GSM Prepared by : Eng. Mahmoud Yousry
Sep 06, 2015
GSMPrepared by :
Eng. Mahmoud Yousry
Contents
Introduction
Basic GSM Network Architecture
Air interface
Traffic cases
Planning
Introduction
History of GSM system.
Wireless transmission
Multiple Access Techniques
Wireless Challenges
What is the GSM?
GSM is the Global System for Mobile telecommunications.
It is the European standard for the Mobile telecommunications and it is considered as one of the most popular standard worldwide.
It is known as the second generation mobile telecommunications system 2G system.
It is used in Egypt by the three existing operators; Mobinil and Vodafone; Etisalat.
Evolution of GSM
1982, CEPT conference is held to specify a common European mobile telecommunications in the band of 900 MHz.
1987, GSM specs drafted TDMA+FDMA, Time multiplexing of order 8 with slow frequency hopping.
1992, first GSM network was commercial in Finland .
Global System for Mobile (GSM) is a second-generation digital cellular telephone system.
Evolution of GSM
1996, first GSM operator in Egypt owned by the government.
1998, Egypt privatizes its GSM operator.
1998, Click GSM commercial launch.
GSM became the world's leading and fastest growing mobile standard, spanning over 174 countries, serving more than one in ten of the world's population.
2000, First GPRS roll out.
What is the GPRS?
GPRS is the General Packet Radio Service.
GPRS is defined by 3GPP (Third Generation Partnership Project) and is employed to connect mobile cellular users to PDN (Public Data Network).
Within the GSM network it shares the network databases and radio access network.
It is known as the 2.5 generation mobiletelecommunications system 2G system.
GSM to GPRS development
GSM offers circuit-switched with good voice quality, but it is providing data rates of 9.6 kbps which is too slow.
In 1999 General Packet Radio Service (GPRS) reuses the existing GSM infrastructure to provide higher data rate
It was lunched to increase the data rate to 115 kbps by using the packet-switched in data transmission and defining new coding scheme.
In 2001 Evolved Data rate for GSM Evolution (EDGE) offers data rate of 384 kbps by using new modulation scheme.
Now, EDGE is used in Mobinil the first operator in Egypt.
3G Systems
In 2000 the ITU-T was responsible for the IMT-2000 specification, which is meant to be a guideline for every 3G standard
Universal Mobile Telecommunication Service (UMTS) is the marketing name for the 3G has two standardization bodies:
1- 3GPP which uses the W-CDMA technology.
2- 3GPP2 which uses the CDMA2000 technology.
WCDMA as a 3G Approach
The 3G solution for GSM is called WCDMA (Wideband CDMA).
WCDMA requires a new radio spectrum as it operates in ultra wide 5-MHz radio channels.
WCDMA meets the IMT-2000 requirements of 384 kbps outdoors and 2 Mbps indoors.
Evolution Vs. Time
Functionality
& capabilities
Speech
Circuit
data
9.6 kbps
UMTS capable systems
EDGE
384
kbps
WCDMA2 Mbps
1998 1999 2000 2001 20021997 Time
HSCSD
57.6
kbpsCircuit Switched
GPRS
115 kbpsPacket
Switched
Fig 9-1 GSM Data Rates Evolution
Introduction
History of GSM system.
Wireless transmission
Multiple Access Techniques
Wireless Challenges
Wireless transmission
source DestinationTransmission medium
Wireless transmission
The kinds of transmission medium :
1- Twisted-pair: It has very low bandwidth and it is easily tapped either physically or by monitoring its electromagnetic radiation
2- Coaxial cable:
It has greater bandwidth than twisted-pair but it is very expensive.
3- optical fibers: It is very high bandwidth , very high bit rate and inherently transmission medium.
Wireless transmission
4- Radio (wireless):
It is greatly depending on the particular frequency of the electromagnetic wave
Some of their advantages :
a- They are very flexible and suitable for all terrain.
b- Portable system can be installed very quickly
c- There are often the most cost-effective solution
Wireless transmission
As a summary, the radio transmission is used in case of mobile telecommunications. Also, the using of Radio transmission is combined with many problems that may cause the degradation of the service quality.
Introduction
History of GSM system.
Wireless transmission
Multiple Access Techniques
Wireless Challenges
Multiple Access Techniques
What is the Multiple Access Technique?
It is the use of the same transmission media by more than one user.
It is used because the limitation of transmission resources comparing with
the number of users
Multiple Access Techniques
Multiple Access Techniques
Three types of Multiple Access Technique are available:
Frequency Division Multiple Access (FDMA)
Time Division Multiple Access (TDMA)
Code Division Multiple Access (CDMA)
Multiple Access Techniques
Frequency Division Multiple Access (FDMA), divide the available bandwidth into smaller bandwidths and allocated to the users.
Strength
f1 f2 f3
Multiple Access Techniques
Time Division Multiple Access (TDMA), allocate the whole available bandwidth to one subscriber for a certain period of time
Strength
User 1, time=0User 2, time=t0
Multiple Access Techniques
Time Division Multiple Access (TDMA) and Frequency Division Multiple Access.
Strength
User 7User 2
User 1
Multiple Access Techniques
Code Division Multiple Access (CDMA).
Frequency
Strength
Introduction
History of GSM system.
Wireless transmission
Multiple Access Techniques
Wireless Challenges
Wireless Challenges
Radio Communication applied to mobile telephony has the following limitations :
Powerful transmitters & huge equipment are required.
Capacity is limited to the frequency band allocated.
So, in order to get rid of these shortages the cellular system is being used in the mobile communications.
