Core Network. Both Release 5 and 6 change the
architecture by introducing the IP Multimedia Subsystem which provides all IP-
Based services, including voice.
UMTS Release 6 takes a radical approach to the introduction of conversational and
real time interactive multimedia services over an end-to-end IP transport provided by
an enhanced general packet radio service in the packet switched domain.
1.1.2 UMTS Services and Features
UMTS offers teleservices (like speech or SMS) and bearer services, which provide
the capability for information transfer between access points. It is possible to
negotiate and renegotiate the characteristics of a bearer service at session or
connection establishment and during ongoing session or connection. Both
connection-oriented and connectionless services are offered for Point-to-Point and
Point-to-Multipoint communication.
Bearer services have different QoS parameters for maximum transfer delay, delay
variation and bit error rate.
Technical Overview
12
The main UMTS feature that distinguishes UMTS from GSM/GPRS is the maximum
achievable bit rate. Offered data rate targets depend on the coverage area and they
are:
• 144 kbits/s satellite and rural outdoor
• 384 kbits/s urban outdoor
• 2048 kbits/s indoor and low range outdoor
UMTS network services have different QoS classes for four types of traffic:
• Conversational class (voice, video telephony, video gaming)
• Streaming class (multimedia, video on demand, webcast)
• Interactive class (web browsing, network gaming, database access)
• Background class (email, SMS, downloading)
UMTS will also have a Virtual Home Environment (VHE). It is a concept for
personal service environment portability across network boundaries and between
terminals. Personal service environment means that users are consistently presented
with the same personalized features, User Interface customization and services in
whatever network or terminal, wherever the user may be located. UMTS also has
improved network security and location based services.
In order to provide such services UMTS has to be very flexible.
UMTS Release 6 introduces new features and services thanks to the IMS Core
Network. It is allowed network sharing (i.e. multiple radio access networks sharing
common core network) and WLAN interworking (use WLAN as access network for
IMS instead of PS domain). New services are:
• MBMS (Multimedia Broadcast and Multicast Service): downstream
broadcasting and multicast support enable resource and cost efficient data
transfer to many users in parallel
• Push Service: pushing of information from network to UE
• IMS Group Management: supporting service for other services
• IMS Presence Services: user can find out presence of others and is visibility
is defined to others
• IMS Messaging: Instant Messaging interworks with Presence Service
Technical Overview
13
1.1.3 UMTS Architecture
A UMTS network consist of three interacting domain:
• Core Network (CN)
• UMTS Terrestrial Radio Access Network (UTRAN)
• User Equipment (UE)
The main function of the Core Network is to provide switching, routing and transit
for user traffic. Core Network also contains the databases and network management
functions.
The basic Core Network architecture for UMTS is based on GSM network with
GPRS. All equipment has to be modified for UMTS operation and services.
The UTRAN provides the air interface access method for User Equipment. Base
Station is referred as Node-B and control equipment for Node-B's is called Radio
Network Controller (RNC).
It is necessary for a network to know the approximate location in order to be able to
page user equipment. Here is the list of system areas from largest to smallest:
• UMTS systems (including satellite)
• Public Land Mobile Network (PLMN)
• MSC/VLR or SGSN
• Location Area
• Routing Area (PS domain)
• UTRAN Registration Area (PS domain)
• Cell
• Sub cell
In particular, the architecture of UMTS Release 6 includes the UMTS Terrestrial
Access Network (UTRAN), the GPRS domain and the Internet protocol Multimedia
Subsystem Core Network (IMS CN). This architecture is shown in Figure 2.
Technical Overview
14
Figure 2: UMTS Release 6 Architecture
1.1.3.1 Core Network
The Core Network in the earlier releases (Rel99 and Rel4) is divided in circuit
switched and packet switched domains. Some of the circuit switched elements are
Mobile services Switching Centre (MSC), Visitor location register (VLR) and
Gateway MSC. Packet switched elements are Serving GPRS Support Node (SGSN)
and Gateway GPRS Support Node (GGSN). Some network elements, like EIR, HLR,
VLR and AUC are shared by both domains. The Asynchronous Transfer Mode
(ATM) is defined for UMTS core transmission. ATM Adaptation Layer type 2
(AAL2) handles circuit switched connection and packet connection protocol AAL5
is designed for data delivery. The architecture of the Core Network may change
when new services and features are introduced. Number Portability DataBase
(NPDB) will be used to enable user to change the network while keeping their old
phone number. Gateway Location Register (GLR) may be used to optimize the
subscriber handling between network boundaries. MSC, VLR and SGSN can merge
to become a UMTS MSC.
1.1.3.1.1 CS Domain
The CS domain refers to the set of all the CN entities offering "CS type of
connection" for user traffic as well as all the entities supporting the related signaling.
Technical Overview
15
A "CS type of connection" is a connection for which dedicated network resources are
allocated at the connection establishment and released at the connection release.
The entities specific to the CS domain are: MSC, GMSC, VLR. All the other CN
entities not defined as PS domain specific entities are common to the CS and to the
PS domains.
1.1.3.1.2 PS Domain
The PS domain refers to the set of all the CN entities offering "PS type of
connection" for user traffic as well as all the entities supporting the related signaling.
A "PS type of connection" transports the user information using autonomous
concatenation of bits called packets: each packet can be routed independently from
the previous one.
The entities specific to the PS domain are the GPRS specific entities, i.e. SGSN and
GGSN. [§ 4.3]
1.1.3.1.3 IP Multimedia subsystem (IMS)
The IM subsystem comprises all CN elements for provision of IP multimedia
services comprising audio, video, text, chat, etc. and a combination of them delivered
over the PS domain. The entities related to IMS are CSCF, MGCF, MRF, etc.
[Chapter 3].
1.1.3.2 UTRAN
The Universal Terrestrial Radio Access Network does not change in the different
releases. It is a set of RNS which are composed of RNC and Node B. Every element
is linked to another by using an ATM transport protocol.
1.1.3.2.1 Radio Access
Wideband CDMA7 technology was selected to for UTRAN air interface.
UMTS WCDMA is a Direct Sequence CDMA system where user data is multiplied
with pseudo-random bits derived from WCDMA Spreading codes. In UMTS, in
addition to channelization, Codes are used for synchronization and scrambling.
7 W-CDMA is a wideband spread-spectrum 3G mobile telecommunication air interface that utilizes code division multiple access (or CDMA the general multiplexing scheme, not to be confused with CDMA the standard).
Technical Overview
16
WCDMA has two basic modes of operation: Frequency Division Duplex (FDD) and
Time Division Duplex (TDD).
1.1.3.2.2 WCDMA channels
Due to the wide range of information that the UMTS is able to transport, the protocol
architecture of the UTRA FDD is layered in many levels. Because of this, there are
different channels corresponding the different layers of the protocol stack:
• Physical channels
• Transport channels
• Logical channels
Physical channels connect the UE and the Node B, logical and transport channels
transfer information from the RNC to the UE and vice versa.
UE Node B RNC
Figure 3: Physical, Logical and Transport Channels
In Figure 4 are shown the channels involved in the air interface.
Technical Overview
17
Figure 4: Radio Interface Protocol Architecture
1.1.3.3 User Equipment
The UMTS standard does not restrict the functionality of the User Equipment in any
way. Terminals work as an air interface counter part for Node-B and have many
different types of identities.
Most of these UMTS identity types are taken directly from GSM specifications:
• International Mobile Subscriber Identity (IMSI)
• Temporary Mobile Subscriber Identity (TMSI)
• Packet Temporary Mobile Subscriber Identity (P-TMSI)
• Temporary Logical Link Identity (TLLI)
• Mobile station ISDN (MSISDN)
• International Mobile Station Equipment Identity (IMEI)
• International Mobile Station Equipment Identity and Software Number
(IMEISV)
UMTS mobile station can operate in one of three modes of operation:
• PS/CS mode of operation: The MS is attached to both the PS domain and CS
domain, and the MS is capable of simultaneously operating PS services and
CS services.
Technical Overview
18
• PS mode of operation: The MS is attached to the PS domain only and may
only operate services of the PS domain. However, this does not prevent CS-
like services to be offered over the PS domain (like VoIP).
• CS mode of operation: The MS is attached to the CS domain only and may
only operate services of the CS domain.
UMTS IC card has same physical characteristics as GSM SIM card. It has several
functions:
• Support of one User Service Identity Module (USIM) application (optionally
more that one)
• Support of one or more user profile on the USIM
• Update USIM specific information over the air
• Security functions
• User authentication
• Optional inclusion of payment methods
• Optional secure downloading of new applications
1.1.4 General Protocol Architecture
In Figure 5 a simplified UMTS Architecture with the external reference points and
interfaces to the UTRAN is shown.
Figure 5: UMTS Architecture
Two interfaces are illustrated: Uu and Iu interfaces.
Technical Overview
19
The protocols over these interfaces are divided into two structures:
• User plane protocols: These are the protocols implementing the actual radio
access bearer service, i.e. carrying user data through the access stratum. The
radio access bearer service is offered from SAP (Service Access Point) to
SAP by the Access Stratum.
• Control plane protocols: These are the protocols for controlling the radio
access bearers and the connection between the UE and the network from
different aspects (including requesting the service, controlling different
transmission resources, handover & streamlining etc.). Also a mechanism for
transparent transfer of NAS messages is included.
1.2 Mobile WiMAX (Wireless Interoperability for Microwave
Access) [802.16e]
1.2.1 Introduction
In December 2005 the IEEE ratified the 802.16e amendment to the 802.16 standard
with the target of providing mobility to WiMAX (Wireless Interoperability for
Microwave Access) technology. WiMAX is a broadband wireless solution that will
enable the convergence of mobile and fixed broadband network. In order to
distinguish the new standard version from the old one, the IEEE 802.16-2004 is
called as “Fixed WiMAX” and the IEEE 802.16e as “Mobile WiMAX”. The Mobile
WiMAX Air Interface uses OFDMA (Orthogonal Frequency Division Multiple
Access) to improve the efficiency in a NLOS (No Line Of Sight) multipath context.
SOFDMA (Scalable OFDMA) is introduced to support scalable channel bandwidths
from 1.25 to 20 MHz. The IEEE 802.16 standard addresses the air interface
specification, to define end-to-end system solutions for a Mobile WiMAX network
the WiMAX forum Network Working Group (NWG) is developing a higher-level
networking specifications for Mobile WiMAX system solution.
Technical Overview
20
Some of the characteristics provides by the Mobile WiMAX are:
• High Data Rates: Mobile WiMAX in a 10 MHz channel will support peak
downlink (DL) data rates up to 63 Mbps per sector and peak uplink (UL) data
rates up to 28 Mbps per sector. This performance can be achieved thanks to:
- MIMO antenna technology
- Flexible sub-channelization schemes
- Advanced Coding and Modulation
• Quality of Service: IEEE 802.16 MAC architecture defining Service Flow,
which can map to DiffServ code points or MPLS (Multiprotocol Label
Switching) flow labels, enables end-to-end IP based QoS (Quality of
service).
• Scalability: Mobile WiMAX technology is thought to be scalable in order to
work in different channelizations from 1.25 to 20 MHz so it is conformable to
the different worldwide spectrum resource allocations and allows countries
with different economies to realize the configuration shaped to their specific
geographic needs such as providing affordable internet access in rural setting
versus high capacity in metro and suburban areas.
• Security: To provide security Mobile WiMAX to exploit:
- EAP-based authentication
- AES-CCM-based authenticated encryption
- CMAC and HMAC based control message protection schemes
Moreover it supports a diverse set of user credentials such as the following:
- SIM/USIM cards
- Smart Cards
- Digital Certificates
- Username/Password schemes
• Mobility: To provide real-time communications, such as VoIP, Mobile
WiMAX adopts optimized handover schemes (latencies less than 50
milliseconds). Security is maintained during handover thanks to flexible key
management.
Technical Overview
21
1.2.2 Features
1.2.2.1 Physical Layer
1.2.2.1.1 OFDMA
Orthogonal Frequency Division Multiple Access (OFDMA) is a multiple-access
multiplexing scheme for OFDM systems. It works by assigning a subset of
subcarriers to individual users. It provides multiplexing operation of data streams
from multiple users onto the downlink sub-channels and uplink multiple accesses by
means of uplink sub-channel.
OFDMA features are summarized in the following:
• OFDMA is the “multi-user” version of OFDM
• Functions essentially as OFDM-FDMA
• Each OFDMA user transmits symbols using subcarriers that remain
orthogonal to those of other users
• More than one subcarrier can be assigned to one user to support high rate
applications
• Allows simultaneous transmission from several users and so better spectral
efficiency
As the Figure 6 shows, the OFDMA symbol structure consists of three types of sub-
curriers:
• Data sub-carriers for data transmission
• Pilot sub-carriers for estimation and synchronization purposes
• Null sub-carriers for no transmission, used for guard bands and DC carriers
Technical Overview
22
Figure 6: OFDMA Sub-Carriers Structure
A sub-channel is a subset of Active (data end pilot) sub-carriers. Sub-channelization
is made both in DL and UL. The minimum frequency-time resource unit of sub-
channelization is one slot, which is equal to 48 data tones (sub-carriers).
