gsm architecture v2

7/31/2019 GSM Architecture v2

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Mobile Communications and Wireless Sensor Networks

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Fig. 1.3. General Architectur of GSM Systems

Chapter 3

GSM SISTEMS ARCHITECTURE

3.1 Overview

A mobile telephony network (Public Land Mobile Network – PLMN) as GSM can be

split in three big subsystems:

- Base Station Sub-System (BSS ) which ensures the radio-electric transmissions and

manages the radio resources

- Network Sub-System (NSS) which consists of the set of functions needed for setting a

call and for mobility; (in the second phase of the GSM specifications the used terms is

SMSS from Switching and Management Sub-System; of course this term was very

close to NSS which is largely distributed and still in use);

- Operation Sub-System (OSS) which let the administrator manage the network

The terminal equipment (MS) is alternatively included and excluded from the radio

subsystem according to the context.

The functions separation between BSS and NSS requires making the distinction

between the itineration aspect and the radio mobility one:




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♦ Itineration is defined throw the possibility of using a telecommunication terminal in a

random point. It is necessary to take into consideration the itineration, but it is useless

considering the handover. This is how NSS administrates itineration.

♦ Radio mobility offers supplementary the possibility of movement during communication

implying also the handover function. The BSS administrates at its level the radio specific

aspect of the GSM network, having as tasks the adapted to the radio channel transmission,the radio channels allocation and handover decision taking.

3.2 Canonical Architecture

The BSS contains:

- Base Transceiver Station (BTS) being the emitters –receivers with a minimum of

intelligence,

- Base Station Controller (BSC) which control a set of BTS`s and permit a primary

circuit concentration.

The NSS contains databases and commuters:

- Mobile-services Switching Center (MSC) which are mobile commuters associated in

general to the VRL`s databases (Visitor Location Register )

- Home Location Register (HLR) is a database for localization and subscribers

characterization.




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BTS

BTS

BTS

BTS

BTS

BTS

BTS

BSC

BSC

BSC MSC

MSC

VLR

VLR

HLR

BSS NSS (SMSS)

Abis

Abis

Abis A

A

A

C

D

B

E

B

G

D

Signaling

Data circuit

Fig. 2.3 Principal Elements of the GSM Network without OSS




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3.3. Structural architecture of radio subsystems

3.3.1. BTS functions

♦ BTS is a set of emitters-receivers called TRX having the role of radio transmission:

modulation, demodulation, equalization, error coding correction.♦ It generally administrates the entire physical layer: TDMA multiplexing, frequencies

slow jumps, ciphering. It also realizes the set of radio measurements necessary to

ensure the communication in progress is done properly. These measurements are not

interpreted throw the BTS, but are directly sent to the BSC.

♦ The BTS administrates the level of data connection for the signaling changes between

the mobiles and the infrastructure (LAP Dm) and also the one with BSC for ensuring

the dialog viability (LAP D).

♦ The maximum capacity of a BTS is typically of 16 gates, which means it can support

more than 100 simultaneous communications. In the countryside area, BTS can be

restricted to using only one gate which can support up to 7 simultaneous

communications. In the urban areas, the BTS has generally from 2 to 4 TRX, being

able to support from 14 to 28 simultaneous communications. In order to give an

overview, in Paris, in the urban area at the end of 94there where at leas 100 BTS`s

each serving a area of approximately 1km2.

♦ The norm is the one that distinguishes the so called normal MTS`s from the micro-

BTS. The first ones correspond to the classical cellular systems base stations installed

in technical locations and to roof antennas connected throw cables. The coupling

devices permit having only one antenna for more TRX`s, but can also reduce

considerably the available power at the entrance of the antenna. The norm specifies

the sensibility and the maximal powers of the TRX`s without taking into consideration

the coupling devices (cf. fig. 3.2 and 3.4). So the values indicated are greater than the

ones in the figure for mobiles. A small tolerance is accepted for the maximum

effective power.

GSM 900 DCS1800

Class Number Maximum

Power (W)

Limit of Maximum

Power (W)

Maximum

Power (W)

Limit of Maximum

Power (W)

320 640 20 40

2 160 320 10 20

3 80 160 5 104 40 80 2,5 5

5 20 40

6 10 20

7 5 10

8 2,5 5




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Fig 3.3 power classes of the normal BTS`s before coupling

♦ The Micro – BTS`s are designed for ensuring connection in the dense urban area with

the help of microcells microcellular. This are small size equipments, including

coupling devices and having a lower price than normal BTS`s. They are located in the

exterior, in a very large number, the norm proposes small powers for having limited

number of ports and it also implies sever restrictions for allowing the functioning of two nearby BTS`s [GSM 05.05 /3]. The different classes of powers and sensibility are

shown in the table below

♦ When a mobile approaches a BTS the recommendations imply that it can reduce the

effective transmission power to its mobile. So the power classes define well the

maximal power values.

GSM 900 DCS 1800

Class Number Maximum

Power (W)

Limit of Maximum

Power (W)

Maximum

Power (W)

Limit of Maximum

Power (W)

M1 0,08 0,25 0,5 1,6

M2 0,03 0,08 0,16 0,5

M3 0,01 0,03 0,05 0,16

Fig. 3.4 Power classes of the micro-BTS after coupling

Type of BTS GSM 900 DCS 1800

Micro BTS M1 -97 dBm -102 dBm

Micro BTS M2 -92 dBm -97 dBmMicro BTS M3 -87 dBm -92 dBm

Normal BTS -104dBm -104dBm

Fig. 3.5 BTS Sensibility

There exist different BTS-BSC configurations. The BTS`s are in general connected to

BSC multi-drop or star network configurration. The BSC can be placed in the same place

with the BTS.

