cellular mobile systems and services 07

8/3/2019 Cellular Mobile Systems and Services 07

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GSM ARCHITECTURE

A GSM system is basically designed as a combination of three major subsystems: the network

subsystem, the radio subsystem, and the operation support subsystem. In order to ensure that

network operators will have several sources of cellular infrastructure equipment, GSM decided

to specify not only the air interface, but also the main interfaces that identify different parts.

There are three dominant interfaces, namely, an interface between MSC and base transceiver

station, and an Um interface between the BTS and MS.

GSM Network Structure

Every telephone network need a well designed structure in order to route the incoming call to

the correct exchange and finally to the called subscriber. In a mobile network this structure is

of great importance because of the mobility of all its subscribers. In the GSM system the

network is divided in to the following partitioned areas.

y GSM service area

ell

Location Area

M

S

C

PL

M

N

GSM service Area



y PLMN service area

y MSC service area

y Location area

y Cells

The Mobile Station is carried by the subscriber; the Base Station Subsystem controls the radio

link with the Mobile Station. The Network Subsystem, the main part of which is the Mobile

services Switching Center, performs the switching of calls between the mobile and other fixed

or mobile network users, as well as management of mobile services, such as

authentication. Not shown is the Operations and Maintenance center, which oversees the

proper operation and setup of the network. The Mobile Station and the Base Station

Subsystem communicate across the Um interface, also known as the air interface or radio

link. The Base Station Subsystem communicates with the Mobile service Switching Center

across the A interface.



The above figure depicts a typical GSM network (called, Public Land Mobile Network or

PLMN) infrastructure.

AUC- Authentication Center

BSC -Base Station Controller

BSS -Base Station Subsystem

BTS -Base Transceiver System (Antenna System + Radio Base Station)

EIR -Equipment Identification Register (for IMEI verification)

IMEI -International Mobile Equipment Identity

FNR -Flexible Numbering Register (for number portability)

GMSC- Gateway MSC

HLR -Home Location Register

ISDN -Integrated Services Digital Network

IWF -Interworking Function

ILR -Interworking Location Register (for roaming between AMPS and GSM system)

IWMSC -Interworking MSC

MS -Mobile Station

MSC- Mobile Switching Center

NSS -Network Switching Subsystem

OSS -Operation and Support System

PDN -Public Data Network

PSTN -Public Switched Telephone Network

SMS -Short Message Service

VLR -Visitor Location Register

The GSM divides the infrastructure into the following three parts.

y Network Switching Subsystems (NSS)

y Base Station Subsystem (BSS)

y Network Management Subsystem (NMS)

If we count the Mobile Station (MS) or cell-phone is the 4th element.

The figure below depicts only the basic elements of the network architecture.



.

Any telecommunications network requires some kind of NMS. A part of NMS is generic for

any telecom system. The billing and messaging are two examples. The core of the NSS is the

MSC (Mobile Switching Center) which is basically a PSTN switch with mobility management

related enhancement/add-on. The BSS is entirely new (compared to PSTN) that are required

for wireless access and mobility. The following sections of this document provide an overview

of the network elements and their functions. The role of these elements will be clearer as we

learn more.

Network Switching Subsystem (NSS)

Fundamentally, the network and switching subsystems (NSS) is responsible for call

connection, supervision and release operations between calling and called stations, where oneor both of them are mobile stations (MS). Other functions include:

y Handling short messages and packet data (email, fax and a variety of notifications)

y Providing µbearer¶ channel for data communications

y Maintaining database of its own users as well as visitors



y Variety of authentication and encryption

y Gateway to PSTN, other mobile networks and data networks including the Internet

Home Location Register (HLR)

The home location register (HLR) is a database used for storing and managing subscriptions.

