Gsm

Wireless communication and mobile Programming

Overview of GSM

GSM Stands for global system for mobile Communications

GSM is based on set of standards formulated in early 1980.

GSM was developed by ETSI(European Telecommunications Standards Institute)

GSM offers data services with maximum bit rate 14.4 kb/s GSM uses combination of FDMA ,TDMA

FDMA/TDMA

Uplink/Downlink of 25MHz 890 -915 MHz for Up link 935 - 960 MHz for Down link

Combination of frequency division and time division multiplexing

FDMA 124 channels of 200 kHz

TDMA Burst

Air Interface: MS to BTS

Number of channels in GSM

Freq. Carrier: 200 kHz TDMA: 8 time slots per freq carrier

No. of carriers = 25 MHz / 200 kHz = 125 Max no. of user channels = 125 * 8 = 1000

Considering guard bands = 124 * 8 = 992 channels

Cellular Concept

Cellular Concept Base stations (BS): implement space division multiplex

Each BS covers a certain transmission area (cell) Each BS is allocated a portion of the total number of

channels available Cluster: group of nearby BSs that together use all

available channels Mobile stations communicate only via the base station,

using FDMA, TDMA, CDMA…

Advantage of FDMA Simple algorithmically, and from a hardware standpoint Fairly efficient when the number of stations is small and

the traffic is uniformly constant No need for network timing Capacity increase can be obtained by reducing the

information bit rate and using efficient digital code

Advantage of TDMA Flexible bit rate No frequency guard band required No need for precise narrowband filters Easy for mobile or base stations to initiate and execute

hands off Extended battery life

GSM System Hieararchy

Architecture of GSM

Mobile Station (MS)

MS consists of following two components Mobile Equipment (ME) Mobile Subscriber Identity Module (SIM)

Removable plastic card Stores Network Specific Data such as list of

carrier frequencies and current Location Area ID (LAI).

Stores International Mobile Subscriber Identity (IMSI) + ISDN

Stores Personal Identification Number (PIN) & Authentication Keys.

Also stores short messages, charging information, telephone book etc.

Base Transceiver Station (BTS)

A Base Station Transceiver (BTS) is a radio transceivers station that communicates with the mobile stations. Its backend is connected to the BSC

One per cell Consists of high speed transmitter and receiver Function of BTS

Provides two channels

Signalling and Data Channel

Base Station Controller (BSC)

Controls multiple BTS A Base Station Controller (BSC) is a high-capacity switch

with radio communication and mobility control capabilities.

The functions of a BSC include radio channel allocation, location update, handover, timing advance, power control and paging.

Time and frequency synchronization signals to BTSs Time Delay Measurement and notification of an MS to

BTS Power Management of BTS and MS

The mobile switching center (MSC) performs the telephony switching function.

Switching node of a PLMN (Public Land Mobile Network)

Allocation of radio resource (RR) Handoff

Mobility of subscribers Location registration of subscriber

There can be several MSCs in a PLMN

Mobile Switching Centre (MSC)

Connects mobile network to a fixed network Entry point to a PLMN

Usually one per PLMN Request routing information from the HLR and routes the

connection to the local MSC

Gateway MSC (GMSC)

HLR/VLR

HLR - Home Location Register Contains semi-permanent subscriber information For all users registered with the network, HLR keeps user profile MSCs exchange information with HLR When MS registers with a new GMSC, the HLR sends the user

profile to the new MSC VLR - Visitor Location Register

Contains temporary info about mobile subscribers that are currently located in the MSC service area but whose HLR are elsewhere

Copies relevant information for new users (of this HLR or of foreign HLR) from the HLR

VLR is responsible for a group of location areas, typically associated with an MSC

AuC/EIR

AuC: Authentication Center The authentication center (AUC) provides authentication

and encryption parameters that verify the user's identity and ensure the confidentiality of each call.

The AUC protects network operators from different types of fraud found in today's cellular world.