Wireless Challenges
The Area to be covered is
divided into small cells.
So,
Low Transmission power.
Smaller equipment size.
Capacity of the system can be increased by reuse the frequency.
Cell Geometry
The simplest shape is the circular shape
So, why not using this shape???????
Dead Spots
Cell Geometry
So, what are the possible shape and what is the used shape???
RRR
Types of Antennas
Two types of antennas are being commonly used; the Omni-directional antenna and the directional antenna.
A site is the place where the transmission equipment is placed; this maybe on the top of the buildings or on the top of the advertising.
In case of directional type antenna; one site is corresponding to 3 cells and in case of Omni-directional antenna one site is corresponding to one cell and one antenna.
Sectorial CellsOmni-Directional Cell
Frequency Reuse
As the frequency resources that are given to an operator Vodafone or Mobinil are limited; the frequency used in a cell should be reused again in a cell that is away from this cell in order to increase the capacity of the system. Moreover, for a certain frequency 8 time slots are used for more capacity of the system.
So, the access technique that is used is FDMA+TDMA with frequency reused.
Cellular System Concepts
Reuse Pattern(Cluster):
Cells are grouped into Clusters
Available Band is distributed among the cells of the cluster
N is the number of cells in a cluster .
Each frequency is reused after the same distance D
Reuse Plan = (D/R)= 3N
Where R is the cell radius
5
2
3
4
7
1
6
5
N=7 Cell ClusterN=7 Cell Cluster
7 Cell Reuse Plan7 Cell Reuse Plan
2
3
4
7
1
6
5
2
3
4
7
1
6
5
2
3
4
7
1
6
5
2
3
4
7
1
6
5
D
Contents
Introduction
Basic GSM Network Architecture
Air interface
Traffic cases
Planning
Basic GSM Network Structure
Mobil station and SIM card
BSS part
NSS part
OSS
GSM subscriber identifications
Basic GSM Network Structure
BSC
BSC
MSC
MS
MS
MS BTS
BTS
BTS
GMSC
EIR
AUC
HLR
VLR
PSTNISDNPDN
EIR
AUC
HLR
VLR
Basic GSM Network Structure
Mobil station and SIM card
BSS part
NSS part
OSS
GSM subscriber identifications
Mobile Station (MS)
The Mobile Station (MS) is the interface between the user and the network. The MS consists of two independent parts:
Subscriber Identity Module (SIM) card
Mobile Equipment (ME)
+
The Mobile Equipment is said to be a Mobile Station if the Subscriber Identity Module ( SIM Card ) is added to it.
Mobile Station (MS)
MS features are defined as mandatory or optional
Mandatory features are:
Display of called number: Check the dialed number before call setup.
Support of A5/1 and A5/2: Provisions are made for support of up to seven algorithms as well as the support of no encryption. It is mandatory for A5/1 and A5/2 and non-encrypted mode to be implemented, but other algorithms are optional.
Short message indication and acknowledgment: This feature allows the reception of the delivery report of short messages to a MS from a service center.
Emergency call capabilities: It must be possible to make an emergency call even without a valid SIM.
Mobile Equipment (ME)
The MS sends these features to the network when it is powered on.
Optional features are:
On/Off switch: The MS can be equipped with the means of switching its power supply on and off.
Short Message Service Cell Broadcast (SMSCB) screening
Subscriber Identity Module (SIM) card
A SIM card is an electronic smart card, which stores information about the subscription.
Thus it contains a microprocessor and a memory
The SIM stores three types of subscriber related information:
Fixed data stored before the subscription: e.g. IMSI, authentication key and security algorithms.
Temporary network data: e.g. the location area of the subscriber and other data of the subscriber.
Service data
Basic GSM Network Structure
Mobil station and SIM card
BSS part
NSS part
OSS
GSM subscriber identifications
Base Station Subsystem (BSS)
MS
BTS BSC
The Base Station Sub-system (BSS) is responsible for all the radio-related functions in the system.
BSS
Base Transceiver Station (BTS)
A BTS acts as the interface between MSs and the network, by providing radio access to the network subscribers.
Converts the GSM radio signals into a format that can be recognized by the BSC.
Ciphering using the ciphering key.
Channel coding and interleaving.
Records and passes to the BSC the Signal strength measurements.
Base Station Controller (BSC)
The BSC is the central node within a BSS and co-ordinates the actions of Base Stations. The BSC controls a major part of the radio network.
Its main functions can be divided into two types:
During Call Set Up:
Finding the called mobile station by paging.
Allocate the frequency for setting the call.
During Call Progress:
Monitoring the call quality.
Controlling the transmitted power to the MS depending on the location of the MS.
Control the handover for the MS after receiving the power measurements from the MS and from the BTS.
Base Station Controller (BSC)
TRC Handling
The TRC provides the BSS with rate adaptation capabilities. This is necessary because the rate used over the air interface and that used by MSC/VLRs are different -33.8 Kbit/s and 64 Kbit/s respectively. A device, which performs rate adaptation is called a transcoder.
LAI and CGI
LAI: Location Area Identification.
A location area is determined by the operator and it consists of one or more BSC coverage area. It is used to control the subscribers within a coverage area larger than the cell area and smaller than the MSC coverage area.
LAI consists of MCC (Mobile Country Code) + MNC (Mobile Network Code) + LAC (Location Area Code 4 hex digit); for example for Vodafone, Egypt one LAI can be as following: 602 02 8f65
LAI and CGI
CGI: Cell Global Identification.
It is an identifier for the cell of GSM system.