Generally a PLMN (Public Land Mobile Network) consists of several HLRs. The first two

digits of the mobile directory number (e.g. 0171 2620757) are the number of the HLR where

the mobile subscriber is stored. The data includes permanent data on subscribers (such as

subscriber's service profile) as well as dynamic data (such as current location and activity

status). When an individual buys a subscription from one of the GSM operators, he or she is

registered in the HLR of that operator.Data Elements (Subscriber) Examples:

Mobile Station¶s Identities:

y IMSI (International Mobile Subscriber Identity) (the primary Key),

y Current TMSI (Temporary IMSI)

y IMEI (International Mobile Equipment Identity)

y Mobile Station¶s Telephone number

y MSISDN (Mobile Stations ISDN number)

y Current MSRN (Mobile Station Roaming Number), if assigned

y Name and address of the subscriber

y Current service subscription profile

y Current location (MSC/VRL address)

y Authentication and encryption keys

y Individual Subscriber Authentication Key (KI)

y Mobile Country Code (MCC) and MNC (Mobile Network Code)

y List of MSC/VLR that belongs to this HLR



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

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

station must be attached to a single MSC at a time (either homed or visitor), if it is currently

active (not switched off). The visitor location register (VLR) is a database attached to an MSC

to contain information about its currently associated mobile stations (not just for visitors). A

basic switch (that is a PSTN/ISDN switch) already has a database for its telephone

connections. However, it is not designed to include visitors since a visitor has telephone

number that does not belong to this switch. That is why a separate VLR is needed. An MSC,

with the help of the HLR, allocates a visitor a µlocal¶ telephone number (the MSRN), which is

not currently allocated to anyone. This allocation is temporary (like visitor ID card). The VLR

stores the MSRN as mobile station¶s telephone number (along with other information).

However, VLR also stores some information like µsecurity triple¶ (authentication and

encryption information) for each mobile station that are currently attached to the MSC. A VLR

stores such information not only for its visitors but also for the homed mobile stations. From

this perspective VLR is for homed mobile stations as well.

Visitor Location Register contains selected administrative information from the HLR,

necessary for call control and provision of the subscribed services, for each mobile currently

located in the geographical area controlled by the VLR. Although each functional entity can be

implemented as an independent unit, most manufacturers of switching equipment implement

one VLR together with one MSC, so that the geographical area controlled by the MSC

corresponds to that controlled by the VLR, simplifying the signaling required.

Information of currently attached mobile stations

y IMSI/TMSI numbers

y MSISDN/MSRN numbers

y Security triples (authentication and encryption information)

y Location Area Identity (where the mobile station is currently located)

y List of base stations that belong to this MSC/VLR (by their BSIC or Base Station

Identity Code)



y List of location areas that belong to this MSC/VLR (by their LAI or Location Area

Identity code)

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. In the authentication procedure, the key Ki is never

transmitted to the mobile over the air path, only a random number is sent. In order to gain

access to the system, the mobile must provided the correct Signed Response (SRES) in answer

to a random number (RAND) generated by AUC.The HLR is also responsible for the ³authentication´ of the subscriber each time he makes or

receives a call. The AUC, which performs this function, is a separate GSM entity that will

often be physically included with the HLR. Being separate, it will use separate processing

equipment for the AUC database functions.

Equipment Identity Register (EIR)

The equipment identity register (EIR) is a database that contains information about the identity

of mobile equipment (ME) that prevents calls from stolen, unauthorized, or defective mobile

stations. The AUC and EIR can be implemented as stand-alone nodes or as a combined

AUC/EIR node. There is generally one EIR per PLMN. There are three classes of ME that are

stored in the database, and each group has different characteristics.

y White List: contains those IMEIs that are known to have been assigned to valid MS¶s.

This is the category of genuine equipment.

y Black List: contains IMEIs of mobiles that have been reported stolen.

y Gray List: contains IMEIs of mobiles that have problems (faulty software, wrong make

of the equipment etc). This list contains all MEs with faults not important enough for

barring.

Gateway MSC (GMSC)



The Gateway MSC (GMSC) is an MSC that connects the PLMN (Public Land Mobile

Network) to a PSTN/ISDN. This is the only MSC in the network connected to the HLR. If a

network delivering a call to the PLMN cannot interrogate the HLR, the call is routed to the

GMSC. The GMSC will interrogate the HLR and then route the call to the MSC where the MS

is currently located.

Base Station Subsystem (BSS)

All radio-related functions between mobile stations and network are performed in the base

station subsystem (BSS). The radio equipment of BSS may be composed of one or more cells.