1.2.2.1.2 Scalable OFDMA
The key difference between the Fixed and Mobile WiMAX standards is the more
efficient S-OFDMA modulation scheme. S-OFDMA (Scalable OFDM Access) can
assign a subset of sub-carriers to individual users. By using different sub-carriers
multiple people can connect at the same time on the same frequency without
interference. The number of sub-carriers can adjust dynamically for different
conditions. Unlike many other OFDM-based systems such as WLAN, the 802.16
standard supports variable bandwidth sizes, this wide range of bandwidths is thought
to flexibly address the need for various spectrum allocation and usage model
requirements. A scalable physical layer enables standard-based solutions to deliver
optimum performance in channel bandwidths ranging from 1.25 MHz to 20 MHz
with fixed sub-carrier spacing. The S-OFDMA’s architecture is based on a scalable
sub-channelization structure with variable Fast Fourier Transform (FFT) sizes
according to the channel bandwidth. The scalability is supported by adjusting the
FFT size while fixing the sub-carrier frequency spacing at 10.94 kHz, this is made to
meet the optimal operation point balancing protection against multipath and Doppler
shift. Since the resource unit sub-carrier bandwidth and symbol duration is fixed, the
impact to higher layers is minimal when scaling the bandwidth. The SOFDMA
parameters are listed in Table 1.
Technical Overview
23
Table 1: OFDMA Scalability Parameters
1.2.2.1.3 TDD Frame Structure
The 802.16e PYH supports TDD, FDD, and Half-Duplex FDD operation but the
initial release of Mobile certification profile will only include TDD. Some of the
reasons for preferring TDD are the following:
• It has a strong advantage in the case where the asymmetry of the uplink and
downlink data is variable. As the amount of uplink data increases, more
bandwidth can be allocated to that and as it shrinks it can be taken away so it
enables adjustments of the downlink/uplink ratio while with FDD both
downlink and uplink always have a fixed and generically equal DL and UL.
• TDD assures channel reciprocity for better support of link adaptation, MIMO
and other closed loop advanced antenna technologies.
• Unlike FDD, which requires a pair of channels, TDD only requires a single
channel for both downlink and uplink providing greater flexibility for
adaptation to varied global spectrum allocations.
• Transceiver designs for TDD implementations are less complex and therefore
less expensive.
The frames are divided into DL and UL sub-frames. To prevent DL and UL
transmission collision the sub-frames are separated by Transmit/Receive and
Receive/Transmit Gaps. The Figure 7 shows the OFDM frame structure for TDD
implementation.
Technical Overview
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The fields are:
• Preamble: The preamble, used for synchronization, is the first OFDM symbol
of the frame.
• Frame Control Head (FCH): The FCH follows the preamble. It provides the
frame configuration information such as MAP message length and coding
scheme and usable sub-channels.
• DL-MAP and UL-MAP: The DL-MAP and UL-MAP provide sub-channel
allocation and other control information for the DL and UL sub-frames
respectively.
• UL Ranging: The UL ranging sub-channel is allocated for mobile stations
(MS) to perform closed-loop time, frequency, and power adjustment as well
as bandwidth requests.
• UL CQICH: The UL CQICH channel is allocated for the MS to feedback
channel state information.
• UL ACK: The UL ACK is allocated for the MS to feedback DL HARQ
acknowledgement.
Figure 7: OFDMA Frame Structure
Technical Overview
25
1.2.2.1.4 Other Advanced PHY Layer Features
To best meet the coverage and capacity in mobile applications the 802.16e introduces
the following features:
• Adaptive modulation and coding (AMC): The core idea of AMC is to
dynamically change the Modulation and Coding Scheme (MCS) in
subsequent frames with the objective of adapting the overall spectral
efficiency of the channel condition. The decision concerning selecting the
appropriate MCS is performed at the receiver side according to the observed
channel condition. Then the information is fed back to the transmitter in each
frame.
• Hybrid Automatic Request (HARQ): it is a variation of the ARQ error control
method. When the coded data block is received, the receiver first decodes the
error-correction code. If the channel quality is good enough, all transmission
errors should be ok, and the receiver can obtain the correct data block. If the
channel quality is bad and some transmission errors can not be corrected, the
receiver will detect this situation using the error-correction code and then the
received coded data block is discarded and a retransmission is requested by
the receiver, similar to ARQ.
• Fast Channel Feedback (CQICH).
Table 2: Code and Modulation options
As can be seen in Table 2, WiMAX supports different digital modulation schemes, in
the DL QPSK, 16QAM and 64QAM are mandatory while in the UL 64QAM is
optional. It can support Convolutional Code (CC) and Convolutional Turbo Code
(CTC) with variable code rate and repetition coding. Optionally, it can support Block
Turbo Code and Low Density Parity Check Code (LDPC).
Combining the various modulation and code rates WiMAX can provide a variety of
data rates that are adaptable to different needs.
Technical Overview
26
1.2.2.2 MAC Layer
The WiMAX standard wants to reach the goal of providing a set of heterogeneous
broadband services like voice, data and video. To achieve this target it may be able to
supporting simultaneously on the same channel bursty data traffic with high peak
rate demand, streaming video and latency sensitive voice traffic. A MAC scheduler
has the capability of changing dynamically the throughput by varying the resources
allocated to one terminal from a single time slot to an entire frame. The allocation
information is transmitted by the MAP messages at the beginning of each frame. In
this way, the scheduler can adapt the resource allocation frame-by-frame.
1.2.2.2.1 Quality of Service
To provide the different types of services, Mobile WiMAX needs to support different
QoS requirements. Some of the characteristics that enable it to achieve this
peculiarity are:
• Fast air link
• Symmetric UL/DL capacity
• Fine resource granularity
• Flexible resource allocation mechanism
To manage the QoS the Mobile WiMAX MAC layer uses a service flow that is a
unidirectional flow of packets that contain the particular set of QoS parameters.
First of all the base station and the user terminal establish a unidirectional logic link
called a connection between the peer MACs. The outbound MAC then associates
packets crossing the MAC interface into a service flow to be delivered over the
connection. The QoS parameters associated with the service flow define the
transmission ordering scheduling on the air interface.
The service flow parameters can be dynamically managed through MAC messages to
accommodate the dynamic service demand. The service flow-based QoS mechanism
applies to both DL and UL and provides improved QoS in both directions. Mobile
WiMAX supports a wide range of data services and applications with varied QoS
requirements. These are summarized in Table 3.
Technical Overview
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Table 3: Quality of service options
1.2.2.2.2 MAC Scheduling Service
The goal of scheduling and link adaptation is to provide the desired QoS treatment to
the traffic traversing the airlink, while optimally utilizing the resources of the airlink.
Some of the most important scheduler’s characteristics are the following:
• It is located at each base station to enable rapid response to traffic
requirements and channel condition.
• It can correctly determine the packet’s transmission ordering, thanks to the
data packets association of the service flow, with well defined QoS
parameters in the MAC layer.
• It can choose the appropriate coding and modulation for each allocation
taking advantage of the CQICH channel fast feedback.
The scheduling is provided for both DL and UL traffic. In order for the MAC
scheduler to make an efficient resource allocation and provide the desired QoS in the
UL, the UL must feedback accurate and timely information reaching the traffic
condition and QoS requirements. The UL service flow defines the feedback
mechanism for each uplink connection. The MAC scheduler manages the data
Technical Overview
28
transport on a connection-by-connection basis. Each connection is associated with a
single data service with a set of QoS parameters that quantify the aspects of its
behavior. With the ability to dynamically allocate resources in both DL and UL, the
scheduler can provide superior QoS for both DL and UL traffic. Particularly with
uplink scheduling, the uplink resources are more efficiently allocated, performance is
more predictable, and QoS is better enforced.
1.2.2.2.3 Mobility Management
To enable power-efficient MS operation and save battery Mobile WiMAX works in
two way of working:
• Sleep Mode: When working in Sleep Mode state the MS conducts pre-
negotiated periods of absence from the Serving Base Station air interface.
• Idle Mode: When working in Idle Mode state, the MS is periodically
available for DL broadcast traffic messaging without registering at a specific
base station.
To manage the handoff Mobile WiMAX supports three different methods:
• Hard Handoff (HHO): It is the only mandatory. With hard handoff, the link
to the prior base station is terminated before or as the user is transferred to the
new cell’s base station. That is to say that the mobile is linked to no more
than one base station at a given time. Initiation of the handoff may begin
when the signal strength received on the mobile from next base station is
greater than that of the prior base station.
• Fast Base Station Switching (FBSS): With FBSS the MS and the BS have a
list called Active set in which they maintain the BSs involved in FBSS with
the MS. The MS continuously monitors the BS in the Active Set among
which defines the Anchor BS. MS only communicates with the Anchor BS
for uplink and downlink messages including management and traffic
connections. The Anchor BS switching is performed without of explicit HO
signaling messages, only communicating the BS signal strength via the
Channel Quality Indicator (CQI). A FBSS handover begins with a decision
by the BS to receive or transmit data from the Anchor BS that may change
Technical Overview
29
within the Active set. The MS scans the neighbor BSs and selects those that
are suitable to be included in the active set. The MS reports the selected BSs
and the active set update procedure is performed by the BS and MS. The MS
continuously monitors the signal strength of the BSs that are in the active set
and selects one BS from the set to be the Anchor BS. The MS reports the
selected Anchor BS on CQICH or MS initiated HO request message. An
important requirement of FBSS is that the data is simultaneously transmitted
to all members of an active set of BSs that are able to serve the MS.
• Macro Diversity Handover (MDHO): Also in this case the BS has a list called
Active set in which they maintain the BSs involved in MDHO with the MS.
The communications either in uplink or in downlink. A MDHO begins when
a MS decides to transmit or receive unicast messages and traffic from
multiple BSs at the same time. For downlink MDHO, two or more BSs
provide synchronized transmission of MS downlink data such that diversity
combining is performed at the MS. For uplink MDHO, the transmission from
a MS is received by multiple BSs where selection diversity of the information
received is performed. Selection diversity is the simplest diversity approach.
Using multiple antennas with overlapping coverage, this approach selects the
antenna with the highest received signal power, mitigating fading.
The sleep Mode provides battery economizing and seamless handoff enables
switching from one base station to another without interrupting the connection. In
this way it tries to solve two of the most important barriers to the mobility.
1.2.2.2.4 Security
Mobile WiMAX supports best in class security features by adopting the best
technologies available today. Support exists for mutual device/user authentication,
flexible key management protocol, strong traffic encryption, control and
management plane message protection and security protocol optimization for fast
handovers. The usage aspects of the security features are:
• Key Management Protocol: Privacy and Key Management Protocol Version 2
(PKMv2) is the basis of Mobile WiMAX security as defined in 802.16e. This
protocol manages the MAC security using PKM-REQ/RSP messages. PKM
• delivery Radius/Diameter messaging to selected CSN AAA
• Mobility Tunneling establishment and management with BSs
• Session/mobility management (client)
• QoS and Policy Enforcement
• Foreign Agent (FA) (with Proxy MIP)
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• Routing to selected CSN
Finally, the interface between the Base Station and the ASN GW is standardized and
is called R6. It consists of a set of control and bearer plane protocols for
communication between the BS and the ASN GW. The bearer plane consists of intra-
ASN data path or inter-ASN tunnels between the BS and ASN GW. The control
plane includes protocols for IP tunnel management (establish, modify, and release) in
accordance with the MS mobility events. R6 may also serve as a conduit for
exchange of MAC states information between neighboring BSs.
2.3.3 Functions
Some general tenets have guided the development of Mobile WiMAX Network
Architecture include the following features:
• Provision of logical separation between such procedures and IP addressing,
routing and connectivity management procedures and protocols to enable use
of the access architecture primitives in standalone and interworking
deployment scenarios,
• Support for sharing of ASN(s) of a Network Access Provider (NAP) among
multiple NSPs,
• Support of a single NSP providing service over multiple ASN(s) – managed
by one or more NAPs,
• Support for the discovery and selection of accessible NSPs by an MS or SS
• Support of NAPs that employ one or more ASN topologies,
• Support of access to incumbent operator services through internetworking
functions as needed,
• Specification of open and well-defined reference points between various
groups of network functional entities (within an ASN, between ASNs,
between an ASN and a CSN, and between CSNs), and in particular between
an MS, ASN and CSN to enable multi-vendor interoperability
• Support for evolution paths between the various usage models subject to
reasonable technical assumptions and constraints,
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• Enabling different vendor implementations based on different combinations
of functional entities on physical network entities, as long as these
implementations comply with the normative protocols and procedures across
applicable reference points, as defined in the network specifications
• Support for the simplest scenario of a single operator deploying an ASN
together with a limited set of CSN functions, so that the operator can offer
basic Internet access service without consideration for roaming or
interworking.
The WIMAX architecture also allows both IP and Ethernet services, in a standard
mobile IP compliant network. The flexibility and interoperability supported by the
WiMAX network provides operators with a multi-vendor low cost implementation of
a WiMAX network even with a mixed deployment of distributed and centralized
ASN’s in the network.