CGI consists of LAI+CI (Cell Identity, 4 hex digit)
Basic GSM Network Structure
Mobil station and SIM card
BSS part
NSS part
OSS
GSM subscriber identifications
Network Switching Subsystem (NSS)
MSC/VLR
HLR
AUC
BSC
NSS
BTS
BTS
Mobile Switching Center (MSC)
The primary node in a GSM network is the MSC. It is the node which controls calls establishment. The primary functions of an MSC include the following:
Switching and call routing to or from MS.
Charging.
Service provisioning.
Control of connected BSCs.
Access to PSTN.
Provides the gateway functionality to other networks.
One MSC controls more than one BSC.
Types of the MSC
VMSC: Visited MSC
There are three types of the MSC, the difference just in the function.
GMSC: Gateway MSC
TMSC: Transit MSC
Visited Mobile Switching Center
Its function is to switch in the level of BSCs and it is combined with a VLR.
MSC/VLR
BSC BSC
Transit Mobile Switching Center
Its function is to switch between the different VMSC. It is not combined with a VLR.
TMSC
VMSC VMSC
Gateway Mobile Switching Center (GMSC)
Its function is to connect the PLMN to the PSTN or to the other PLMN existing in the country.
TMSC
VMSCVMSC
TMSC
VMSCVMSC
Mobinil GMSC
PSTNVodafone
Home Location Register (HLR)
The HLR is a centralized network database that stores and manages all mobile subscriptions belonging to a specific operator.
It acts as a permanent store for a persons subscription information until that subscription is cancelled.
The primary functions of the HLR include: Stores for each mobile subscriber:
Basic subscriber categories. Supplementary services. Current location. Allowed/barred services. Authentication data.
Subscription database management Controls the routing of mobile terminated calls and SMS.
Visitor Location Register (VLR)
The role of a VLR in a GSM network is to act as a temporary storage location for subscription information for MSs, which are within a particular MSC service area.
Thus, there is one VLR for each MSC service area. This means that the MSC does not have to contact the HLR (which may be located in another country) every time the subscriber uses a service or changes its status.
The VLR is always integrated with the MSC.
Visitor Location Register (VLR)
For the duration when the MS is within one MSC service area, then the VLR contains a complete copy of the necessary subscription details, including the following information:
Identity numbers for the subscriber
Supplementary service information (e.g. Does the subscriber has call waiting activated or not)
Activity of MS (e.g. idle or busy)
Current Location Area of MS
Authentication Center (AUC)
To protect GSM systems, the following security functions have been defined:Subscriber authentication: by performing authentication,
the network ensures that no unauthorized users can access the network, including those that are attempting to impersonate others.
Radio information ciphering: the information sent between the network and an MS is ciphered. An MS can only decipher information intended for it.
Authentication Center (AUC)
The information provided by the AUC to ensure authentication processing is called a triplet and consists of:
A non predictable random number (RAND)
A Signed Response (SRES)
A ciphering Key (Kc)
Triplets
Authentication Center
Triplet
RAND SRES Kc
Home Location Register
Triplet
RAND SRES Kc
Visitor Location Register
RAND SRES Kc
The Ciphering Procedure
Base Transceiver StationDecryption
Algorithm A5
Encryption
Algorithm A5
From the BSCTo the BSC
Mobile Station
TDMA
Frame Number
DATA
En
cry
pte
d
DA
TA
DATA
En
cry
pte
d
DA
TA
Encryption
Algorithm A5
Decryption
Algorithm A5
Kc
TDMA
Frame Number
DATA DATA
Kc
Air Interface
The Authentication Procedure
Visitor Location RegisterRand SRES AUC Kc
SRES AUC SRES MS
Barred
Base Station Subsystem
SIM Card
A8
SRES MS
= Access
Kc
SRES MSRand
Rand
A3 KiRand
VLR
Equipment Identification Register(EIR)
In order to block the stolen mobiles equipments; the EIR equipment is used; also in case of the Mobile operator wants to block a certain type of Mobile phones.
Example, In Turkey all the mobile phones bought from outside Turkey are blocked and can not be used before paying fees.
The Mobile equipment is identified by a number called International Mobile Equipment Identity (IMEI). This number is uniquely identifies the MS worldwide.
Equipment Identification Register(EIR)
Because the subscriber and equipment are separate in GSM, it is necessary to have a separate authentication process for the MS equipment.
The equipment identification procedure uses the identity of the equipment itself (IMEI) to ensure that the MS terminal equipment is valid.
EIR
1. IMEI Request
2. IMEI 3. IMEI Check
4. Access/ Barring Data
MSC / VLR
Equipment Identification Register(EIR)
International Mobile Equipment Identity (IMEI)
IMEI
6 Digits
TAC
2 Digits
FAC
6 Digits
SN
TAC: Type Approval Code,
The first two digits are the
code for the country
approval
FAC: Final Assembly Code
SN: Serial Number
Short Message Center (SMC)
An SMC is added to a GSM network to provide one or more of the following messaging services:
Short Message Service (SMS) text messages.
Any service that combined with SMS; like the missed call notification SMS and the voice mail notification SMS.
Basic GSM Network Structure
Mobil station and SIM card
BSS part
NSS part
OSS
GSM subscriber identifications
OSS
The operation and Maintenance center (OMC) is connected to all equipment (the GMSC, MSC, HLR, VLR, AUC, EIR and the BSC).
It can be viewed as a computerized monitoring center were staff can monitor and control the network remotely.
MSC
SMCBSC
HLROMC LAN
OSS
In the GSM network, the implementation of the OMC is called the operation and support sub-system (OSS).