The GSM has standard encryption and authentication algorithm which are used to dynamically compute challenge keys and encryptions keys for a call.

is accessed by HLR to authenticate a user for service Contains authentication and encryption keys for

subscribers EIR: Equipment Identity Register The equipment identity register (EIR) is a database that

contains information about the identity of mobile equipment that prevents calls from stolen, unauthorized, or defective mobile stations

Registration Process

Call Establishment

Example: Outgoing call setup

User keys in the number and presses send Mobile transmits Set Up message on uplink signaling channel

(RACH) to the MSC MSC requests HLR/VLR to get subscriber parameters

necessary for handling the call. VLR/HLR sends Complete Call msg to the MSC MSC sends an Assignment message to the BSS and asks it to

assign TCH for the MS BSS allocates a radio channel (TCH) and sends an Assignment

message to MS over SDCCH MS tunes to the radio channel (TCH) and sends an Assignment

Complete message to the BSS. BSS deallocates SDCCH. Now voice path is established

between MS and MSC MSC completes the PSTN side of the signaling.

Mobility management Why mobility and location management is required?

Very nature of mobile communications implies that the MS is constantly changing its location, a need for tracking mobile and restructuring existing connection as its moves, mobility management and location management handle the operation require for this purpose.

Location management Location management involve tracking of the location of

the MS as its moves for delivery of voice and data service. Basic operation:- Location update, paging, location

information Dissemination Location updates are the message sent by the MS

regarding its changing points of access to the fix network Updates may have varying frequency Types of Location update:-

Location registration:-

Location registration takes place when MS is turned on Network send TMSI and LAI to MS store in SIM Generic Location update perform when LAI is different Chanel request message sent to target MSC/VLR include

TMSI,LAI

Generic location update procedure

Periodic:-periodic location update is carried out when network does not receive any location update request from MS in specified time

Issue:- Unnecessary cost in terms of both usage of spectrum and

network resource if location updates are too frequent Large area to be paged if location updates are few. Location update algorithms :- Static and Dynamic Static:- In static location updates the topology of cellular

network decides when the location updates need to be initiated

Each group of cell assign LAI BS broad cast LAI periodically Problem:-ping-pong effect Solution:-distance based, timer based Dynamic:- in dynamic location update mobility of users

and call pattern is used in initiating the location update Dynamic location update is state based and user profile

based State based decision are made based on current state

information

state information include time elapsed, distance travel, no of LA crossed

Profile based maintain sequential list of LA

Paging scheme:-Paging is broadcasting message in cell or group of cell to bring out response from MS

Blanket paging:-paging the MS in all cell of LA simultaneously

Location information Dissemination:- Every MS associated with home network and home database

Handover Handoff management required to handle on going

connection when mobile terminal moves from coverage of one point of access to another

Maintaining the traffic connection with a moving user when crossing cell boundaries

Occurs when the quality or the strength of the radio signal falls below certain parameters (signal quality reason)

Occurs when the traffic capacity of a cell has reached its maximum or is approaching (traffic reason)

Relative Signal Strength

Relative Signal Strength with Threshold

For a high threshold (e.g., Th1), this scheme performs the same as the relative signal strength scheme

If the threshold is set quite low (e.g., Th3), the mobile may move far into the new cell

Threshold should not be used alone because its effectiveness depends on prior knowledge of the crossover signal strength between the current and the candidate base stations

Relative Signal Strength with Threshold and Hysteresis

Handover occurs only if the current signal level drops below a threshold, and

the target base station is stronger than the current one by a hysteresis margin H

Hard handover

“Break before make” Connection is released before making the new connection Causes a short cut in the connection The terminal is linked to no more than one base station at

any given time Primarily used in FDMA and TDMA, where different

frequency ranges are used in adjacent cells

Soft handover

“Make before break” New connection is established before the old connection is

released, avoiding a cut in the connection during handover After the successful handover, the old connection is

released Used in CDMA, where adjacent cells use the same

frequency range

Control over Handover

Network-controlled Handover (NCHO) Network measures the transmission quality via base

stations and decides when handover should be executed Handover process (including data transmission, channel

and network switching) takes 100-200ms Mobile-assisted Handover (MAHO) Mobile terminal continuously measures signal strength

from serving and neighboring base stations and sends the recorded values to the serving base station

On the basis of these values, the network decides when handover should take place

Handover time between handover decision and execution is approximately 1 second

Mobile-controlled Handover (MCHO) Mobile terminal is completely in control of the handover

process, i.e., it measures signal strength and decides on handover

Very short reaction time (on the order of 0.1 seconds)

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