So, the major functions performed by the WiMAX Access Network can be
summarized as:
• Support for Service and Applications: The WiMAX Architecture includes the
support for:
- Voice, multimedia services and other mandated regulatory services
such as emergency services and lawful interception
- Access to a variety of independent Application Service Provider (ASP)
networks in an agnostic manner
- Mobile telephony communications using VoIP
- Support interfacing with various interworking and media gateways
permitting delivery of incumbent/inherit services translated over IP (for
example, SMS over IP, MMS, WAP) to WiMAX access networks
- Support delivery of IP Broadcast and Multicast services over WiMAX
access networks
• Security: The end-to-end WiMAX Network Architecture is based on a
security framework that is agnostic to the operator type and ASN topology
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and applies consistently internetworking deployment models and usage
scenarios. In particular there is support for:
- Strong mutual device authentication between an MS and the WiMAX
network, based on the IEEE 802.16 security framework,
- All commonly deployed authentication mechanisms and authentication
in home and visited operator network scenarios based on a consistent
and extensible authentication framework,
- Data integrity, replay protection, confidentiality and non-repudiation
using applicable key lengths,
- Use of MS initiated/terminated security mechanisms such as Virtual
Private Networks (VPNs),
- Standard secure IP address management mechanisms between the
MS/SS and its home or visited NSP.
• Mobility and Handover: The end-to-end WiMAX Network Architecture has
extensive capability to support mobility and handovers. It will:
- Include vertical or inter-technology handovers— e.g., to Wi-Fi, 3GPP,
3GPP2, DSL, or MSO – when such capability is enabled in multi-mode
MS
- Support IPv4 or IPv6 based mobility management. Within this
framework, and as applicable, the architecture will accommodate MS
with multiple IP addresses and simultaneous IPv4 and IPv6
connections,
- Support roaming between NSPs,
- Utilize mechanisms to support seamless handovers at up to vehicular
speeds— satisfying well defined.
Some of the additional capabilities in support of mobility include the support of:
- Dynamic and static home address configurations,
- Dynamic assignment of the Home Agent in the service provider
network as a form of route optimization, as well as in the home IP
network as a form of load balancing,
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- Dynamic assignment of the Home Agent based on policies.
• Scalability, Extensibility, Coverage and Operator Selection: The end-to-end
WiMAX Network Architecture has useful support for scalable, extensible
operation and flexibility in operator selection. In particular, it will:
- Enable a user to manually or automatically select from available NAPs
and NSPs,
- Enable ASN and CSN system designs that easily scale upward and
downward – in terms of coverage, range or capacity
- Accommodate a variety of ASN topologies – including hub-and-spoke,
hierarchical, and/or multi-hop interconnects
- Accommodate a variety of backhaul links, both wireline and wireless
with different latency and throughput characteristics
- Support incremental infrastructure deployment
- Support phased introduction of IP services that in turn scale with
increasing number of active users and concurrent IP services per user
- Support the integration of base stations of varying coverage and
capacity - for example, pico, micro, and macro base stations
- Support flexible decomposition and integration of ASN functions in
ASN network deployments in order to enable use of load balancing
schemes for efficient use of radio spectrum and network resources
Additional features pertaining to manageability and performance of WiMAX
Network Architecture include:
- Support a variety of online and offline client provisioning, enrollment,
and management schemes based on open, broadly deployable, IP-based,
industry standards
- Accommodation of Over-The-Air (OTA) services for MS terminal
provisioning and software upgrades
- Accommodation of use of header compression/suppression and/or
payload compression for efficient use of the WiMAX radio resources.
• Multi-Vendor Interoperability: Another key aspect of the WiMAX Network
Architecture is the support of interoperability between equipment from
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different manufacturers within an ASN and across ASNs. Such
interoperability will include interoperability between:
- BS and backhaul equipment within an ASN,
- Various ASN elements (possibly from different vendors) and CSN,
with minimal or no degradation in functionality or capability of the
ASN.
The IEEE 802.16 standard defines multiple convergence sub-layers. The
WiMAX Network Architecture framework supports a variety of CS types
including: Ethernet CS, IPv4 CS and IPv6 CS.
• Quality of Service: The WiMAX Network Architecture has provisions for
support of QoS mechanisms. In particular, it enables flexible support of
simultaneous use of a diverse set of IP services. The architecture supports:
- Differentiated levels of QoS: coarse-grained (per user/terminal) and/or
fine-grained (per service flow per user/terminal),
- Admission control,
- Bandwidth management
- Implementation of policies as defined by various operators for QoS-
based on their SLAs (including policy enforcement per user and user
group as well as factors such as location, time of day, etc.). Extensive
use is made of standard IETF mechanisms for managing policy
definition and policy enforcement between operators. The flexible
WiMAX network specifications allows different implementations of
Access Service Network (ASN) configurations namely ASN profiles,
including both distributed/collapsed as well as centralized architectures.
Furthermore, the WiMAX forum is developing an interoperability
framework in which intra-ASN and inter-ASN interoperability across
different vendors is ensured.
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2.4 Access Networks Analysis: DVB-H
DVB-H is principally a transmission system allowing reception of broadcast
information on single antenna handheld mobile devices. It provides an efficient way
of carrying multimedia services over digital terrestrial broadcasting networks to
handheld terminals (DVB-H). So, the DVB-H network can not be considered as an
Access Network in the way that is usually meant. DVB-H is a really delivering
network. No channel access method is used to share a communications channel or
physical communications medium between multiple users.
According to the scope of this thesis, in this section the whole DVB-H network is
considered as the Access Network, always keeping in mind that the communication
is unidirectional (only in downlink).
Because DVB-H is a broadcasting technology, the functionalities performed by
DVB-H devices are regarding exclusively on the signal transmission and on the
carousel information synchronization. In addition to these (based on DVB-T
transmission), a DVB-H network provides some functionalities related to the
mobility. It is very important to underline that these functionalities just allow the
mobility but they do not manage it. The Mobility Management is not possible
because there is not a return channel and, most of all, User Localization and
Identification are not broadcast transmission requirements.
There are three possible architectures for a DVB-H network: DVB-H Standalone and
Shared network DVB-H/T by multiplexing or hierarchical modulation.
In the first one the DVB-H system has a dedicated multiplexer and the elements
composing this kind of architecture are shown in Figure 25.
Figure 25: Headend Construction for Dedicated Multiplex
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The IP Encapsulator is assumed to take responsibility for generating MPE sections
from incoming IP datagrams, as well as to add the required PSI/SI data. Also, MPE-
FEC Frames, when used, are generated in the IP Encapsulator. The output stream of
the IP Encapsulator is composed of MPEG-2 transport packets.
Instead, a shared network is a network of DVB-T transmitters is serving both DVB-
H and DVB-T terminals. The existing DVB-T network has to be, however, designed
for portable indoor reception so that it can provide high enough field strength for the
hand-held terminals inside the wanted service area. The only required modification
in the transmitters is an update so that the DVB-H signaling bits and Cell ID bits are
added to the TPS information of the transmitter.
The actual sharing is done at the multiplex level. DVB-H offers a full flexibility to
select the wanted portion of the multiplex to DVB-H services.
The key DVB-H component in the network is the IP-Encapsulator, where the MPE
of IP data, time slicing, and MPE-FEC are implemented.
Another possibility to share the network is to use the DVB-T hierarchical
modulation. In that case the MPEG-2 and DVB-H IP services will have their own
independent TS inputs in the DVB-T transmitters. The DVB-H services would use
the high-priority part, which would offer increased robustness over the low-priority
input, which is then used for the normal digital TV services.
In Figure 26 a shared network by multiplexing is illustrated and its components are
highlighted.
Figure 26: Headend Construction for Mixed Multiplex
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In order to introduce DVB-H services into an existing DVB-T network using
multiplexing, the following steps are required, in any order:
• Timeslice-capable IP Encapsulators are connected to the last-hop multiplexer,
which is ideally located in each coverage area (MFN or SFN), and a fixed
amount of bitrate is reserved for DVB-H services
• The last-hop-multiplexers are upgraded for better DVB-H support (smoothing
of reinserted PSI/SI tables, management of INT table)
• If necessary, improve the coverage of the DVB-T network (more cells,
upgrade of single-transmitter cells to SFN-areas, addition of radio frequency
repeaters)
The possibility to have global and local IP services is the same as in the case of a
dedicated DVB-H network, and the properties of the IP backbone network are the
same. The number of last-hop-multiplexers determines the granularity of service
coverage areas. This is why these multiplexers (and with them the IP encapsulators)
are ideally located locally in each coverage area (MFN or SFN).
For network-wide distribution of IP streams, there is now an additional option: the IP
streams can be encapsulated centrally, and distributed to the sites within a centrally
produced transport stream, which is then re-multiplexed by the last-hop-multiplexer
to produce the final transport stream that is broadcast.
Whether or not this is a good option depends on many factors. IP networks can be
expected to be cheaper, more scalable, and simpler to manage than transport stream
distribution networks. But if there is capacity available in an existing transport
stream distribution network, why not use it, especially if there is no IP network
available.
In this case, the centrally encapsulated IP streams should not be timesliced, but
simply embedded in the transport stream using normal multi-protocol encapsulation.
The local IP Encapsulator can then decapsulate these IP streams, and timeslice them
as any other IP stream that is received over the IP backbone network.
It would be technically possible and allowed by the standard to timeslice also the
centrally encapsulated IP streams, and to add locally another set of timesliced IP
streams. However, this would not be optimal from power-saving perspective.
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As timeslicing is a technology for reduction of power consumption of a mobile
handheld terminal, there is no need for central timeslicing.
A shared network by hierarchical modulation is quite different as it is shown in
Figure 27.
Figure 27: Headend Construction for Hierarchical Tranmission
In order to introduce DVB-H services into an existing DVB-T network using
hierarchical modulation, the following steps are required:
1. If necessary, replace modulators with models that support hierarchical mode and
put a 2nd synchronized transport stream distribution system in place for
modulators in SFN-areas
2. Timeslice-capable IP Encapsulators are connected to the modulators, or, in case
of SFN-areas, to the SFN timestamp inserter
If necessary, improve the coverage of the DVB-T network (more cells, upgrade of
single-transmitter cells to SFN-areas, addition of radio frequency repeaters).
From DVB-H perspective, this case is identical to having a dedicated DVB-H
network, so all the comments on how to construct an IP backbone network and how
to mix global and local IP streams are the same.
Hence, the main elements of a DVB-H network are the IP Encapsulator, the
Multiplexer and the DVB-H Modulator.
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2.4.1 IP Encapsulator
The IP Encapsulator puts the IP packets into TS packets using the MPE protocol. It is
composed by the Time-Slicing Module and MPE-FEC Module.
The Time-Slicing Module controls the receiver/transmitter to decode the requested
service and shut off during the other service bits. It aims to reduce power
consumption while also enabling a smooth and seamless frequency handover. The
MPE-FEC module offers, in addition to the error correction in the physical layer
transmission, a complementary FEC function that allows the receiver/transmitter to
cope with particularly difficult reception situations. As we said before, DVB has
introduced multiprotocol encapsulation (MPE) for encoding OSI-model layer 3
(Network Layer) datagrams into TS packets. Each IP datagram is encoded into a
single MPE section. A Single Elementary Stream may contain multiple MPE section
streams. The IPDC DVB-H Receiver may differentiate MPE encoded IP streams by
checking the IP source and/or destination address in the IP datagram carried within
an MPE section. In such case, the Receiver does not differentiate MPE section
streams, but is directly filtering IP streams. For such a Receiver, there is no need to
differentiate MPE section streams within an Elementary Stream.
2.4.2 Multiplexer
A DVB network consists of one or more Transport Streams (TS) each carrying a
multiplex and being transmitted by one or more DVB signals.
A MUX multiplexes a set of DVB services together, and then these services are
carried over a Transport Stream. A TS carries exactly one multiplex (set of services).
If one multiplex is transmitted on two different radio signals (i.e. DVB signals)
within a DVB network, the DVB signals carry the same TS. However, if the DVB
signals belong to different DVB networks, the TSs are different. In both cases, the set
of DVB signals, PSI information and multiplex identifiers are identical. However, in
later case, SI information (particularly the information about the actual DVB
network) is different. In such a case, only one multiplex occurs, even though the
information was carried on two different TSs.
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Therefore, a multiplex is a set of DVB services, while a TS is a bitstream carrying a
multiplex and related PSI/SI information. A multiplex may be delivered on multiple
DVB networks, while a TS belongs to exactly one DVB network.
Within a DVB network, a TS may be carried on multiple DVB signals. A DVB
signal using non-hierarchical modulation carries one TS, while a DVB signal using
hierarchical modulation carries two TSs.
A DVB service is a sequence of program events, each of which groups together a set
of components, each carried in its own Elementary Stream.
2.4.3 Modulator
The Modulator works on the physical layer. It performs the modulation of the signal
using OFDM symbols. In compliance with the DVB-T, the DVB-H Modulator can
work in 2k and 8k mode. Furthermore it can operate in the 4k mode. The additional
4k transmission mode is a scaled set of the parameters defined for the 2k and 8k
modes. In order to further improve robustness of the DVB-T 2k and 4k modes in a
mobile environment and impulse noise reception conditions, an in-depth symbol
interleaver is also standardized. In the Modulator, the TPS-bit signaling provides
robust multiplex level signaling capability to the DVB-T transmission system.
