2.GSM Archticturegf

8/10/2019 2.GSM Archticturegf

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Basic GSM Network Structure


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SS

SS

D T I

BTSBT

S

O S S

ISDN

PLMN

PSTN

ISDN

ISDN

PLMN

PLMN

PSTNM I N

S C

B G W

H L R

AUC

SMSC-GMSC

SMS-IWMSC

E I R

GWMSC

GWMSC

MSC/VLR

MSC/VLR

MSMS

Air I/fGPRS

SGSN

BSC/TRC

BSC/TRC

BSS

BSS


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Mobile Station (MS)

The Mobile Station is the interface between the user and the network. TheMS consists of two independent parts:

Subscriber Identity Module (SIM) card

Mobile Equipment (ME)

+

TheMobile Equipment is said to be a Mobile Station if the Subscriber Identity

Module ( SIM Card ) is added to it.


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Subscriber Identity Module (SIM) card

A SIM card is an electronic smart card, which stores informationabout the subscription.

Thus it contains a microprocessor and a memory

The SIM stores three types of subscriber related information:

Fixed datastored before the subscription is sold: e.g. IMSI,

authentication key (Ki) and security algorithms (A3,A8).

Temporary network data: e.g. the location area of thesubscriber and forbidden PLMNs

Service data: e.g. language preference also SIMscontain

all necessary network control information,


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Mobile Identification Numbers in SIM card

1. International Mobile Subscriber Identity (IMSI)

MCC : Mobile Country Code

MNC : Mobile Network Code

MSIN : Mobile Station Identification Number


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2. Mobile Station ISDN Number (MSISDN)

CC : Country Code

NDC : Network Destination Code

SN : Subscriber Number


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3. Temporary Mobile Subscriber Identity (TMSI)

The TMSI can be allocated to the mobile subscriber in order to be used

instead of his IMSI during all radio communications. The purpose is tokeep subscriber information confidential on the air interface.

The TMSI is relevant on the local MSC/VLR level only and is changed at

certain events or time intervals.

Each local operator can define its own TMSI structure.


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4. The Mobile Station Roaming Number (MSRN) The MSRN is allocated on a temporary basis when the MS roams into

another numbering area.

The MSRN number is used by the HLR for rerouting calls to the MS. It isassigned upon demand by the HLR on a per-call basis.

The MSRN for PSTN/ISDN routing shall have the same structure as

international ISDN numbers in the area in which the MSRN is allocated. At the reception of the MSRN, HLR sends it to the GMSC, which can now

route the call to the MSCNLR exchange where the called subscriber is

currently registered.


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4. The Mobile Station Roaming Number (MSRN)


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5. International Mobile Equipment Identity (IMEI)

IMEI

6 Digits

TAC

2 Digits

FAC

6 Digits

SN

TAC: Type Approval Code, The firsttwo digits are the code for the

country approval

SN: Serial Number

Final Assembly Codes (FAC)

01,02 AEG07,40 Motorola

10,20 Nokia

40,41,44 Siemens

47 Optional International51 Sony

51 Siemens

51 Ericsson60 Alcatel

80 Philips

85 Panasonic



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ME features are defined as mandatory or optional

Mandatory features are:

Display of called number: Check the dialed numberbefore call setup.

Dual Tone Multi Frequency function (DTMF)

Support of A3, A8 and Ki

Support of A5/1 and A5/2: Provisions are made forsupport of up to seven algorithms as well as the supportof no encryption. It is mandatory for A5/1 and A5/2 andnon-encrypted mode to be implemented, but otheralgorithms are optional.


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Mandatory features are: Short message indication and acknowledgment: This feature

allows delivery 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.

Optional features are:

On/Off switch: The MS can be equipped with the means ofswitching its power supply on and off.

Short Message Service Cell Broadcast (SMSCB) screening


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Base Station System (BSS)

MS

BTSBSC

The Base Station System (BSS) is responsible for all the radio related functions in

the system.


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Base Transceiver Station (BTS)

The primary responsibility of the BTS is to transmit andreceive radio signals from a mobile unit over an air

interface.

