basic gsm

GSMWireless Technologies

2The contents here are for Aricent internal training purposes only and do not carry any commercial value

Objectives

At the end of this module, the trainee should be able to:– Define wireless technology– Define basic terms like :

• cell, frequency reuse, handover, Access scheme – Explain the network components in GSM network

Draw flows for key GSM procedures like• Location Update• Originating Call• Terminating Call


What is Wireless network?

Communication between two network elements via a radio interface.

Switch

Access Network


Access without wire

How does user access network in absence of wire ?


Radio Access Schemes

TDMA: Time Division Multiple Access

Time

Frequency

Single User Time Slice



• FDMA: Frequency Division Multiple Access

Time

Frequency

Single User Frequency Slice



• Hybrid FDMA/TDMA

Time

Frequency

Single User Bandwidth


Concept of Cell/Frequency reuse

– Cell is a small geographic area– Every cell is covered by one base station having a set of frequencies– With usage of multiple cells a large geographic area can be covered – Frequency is reused to maximize the use of allotted spectrum in such

a way that interference is minimized


Categorization of Cells

Depending upon cell size, the cell is given a specific name:

– Large cell: 10 - 35 KM• In rural areas, coverage is key

– Small Cell: 1 - 3 KM• In urban areas, capacity is key

– Micro Cell: 100 - 300 M

– Pico Cell: 10 - 30 M


Consequences of removing wire -Mobility

Location Management

• Handover • When a mobile moves into a different cell while a

conversation is in progress, the call is transferred to new cell

• Roaming


MSC/VLR

BSC

BSC

BSC

MSC/VLR

BTS

BTS

BTS

BTS

Handover


Soft Handover :- Make-before-break type Connection is established first. After that old channel is released

Hard Handover :- Break-before-make type

Connection with old cell is broken before establishing new connection

Type Of HandOver (HO)


Intra Cell Handover :- Handover from one channel/timeslot to another within same cell

Type Of Handover

Inter Cell Handover :- Handover from one cell to another cell

Inter BSC Handover :-Handover when both the source and destination cells are controlled by different BSC

Inter MSC Handover :- Handover when both the source and destination cells are controlled by different MSC


Roaming

Network A Network B

When subscriber of network A moves to another geographical area and receives services from another network B


Wireless Networks

Started with First Generation Networks ( Early1980s )– No or minimum standardization.– Analog– Deployed and used within regional boundaries

Evolved to Second Generation Networks– Standards defined. – Digital– Roaming across network boundaries possible.– GSM: one of the most prevalent standard for

2G mobile networks


Issues in wireless networks

Multipath propagation Path loss Radio Signal Interference Network Security


Advantages

Wireless Network Advantages:– Provides Mobility– Easy installation. No cabling required.


GSM Radio interface

GSM radio interface is mix of FDMA and TDMA

FDMA divides the frequency spectrum into small slices– Carrier Separation is 200 kHz

TDMA divides each channel into 8 timeslots such that each carrier is shared by 8 users

The basic radio resource is a time slot with a duration of 577 µs


Various GSM based Systems Bands

P-GSM ( Primary GSM ) up-link 890-915MHz 200KHz Carrier down-link 935-960MHz

E-GSM ( Extended GSM ) up-link 880-915MHz 200KHz Carrier down-link 925-960MHz


Services provided by GSM:User perspective

Speech Services Data services SMS

– PTP– Cell Broadcast

Supplementary services– Call barring ,call forwarding etc


GSM Network Architecture - Nodes

MS

MS

BTS

BTS

BTS

BSC

BSC

MSC

VLR GMSC

HLR

PSTN

EIR

AuC


GSM Network Components

Mobile Station– SIM– Mobile Equipment

Access Network – Base Transceiver Station (BTS)– BSC (Base Station Controller)

Core Network– Home Location Register (HLR)– Authentication Center (AuC)– Equipment Identity Register (EIR)– Short Message – Service Center (SM-SC)– Mobile Switching Center (MSC)/Visitor Location Register (VLR) – Gateway Mobile Switching Center (GMSC)


Mobile Station

Comprises of two separate units, viz. SIM and ME. The Structure provides terminal mobility. Subscriber Identity Module (SIM): Belongs to the

operator (e.g. Bharti, Hutch). – Contains the permanent data of subscriber (e.g. IMSI).

Mobile Equipment is developed by equipment manufacturers (e.g. Nokia, Alcatel).– Contains Mobile Termination and User Interface functions.– Communicates with SIM


Access Network: Overview

All the radio-related functions activities have been separated from the MSC and concentrated in the BSS.

The BSS consists of two components– Base Transceiver Station – Base Station Controller


Base Transceiver Station (BTS)

It takes care of – air interface signaling – speech processing – ciphering

It consists of – radio transmitter & receiver,– antennas and signal processing specific to

radio interface


Base Station Controller (BSC)

Controls the radio network Main responsibilities are

– Controls all BTS under its purview. – Connection establishment between MS and

NSS– Handover Management


Core Network: Overview

Core Network provides core functionality including:– Mobility management,

– Call handling,

– Subscriber data management

– Authentication, etc.

Includes– Home Location Register (HLR)

– Authentication Center (AuC)

– Equipment Identity Register (EIR)

– Short Message Service Center (SMSC)

– Mobile Switching Center (MSC)/Visitor Location Register (VLR)

– Gateway Mobile Switching Center (GMSC)


Home Location Register: Overview

HLR is the master database for a subscriber (i.e. it holds the subscription data).

– Each subscriber is assigned to only one HLR.

HLR maintains and provides subscriber data to other network entities on demand (i.e. pulled by network entities).