OSS is the functional entity from which the network operator can monitor and control the entire mobile network.
Other functions provided by the OSS include provision of network maintenance support as well as customer support.
Basic GSM Network Structure
Mobil station and SIM card
BSS part
NSS part
OSS
GSM subscriber identifications
Subscribers Identities in GSM
In order to identify the subscribers of the GSM system; the standard defined identities to be assigned to the subscribers uniquely worldwide.
MSISDN: Mobile Subscriber Integrated Service Digital Number
IMSI: International Mobile Subscriber Identity
MSRN: Mobile Subscriber Roaming number
TMSI: Temporary Mobile Subscriber Identity
GSM Identities
CC : Country Code
NDC : National Destination Code
SN : Subscriber Number
Vodafone Egypt MSISDN
20
CC
10
NDC
1100477
SN
Mobile Station ISDN Number (MSISDN)
Vodafone UK MSISDN
44
CC
385
NDC
196099
SN
GSM Identities
International Mobile Subscriber Identity (IMSI)
MCC : Mobile Country Code
MNC : Mobile Network Code
MSIN : Mobile Station Identification Number
Vodafone UK IMSI
234
MCC
15
MNC
1234567890
MSIN
Vodafone Egypt IMSI
602
MCC
02
MNC
1234567890
MSIN
GSM Identities
Temporary Mobile Subscriber Identity Number (TMSI)
The TMSI is allocated to the mobile subscriber in order to be used instead of his IMSI during all radio communications. The purpose is to keep subscriber information confidential on the air interface. So that this make the call is so difficult to be traced.
The VLR is the device responsible for the allocation of the TMSI for the mobile subscriber.
This TMSI is changed at certain events or after a certain time.
The TMSI consists of 4 bytes and determined by the telecom operator.
GSM Identities
Mobile Station Roaming Number (MSRN)
When a mobile terminating call is to be set-up, the HLR of the called subscriber requests the MSC/VLR to allocate an MSRN to the called subscriber.
This MSRN is returned via the HLR to the MSC where the calling subscriber exists.
The routing is done using the MSRN. When the routing is completed, the MSRN is released so it can be used in the routing of another call.
The MSRN is in the same format like that of the MSISDN.
GSM Identities
On the SIM card the following data is included:
IMSI
TMSI
KI
KC
A3 Algorithm
A8 Algorithm
Contents
Introduction
Basic GSM Network Architecture
Air interface
Traffic cases
Planning
Communication Channels types
ApplicationsPropertiesChannel Type
FM radio, televisionOne-way onlySimplex
Police radio, push-to-talkTwo-way, only one at a timeHalf duplex
PSTN, Mobile systemsTwo-way, both at the same
timeFull duplex
A duplex channel, such as that used during a mobile call, uses two frequencies: one to the MS and one from the MS. The direction from the
MS to the network is referred to as uplink. The direction from the network to the MS is referred to as downlink.
Because it requires less power to transmit at lower frequency over a given distance, uplink frequencies in mobile systems are always the lower
band of frequencies this saves valuable battery power of the Mobile station.
Frequency Allocation
GSM 1900GSM 1800GSM 900
1850-1910 MHz1710-1785 MHz890 -915 MHzUplinkFrequency
1930-1990MHz1805-1880 MHz935-960MHzDownlink
60 MHz75 MHz25 MHzBandwidth
80 MHz95 MHz45MHzDuplex Distance
200KHz200 KHz200 KHzCarrier Separation
299374124Radio Channels
FDMA in GSM
Separation between carriers Frequency gap must be sufficient to eliminate interference between adjacent channels.
Where The more the separation the less the co-channel interference but the less the available channels suited in the bandwidth.
It is found that a 200 kHz channel separation is suitable for all systems.
Spectrum Allocation (GSM 900)
GSM 900 Frequency Allocation
F (MHz)915890
Uplink1 2 3 4 121 122 123 124
F (MHz)
Downlink
960935
1 2 3 4 121 122 123 124
890.2
890.4
890.6
935.2
935.4
935.6
200 KHz
1
1
121
121
Downlink 935 960 MHz
Uplink 890 915 MHz
TDMA in GSM
With TDMA, one carrier is used to carry a number of calls, each call using that carrier at designated periods in time .
These periods of time are referred to as time slots .
Each MS on a call is assigned one time slot on the uplink frequency and one on the downlink frequency, and both the same.
It is found that a 8 Time Slots per carrier, called physical channels is suitable for all systems.
Information sent during one time slot is called a burst, and depending on information sent we named what called logical Channels
Physical Channels
GSM band is divided into 124 RF channels, and each channel is divided into 8 time slots using TDMA.
These time slots are called physical channels.
CH 1 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
CH 2 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
CH 3 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
CH 124 0 1 2 3 4 5 6 7 0 1 2 3 4 5
Logical Channels-Traffic Channels
TCH
Traffic Channels
Speech
TCH/FS
Data
TCH/HSTCH/9.6 TCH/2.4
TCH/4.8
Normal Burst
TCH Traffic Channel
TCH/FS Full rate Speech Channel
TCH/HS Half rate Speech Channel
TCH/9.6 Data Channel 9.6kb/s
TCH/4.8 Data Channel 4.8kb/s
TCH/2.4 Data Channel 2.4Kb/s
Traffic Channels
Carries either encoded speech or user data up and down link between a single mobile and a single BTS.
Types of traffic channel:
Full rate (TCH)
Transmits full rate speech (13 Kbits/s). A full rate TCH occupies one physical channel.
Half rate (TCH/2)
Transmits half rate speech (6.5 Kbits/s).