To perform this function completely, the signals areencoded, encrypted, multiplexed, modulated, and then fed

to the antenna system at the cell site.

Transcoding to bring 13-Kbps speech to a standard data

rate of 16 Kbps and then combining four of these signals to

64 Kbps is essentially a part of BTS, (though, it can be

done at BSC or at MSC).

The received signal from the mobile is decoded, decrypted,

and equalized for channel impairments by the BTS.


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The BTS can support the voice communication be eitherat a full or half rate over logical speech channel.

In order to keep the mobile synchronized, BTS transmits

frequency and time synchronization signals overFrequency Correction Channel (FCCH) and BCCH

logical channels.

Since the GSM signals are supposed to be frequencyhopped, the control within the cell is actually exercised

such that no two subscribers hop to the same

frequency. Thus it is the responsibility of the BTS tomake sure that hopping signals are kept orthogonal

within the BSC serving area.


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Real Solutions

Outdoor BTS Indoor BTS


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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

BTS configuration: This involves the allocation offrequencies to channel combinations and power

levels for each cell according to available equipment.

Cell Description Data (e.g. cell identity, BCCH

channel number, maximum and minimum output

powers in the cell).

Supervises the transmission network and the

operation of each BTS

Base Station Controller (BSC)

S C ( SC)


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Handling of MS connections : During Call Set Up

Paging:

Signaling set-up Assignment of traffic channel

During a Call:

Dynamic power control in MS and BTS

Locating

Handover

Frequency Hopping


B St ti C t ll (BSC)


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TRAUHandling

In telephone network, the voice coding mode is PCM

(Pulse Code Modulation), with the rate of 64kbit/s.

But in GSM system, voice coding mode RPE-LTP orCELP with much lower rate is adopted to save

wireless channel resource, with rate of 16kbit/s.

If a fixed telephone subscriber wants to communicate

with GSM subscriber, code speed of the voice mustbe adjusted

Trans-coding RateAdapting Unit (TRAU) is used to

accomplish this function.

Motorola TRAU



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TRAUHandling

In addition, since the rate of each channel of existing

terrestrial lines is 64kbit/s, transmission line resources

are greatly wasted if such channel serves as a 16kbit/schannel of GSM.

To save terrestrial line resources, sub-multiplex

equipment (SMUX) is generally adopted between MSC

and BSC to combine four 16kbit/s channels in GSM

into one 64kbit/s channel for transmission.

Motorola TRAU



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N t k t l b t BSC dBTS


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Network topology between BSC and BTS

line topology Only one BTS, for the line topology, is physically connected to the BSC.

For the network operator, the advantage of the serial approach over the star

configuration is that it saves line costs. Furthermore, the serial connection allows

for more efficient use of resources,

The disadvantage, however, is that a single link failure causes the loss of the

connection to a large number of BTSs

This kind of networking usually uses in along highways and train tracks where thepopulation density is very low.

Network topologybetweenBSCandBTS


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Ring topology The use of a ring configuration provides some redundancy in which the signal

can always go in one of two directions, so that in the event of a link failure, it is

still possible to provide an alternative connection.



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Star topology The feature of star networking is each site is

connected directly to BSC with an E1 link.

The advantage is that signal pass through

very few nodes, which means that no BTS

depends on another. So in the case of the

failure of one BTS, another BTS will not be

affected, leading to high link reliability. Concerning its disadvantage is that this type

of networking requires large quantity of

transmission links.

This networking mode is usually applied in

densely populated urban areas to take full

advantage of easy expansion



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Tree topology In tree networking, Signals pass

through many nodes, any abnormality

in the superior site will affect

subordinate sites, which leads to low

reliability.

This type of networking is applicable in

vast areas with relatively low

population density.

In such configuration, furtherexpansion is quite difficult because it

requires reconstructing of the network.

BTS1

BSC

BIE

BIE

BIE

BTS3

BIE

BTS2

BIE

BTS4

NetworkSwitchingSystem(NSS)


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Network Switching System (NSS)

MSC/VLR

HLR

AUC

BSS GMSC

EIR

Mobile Switching Center (MSC)


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The primary node in a GSM network is the MSC. It is the node, which controlscalls both to MSs and from MSs.