2.4.4 Features
The main features of a DVB-H system are summarized in Figure 28.
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Figure 28: Main Features of a DVB-H System
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2.5 Access Networks Comparison
After studying the three access networks considered in the previous paragraphs, this
section will be focused on the different functionalities of each element, within each
network, in order to find similarities and differences between them. Finding common
functionalities of the different Access Networks devices is needed to reach a
Convergent Access Network between different architectures, in particular the UMTS
Release 6, Mobile WiMAX and DVB-H ones.
It is important to underline that DVB-H is a broadcast technology, so it does not
really have an access network. Because the transmission is not bidirectional and there
is not the need of knowing who and where the users are, it is more difficult to find
similarities with the other two technologies. Hence, the parallelism is less obvious.
Figure 29: DVB-H “Access Network”
Figure 29 shows which are the components of a DVB-H Access Network, described
in the previous paragraph.
On the other side, UMTS and WiMAX are point-multipoint technologies. They can
be matched easier because of their comparable features, although they work in quite
different ways. For example, they use different multiple access schemes, modulation
modes, channel management, etc...
The UMTS and WiMAX Access Network are respectively illustrated in Figure 30
and in Figure 31.
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Figure 30: UMTS Access Network
As these pictures show, I have supposed an IP Core Network not specified yet in this
section, in order to analyze these three Access Networks and to study their
functionalities comparing their devices. The Core Network will be introduced in the
next chapter.
Figure 31: WiMAX Access Network
For the reasons listed before, the result of the Access Network Analysis is that it is
not possible to reach a physical convergence for the access networks. In fact, kinds
of features like frequency, bandwidth, modulation mode, channel management,
transport protocol, policy rules, etc are peculiar to each technology.
Thus, I have looked for a unique architectural block model, that although every
technology has its own specific features, it simplifies their analogous functionalities.
This effort is summarized and shown in Table 8.
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Table 8: Devices Functionality Comparison
The Table 8 shows that the DVB-H Modulator, the UMTS Node B and the WiMAX
Base Station play similar roles, as does the DVB-H Encapsulator, the UMTS RNC
and the WiMAX ASN-GW. However, it is evident that this association is forced
because we can see that some functions are performed by the RNC and the Base
Station, relative to the UMTS and WiMAX.
Some of the functionalities needed for the comparison are not performed by any
DVB-H device. The reason for this is that DVB-H is broadcast, so it does not provide
all the functions related to the Location and Radio Resource Management, Channel
Allocation, Admission Control, Ciphering, Scheduling and QoS Management.
Considering that each technology (DVB-H, UMTS and WiMAX) has a mobile user
terminal, the user mobility is always possible but the Mobility Management is
supported only by the UMTS and WiMAX.
From this study results also that the DVB-H MUX is not involved in the comparison
because it does not perform any functionality related to the signal processing,
furthermore its role depends on the DVB-H network architecture used.
Let’s now consider each function in detail. The Handover is a function allowed in the
DVB-H thanks to the time-slicing performed inside the Encapsulator, while in the
UMTS and WiMAX handover is based on radio measurements performed by Node B
and the Base Station together with UE. On the other side, the Handover Control is
managed by the RNC and the ASN-GW. It is the same situation concerning the
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Power Control, in fact in the DVB-H there is not a dynamic power management
because in transmission the power is determined in advance. However, the Time-
Slicing Module in the DVB-H Modulator allows power saving in the UE
(requirement needed for mobile terminals). In the UMTS (WCDMA system) the
Power Control plays a fundamental role because the mobile terminals work at the
same frequency and time inside a cell8. So, the power in both downlink and uplink
needs to be the minimum to ensure the Signal-to-Ratio (SIR) required by the service.
During a communication, UE and UTRAN exchange power control messages.
The Air Interface is distinguished for each technology [Chapter 1]. Mainly the DVB-
H Air Interface works only in transmission while the UMTS and WiMAX ones can
act also as receivers.
The Modulation/Demodulation are functions performed by the DVB-H Modulator,
UMTS Node B and WiMAX Base Station but the modulation mode used changes. In
the DVB-H system the most usable modulation scheme for a mobile and portable
reception is 16-QAM with a code rate of ½ or 2/3 (requiring a moderate C/N)9. This
modulation scheme is only in DL (downlink), in fact the DVB-H Modulator is able
to perform only the Modulation while the Demodulation is a own function of the
User Terminal (DVB-H Receiver). On the contrary, WiMAX and UMTS support
modulation schemes both for DL (Modulation) and UL (Demodulation). WiMAX
provides different modulation: QPSK, 16-QAM and 64-QAM (mandatory in DL and
optional in UL) with 1/2, 2/3, 3/4 and 5/6 convolutional code or convolutional turbo
code. The modulation used by the UMTS is QPSK and spreading and scrambling
codes are applied.
Other kinds of functions like Ciphering, Scheduling, Channel Allocation and so on
are performed only by UMTS and WiMAX, because they are bidirectional point-
multipoint technologies. Anyway, these functions are performed by UMTS and
WiMAX in different ways [Chapter 1].
8 The near-far problem makes the Power Control in UMTS a critical issue 9 Providing enough capacity to meet commercial requirements, the available constellations for a DVB-H system are QPSK, 16-QAM and eventually, although not recommended, 64-QAM. The FEC code rate can be 1/2, 2/3, 3/4, 5/6 and 7/8.
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2.6 A UNIQUE ARCHITECTURAL BLOCK MODEL FOR
ACCESS NETWORK ANALYSIS AND COMPARISON
Starting from the considerations made in the previous paragraph I have built a model
of the access network. Table 9 proves that the Access Networks can be modeled as a
unique logical access network, but the physical convergence is unachievable.
Table 9: Logical Access Network
As different blocks could be joined into one physical device or a physical device
could be split into several functional blocks, a one-to-one correspondence with the
real architectures is not always possible.
Figure 32: Architectural Block Model
Figure 32 shows the unique architectural block model that I have proposed in order
to represent the access network. It is composed of three main functional blocks
which are the Air Interface Control Module, Enforcement Module and
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Decision&Control Module. Each function of DVB-H, WiMAX and UMTS Access
Network elements can be mapped into these blocks. As said before, some elements
are split up and others are grouped together.
This block model could be seen as a tree where the Decision&Control Module
controls more Enforcement Modules and each Enforcement Module is responsible of
many Air Interfaces Control Modules.
The logical blocks that I have identified are explained in the following paragraphs.
2.6.1 Air Interface Control Module
This module is the access network end-point and it manages the physical channel
with the user. It is related to just one cell.
Some of its most important functions are:
• Handover
• Air Interface Transmission/Reception
• Power Control
• Modulation/Demodulation
The devices that compose the Air Interface Control Module are the Modulator for the
DVB-H, the Node B for the UMTS and the Base Station for the WiMAX.
2.6.2 Enforcement Module
The Enforcement Module is responsible for the resource allocation and distribution.
It mainly enforces functions controlled by the Decision&Control Module.
Some of the major functions are:
• Radio Resource Management Update
• Active Set Update
• Ciphering
• Scheduling
• Channel Allocation
This module does not hold any DVB-H device because the functions it performs are
not needed for the broadcasting transmission. This module is the one that makes the
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parallelism between UMTS and WiMAX devices less clear, considering that it
comprises the RNC and the Base Station. So, the Enforcement Module is the critical
issue in order to reach a unique Access Network.
2.6.3 Decision & Control Module
It is the module that manages all of the functions in the access network. It is the
supervisor of a set of cells.
It is responsible for:
• Power Control
• UE Location Management
• Radio Resource Control
• Handover Control
• Admission Control
• Mobility Management
• QoS Management
• Interface to the Core Network
The UMTS RNC and WiMAX ASN GW are placed inside the Decision&Control
Module. The DVB-H Encapsulator also belongs to this module but it does not
performs all the functions related to the Location, Radio Resource, Mobility and
Admission Control for the reasons explained before. Handover Control is signed as
function provided by the DVB-H Encapsulator but it is performed in a quite different
way compared with the UMTS or WIMAX one.
The Interface to the Core Network is the only one functionality provided by all three
of these devices and it distinguishes each technology.
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CCHHAAPPTTEERR TTHHRREEEE
3. CORE NETWORK: IMS TECHNICAL
DESCRIPTION
3.1 Introduction
In the previous chapter the Access Network was described and the accomplishment
was a unique architectural block model that needs to converge to one single Core
Network. Thus, in this section the Core Network is analyzed. The IMS is the best
choice for the Core Network because is the standard one that gathers all the features
needed to the convergence.
The IMS (IP Multimedia Subsystem) is a standard that defines a generic architecture
for multimedia services. IMS standard supporting multiple access types has the goal
of offering Internet services everywhere and at any time on different devices, so it is
the key for implementing fixed-mobile convergence.
As the cellular networks already provide a wide range of services (in fact any
cellular user can access the Internet using a data connection), all the power of the
Internet is already available for 3G users through the packet-switched domain, but
IMS is necessary to achieve three purposes: QoS (Quality of Service), charging, and
integration of different services. So, the reason for creating the IMS was to provide
the QoS required for real time multimedia sessions (packet-switched domain
provides a best effort service without QoS), to be able to charge multimedia sessions
appropriately (a multimedia session in the packet-switched domain usually transfers
a large amount of data that may generate large expenses to the user) and, finally, to
provide integrated services to users.
Operators want to be able to use services developed by third parties, combine them,
integrate them with services they already have, and provide the user with a
completely new service. Furthermore, the aim of the IMS is not only to provide new
services but to provide all the services that Internet provides. IMS achieves this by
using a layered and horizontal architecture where service enablers and common
function can be reused for multiple applications. This simplifies the interoperability
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and roaming, and providing bearer control, charging, and security. From an
architectural point of view IMS uses SIP protocol for signaling.
The 3GPP IMS, that is a part of 3GPP Release 5, is a standard implemented by the
Third Generation Partnership Project. It uses IPv6 and other advanced IP
technologies. Many of the capabilities of these technologies have been incorporated
into the design of the IMS architecture. This includes nearly limitless addresses, QoS
control, access independence (WLAN etc), IPSec and IPv6 routing efficiency.
The migration to an all IP context lowers maintenance costs (this is partly because
the maintenance platforms are based on open standards). It also establishes the
operation of new services that can be accessed by different types of terminal devices.
Today IP implementations are almost all IPv4 based and many of the advanced IP
technologies that have been assumed in the design of the IMS architecture are
currently not deployed on a large scale. Introducing IMS services into existing 3GPP
networks therefore requires upgrades in many parts of the system. New or enhanced
features have to be available in the different components that are involved in the end-
to-end service provisioning.
Implementing IMS will have a large impact on network structures, terminals, packet-
switched domain nodes, nodes for IP support (e.g. DNS and AAA servers), IMS
servers and application server, therefore there is doubt on the wide-spread
availability of the required critical components for 3GPP compliant IMS deployment.
3.1.1 Why IMS?
As said before, IMS provides QoS benefits, cost savings and the deployment of new
applications. The main problem with using the packet-switched domain is that it
performs a “best effort” service, which is inadequate for real-time communication,
because of the bandwidth and delay variations. But, IMS fixes this problem.
Furthermore, IMS enables the operator to know the data type and the best way to
charge the data stream (not only based on the number of byte transferred). For
example, a video conference in the packet-switched domain needs a lot of audio-
video data and this can be very expensive if the operator charges the service
proportionally to the transferred bytes. Instead, knowing the type of data thanks to
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IMS, the operator can select a different charging criteria based on time, QoS or other
service features.
IMS allows the operators to use services developed by others and combine them,
which provides innovative services for users. For example, if an operator has voice-
mail service, it can buy a test-speech conversion service and combine them to give a
speech version of the messages to blind people.
Because IMS uses Internet technology and protocols it can provide all of the Internet
services also to mobile users. Users can enjoy these services in roaming as well as in
their home network.
IMS based on packet switching can provide more efficient services than UMTS
based on circuit switching. Every service, thanks to the SIP protocol, knows all the
changes of the session, and therefore can provide new functionality. For example it
can give off an alert when the presence state of a colleague changes from busy to free
and allows the user to invite the colleague to take part in a video conference. Because
the service can know all the session features, it can make a lot of operations without
sending data which saves bandwidth. So, it can provide better QoS to the user or
serve more users with the same QoS.
IMS employs devices that don’t access to the circuit switched domain so the number
of IMS users who can connect increases considerably.
3.1.2 The Introduction of IMS
The introduction of IMS is an innovation regarding the whole network: both the core
network and the radio access network, creating an integration of different core
networks into one based on packet switching. It will be able to use just one platform
to provide multimedia services. The objective of the introduction of IMS is the
unification of the platforms that supply the services. This allows the definitions of
the services to be the same quality for all users, independent of the technology that
they use to access the network. As signaling is needed for multimedia sessions
control, it is placed on the application layer. Signaling goes through the same path as
the user traffic, instead of separated from it, as it was before. That has a considerable
effect on channel configuration because the channels also have to administrate a
signaling filter.
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In order to manage communication session and packet-switched media, assuring
traditional network compatibility (as ISDN or PSTN), new nodes need to be
introduced.