Two half rate TCHs can share one physical channel, thus doubling the capacity of a cell.
Logical Channels-Control Channels
FCCHSCH
CCH Control Channels
DCCH
SDCCH
BCH
BCCH Synch. CH.ACCH
SACCHFACCH CCCH
RACH
CBCH
PCH/AGCH
Control Channels
These are used to carry signaling or synchronization data. They are divided into three types:-
Broadcast CHannels (BCH)
Common Control CHannels (CCCH)
Dedicated Control CHannels (DCCH)
Broadcast Channels
From Single BTS to all the mobiles in the area
Frequency Correction Control CHannel (FCCH) Carries information for frequency correction of the mobile
Synchronization CHannel (SCH) Carries 2 important pieces of information
TDMA frame number (max = 2715684 ) Base station identity Code (BSIC)
Broadcast Control CHannel (BCCH) Broadcasts some general cell information such as:
Location Area Identity (LAI), maximum output power allowed in the cell and the identity of BCCH carriers for neighboring cells.
Common Control Channels
To or from a certain BTS to a single mobile
Paging Channel (PCH)
BTS Transmits a paging message to indicate an incoming call or short message. The paging message contains the identity number of the mobile subscriber that the network wishes to contact.
Random Access Channel (RACH)
MS Answers paging message on the RACH by requesting a signaling channel of SDCCH.
Access Grant Channel (AGCH)
Assigns a signaling channel (SDCCH) to the MS.
Dedicated Control Channels
Stand alone Dedicated Control Channel (SDCCH) The BTS switches to the assigned SDCCH. The call set-up procedure is performed in
idle mode. The BSC assigns a TCH (carrier and time slot) and the MS switches to the assigned SDCCH.
SDCCH is also used to Registration & Authentication
Slow Associated Control Channel (SACCH) BTS Instructs the MS the transmitting power to use and gives instructions on
timing advance (TA).
MS Sends averaged measurements on its own BTS (signal strength and quality) and neighboring BTSs (signal strength). The MS continues to use SACCH for this purpose during a call.
Fast Associated Control Channel (FACCH) Transmits handover information.
Transmits necessary handover information
Burst Structure Types
Normal Burst Structure
Tail Bits
Encrypted Bits
Training Sequenc
e
Encrypted Bits
Tail Bits
Guard Period
3 57 1 26 1 57 3 8.25
0 1 2 3 4 5 6 7
TDMA Frame
156.25 bits in 577 u Sec
The tail bits help the equalizer to determine the start and stop points of the
transmitted bits. They are three bits at the beginning and at the end of the burst
and they are always zeros
Burst Structure Types
Normal Burst Structure
Tail Bits
Encrypted Bits
Training Sequenc
e
Encrypted Bits
Tail Bits
Guard Period
3 57 1 26 1 57 3 8.25
0 1 2 3 4 5 6 7
TDMA Frame
156.25 bits in 577 u Sec
The subscriber speech or data is encrypted into 57 bit blocks. Each burst will
contain two 57 bits blocks from two different speech segments(20m Sec).
Burst Structure Types
Normal Burst Structure
Tail Bits
Encrypted Bits
Training Sequenc
e
Encrypted Bits
Tail Bits
Guard Period
3 57 1 26 1 57 3 8.25
0 1 2 3 4 5 6 7
TDMA Frame
156.25 bits in 577 u Sec
One bit stealing flag will be added to each block to indicate whether the burst is stolen for the FACCH
signaling or used as a normal traffic channel
Burst Structure Types
Normal Burst Structure
Tail Bits
Encrypted Bits
Training Sequenc
e
Encrypted Bits
Tail Bits
Guard Period
3 57 1 26 1 57 3 8.25
0 1 2 3 4 5 6 7
TDMA Frame
156.25 bits in 577 u Sec
The guard period of 8.25 bits length, which is equivalent to about 30 s,
is left at the end of each burst, to prevent overlapping between
consecutive bursts and to facilitate burst synchronization
Burst Structure Types
Normal Burst Structure
It carries information of all logical channels except RACH, SCH and FCCH
Tail Bits
Encrypted Bits
Training Sequenc
e
Encrypted Bits
Tail Bits
Guard Period
3 57 1 26 1 57 3 8.25
0 1 2 3 4 5 6 7
TDMA Frame
156.25 bits in 577 u Sec
Burst Structure Types
Tail Bits Fixed Bits
Tail Bits
Guard Period
3 142 3 8.25
This is the one used by the channel (FCH) for frequency correction of the mobile.
It consists of a long sequence of bits called the fixed bits which are all equal to
zeros, leading to a constant frequency output from the GMSK modulator
0 1 2 3 4 5 6 7
TDMA Frame
156.25 bits in 577 u Sec
Frequency Correction Burst Structure
Burst Structure Types
0 1 2 3 4 5 6 7
TDMA Frame
156.25 bits in 577 u Sec
Tail Bits
Encrypted Bits
Synchronization Sequence
Encrypted Bits
Tail Bits
Guard Period
3 39 64 39 3 8.25
The SCH burst consists of a long synchronization sequence along with the important
information being encrypted and divided into two blocks. The TDMA frame number is
sent on the SCH channel, which carries also the Base station Identity code (BSIC).
The TDMA frame number is used by the mobile to determine which control channels
will be transmitted on that frame.
Synchronization Burst Structure
Burst Structure Types
Tail Bits
Synchronization Sequence
Encrypted Bits
Tail Bits Guard Period
8 41 36 3 68.25
The Access Burst is used by the RACH channel. The mobile sends this burst when
it does not know the distance to its serving BTS, which is the case when the
mobile is switched on or after it makes a handover to a new cell. So this burst
must be shorter in order to prevent it from overlapping with the burst on the next
time slot.