The BSS consists of:

y One base station controller (BSC) and

y All base transceiver stations (BTS) under the BSC

Base Transceiver Station

A Base Station Transceiver (BTS) is a radio transceivers station that communicates with the

mobile stations. Its backend is connected to the BSC. A BTS is usually placed at the center of a

cell. Its transmitting power defines the size of a cell. The primary responsibility of BTS is to

transmit and receive radio signals from a mobile unit over an air interface. To perform this

function completely, the signals are encoded, encrypted, multiplexed, modulated, and then fed

to the antenna system at the cell site. Random access detection is made by BTS which then

sends the message to BSC. Timing advance is determined by BTS. BTS signals the mobile for

proper timing adjustments. Uplink radio channel measurement corresponding to the downlink

measurements made by MS has to be made by BTS.

Base Station Controller

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.



Database associated with Subscriber

y Associated IMSI (Primary key)

y Paging Groups

Database associated with Network

y MSC Address

y List of Paging Groups

y List of LAI

y List of BSIC

y List of Cell ID (CI)

The BSC performs the inter cell hand over for MS moving between BTS in its control. It also

reallocates frequencies to the BTSs in its area to meet locally heavy demands during peak

hours or on special events. It controls the power transmission of both BSSs and MSs in its area.

The BSC also measures the time delay of received MS signals relative to the BTS clock. The

BSC may also perform traffic concentration to reduce the number of transmission lines from

the BSC to its BTSs.

Transcoder Rate Adapter Unit

The Transcoder/Rate Adaptation Unit (TRAU) is the data rate conversion unit. The

PSTN/ISDN switch is a switch for 64 kbps voice. Current technology permits to decrease the

bit-rate (in GSM radio interface it is 13 kbps for full rate and 6.5 kbps for half rate). Since

MSC is basically a PSTN/ISDN switch its bit-rate is still 64 kbps. That is why a rate

conversion is required in between the BSC and MSC.



Mobile Station

The mobile station (MS) consists of the physical equipment, such as the radio transceiver,

display and digital signal processors, and a smart card called the Subscriber Identity Module

(SIM). The SIM provides personal mobility, so that the user can have access to all subscribed

services irrespective of both the location of the terminal and the use of a specific terminal. By

inserting the SIM card into another GSM cellular phone, the user is able to receive calls at that

phone, make calls from that phone, or receive other subscribed services.

The mobile equipment is uniquely identified by the International Mobile Equipment Identity

(IMEI). The SIM card contains the International Mobile Subscriber Identity (IMSI),

identifying the subscriber, a secret key for authentication, and other user information. The

IMEI and the IMSI are independent, thereby providing personal mobility. The SIM card may

be protected against unauthorized use by a password or personal identity number. The MS

monitors the power level and signal quality for known receiver bit sequence from its current

BTS and up to six surrounding BTSs.



GSM INTERFACES

The Radio interface (MS to BTS)

The Um radio interface (between MS and base transceiver stations [BTS]) is the most

important in any mobile radio system, in that it addresses the demanding characteristics of the

radio environment. The physical layer interfaces to the data link layer and radio resource

management sub layer in the MS and BS and to other functional units in the MS and network

subsystem (which includes the BSS and MSC) for supporting traffic channels. The physical

interface comprises a set of physical channels accessible through FDMA and TDMA. Each

physical channel supports a number of logical channels used for user traffic and signaling. The

physical layer (or layer 1) supports the functions required for the transmission of bit streams onthe air interface. Layer 1 also provides access capabilities to upper layers. The physical layer is

described in the GSM Recommendation 05 series (part of the ETSI documentation for GSM).

At the physical level, most signaling messages carried on the radio path are in 23-octet blocks.

The data link layer functions are multiplexing, error detection and correction, flow control, and

segmentation to allow for long messages on the upper layers. The radio interface uses the Link

Access Protocol on Dm channel (LAPDm). This protocol is based on the principles of the

ISDN Link Access Protocol on the D channel (LAPD) protocol. Layer 2 is described in GSM

Recommendations. The following logical channels are supported.