The impact that this new technology has on the network is tied to the introduction of
new protocols and network nodes. In order to realize and manage the sessions
(SIP/SDP protocols), an evolution of the signaling gateway is necessary. This will
allow the realization of all the service configurations that are based on both the
terminals and the access network.
We have to also consider that we will need to introduce the protocols that are
necessary to manage the real-time services (i.e. RTP/RTCP). They will allow
appropriate packet switching that meets the requirements of the imperceptible delays
(delays not perceived by the user).
The migration from a circuit-switched platform to a packet-switched one also needs
to take into consideration the management of the additional overload that is created
by the transmission. In fact, in circuit switching the voice, or otherwise the coded
stream of the voice, is transported by the network in a transparent mode. The
migration to the packet switching requires both payload (real content and voice
communication) and header (additional data). Because of the lower bitrate of this
service (12.2 kbps for the voice), the overload (the additional load of transport data)
can become enormous—up to almost 100%.
It is therefore necessary to determine the optimal configuration of the radio channels
in order to assure the quality of service for the flow of the signaling (SIP/SDP),
which is something that does not exist in circuit switching, and also for the protocols
that serve to transport voice and control data.
3.2 The Architecture of IMS
When one thinks of IMS architecture, it is important to keep in mind that the Third
Generation Partnership Project (3GPP) does not standardize roles, but instead
functions. The importance of the requirements is in the standardized interfaces. This
creates a flexible architecture where implementers can combine different functions
into a single node or split that function into multiple nodes. In order to access the
IMS network, the end user needs a terminal which is referred to as a User Equipment
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(UE). This terminal could be a mobile device, Personal Digital Assistant (PDA), or
even a computer. In addition, the modes of access to the network are variable. The
terminals can not only connect using a radio link, but can also use WLAN or ADSL.
The nodes included in the IP Multimedia Core Network Subsystem are:
• User Databases, called HSS (Home Subscriber Service) and SLF (Subscriber
Location Function)
• SIP servers, called CSCF (Call Section Control Function)
• AS (Application Service)
• MRF (Media Resource Functions) each one further divided into:
- MRFC (Media Resource Function Controller)
- MRFP (Media Resource Function Processor)
• BGCF (Breakout Gateway Control Function)
• PSTN gateway, each divided into:
- SGW (Signaling Gateway)
- MGCF (Media Gateway Controller Function)
- MGW (Media Gateway)
Figure 33: Architecture of IMS
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3.2.1 The HSS and SLF Databases
The Home Subscriber Subsystem (HSS) holds all of the user’s personal information.
This allows the user to log on to the network by holding his or her subscription data
that is required to run the multimedia sessions.
Possible Data:
• Location Information
• Security Information (Authentication and Authorization)
• User Profile Information (Service that User is Subscribed to)
• Serving-CSCF allocated to user
A network may contain more than one HSS, but the information for a single user is
stored in a single HSS. If a network has more than one HSS it needs a SLF, which is
a database that maps the users’ addresses. When a node queries the SLF to obtain
information related to a particular user, the SLF locates the correct HSS containing
the user’s information.
3.2.2 The CSCF
The CSCF (Call/Session Control Function) is a SIP server that is essential to IMS.
The three types of this server are:
• P-CSCF (Proxy-CSCF)
• I-CSCF (Interrogating-CSCF)
• S-CSCF (Serving-CSCF)
3.2.2.1 P-CSCF
P-CSCF is the first point of contact (in the signaling place) between the IMS terminal
and the IMS network. This means that all requests initiated by the IMS terminal or
destined to the IMS terminal traverse the P-CSCF. P-CSCF is necessary to receive
the requests and forward them in the appropriate direction, either towards the IMS
terminal or towards the IMS network. During IMS registration, a particular P-CSCF
is allocated to each IMS terminal. That P-CSCF never changes for the duration of the
registration.
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The following are some of the most important functions of the P-CSCF:
• Security: One of the functions of P-CSCF is security. It first establishes some
IPsec security associations toward the IMS terminal. The IPsec is a security
standard that allows the encryption and authentication of IP packets. P-CSCF
authenticates the user and asserts his or her identity to the other nodes in the
network. This means it is not necessary for the other nodes to further
authenticate the user, because they trust the P-CSCF.
• SIP Verification: Another function of the P-CSCF is to verify the SIP
requests to the terminal in order verify that they are appropriate for the SIP
rules that govern that terminal.
• SIP Message Compressor and Decompressor: This compresses the SIP
message in order to gain bandwidth, and then decompresses it at the receiving
end.
• PDF: The P-CSCF may include a PDF (Policy Decision Function) that
authorizes and media planes and manages QoS over the media plane.
• Charging: Generates charging information toward a charging collection node.
The P-CSCF may be located either in the visited network or in the home network.
When the packet network is based on GPRS, the P-CSCF is always located in the
same network where the GGSN (Gateway GPRS Support Node) is located.
3.2.2.2 The I-CSCF
The I-CSCF is a SIP proxy server located at the edge of an administrative domain.
The DSN (Domain Name System) records of the domain hold the address of the I-
CSCF. When a SIP server looks for the next hop for a message it obtains the address
of an I-CSCF belonging to the destination domain. I-CSCF interfaces with HSS and
SLF. It finds the user location information and routes the SIP request to the
appropriate destination.
Optionally I-CSCF may encrypt part of the messages if it has sensitive information
about the domain.
Typically, I-CSCF is located in the home network.
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3.2.2.3 The S-CSCF
Although the S-CSCF is basically a SIP server it also performs session control. It
works as a SIP registrar as well so it can connect the user location (IP address of the
user’s terminal) with Public User Identity (the user’s SIP address of record).
When a user is connecting to the IMS network, S-CSCF HSS’s interface informs
HSS that it is the allocated S-CSCF for the registered user and gets user
authentication vector.
S-CSCF has a central role in the network: all the SIP signaling to or from the IMS
terminal passes through the allocated S-CSCF. It analyzes the SIP messages to
determinate the application servers that they have to cross. Each of these servers will
be able to provide its services to the user. If the user dials a telephone number instead
of a SIP URI the S-CSCF provides translation the service.
S-CSCF has a policy role which inhibits the user from performing an unauthorized
session.
S-CSCF is located in the home network.
3.2.3 The AS
The AS (Application Server) is a SIP entity interfaced with S-CSCF that hosts and
executes services. It can operate like the following:
• SIP proxy
• SIP UA (User Agent): endpoint
• SIP B2BUA (Back-to-Back User Agent): concatenation of two SIP User
Agents
There are three types of AS:
• SIP AS (Application Server): the native AS that hosts and executes IP
Multimedia Services based on SIP
• OSA-SCS (Open Service Access – Service Capability Server): this
application server provides an interface to the OSA framework Application
Server. It has all the OSA capabilities, including the ability to access securely
to IMS from external network.
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• IM-SSF (IP Multimedia Switching Function): this server permits the reuse of
CAMEL (Customized Application for Mobile network Enhanced Logic)
services developed for GSM and it allows a gsmSCF (GSM Service Control
Function).
The AS may optionally interface to the HSS but only when the AS is located in the
Home Network.
3.2.4 The MRF
The MRF (Media Resource Network) is located in the home network and provides it
with a source of media. This allows the home network to play announcements, mixed
media streams (e.g. in centralized conference bridge), transcode between different
codes, obtain statistics, and perform analyses.
It is divided into:
• MRFC (MRF Controller): a signaling plane node that acts as a User Agent. It
contains a SIP interface towards the S-CSCF and controls the resources in
MSFP.
• MRFP (MRF Processor): a media plane node
3.2.5 The BGCF
The BGCF is a SIP server with routing functionality based on telephone numbers. It
is used in sessions that are initiated by an IMS terminal and addressed to a user in the
circuit-switched network (like PSTN or PLMN).
Depending on where the destination user BGCF is located:
• if this network isn’t the one where the BGCF is located, it selects an
appropriate network where interworking with the circuit-switched domain is
to occur;
• if interworking is to occur in the same network where the BGCF is located, it
selects an appropriate PSTN/CS gateway.
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3.2.6 The PSTN/CS Gateway
The PSTN/CS gateway provides an interface toward a circuit-switched network,
allowing IMS terminals to make and receive calls to and from the PSTN.
The PSTN/CS gateway is further divided into three functional elements:
• SGW (Signaling Gateway): interfaces with the signaling plane of the CS
network and performs lower layer protocol conversion.
• MGCF (Media Gateway Control Function): central node of the PSTN/CS
gateway, witch implements a state machine that does protocol conversion and
maps SIP (the call control protocol on the IMS side) to either ISUP over IP or
BICC over IP (the call control protocol in circuit-switched networks). In
addition it controls the resources in an MGW.
• MGW (Media Gateway): interfaces the media plane of the PSTN or CS
network and:
- It is able to send and receive IMS media over the Real-Time Protocol
(RTP),
- It uses PCM (Pulse Code Modulation) time slots to connect to the CS
network,
- It performs transcodification when the IMS terminal does not support
the codec used by the CS side.
3.2.7 Home and Visited Networks
IMS inherits the concepts of home networks (our operator’s network) and visited
networks (the network of another operator who manage our operator’s no covered
area). If there is a roaming agreement between the home and visited network, where
some aspects like costs or quality of services are defined, the user benefits from the
same service that is provided by his home network.
3.2.8 Identification in the IMS
As in the PSTN (Public Switched Telephone Network) users are identified with their
telephone numbers and the service with some special number like 800, in IMS we
need some identification method too.
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3.2.8.1 Public User Identification
An IMS user is allocated with one or more Public User Identities that identify him in
a deterministic way like the MSISDN in GSM.
A public User Identity is either a SIP URI (e.g. sip: [email protected]) or a
TEL URL (+39-06-123456). Additionally, it is possible to include a telephone
number in a SIP URL (sip: [email protected]; user=phone). The Public
User Identities are needful to route the SIP signaling.
We need all this type of format for the Public User Identities in order to allow the
interaction between different type of terminals like computers and telephones.
There are reasons for allocating more than one Public User Identity to a user, such us
having the ability to differentiate personal identities like the private and the business
one or for triggering a different set of services.
3.2.8.2 Private User Identities
Each IMS subscriber is assigned a Private User Identity that isn’t SIP URI or TEL
URL but takes the format of NAI (Network Access Identifier). Private User Identities
are exclusively used for subscription purposes and not for routing SIP requests so the
users don’t need to know it. They are stored in a smart card like IMSI in SIM for
GSM.
3.2.8.3 The relation between Public and Private User Identities
Every user has a Private User Identity and one or more Public User Identity, this data
are stored in the HSS.
3.2.8.4 Public Service Identities
The Public Service Identity is an identity allocated to a service that can take the
format of a SIP URI or a TEL URL.
3.2.9 SIM, USIM and ISIM in 3GPP
The UICC (Universal Integrated Circuit Card) is a removable card that contains a
limited storage of data, including subscription information, authentication keys, a
phone book, and messages. Removing the smart card from a terminal and inserting it
into another one the user can easily move his subscription.
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UICC is a generic term that defines the physical characteristics of the smart card, like
the number and the positions of the pins or the voltage values that define a standard
interface between the UICC and the terminal.
The UICC may contain several applications:
• SIM (Subscriber Identity Module): provides storage for a collection of
parameters for the identification of the user and the service, which are
essential for operating in a GSM network. Sometime it is used to refer to the
physical card whereas in this case UICC is more appropriated.
• USIM (Universal Subscriber Identity Module): is an application that resides
in third-generation UICCs. It provide another set of parameters, similar in
nature but different from those provided by SIM, useful for the identification
of the user and the service in a UMTS (Universal Mobile
Telecommunication) network.
• ISIM (IP Multimedia Service Identity Module): contains the collection of
parameters that are used for user identification, user authentication, and
terminal configuration when the terminal operates in the IMS. Among other
things it will contain Private and Public User Identity, Home Network
Domain URL (used to find the home network during the registration
procedure) and Long-term secret (a key for authentication, encrypt/decrypt or
integrity purpose). The user can not modify these parameters.
ISIM, USIM and SIM can co-exist in the same UICC.
Access to the 3GPP IMS network relies on presence of either an ISIM or a USIM
application in the UICC although ISIM is preferred because it is tailored to the ISIM.
Non-3GPP IMS networks that do not support UICC in the IMS terminals store the
parameters contained in the ISIM as a part of terminal’s configuration or in the
terminal’s build-in memory.
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3.3 Protocols
The protocols that are defined for IMS are classified into three extensive categories:
1. protocols needed for signaling and session control
2. protocols used in the “media plane”
3. authentication and security protocols
3.3.1 Session Control Protocol
A session is an exchange of data in a communication. Many Internet applications
need the creation and the management of a session. The protocol which is dedicated
to the session control in IMS is SIP [§ 3.4].
3.3.2 Media Plane Protocol
IMS employs RTP (Real Time Protocol) and RTCP (Real Time Control Protocol,
which provides statistics and information about the media stream) for delivering
multimedia contents.
Multimedia contents can be transported through an IP network, so it is possible for
packets to accumulate delays. Due to this, packets may arrive at the destination late
or out of sequence because of the IP network jitter. RTP timestamps are put inside
the packets to allow the receiver to get the correct content. All the packets are also
numbered in order to verify those that were misplaced during the transmission. If the
number of lost packets increases considerably, we can choose different codes for
improving the quality of service.