0 1 2 3 4 5 6 7
TDMA Frame
156.25 bits in 577 u Sec
Access Burst Structure
Burst Structure Types
0 1 2 3 4 5 6 7
TDMA Frame
156.25 bits in 577 u Sec
Tail Bits
Encrypted Bits
Training Sequenc
e
Encrypted Bits
Tail Bits
Guard Period
3 58 26 58 3 8.25
The dummy burst is sent from the BTS when there is nothing else to be sent. It
carries no information and it has the same structure of a normal burst with the
encrypted bits replaced by a known bit pattern to the mobile.
Dummy Burst Structure
3 57 1 26 1 57 3 8.25
0 1 2 3 4 5 6 7
156.25 bits in 577 m Sec
Normal burst
1 2 e.g. TCH 26 1 2 e.g. BCCH 51
1 e.g. BCCH 26
1 2 e.g. TCH 51
1 Hyper frame = 2048 Super frames = 2,715,648 TDMA Frames = 3hrs 28 min and 53.76 s
Hyper Frame
12
1
2
2
1 Super frame = 51 TCH Multi frames
1 Super frame = 26 BCCH Multi frames
1 BCCH Multi frame = 51 TDMA Frames1 TCH Multi frame = 26 TDMA Frames
Power Measurements in Active Mode
1. To enable the mobile from making power measurements during a call, the uplink time slot will be delayed by an offset of three time slots from the down link time slot. (The mobile will try to measure the signal strength of these carriers one by one during the time between transmission and reception of the allocated traffic channel)
2. The mobile is informed on the SACCH channel which BCCH frequencies to be measured.
3. To make sure that the measured carriers do not belong to co-channel cells, the mobile will have to check the identity of the adjacent cells by reading the BSIC value sent on the SCH of each cell. This will take place during the idle frame number 26.
(Note) The signal strength of the serving cell is measured during reception of the allocated traffic channel.
4. The mobile will make a list of the strongest six carriers and their BSIC values along with the signal strength of its cell, and reports this list to the BSC via the uplink SACCH channel which is repeated once every 26 frame.
Radio Transmission problems
As it was stated before that the mobile telecommunications will use radio transmission as the transmission technique; the radio transmission is suffering
from many problems which causing unacceptable degradation of the service
quality.
We will discuss these problems in details during our course.
Radio Transmission problems
1. Path Loss
2. Shadowing (Normal Fading)
3. Multipath Fading
a. Rayleigh Fading
b. Time Dispersion
4. Time Alignment
Radio Transmission problems
1. Path Loss
Cause
Due to Increasing distance between MS, and BTS.
Solution
Handover
Radio Transmission problems
2. Shadowing (Normal Fading)
Cause
Due to Obstacles between MS, and BTS.
Solution
increase the fading margin for the receiver in the mobile
station and at the Transceiver station of the mobile network; so that how
weak the signal received is the receiver will still can receive and regenerate
this signal.
Radio Transmission problems
3. Multipath Fading
a. Rayleigh Fading
Cause
Due to different paths of signals between MS, and BTS.
Which cause fading dips as a result of different in phase and amplitude
Solution
Diversity
a. Space Diversity
b. Polarization Diversity
c. Frequency Diversity ( Frequency Hopping)
Radio Transmission problems
3. Multipath Fading
b. Time Dispersion
Cause
Due to different paths of signals between MS, and BTS.
Which cause Intersymbol Interference (ISI)
Solution
Adaptive Equalizer, by using training sequence in bursts
Radio Transmission problems
4. Time Alignment
Cause
Due to different distance of different MSs from BTS (Near-Far)
Solution
Time Advance
Contents
Introduction
Basic GSM Network Architecture
Air interface
Traffic cases
Planning
Traffic cases
Location update
Handover
Mobile originating a call
Mobile terminating a call
SMS
Location Update
Why do we need to update our location data ?
Actually, the location update process is done in aim to exactly identify the location of the subscribers within the network so that any incoming call goes directly to the called subscriber.
To fulfill this aim, one can say that we may update the system with the cell ID each time the subscriber changes his serving cell. The MSC/VLR will now know the exact cell you are roaming in. This will result in a huge amount of location update messages.
An extreme is never to make a location update and to be paged in all the network. This will cause huge amount of paging messages.
Location Update
Types of Location Update
1. Normal Location update within same MSC/VLR service area
2. Normal Location update between 2 different MSC/VLR service areas
3. IMSI attach/detach
4. Periodic Location Update
Normal Location within the same MSC/VLR Service area
BSC
1. The Mobile sends an SDCCH allocation
request message to the BTS on RACCH
2. The BTS responds with the SDCCH
allocation message on AGCH .
3. The mobile sends a location update
request message with its IMSI to the
MSC/VLR on SDCCH
4
4. The MSC/VLR updates the location
information and sends a Location Update
confirmation message on SDCCH
MSC/VLR
Updates
LA Record
5. SDCCH is released
Normal Location Update between 2 different MSC/VLR service areas
Old MSC/VLR New MSC/VLR
NEW BSCOld BSC
LA 1
LA 2
1. The mobile sends a location
update request to the MSC.
2. The new MSC/VLR receives the IMSI
and conclude the its HLR address.