The primary functions of an MSC include the following: Switching and call routing to or from MS.

Charging.

Service provisioning. Control of connected BSCs.

Direct access to Internet services.

Provides the gateway functionality to other networks.

Mobile Switching Center (MSC)

Gateway Mobile Switching Center (GMSC)


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Gateway functionality enables an MSC to interrogatea HLR in order to route a mobile terminating call. It isnot used in calls from MSs to any terminal otherthan another MS.

For example, if a person connected to the PSTNwants to make a call to a GSM mobile subscriber,then the PSTN exchange will access the GSMnetwork by first connecting the call to a GMSC

Gateway Mobile Switching Center (GMSC)

HomeLocationRegister (HLR)


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The HLR is a centralized network database that stores andmanages all mobile subscriptions belonging to a specificoperator.

It acts as a permanent store for a persons subscription

information until that subscription is cancelled. The primary functions of the HLR include:

1. Stores for each mobile subscriber:

Basic subscriber categories.

Supplementary services.

Current location.

Allowed/barred services.

Authentication data.2. Subscription database management

3. Controls the routing of mobile terminated calls and SMS.

Home Location Register (HLR)

Visitor Location Register (VLR)


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The role of a VLR in a GSM network is to act as atemporary 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 anothercountry) every time the subscriber uses a service

or changes its status.

The VLR is always integrated with the MSC.




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For the duration when the MS is within one MSC service area, then the VLRcontains 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 (LA) of MS


AuthenticationCenter (AUC)


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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 thosethat are attempting to impersonate others.

Radio information ciphering: the information sent between the network andan MS is ciphered. An MS can only decipher information intended for it.

Authentication Center (AUC)


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DataTransmission Inter-working (DTI)


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It performs data handling functions such as data rate conversionand providesthe functions necessary for data inter-working between GSM networks andother networks, including:

Data Traffic between mobiles

Data Traffic to/from ISDN

Data Traffic to/from PDNs

Data Traffic to/from PSTN

Data Transmission Inter working (DTI)

Message Center (MC)


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essageCe e ( C)

An MC may be added to a GSM network to provide one or more of the followingmessaging services:

Voice mail

Fax mail

Short Message Service (SMS) text messages

SMS Cell Broadcast (SMSCB) text messages

Operation & Support Subsystem (OSS)


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p pp y ( )

The operation and Maintenance center (OMC) is connected to all equipment inthe SS (the GMSC, MSCs, HLR, VLR, AUC, EIR and the BSCs).

It can be viewed as a computerized monitoring center were staff can monitor and

control the network remotely

In the GSM network, the implementation of the OMC is called the operation andsupport system (OSS).

OSS is the functional entity from which the networkoperator can monitor andcontrol the entire mobile network.

Other functions provided by the OSS include provision of network maintenancesupport as well as customer support.



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SSSS

D T I

BTSBTS

O S S

ISDN

PLMN

PSTN

ISDNISDN

PLMNPLMN

PSTNM I N

S C

B G W

H L R

AUC

SMSC-GMSC

SMS-IWMSC

E I R

GWMSCGWMSC

MSC/VLRMSC/VLR

MSMS

Air I/fGPRS

SGSN

BSC/TRCBSC/TRC

BSSBSS

GSM interfaces


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1. Um interface (Air interface): Um interface is defined as the communication interface between MS and

BTS, and is used for the interworkingbetween MS and the fixed part of

the GSM system. The physical link is a radio link.

The information transferred on this interface includes the information of

radio resource management, mobility management, connectionmanagement, etc.

GSM interfaces


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2. Abis interface: Abis interface defines the standard of communication between BSCand

BTS in BSS, and is used in remote interconnection mode.

This interface supports all MS-oriented services, and supports the controlof BTS radio equipment and the allocation of radio frequencies.

GSM interfaces


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3. A interface: A interface is the standard interface between BSS and MSC.

The information transferred on this interface includes MS management,

BTS management, mobility management, connection management, etc.

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