RTCP gives statistics about QoS and permits synchronization among the media.
One of the most important features of RTCP is that it establishes an association
between the RTP timestamps and the reference clock. So, is possible to achieve
synchronization among the media. The synchronization is of vital importance in
applications like video conferences.
3.3.3 Security and Authentication Protocol
In the IMS architecture there are three interfaces where authentication functions are
performed. The authentication protocol used within this interfaces is DIAMETER. It
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works over safe protocols as TCP and STCP and is an evolution of the previous
RADIUS protocol.
3.4 SIP Protocol
SIP (Session Initiation Protocol) is a signaling and control protocol on the
application layer. This protocol needs a transport network to be IP based.
SIP guarantees establishing, changing and ending communication sessions. Sessions
can be single or multi user, regardless of the session communication type.
Furthermore, SIP allows the achievement of a complete integration between data and
voice in real-time communication, regardless of the device type.
SIP is also independent from the transport layer, so the transport protocol can be
UDP, TCP or STCP. Because of this, SIP has got its own dependability and
protection mechanism.
SIP signaling messages are similar to the HTTP ones, so we can say that SIP is a
textual protocol.
In summary, SIP is distinguished by these main features:
• Based on the Client/Server model: every transaction is originated from a node
of the network able to act as client and it is directed to another entity that
involves from server;
• Simple: signaling is composed by a sequence of textual message (header and
body);
• Internet-oriented: it provides a complete integration with all the Internet open
standards like HTTP, URI (Uniform Resource Indicators), DNS (Domain
Name System) and MIME (Multipurpose Internet Mail Extension);
• Terminal-based: user terminals have the complete control of the session (or of
the call), while in the telephonic network (circuit-switched) the control is
decentralized;
• Text-based: the protocol is managed through text-based messages, so it
allows the adaptation the different situations;
• Based on a Request/Response model: every transaction consists of a request
and one or more answers (provisional or final).
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3.4.1 SIP functionality
As said before, the main goal of SIP is to deliver a session description to a user at
their current location. Once the user has been located and the initial session
description delivered, SIP can deliver new descriptions to modify the characteristics
of the outgoing sessions and terminate the session whenever the user wants. A
session description is, as its name indicates, a description of the session to be
established. It contains enough information for the remote user to join the session. In
multimedia sessions over the Internet this information includes the IP address and
port number where the media needs to be sent and the codes used to encode the voice
and the images of the participants.
In addition, in order to establish and close multimedia communication, the SIP
protocol supports five fundamental functions:
• User location: it determinates which terminal system we have to use for the
multimedia communication,
• User Availability: it determinates if a part wants to be involved in the
communication,
• User Capability: it determinates which kind of the media are involved in the
communication and its descriptive parameters,
• Session Set Up: it establishes the session, both on the originating and
terminating side,
• Session Management: session management foresees the changing of the
parameters of a session in course, the invocation of services on behalf of the
user and the closing of the session.
Finally, we have to consider that SIP is a layered protocol, that is, each functionality
is realized like independent elaboration stages.
SIP is used combined with other protocols:
1. RSTVP (Resource Reservation Protocol), it reserves network resources
2. RTP (Real Time Protocol) and RTCP (real Time Transfer Control Protocol), they
transport data in real time and provide QoS using feedback
3. RTSP (Real Time Streaming Protocol), it controls the media flow transmission
4. SAP (Session Announcement Protocol), it delivers multicast communication
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5. SDP (Session Description Protocol), it describes multimedia session
3.4.2 SIP Entities
A SIP network is composed of five types of logical SIP entities. Each entity has
specific functions and participates in SIP communication as a client (initiates
requests), as a server (responds to requests), or as both.
One physical device can have the functionality of more than one logical SIP entity.
The logical SIP entities are:
• User Agent (UA): it is an endpoint entity which initiates and terminates
sessions by exchanging requests and responses. UA is defined as an
application which contains both a User Agent Client (UAC, a client
application that initiates SIP requests) and a User Agent Server (UAS, is a
server application that contacts the user when a SIP request is received and
that returns a response on behalf of the user. During a session these entities
can dynamically interchange their roles while preserving them during a
transaction.
• Proxy Server: it is an intermediary entity that acts as both a server and a
client, for the purpose of making requests on behalf of other clients. Requests
are serviced either internally or by passing them on, possibly after translation,
to other servers. A Proxy interprets, and, if necessary, rewrites a request
message before forwarding it. Proxy Servers can be Stateless (logical entity
which does not keep memory of the transaction) or Stateful (proxy which
keeps the record of the transaction).
• Redirected Server: it is a server that accepts a SIP request, maps the SIP
address of the called party into zero (if there is no known address) or more
new addresses and returns them to the client. Unlike Proxy servers, Redirect
Servers do not pass the request onto other servers. It uses the Location
service.
• Registrar: it is a server that accepts REGISTER requests for the purpose of
updating a location database with the contact information of the user
specified in the request.
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• B2BUA (Back-to-Back User Agent): it is a logical entity that receives a
request, processes it as a User Agent Server (UAS) and, in order to determine
how the request should be answered, acts as a User Agent Client (UAC) and
generates requests. A B2BUA must maintain call state and actively
participate in sending requests and responses for dialogs in which it is
involved. The B2BUA has tighter control of the call than a Proxy. For
example, a Proxy cannot disconnect a call or alter the messages.
3.4.3 Messages
SIP is based on HTTP and, so, is a textual request-response protocol. There are two
types of SIP messages:
• Requests: they are sent from the client to the server. They contain a Request
Line, a Header, and a Message Body.
• Responses: they are sent from the server to the client. They contain a Status
Line, a Header, and a Message Body.
In general, SIP messages are composed of three parts: Start line, Headers and
Message Body.
3.4.3.1 Start Line
Every SIP message begins with a Start Line. The Start Line conveys the message
type (method type in requests, and response code in responses) and the protocol
version. Table 10 shows the methods that are currently defined in SIP and their
meaning.
The Start Line may be either a Request-line (requests) or a Status-line (responses), as
follows:
• The Request-line includes a Request-URI, which indicates the user or service
to which this request is being addressed.
• The Status-line holds the numeric Status-code and its associated textual
phrase.
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Table 10: SIP Methods
Method name Meaning
ACK
BYE
CANCEL
INFO
INVITE
NOTIFY
OPTIONS
PRACK
PUBLISH
REGISTER
SUBSCRIBE
UPDATE
MESSAGE
REFER
Acknowledges the establishment of a session
Terminates a session
Cancels a pending request
Transports PSTN telephony signaling
Establishes a session
Notifies the user agent about a particular event
Queries a server about its capabilities
Acknowledges the reception of a provisional response
Uploads information to a server
Maps a public URI with the current location of the user
Requests to be notified about a particular event
Modifies some characteristics of a session
Carries an instant message
Instructs a server to send a request
3.4.3.2 Headers
SIP header fields convey message attributes that provide additional information
about the message. They are similar in syntax and semantics to HTTP header fields
(in fact, some headers are borrowed from HTTP) and thus always take the format:
<name>:<value>.
Headers can span multiple lines. Some SIP headers such as Via, Contact, Route and
Record-Route can appear multiple times in a message or, alternatively, can take
multiple comma-separated values in a single header occurrence.
3.4.3.3 Message Body
A message body is used to describe the session to be initiated (for example, in a
multimedia session this may include audio and video codec types and sampling
rates), or alternatively it may be used to contain opaque textual or binary data of any
type which relates in some way to the session. Message bodies can appear both in
request and in response messages. SIP makes a clear distinction between signaling
information, conveyed in the SIP Start Line and headers, and the session description
information, which is outside the scope of SIP.
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Possible body types include:
• SDP (Session Description Protocol)
• Multipurpose Internet Mail Extensions (MIME)
• Others, to be defined in the IETF and in specific implementations.
3.4.4 Management of a SIP session
3.4.4.1 Session Establishment and Termination
Figure 34 shows the interaction between two user agents during trivial session
SGSN also performs the functionalities related to security.
UMTS cells are grouped in Routing Areas, which are contained in Location Areas.
When a mobile that is registered in a SGSN moves from one Routing Area to
another, it informs the SGSN in order to update its location.
Thus, the location register function in the SGSN stores two types of subscriber data
needed to handle originating and terminating packet data transfer (in order to manage
user mobility and traffic):
• Subscription Information
- The IMSI
- One or more temporary identities
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- Zero or more IP addresses (or PDP addresses)
• Location Information
- The cell or the routing area where the MS is registered, depending on
the operating mode and state of the MS
- The VLR number of the associated VLR (if the Gs interface to the
MSC/VLR in the CS-Domain is implemented)
- The GGSN address of each GGSN for which an IP virtual channel ism
active (an active PDP context exists).
4.3.2 Gateway GPRS Support Node (GGSN)
The GGSN is the node that interfaces the external PS-Domain networks, as Internet,
and IMS.
When a mobile wants to transfer some packet data, it asks to the SGSN to establish a
PDP Context. The SGSN, together with the GGSN, creates a virtual channel which
connects the UE to the desired network. The GGSN is the gateway which gives the
access to the external network. So the GGSN stores user data needed to manage the
user traffic.
The location register function in the GGSN stores subscriber data received from the
HLR and the SGSN. There are two types of subscriber data needed to handle
originating and terminating packet data transfer:
• Subscription Information
- The IMSI
- Zero or more IP addresses (PDP addresses)
• Location Information
- The SGSN address for the SGSN where the MS is registered
4.3.3 Data Bases
4.3.3.1 Home Subscriber Server (HSS)
The HSS is the master database for a given user. It is the entity containing the
subscription-related information to support the network entities actually handling
calls/sessions.
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A Home Network may contain one or several HSSs: it depends on the number of
mobile subscribers, on the capacity of the equipment and on the organization of the
network.
For example, the HSS provides support to the call control servers in order to
complete the routing/roaming procedures by solving authentication, authorization,
naming/addressing resolution, location dependencies, etc.
The HSS is responsible for holding the following user-related information:
• User Identification, Numbering and addressing information
• User Security information: Network access control information for
authentication and authorization
• User Location information at inter-system level: the HSS supports the user
registration, and stores inter-system location information, etc.
• User profile information.
The HSS also generates User Security information for mutual authentication,
communication integrity check and ciphering.
Based on this information, the HSS also is responsible for supporting the call control
and session management entities of the different Domains and Subsystems of the
operator as shown in Figure 43.
Rp Gr Gc
Location information
Subscription information
HSS
Cx D C
GUP Server CSCF GGSN SGSN MSC Server GMSC Server
Figure 43: Example of a Generic HSS structure and basic interfaces
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The HSS may integrate heterogeneous information, and enable enhanced features in
the core network to be offered to the application & services domain, at the same time
hiding the heterogeneity.
The HSS consists of the following functionalities:
• IP multimedia functionality to provide support to control functions of the IM
subsystem such as the CSCF. It is needed to enable subscriber usage of the
IM CN subsystem services. This IP multimedia functionality is independent
of the access network used to access the IM CN subsystem
• The subset of the HLR/AUC functionality required by the PS Domain
• The subset of the HLR/AUC functionality required by the CS Domain, if it is
desired to enable subscriber access to the CS Domain or to support roaming
to legacy GSM/UMTS CS Domain networks
The HSS is considered as GUP Data Repository for IM CN Subsystem user related
data. The RAF (Repository Access Function) provides the Rp reference point.
4.3.3.2 The Home Location Register (HLR)
The HLR can be considered a subset of the HSS that has the following functions:
• The function required to provide support to PS Domain entities such as the
SGSN and GGSN, through the Gr and Gc interfaces and the 3GPP AAA
Server for the I-WLAN through the D'/Gr' interface. It is needed to enable
subscriber access to the PS Domain services
• The function required to provide support to CS Domain entities such as the
MSC/MSC server and GMSC/GMSC server, through the C and D interfaces.
It is needed to enable subscriber access to the CS Domain services and to
support roaming to legacy GSM/UMTS CS Domain networks.
4.3.3.3 The Authentication Centre (AuC)
The AuC can be considered a subset of the HSS that holds the following
functionality for the CS Domain and PS Domain:
• The AuC is associated with an HLR and stores an identity key for each
mobile subscriber registered with the associated HLR. This key is used to
generate security data for each mobile subscriber:
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- data which are used for mutual authentication of the International
Mobile Subscriber Identity (IMSI) and the network
- a key used to check the integrity of the communication over the radio
path between the mobile station and the network
- a key used to cipher communication over the radio path between the
mobile station and the network.
• The AuC communicates only with its associated HLR over a non-
standardized interface denoted the H-interface. The HLR requests the data
needed for authentication and ciphering from the AuC via the H-interface,
stores them and delivers them to the VLR and SGSN which need them to
perform the security functions for a mobile station.