IMSI to MGT
translation
3. The MSC/VLR sends a subscriber
information request with the IMSI
to the proper HLR
4. The HLR stores the address of
the new MSC/VLR
VLR Address
=
Old MSC
VLR Address
=
New VLR
5. The HLR sends the data to the
new MSC/VLR and it is kept there
6. The HLR sends a location
cancellation message to the old
MSC/VLR to remove the data
HLR
7. The new MSC/VLR sends a location
updating confirmation message to
the mobile
IMSI Detach
1. At power off, the MS asks for a signaling SDCCH channel.
2. The MS uses this signaling channel to send the IMSI detach message to the
MSC/VLR.
3. In the VLR, an IMSI detach flag is set for the subscriber. This is used to reject
incoming calls to the MS.
4. The subscribers record is kept in the VLR for a certain time; i.e. 24 hour then it is removed if the subscriber doesnt switch on his mobile.
IMSI Attach
IMSI attach is a complement to the IMSI detach procedure. It is used by the
mobile subscriber to inform the network that it has re-entered an active state
and is still in the same location area. If the MS changes location area while
being switched off, a normal location update takes place.
1. The MS requests a signaling SDCCH channel.
2. The MSC/VLR receives the IMSI attach message from the MS.
3. The MSC/VLR sets the IMSI attach in the VLR. The mobile is now ready
for normal call handling.
4. The VLR returns an acknowledgment to the MS.
IMSI Attach
MSC/VLRBSC
1
2 3
4
Periodic Location Update
Periodic location update is a routine task performed by the network if the MS doesnt make any network action (sets a call, sends SMS, location
update, receives a call,. etc)
If the MS doesnt respond to this periodic location update, it will be marked as implicitly detached. ( Temporarily out of service )
Traffic cases
Location update
Handover
Mobile originating a call
Mobile terminating a call
SMS
Handover
Handover is to keep continuity of the call when the subscriber is roaming along the network moving from one cell to another and moving between different nodes in the network.
During call, the MS is continuously measuring transmission quality of neighboring cells and reports this results to the BSC through the BTS.
The BSC, being responsible on supervising the cells, is responsible of handover initiation.
Good neighbor relations between cells is an important factor in keeping the network performance in the accepted level.
Types of Handover
1. Intra BSC Handover:
When the cell to which the call will be handed over belongs to the same BSC of the serving cell.
2. Inter BSC / Intra MSC Handover:
When the cell to which the call will be handed over belongs to the different BSCs but to the same serving MSC.
3. Inter MSC
When the cell to which the call will be handed over belongs to the different BSC and different MSC.
BSC
New Cell
1. The BSC decides from the power measurement
reports that the call must be handed over to
another cell
2
2. The BSC checks for an vacant traffic TCH in the new cell and orders this cell to activate the TCH
3
3. The BSC orders the serving cell to send a message to the MS on FACCH telling the
information
of new TCH
4. The MS tunes to the new frequency and Sends handover access burst
45. The new cell detects the handover burstand sends information about the suitable
timing advance to the MS
5 6. The MS sends a HO complete message to the new cell
6
7. The new cell sends a message to the BSC that the handover is successful
7
8. The BSC orders the old Cell to release the TCH
8
Old BSC New BSC
MSC/VLR
Old MSC
Old BSC
New MSC
New BSC
Traffic cases
Location update
Handover
Mobile originating a call
Mobile terminating a call
SMS
1. The MS uses RACH to ask for a signaling channel.
2. The BSC/TRC allocates a signaling channel, using
AGCH.
3. The MS sends a call set-up request via SDCCH to the
MSC/VLR. Over SDCCH all signaling preceding a
call takes place. This includes:
Marking the MS as active in the VLR The authentication procedure Start ciphering Equipment identification Sending the B-subscribers number to the network Checking if the subscriber has the service Barring
of outgoing calls activated4. The MSC/VLR instructs the BSC/TRC to allocate an
idle TCH. The BTS and MS are told to tune to the
TCH.
5. The MSC/VLR forwards the Bnumber to an exchange in the PSTN, which establishes a connection to the
subscriber.
6. If the B-subscriber answers, the connection is
established.
BSC
MSC
Traffic cases
Location update
Handover
Mobile originating a call
Mobile terminating a call
SMS
1. Calling a GSM subscribers
2. Forwarding call to GMSC
3. Signal Setup to HLR
4. 5. Request MSRN from VLR
6. Forward responsible MSC to GMSC
7. Forward Call to current MSC
8. 9. Get current status of MS
10.11. Paging of MS
12.13. MS answers
14.15. Security checks
16.17. Set up connection
Traffic cases
Location update
Handover
Mobile originating a call
Mobile terminating a call
SMS
Short Messages
The Short Message Service (SMS) allows a mobile subscriber to send and receive text messages composed of 160 characters at most.
These messages can be read on the phones LCD panel and they are delivered using the signaling channel SACCH; so short messages can be received while a call is in progress.
The short messages sent or received are handled by the Short Message Service Center (SMSC), which consists of three parts:
Service Center SC
Short Message Service GMSC SMS-GMSC
Short Message Service Inter-working MSC SMS-IWMSC
Short Message Delivery
Process
1
MSC1
HLR
3
SMS-GMSC
SC
SMS-IWMSC
4
2
MSC25
6
7
8
MTMOMT
Unsuccessful delivery
HLR
SMS-GMSC
SC
SMS-IWMSC
MSC2
1
2
5
6
MT
43
Retry method
Usage of Alert Service Center in the HLR
Contents
Introduction
Basic GSM Network Architecture
Air interface
Traffic cases
Planning
GSM Coverage Plan
To provide coverage for a large service area of a mobile network we have two Options:
(A) Install one transceiver with high radio power at the center of the service area
Drawbacks: The mobile equipments used in this network should have
high output power in order to be able to transmit signals across the coverage area.