– E.g. During Location Update In certain cases, the subscriber data is sent to the

network entities (e.g. when the data is modified by operator, the HLR pushes the data to network entities).


Authentication Center (AuC)

AuC holds authentication information. – This information is used for authentication and other security functions.

At the center of all AuC functions is a secret key that is shared between the AuC and the SIM of the UE. – This secret key facilitates roaming of a subscriber because even in a

visited network, the MSC/VLR can authenticate the user by obtaining security information from AuC.

The AuC itself does not take part in the authentication. – This function is performed by the MSC/VLR. – Role of the AuC is to provide MSC/VLR with the necessary information

that can be used by MSC/VLR for authentication.


Equipment Identity Register (EIR)

EIR monitor the legitimacy of a User Equipment (UE) used in the UMTS network,

EIR holds the list of IMEI used in the GSM network.


Short Message Service Center

SMSC is responsible for relaying store‑and‑forwarding

of short messages between an MS and a Short Message Entity


Mobile Switching Center (MSC)

MSC performs all functions necessary to handle the circuit switched services to and from the MS.

It is customary to represent VLR and MSC as MSC/VLR. – They are generally integrated.

Like HLR, Visitor Location Register (VLR) is a repository of information. VLR tracks the location of the MS


Gateway Mobile Switching Center

Similar to MSC Switches voice cells. Provides external interface with PSTN and other mobile

networks.


Hierarchical Organization of GSM N/w (1)

LA1 LA2

LA3 LA4

PLMN


Hierarchical Organization of GSM N/w(2) Public Land Mobile Network (PLMN): It is at the

highest level of the hierarchy.

PLMN ID = MCC+MNC

Location Area (LA): LA is defined as an area in which an MS may move freely without updating its current location at the VLR. Group of cells are given a single Location Area ID (LAI)

LAI = PLMN ID + LAC


Hierarchical Organization of GSM N/w (3)

Cells: At the lowest level of hierarchy is the cell. Each cell is

identified by the Cell Identity (CI). A CI is unique within a

location area. To identify a cell uniquely across PLMNs, an

identity called the Cell Global Identity (CGI) is defined. CGI

is obtained by the concatenation of LAI and the CI.

– CGI = LAI+CI


Addresses and Identifiers


GSM Procedures

Key Procedures in GSM Location Update & Mobile Registration Mobile Originated Call Paging Mobile Terminated Call


Mobile Registration in the network

MS BTS BSC (G)MSC VLR HLR Action

Channel Request

Channel Assignment

Location Update Request

Authentication Request

Authentication Response

Comparison of Authentication params

Accept LUP and alloc TMSI

Ack of LUP and TMSI

Entry of new area and identity into VLR and HLR

Channel Release


Location Registration

MSC/VLR

HLR

PreviousVLR


Location Registration (2)

MSC/VLR

HLR

PreviousVLR

1. LU request<TMSI, LAI>

2. Request IMSI

5. Subscriber Data req <1..N>

4. Update Location req

3. IMSI


Location Registration (3)

MSC/VLR

HLR

PreviousVLR

8. LU Ack<New TMSI>(Store TMSI)

6. Subscriber Data ack <1..N>

7. Update Location ack

9. Cancel Location


Mobile Termination for Roaming

VMSC

HLR

Gateway MSC

3. Provide Roaming Identification (PRI) req

<IMSI>

2. Send Routing Information req<MSISDN>

4. PRI Ack<MSRN>

1. Initial Address Message (IAM)

<MSISDN>


Mobile Terminations and Paging

VMSC

HLR

Gateway MSC

6. Initial Address Message <MSRN>

5. Send Routing Information ack<MSRN>

7. Paging<TMSI>

8. Paging Resp.<Cell ID> 9. SETUP


Mobile Originated Call

MS

MS

BTS

BTS

BTS

BSC

BSC

MSC

VLR

GMSC

HLR

PSTN

EIR

AuC

Req for dedicated channel for communication (RACH)Alloc of dedicated channel(AGCH)Call establishment request(SDCCH)Authentication request(SDCCH)Authentication response(SDCCH)

Authentication response(SDCCH)

Ciphering command(SDCCH)Ciphering response(SDCCH)

Assignment of TCHCall Setup

Assignment of TCH

Call Setup

Alerting(TCH) Alerting

Alerting(TCH)

Connected(TCH) Connected

Connected(TCH)

Connected

Exchange ofCommunication

Exchange of communication(TCH)

Exchange ofCommunication

Call release

Call release(TCH)

Call release


Mobile Terminated Call

MS

MS

BTS

BTS

BTS

BSC

BSC

MSC

VLR

GMSC

HLR

PSTN

EIR

AuC

Land to Mobile call

Query forVLR info

Reply

Routeto MSC

Query VLR for LAC andTMSIPaging

the area

Paging

Paging


References

The GSM system for mobile communications - Mouly, M., Pautet, M.

GSM System Engineering - Asha Mehrotra ETSI GSM Technical Specifications – www.etsi.org GSM presentation by Ashok Kumar GSM presentation by Sumit/Nishit

http://www.etsi.org/


Acknowledgements


Disclaimer

“Aricent makes no representations or warranties with respect to contents of these slides and the same are being provided “as is”. The content/materials in the slides are of a general nature and are not intended to address the specific circumstances of any particular individual or entity. The material may provide links to internet sites (for the convenience of users) over which Aricent has no control and for which Aricent assumes no responsibility for the availability or content of these external sites. While the attempt has been to acknowledge sources of materials wherever traceable to an individual or an institution; any materials not specifically acknowledged is purely unintentional”

Thank You

basic gsm

Documents

commercial value access

commercial value soft

commercial value consequences

commercial value objectivesat

large cell

different cell

old cell

keymicro cell