4.3.3.4 HSS logical functions
• Mobility Management: This function supports the user mobility through CS
Domain, PS Domain and IM CN subsystem
• Call and/or session establishment support: The HSS supports the call and/or
session establishment procedures in CS Domain, PS Domain and IM CN
subsystem. For terminating traffic, it provides information on which call
and/or session control entity currently hosts the user
• User security information generation: The HSS generates user authentication,
integrity and ciphering data for the CS and PS Domains and for the IM CN
subsystem user security support. The HSS supports the authentication
procedures to access CS Domain, PS Domain and IM CN subsystem services
by storing the generated data for authentication, integrity and ciphering and
by providing this data to the appropriate entity in the CN (i.e. MSC/VLR,
SGSN, 3GPP AAA Server or CSCF)
• User identification handling: The HSS provides the appropriate relations
among all the identifiers uniquely determining the user in the system: CS
Domain, PS Domain and IM CN subsystem (e.g. IMSI and MSISDNs for CS
Domain; IMSI, MSISDNs and IP addresses for PS Domain, private identity
and public identities for IM CN subsystem)
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• Access authorization: The HSS authorizes the user for mobile access when
requested by the MSC/VLR, SGSN, 3GPP AAA Server or CSCF, by
checking that the user is allowed to roam to that visited network
• Service authorization support: The HSS provides basic authorization for MT
call/session establishment and service invocation. Besides that the HSS
updates the appropriate serving entities (i.e., MSC/VLR, SGSN, 3GPP AAA
Server, CSCF) with the relevant information related to the services to be
provided to the user
• Service Provisioning Support: The HSS provides access to the service profile
data for use within the CS Domain, PS Domain and/or IM CN subsystem.
Application Services and CAMEL Services Support
• The HSS communicates with the SIP Application Server and the OSA-SCS to
support Application Services in the IM CN subsystem. It communicates with
the IM-SSF to support the CAMEL Services related to the IM CN subsystem.
It communicates with the gsmSCF to support CAMEL Services in the CS
Domain and PS Domain.
• GUP Data Repository: The HSS supports the storage of IM CN Subsystem
user related data, and provides access to these data through the Rp reference
point.
4.3.4 The Equipment Identity Register (EIR)
The Equipment Identity Register (EIR) in the GSM system is the logical entity which
is responsible for storing in the network the International Mobile Equipment
Identities (IMEIs), used in the GSM system. The equipment is classified as "white
listed", "grey listed", "black listed" or it may be unknown.
This functional entity contains one or several databases which store(s) the IMEIs
used in the GSM system. The mobile equipment may be classified as "white listed",
"grey listed" and "black listed" and therefore may be stored in three separate lists. An
IMEI may also be unknown to the EIR. An EIR shall as a minimum contain a "white
list" (Equipment classified as "white listed").
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4.3.5 Interfaces
4.3.5.1 Interface between SGSN and RNS (Iu_PS-interface)
The RNS-SGSN interface is used to carry information concerning:
• Packet data transmission
• Mobility management
4.3.5.2 Interface between SGSN and HLR (Gr-interface)
This interface is used to exchange the data related to the location of the mobile
station and to the management of the subscriber. The main service provided to the
mobile subscriber is the capability to transfer packet data within the whole service
area. The SGSN informs the HLR of the location of a mobile station managed by the
latter. The HLR sends to the SGSN all the data needed to support the service to the
mobile subscriber. Exchanges of data may occur when the mobile subscriber requires
a particular service, when he wants to change some data attached to his subscription
or when some parameters of the subscription are modified by administrative means.
Signaling on this interface uses the Mobile Application Part (MAP), which in turn
uses the services of Transaction Capabilities (TCAP).
4.3.5.3 Interface between SGSN and GGSN (Gn- and Gp-interface)
These interfaces are used to support mobility between the SGSN and GGSN. The Gn
interface is used when GGSN and SGSN are located inside one PLMN. The Gp-
interface is used if GGSN and SGSN are located in different PLMNs. The Gn/Gp
interface also includes a part which allows SGSNs to communicate subscriber and
user data, when changing SGSN. Signaling on this interface uses the User Datagram
Protocol, UDP/IP.
4.3.5.4 Signaling Path between GGSN and HLR (Gc-interface)
This optional signaling path may be used by the GGSN to retrieve information about
the location and supported services for the mobile subscriber, in order to activate a
packet data network address. There are two alternative ways to implement this
signaling path:
• if an SS7 interface is implemented in the GGSN, signaling between the
GGSN and the HLR uses the Mobile Application Part (MAP), which in turn
uses the services of Transaction Capabilities (TCAP)
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• if there is no SS7 interface in the GGSN, any GSN in the same PLMN and
which has an SS7 interface installed can be used as a GTP to MAP protocol
converter, thus forming a signaling path between the GGSN and the HLR.
4.3.5.5 Interface between SGSN and EIR (Gf-interface)
This interface is used between SGSN and EIR to exchange data, in order for the EIR
to be able to verify the status of the IMEI retrieved from the Mobile Station.
Signaling on this interface uses the Mobile Application Part (MAP), which in turn
uses the services of Transaction Capabilities (TCAP).
4.3.6 Protocol Analysis and Definition
Figure 44: UMTS’s Device Protocols
In UMTS each access network element is a transit node for the UE and the IMS CN.
These transit nodes perform all the functionalities of the physical layer and of the
data link layer. Figure 44 shows that the transfer mode used by Node B and the RNC
is ATM. Because of this, the UTRAN cannot connect directly to the IMS CN but an
Edge Network must be taken into consideration. In the Edge Network the transfer
mode uses the IP protocol. The SGSN is the first element of the Edge Network and it
makes a ‘protocol conversion’. In fact, it has a dual protocol stack: on the access
network side it communicates with the RNC by the ATM protocol, while on the
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135
other side it is connected to the GGSN by the Gn interface which supports the UDP
protocol (on the transport layer). The GGSN is the gateway to the IMS core network
and it establishes a virtual channel (PDP Context) between the UE and the IMS core
network. The GGSN supports UDP/TCP on the transport layer and SIP on the
application layer (HTTP/FTP are also supported here).
All of these interfaces and connections are standardized in the UMTS
Release 6.
4.4 The Edge Network for WiMAX
WiMAX is a broadband access technology, and so it should have no problems
connecting to IMS as DSL or WiFi technology are also both able to connect without
problems. Nowadays the WiMAX Network Group (NWG) is still working on the
WiMAX architecture, and the way to connect to IMS is an open issue. It is a
requirement for the standardized architecture but it is has not yet been defined.
WiMAX should be used as transparent IP-bearer connecting the IMS-Client with the
IMS-Network.
In this context it is reasonable to suppose a Wireless Access Gateway (WAG) in the
CSN with SIP protocol on the session layer to interface to the IMS Network. The
WAG will also provide Policy Enforcement, which is a functionality implemented to
ensure that packets coming from or going to the WLAN Access Network are allowed
based on unencrypted data within the packets (e.g. source and destination IP address
and port number).
So, the Edge Network for the WiMAX can be individuated in the NSP (Network
Service Provider) which is implemented by one or more CSN (Connectivity Service
Network). This infrastructure is shown in Figure 45.
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Figure 45: Edge Network in WiMAX
The Network Service Provider (NSP) is a business entity that provides IP
connectivity and WiMAX services to WiMAX subscribers consistent with the
Service Level Agreement it establishes with WiMAX subscribers. Thus, the IMS
may be located in the private network of a network operator (NSP) or at a third-
party provider. The IMS could be connected directly to the provider’s backbone
network, connected via a specific gateway or via the internet.
Another element needed in the NSP to enable a Wireless Access Network to
connect to the IMS is an AAA Server.
4.4.1 CSN-WAG
Connectivity Service Network (CSN) is defined as a set of network functions that
provide IP connectivity services to the WiMAX subscriber(s). Thus, a CSN may
comprise network elements such as routers, AAA proxy/servers, user databases and
Interworking gateway devices. In order to allow the WiMAX Access Network
(ASN) to be linked to the IMS Core Network, the device held in the CSN is a WAG.
A CSN may provide the following functions:
• MS IP address and endpoint parameter allocation for user sessions
• IP address management (based on PoA management)
• Internet access, Connectivity to Internet, ASP and other PLMN sand
Corporate Networks
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• AAA proxy or server, User, equipment and services authentication,
authorization and accounting (AAA)
• Policy and Admission Control based on user subscription profiles
• ASN-CSN tunnelling support
• WiMAX subscriber billing and inter-operator settlement
• Inter-CSN tunnelling for roaming
• Location management between ASNs
• Inter-ASN mobility and roaming between ASNs
• WiMAX services such as location based services, connectivity for peer-to-
peer services, provisioning
• authorization and/or connectivity to IP multimedia services and facilities to
support lawful intercept services
• Policy & QoS management based on the SLA/contract with the user
Interworking gateway devices, such as those compliant with Communications
Assistance Law Enforcement Act (CALEA) procedures.
4.4.2 Interfaces
The interfaces involved in the Edge Network are called R3 and R5.
The R3 is the interface between the ASN and the CSN and it is responsible for
supporting AAA, policy enforcement and mobility management capabilities. It also
encompasses the bearer plane methods (e.g. tunnelling) to transfer IP data between
the ASN and the CSN.
On the other side, the R5 interface consists of a set of control plane and bearer plane
protocols for interworking between CSNs operated by either the home or visited
NSP. Thus, this interface can be considered as the interface between the Edge
Network and the IMS Core Network, i.e. between the CSN-WAG and the P-CSCF.
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4.4.3 Protocol Analysis and Definition
Figure 46: WiMAX’s Device Protocols
Figure 46 shows WiMAX’s device protocol stacks. The user equipment has SIP on
the session Layer, this protocol allows the user to establish a session with other users,
even when they access the network using different technologies. Between the
terminal and the BS the communication is at the Link Layer. The BS manages and
controls the MAC Layer, allocating channels (time and bandwidth) to different users.
On the other side, the terminal sends some essential information, like the CQI, to the
BS. This interface is the one standardized with the 802.16 standard. From this point
on, up to the CN the communication is all IP. The different control layers highlighted
in Figure 46 show how these devices manage traffic across the network. In order to
interface the CSN-WAG to the IMS CN we have decided to use SIP as the session
protocol in the WAG.
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4.5 The Edge Network for DVB-H
I have already said that DVB-H is a broadcasting technology so, without using
another technology to implement a return channel, the DVB-H network is
unidirectional.
A DVB-H unidirectional network together with another broadband network which
provides the return channel is standardized as IP Datacasting (IPDC). IPDC is
needed to achieve any interaction with the user. Indeed, IPDC is an end-to-end DVB
system for services over mobile terminals comprising both a broadcast path through
the DVB-H technology and a bidirectional telecommunication part. This
mobile/cellular technology is standardized to be UMTS/GPRS, but I believe that also
WiMAX will be taken into account thanks to the convergence towards one Core
Network. The IPDC convergence means convergence on transport and applications
layer. Instead, my objective is an improved convergent architecture where DVB-H
system is able to connect to IMS Core Network. In this way the return channel could
not be just the UMTS one but could be chosen dynamically between the best
available access technologies.
However, there is no standard regarding how to connect a DVB-H network to the
IMS. So, my effort was to analyzing how to connect the two networks and how to
realize a whole convergent network involving the DVB-H system.
Some of the advantages that this convergence would give are the following:
• Services synchronization: a synchronization of broadcast services with
additional no-broadcast services, which eventually uses interactivity, is
possible, i.e. interactive advertisement during a football match.
• Presence: it is possible to create presence services related to the broadcast
television programs. In this way, it allows the network owners to know who-
sees-what. This information is very important for the advertising market and
program scheduling (“palinsesto”).
• Service reuse: IMS is a horizontal architecture, so it enables the reuse of
services created by third parties.
• Unique network: allows the user to connect to a unique network and above all
with just one service provider.
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At the top of what is considered as the DVB-H Access Network, i.e. before the IP
Encapsulator, I must suppose a server which I call Content Provider. This Content
Provider server is placed into the Edge Network, as Figure 47 can describe.
Figure 47: Edge Network in DVB-H
This device is the one which interfaces with IMS Core Network. In order to make
this integration possible the Content Provider server must hold the SIP protocol on
the session layer, because IMS is based on SIP. This protocol allows the Content
Provider to establish sessions with other elements of the network. In this way the
Content Provider is able to share contents with other servers. This is the reason of
why I chose to integrate the server which allows the connection to IMS with the one
which stores the multimedia contents to be sent by the broadcast network. It is
possible to perform services with the other technologies reusing the same contents
sent by the broadcast network i.e. TV on-demand by UMTS or WiMAX. SIP also
gives the users (connected to the network via another access network) the possibility
to send some kind of content (i.e. SMS, MMS, etc.) to the Content Provider, which
will transmit them in broadcast together with the shows.
Another advantage of the SIP protocol inside the Content Provider server is that
multimedia can be entered just by establishing a session by a remote server. For that
reason the Content Provisioning shown in Figure 47 is not mandatory inside the Edge
Network.
I assume that the Content Provider server is directly connected to the IMS S-CSCF
via the ISC interface. This interface is standardized to connect the S-CSCF to a SIP
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Application Server or to a third party Application Server. There is not need to
connect the Content Provider to the S-CSCF via P-SCSF because it is not a mobile
IMS terminal. However, the Content Provider can be seen by the IMS network as a
user terminal, i.e. IMS client.
It is important to underline that the Content Provider server performs broadcast
transmission on the air interface side, it is able to establish sessions (bidirectional
transmission) on the other side (towards the Core Network).