The usage of the radio resources would be limited.
GSM Coverage Plan
(B) Divide the service area into smaller areas (cells) Advantages:
Each cell as well as the mobile handsets will have relatively small power transceivers.
The frequency spectrum might be reused in two far separated cells. This yields:
1- Unlimited capacity of the system.
2- Good interference characteristics
Cell Geometry
Problem of omni directional antennas
Dead Spots
Cell Geometrical Shape
Differentiation between these three shapes will be in order to optimize the number of cells required to cover a given service area against the cell transceiver power. By some calculations, you will find that using hexagonal shaped cells achieves the optimum.
RR
R
Cell Geometrical Shape
Umbrella Cell
Normal Cell Normal Cell
Cell Geometrical Shape
Umbrella cell
Macro cell
Slow moving subscribers
Fast moving subscribers
Pico
cellIn buildingcoverage
Clusters
Cluster is a set of cells where the frequency is not being reused within this cluster.
Cluster can be 3, 4, 7 and 9 cells.
Sectorization
sectroized CellsOmni-Directional Cell
3/9 Cluster
A3
A2
A1
B3
B2
B1
C3
C2
C1
A3
A2
A1
B3
B2
B1
C3
C2
C1
A3
A2
A1
B3
B2
B1
C3
C2
C1
A3
A2
A1
B3
B2
B1
C3
C2
C1
A3
A2
A1
B3
B2
B1
C3
C2
C1
A3
A2
A1
B3
B2
B1
C3
C2
C1
A3
A2
A1
B3
B2
B1
C3
C2
C1
A3
A2
A1
B3
B2
B1
C3
C2
C1
A3
A2
A1
B3
B2
B1
C3
C2
C1
4/12 Cluster
A3
A2
A1
B3
B2
B1
C3
C2
C1
D3
D2
D1
A3
A2
A1
B3
B2
B1
C3
C2
C1
D3
D2
D1
A3
A2
A1
B3
B2
B1
C3
C2
C1
D3
D2
D1
A3
A2
A1
B3
B2
B1
C3
C2
C1
D3
D2
D1
A3
A2
A1
B3
B2
B1
C3
C2
C1
D3
D2
D1
A3
A2
A1
B3
B2
B1
C3
C2
C1
D3
D2
D1
A3
A2
A1
B3
B2
B1
C3
C2
C1
D3
D2
D1
A3
A2
A1
B3
B2
B1
C3
C2
C1
D3
D2
D1
A3
A2
A1
B3
B2
B1
C3
C2
C1
D3
D2
D1
A3
A2
A1
B3
B2
B1
C3
C2
C1
D3
D2
D1
A3
A2
A1
B3
B2
B1
C3
C2
C1
D3
D2
D1
7/21 Cluster
A3
A2
A1
C3
C2
C1
D3
D2
D1
B3
B2
B1
E3
E2
E1
F3
F2
F1
G3
G2
G1
A3
A2
A1
C3
C2
C1
D3
D2
D1
B3
B2
B1
E3
E2
E1
F3
F2
F1
G3
G2
G1
A3
A2
A1
C3
C2
C1
D3
D2
D1
B3
B2
B1
E3
E2
E1
F3
F2
F1
G3
G2
G1
A3
A2
A1
C3
C2
C1
D3
D2
D1
B3
B2
B1
E3
E2
E1
F3
F2
F1
G3
G2
G1
A3
A2
A1
C3
C2
C1
D3
D2
D1
B3
B2
B1
E3
E2
E1
F3
F2
F1
G3
G2
G1
Frequency Reuse
If the GSM900 system has 124 Absolute Radio Frequency Traffic Channels, and if we are using only in our network 60 of them, then we can only serve 8 x 60 = 480 Calls if we only use the frequency once.
However, a cellular network overcome this constraint and maximizes the number of subscribers that it can serve by using frequency re-use.
The frequency reuse is performed by dividing the whole available frequencies between a group of neighboring cells which is called frequency reuse pattern or a Cluster, and then repeat this cluster over the whole network
Co- Channel Interference
Co-channel interference is caused by short distance between the cell and other cell that use the same frequency.
To overcome this type of interference. Each frequency is reused after the same distance D
Reuse Plan = (D/R)2 = 3N. Where N is the number of cells per cluster
Adjacent Channel Interference
Adjacent frequencies, that are frequencies shifted 200kHz from the carrier frequency, must be avoided in the same cell and preferably in neighboring cells also .
To overcome this type we must make good planning for the frequencies in the cluster
Frequency Planning
A3A2
A1
B3B2
B1C3
C2C1
A3A2
A1
B3B2
B1C3
C2C1
B3
B2B1
C3C2
C1
A3A2
A1
B3B2
B1C3
C2C1
A3A2
A1
C3B3A3C2B2A2C1B1A1Frequency
group
727170696867666564
Channels 818079787776757473
878685848382
Frequency Planning
In a real network the allocation of channels to cells will not be as uniform as in table, as some cells will require more channels and some will require less.
In this case, a channel may be taken from a cell with low traffic load and moved to one with a higher traffic load.
However, if doing so, it is important to ensure that interference is still minimized.
Which Cluster Size to use?
Carrier to interference ratio
Its the difference in power level between the carrier in a given cell and the same carrier received from the nearest cell that reuses the same frequency.
Number of frequencies per site
Traffic Channels
C/I Ratio
3/9 High High Low
4/12 Medium Medium Medium
7/21 Low Low High
END