Speech traffic channels (TCH)

y Full-rate TCH (TCH/F)

y Half-rate TCH (TCH/H)

Broadcast channels (BCCH)

y Frequency correction channel (FCCH)

y Synchronization channel (SCH)

y Broadcast control channel (BCCH)

Common control channels (CCCH)

y Paging channel (PCH)

y Random access channel (RACH)



y Access grant channel (AGCH)

Cell broadcast channel (CBCH)

y Cell broadcast channel (CBCH) (the CBCH uses the same physical channel as the

DCCH)

Dedicated control channels (DCCH)

y Slow associated control channel (SACCH)

y Stand-alone dedicated control channel (SDCCH)

y Fast associated control channel (FACCH)

y The radio resource layer manages the dialog between

Abis Interface (BTS to BSC)

The interconnection between the BTS and the BSC is through a standard interface, Abis (most

Abis interfaces are vendor specific). The primary functions carried over this interface are

traffic channel transmission, terrestrial channel management, and radio channel management.

This interface supports two types of communications links: traffic channels at 64 kbps carrying

speech or user data for a full- or half-rate radio traffic channel and signaling channels at 16

kbps carrying information for BSC-BTS and BSC-MSC signaling. The BSC handles the LAPD

channel signaling for every BTS carrier. The first three layers are based on the following

OSI/ITU-T recommendations:

y Physical layer: ITU-T Recommendation G.703 and GSM Recommendation 0-8.54

y Data link layer: GSM Recommendation 08.56 (LAPD)

y Network layer: GSM Recommendation 08.58

There are two types of messages handled by the traffic management procedure part of the

signaling interface²transparent and nontransparent. Transparent messages are between the

MS and BSC-MSC and do not require analysis by the BTS. Nontransparent messages do

require BTS analysis.



A Interface (BSC to MSC)

The A interface allows interconnection between the BSS radio base subsystem and the MSC.

The physical layer of the A interface is a 2-Mbps standard Consultative Committee on

Telephone and Telegraph (CCITT) digital connection. The signaling transport uses Message

Transfer Part (MTP) and Signaling Connection Control Part (SCCP) of SS7. Error-free

transport is handled by a subset of the MTP, and logical connection is handled by a subset of

the SCCP. The application parts are divided between the BSS application part (BSSAP) and

BSS operation and maintenance application part (BSSOMAP). The BSSAP is further divided into

Direct Transfer Application Part (DTAP) and BSS management application part (BSSMAP).

The DTAP is used to transfer layer 3 messages between the MS and the MSC without BSC

involvement. The BSSMAP is responsible for all aspects of radio resource handling at the

BSS. The BSSMAP supports all the operation and maintenance communications of BSS.

Interfaces between Other GSM Entities

Information transfer between GSM PLMN entities uses the MAP. The MAP contains a mobile

application and several Application Service Elements (ASEs). It uses the service of the

Transaction Capabilities Application Part (TCAP) of SS7. It employs the SCCP to offer the

necessary signaling functions required to provide services such as setting mobile facilities for

voice and nonvoice application in a mobile network. The major procedures supported by

MAP is

y Location registration and cancellation

y Handover procedures

y Handling supplementary services

y Retrieval of subscriber parameters during call setup

y Authentication procedures.



METHODOLOGY

Hardware Analysis

Analysis of potential hardware problems in the network not detected by µnormal¶ fault

management methods.

Performance Statistics

Analysis of performance statistics with standard graphical information sheet for each cell.

Analysis of potential hardware problems in the network not detected by µnormal¶ fault

management methods.

Call Trace Analysis

Detects problems with antenna tilts.

Detects problems with Base Transceiver Subsystem (BTS) output power.

Frequency Planning Optimization

Re-definition of handovers and assigned frequencies.



NEED FOR OPTIMIZATION

Network issues that may generate a requirement

Perceived reduction in network quality.

- Indications from network performance monitoring.

- Subscriber¶s experience of using the network

Maximizing the use of existing infra structure

- Operators wants to ensure best returns on investment

Introduction of new services

- Maximize existing resources to accommodate new services such as GPRS.

- Change in original design parameters.

1. Flawed original design in format.

2. Original design information has changed.

cellular mobile systems and services 07

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