4.5.1 Protocol Analysis and Definition
The connection of AN DVB-H with the CN IMS was probably the most difficult
point in the research of an architecture that permits the convergence between the
examined technologies. The difficulty comes from the impossibility to create a
session on a Broadcast network. This difficulty was then solved by choosing to
establish the sessions between the Content Provider and the other network terminals.
Figure 48: DVB-H Protocol Stack
Figure 48 shows the typical protocol stack of this technology.
On the Application layer the FLUTE (File Delivery over Unidirectional Transport)
protocol is to be considered if no return channel is available for the DVB-H system.
It is a protocol for the unidirectional delivery of files over the Internet, which is
particularly suited to multicast networks. It is based on a mechanism for signaling
and mapping the properties of files to concepts of ALC (Asynchronous Layered
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Coding) in a way that allows receivers to assign those parameters for received
objects. ALC is a protocol designed for delivery of arbitrary binary objects. It is
especially suitable for massively scalable, unidirectional, multicast or broadcast
distribution.
Figure 49: DVB-H’s Device Protocols
MUX and Modulator work at the Data Link layer. The first, MUX, is for
multiplexing the different IP streams that come from the IP Encapsulator in a unique
Transport Stream and it also adds the SI/PSI and MPE metadata. From this point, the
data is no longer managed at the IP level. The second, the Modulator, is for sending
TS on-air. The Content Provider is an element of the Edge Network that acts as the
interface for the SIP-IMS world and it is the only element that manages data at the
Application layer. Therefore, it is the only one that works in the grey level pictured
in Figure 48. Figure 49 represents this scenario. As you follow the straight path of
information across the broadcast network, the different elements are continually
working at a lower layer than the previous device.
I would like the highlight the necessity of the SIP Protocol in the application layer of
the Content Provider server. This is an indispensable requirement that allows the
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connection to the IMS network, which then offers the benefits and possibilities that
have already been explained in this section.
4.6 CONVERGENT ARCHITECTURE PROPOSAL
All the devices needed to connect the different Access Networks to the IMS Core
Network are now identified. I have called the set of these elements the Edge
Network. The resulting architecture scheme of the whole convergent network is the
one shown in Figure 50.
Figure 50: Convergent Architecture Scheme
Hence, the introduction of the Edge Network consents to realize a convergent
architecture. A convergent architecture is an implementation of technologies aimed
to optimize the network architecture in order to transport video, voice and data on a
single support. The main feature this architecture must provide is access
independency, that is services must be available over different access technologies.
So, a user does not have to take care of which kind of terminal has to be used for a
specific service. In fact there is one network architecture for accommodating all
services. That allows to provide and to require optimized Quality of Service. Such
architecture also consents easier interworking with the Internet.
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As the IP protocol is the reference point for any kind of network transporting video,
voice and data, a convergent architecture is possible assuming an all-IP based
network.
Thus, in my study I have supposed an all-IP backbone and the motivations are
summarized next:
• Enables rich communications combining multiple media or services;
• New IP-based services, easier & faster service creation and execution;
• Smooth evolution from today’s networks and standards:
- Cost efficiency, evolution for current solutions;
• Openness: both specifications and (distributed) architecture.
In order to guarantee one network architecture and access independency with the
existent technologies, the Edge Network is needed. It allows to consider one generic
Access Network which is composed of more access technologies, i.e. UMTS,
WiMAX and DVB-H in this case. Therefore, the Access Network could be any kind
of network because the role of the Edge Network is to fill the gap between Access
and Core Network linking them together.
Figure 51 represents the result of integration between different Access Networks and
one Core Network which is my idea of the convergent network.
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Figure 51: Convergent Architecture
In this figure the Access, Edge and Core Network are highlighted. The Access
Network is intended as unique architectural block model [§ 2.6] which physically
holds the UMTS UTRAN, WiMAX and DVB-H systems.
The Core Network is the basic communications platform IMS which offers easy
integration with other IP protocols and applications and seamless service offering
over various access networks. Thanks to the IMS features (due to the SIP Protocol),
such Convergent Network provides:
• Personalized services aware of desired communication capabilities and
preferences,
• Straightforward integration of voice, images, video and other interactive
communications services,
• Web like service development approach
• Enhanced service inter-working with a client in terminal and a server in
network.
The Edge Network can be considered as the whole of what is needed to connect the
Access Network to the Core Network in order to achieve the convergence. In the
Paragraph 4.2 the Edge Network was introduced, explaining what it is meant for and
which functionality it must hold. While in the successive paragraphs [§ 4.3-4.5] the
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physical elements composing the Edge Network for each technology were described.
In Figure 51 is evident that some elements that were in the Edge Network of each
technology are now not present in the Edge Network of my idea of convergent
architecture. This modification is due to some assessment I have done and it is
another step towards the complete convergence. Everything will be explained in the
next paragraph which gives details about the Edge Network evolutions.
4.6.1 Edge Network Evolutions
The result of my study is a proposal of an Edge Network which can be considered as
the key for achieving a fully convergent architecture. This definition passes through
various phases and the finally convergence, achieved via Edge Network, is
developed. This paragraph is intended to make clear all the stage I have followed to
reach the architecture illustrated in this section.
The starting point of my work is that, currently, there are three distinct Access
Networks standalone. In this environment every access technology needs its own
user terminal, as Figure 52 depicts. Anyway, every technology user equipment needs
to act as IMS Client but the DVB-H Receiver. In the case of DVB-H system the
Content Provider Server is seen as client by the IMS network [§ 4.5]. The main
prerogative an IMS Client must hold is the presence of SIP protocol on the
application layer.
The WiMAX architecture provides unified support of functionality needed in several
usage scenarios ranging from fixed, nomadic, portable, simple mobility to fully
mobile subscribers. In this thesis the Mobile WiMAX is taken into account so the
user terminal is assumed to be a mobile terminal and it is called MS (Mobile
Subscriber). Since the WiMAX network acts as the IP-connectivity Access Network
to IMS based services for the transport of IMS signaling and bearer traffic, the MS
has to run IMS client software to be compatible with IMS at the network side. This
requires Gm interface and Um (3GPP2)10 interfaces between WiMAX client and P-
CSCF, i.e. on the R3/R5 reference point.
10 Gm is the reference point supporting the communication between UE and IMS CN (CSCF), Um is the interface between the Mobile Station and the Base Station
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Summing up, the WiMAX network enables wireless users to access all IP
Multimedia Subsystem (IMS) based services then WiMAX subscribers are able to
establish a WiMAX connection, perform P-CSCF discovery and register to IMS as
defined in the IMS registration procedure.
UMTS UEs are also able to access IMS services, as access to IMS relies on the
presence of either e USIM or an ISIM application in the Universal Integrated Circuit
Card inside the terminal.
Figure 52: Access Networks Standalone
In the first step towards the convergence, the Edge Network is the union of the three
Edge Networks introduced for each Access Network. It is just an interface to the
Core Network. However, it groups all the physical elements and features needed for
the connection to the Core Network by each technology independently. There is not
an interaction between each different technology and the whole network is the result
of three Access Networks put together.
Moreover, some of the functions performed in the Edge Network are the same
performed also by other devices in the IMS Core Network.
Since this issues causes redundancy information and, above all, no scalability a
further effort needs to be made.
Thus, the last evolution is a unique network where the Edge Network is composed
just by the elements strictly needed for connecting each technology to the Core
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Network. In order to optimize the complete convergent network, all the common and
duplicated functions are shifted into the Core Network.
The convergent architecture derived from my considerations is the one in Figure 53.
Figure 53: Convergent Architecture
Some elements that were in the Edge Network of each technology are not present in
the Edge Network of this model of convergent architecture. That is because I have
moved the functionalities those elements perform toward other devices, when I have
found some correspondence between the functions in the Edge Network and the ones
in the IMS network. For example, the HLR in UMTS is a database which stores a
subset of information stored in the HSS within the IMS. So, the same user
information is duplicated and redundant. All the functionalities related to the
Location Register can be centralized into the Core Network, as the IMS devices can
offer them. In addition, the AAA services needed in the WiMAX connection are
performed also by the S-CSCF. I have also centralized the AAA functionalities
performed by WiMAX or UMTS servers and databases dedicated for the same
purpose. These functionalities are performed by IMS CSCF devices which use Core
Network databases. At last, I have chosen to do not put DVB-H content provisioning
inside the Edge Network, it could be in a remote location and it could load contents
by establishing a peer-to-peer SIP session.
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Hence, I have decided to centralize some Edge Network functionalities inside the
IMS Core Network in order to achieve a convergent architecture. In this way, the
Edge Network results to be the bridge between the Access Networks and the Core
Network but not only. Its importance grows up thanks to the fact that it allows a
complete integration between different Access Networks. Similar functions
performed by different technologies (although performed in different ways) are
supposed to be joint together. This means that an interaction between various
elements inside the Edge Network is possible. It follows that the whole network can
be considered as one. Something very important to be taken into account is that the
convergent architecture considered up to now is simple to realize because it holds all
the physical and functional elements currently present, although devices composing
the Edge Network must satisfy some mandatory requirements not needed before
now, regarding each technology. On the contrary, the new opportunity and possible
services introduced by such an architecture are highly developed. All the functions
already existing are able to be combined, due to the Edge Network.
A vertical handover between different technologies can be performed in order to
dynamically choose the best access technology with the respect of the service
requirements (QoS, required bandwidth, coverage area, etc...). For example, the
network can switch the transmission over either UMTS UTRAN or WiMAX Access
Network depending on which kind of service is to be offered.
The same consideration can be done about the User Equipment. Such a network can
be accessed by a unique user terminal regardless of the access technology used.
This User Equipment needs to hold an interchanging radio access interface which is
able to choose either an access mode or another. So the user does not have to care
about which technology is needed to be able to get a specific service. A requirement
the User Equipment must meet is the SIP protocol on the application layer of its
protocol stack. SIP allows session establishing between users, most of all between
users and Content Provider server [Appendix A].
Moreover, a user can access using various kind of terminals and be identified by his
own IP address regardless the kind of terminal. Thanks to the IMS service presence,
the network is able to understand which terminal he is using, then how much
bandwidth is allocable and which bit rate can be reached. Everything is possible
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because the HSS data bases are centralized and the user information are the same for
each devices of every technology.
Some significant functions, like Security, are performed by both Edge Network and
Core Network elements. However, in such case this redundant functionality is not a
bother but a further way to make the system more efficient and robust than the earlier
one. The idea of separate the network into more sections (Access Network, Edge
Network and Core Network) allows a modularization of some functionalities
management. To better understand this assertion, one can think of a protocol stack:
each layer is dedicated to perform some specific functions and it adds other
functionalities to the underlying one.
So, concerning the Handover function:
• it can be performed by the Access Network on a physical level,
• an Handover between different technologies can be considered inside the
Edge Network,
• it can be provided by the one Core Network on a user level.
All these consideration about a developing Edge Network are the basis for an
improved convergent architecture which is supposed to be reached in the future. In
fact, it is assumed that similar management policies for the same functions performed
by different technologies (which work in quite different ways by now) will be
accomplished in the upcoming time. This will enable to integrate various devices in
the Edge Network maintaining one element for different technologies.
4.6.2 Conclusions
In conclusion, it is important to say that a convergent architecture is to be considered
as a development existing networks instead of their revolution. The elements
composing this architecture are the same components as before with improved
features and additional functions, both multiprotocol and multilayer. The
implementation of multiprotocol equipments enables to access to different
technologies.
As a system oriented to the convergence needs a careful valuation of the existing
infrastructures and of the objective to be satisfied, my thesis was aimed to this. So,
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the result of my effort is a Convergent Architecture proposal which takes the
following advantages:
• Integrated and synchronized services are possible: the integration between
broadcast and multicast services is improved as soon as possible;
• All the functions in common between these three different access
technologies (UMTS, WiMAX and DVB-H) are centralized: the Access
Network can be considered as one;
• In the Edge Network are performed just the functionalities related to the need
of connecting different Access Networks: the whole network can develops
with the introduction of new access technologies without changing anything
(Scalability);
• User profiling is allowed because all the Data Bases are centralized in the
HSS.
In order to validate what I have described in this chapter and what is my end result,
the subsequent points have been analyzed:
• Feasibility: the whole system cost is limited because the implementation does
not occur ex novo, but the existing equipments are expanded.
• Flexibility: the whole system can be amplified and adapted introducing new
upcoming access technologies without changing the existing architecture. The
only portion of the network to be modified is the Edge Network.
• Dependability: this network is able to provide required services continuously
due to the centralized Core Network.
• Robustness: critical situations can be supported without failure, in such cases
as traffic increasing, insufficient bandwidth, power lower than a bound in a
coverage area, etc another access technology can be used.
• Maintenance: simple configuration, monitoring and statistics of the system by
software from a remote place because the whole architecture is
interconnected.
• Usability: a user can log on and utilize any service simply and without
knowing which technology is used
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Finally, one of the main issues this kind of convergent architecture introduces is
about Security. As said in the previous paragraph, such convergence makes the entire
system more robust because this feature is performed by different sections of the
network regarding different aspects. Moreover there are centralized AAA server and
HSS. However, it is difficult to ensure protection of information and to save from not
authorized accesses to services because users can establish sessions between other
users and escape somehow authorization or identification. This problem is an open