Depending on the environment, a location can support a single BTS (one-way

configuration, specific in the countryside) or several (district configuration specific for the

urban areas and highways).




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Fig. 3.6 Different BTS-BSC Configurations

3.3.2. BSC`s Functions

The control of the base station BSC is the <smart> assembly of the BSS. It`s main

function is to administrate radio resources. It commands the channel allocation, utilizes the

measurements made by the BTS for controlling the emission power of the mobile or/and of

the BTS and takes the decision of making the handover. More than this, it is also a switch

which transfers the circuit concentration towards the MSC.

The BSC is linked throw one or more MIC connections with BTS and MSC, managingthe data connections with these. The BSC-MSC connection is similar to a RNIS access and

calls LAPD. The connection BSC-MSC utilizes CCITT nr 7 and its level differences.

Initially, not all infrastructure constructors shared the same philosophy regarding BSC.

Some have constructed small capacity BSC`s, preferring to multiply its number in order to

minimize the BTS-BSC distances and also for reducing operators usage costs. Other preferred

the construction of large capacity BSC`s. The first type is more appropriate for the rural areas




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which are not so populated, while the second ones are more likely to be used in the urban

areas which because of the large density per surface unit requires BSC`s capable to take a

great amount of traffic. The BSC`s capacities range between 100 and 900Erlang.

3.4. Structural architecture of fix subsystems

3.4.1. HLR`s functions

HLR or Home Location Register is the database that administrates the subscribers of a

given PLMN. It memorizes the details of each subscriber, data which are returned to the

operator once it leaves its administration system, like:

- IMSI, International Mobile Subscriber Identity

- MSISDN, Mobile Subscriber Integrated Services Digital Network Number

- Subscription profile (authorizes extra services, authorization for international calls)

On the other hand HLR is a localization database. It memorizes for each subscriber the

number of the VLR he is registered to, the same for the case when the subscriber connects to a

foreign PLMN. The localization is performed simultaneously with the emitted information

throw the terminal at crossing the network.

The HLR implementation can be a centralized or decentralized one. In the first case

the HLR can administer thousands of subscribers, acting as a centralized device. In the second

case, it can be integrated in the MSC and so the subscriber s̀ data is physically stored on the

MSC it is preferentially communicating with. In this way the signaling switching is

minimized. In all the implementation cases, each subscriber is uniquely associated to a HLR,

independently from its instantaneous location.

3.4.2. MSC and VLR functions

The MSC, Mobile-services Switching Centre, also called mobile services commuter

administrates the connection establishment between a mobile phone and another MSC, short

message transmission and the transfer execution when it’s recalled. It communicates with the

VLR for managing client’s mobility; verifies the characteristics of subscribers that relate to

call initialization, transfers localization information, and so on.

It also has a gate function, GMSC (Gateway MSC ) which is activated with the start of

each call from a fix subscriber to a mobile one. This function differs with respect to the MSC

and can be implemented in the PSTN. MSC also has a gate function for short messages.

The VLR (Visitor Location Register ) has a database which memorizes the subscriber’s

details from a specific geographic area. MSC`s can be connected to this VLR, usually one for

each. The data stored throw the VLR are similar with the ones from the HLR but contain

more mobile subscribers which are present in the considered area. VLR can have more precise

localization information than HLR, since it has the temporary identity TMSI for help.




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The structural separation between VLR and MSC proposed within the norm is rarely a

respected. Some constructors integrate the VLR in the MSC and so the dialogues needed to

establish a call are simplified. Others set a different coupling between MSC and VLR utilizing

the approximation or “intelligent network”. MSC is in these conditions a simple commuter

without any function for call treatment. RCP ( Radio Control Point ) equipment ensures the

only command function of the MSC and VLR without having any switching function.

An assembly MSC / VLR can manage up to 100.000 subscribers with an average

traffic on subscriber of 0,025 Erlangs. The MSCs are in general transit commuters of the

telephonic network on which there are implanted specific GSM network functions.

3.4.3. EIR functions

The EIR, Equipment Identity Register , is a database that contains the terminals`

identities (IMEI). It can be checked each time subscription services are required in order to

verify if the used terminal is authorized to enter the network. The identity of a terminal

contains a number of homolog’s common to all terminals with the same series, a number that

identifies the factory where it was assembled and a number specific to the terminal.

The access to the network can be refused because the terminal is not homologated,

because it perturbs the network or because it was stolen. The EIR contains a white list with all

the homologated numbers, a black list of stolen equipments with forbidden access and a grey

list with terminals presenting insufficient nonfunctionalities for justifying a total interdiction.

The network can memorize an IMSI identity of a subscriber using a terminal registered

in the black list or grey one and can transfer it to the management system for the illicit access

identification.

It is possible to use each homologated terminal in the entire GSM network in Europe.

The homologation of an organism in agreement with the MoU accreditation is valid of all

MoU assembly. It is though required the settings up of connections between the EIRs of

different PLMNs. Today without mutual agreements between operator EIRs are not usable.

3.4.4. AUC functions

The AUC, Authentication Centre, memorizes for each subscriber a secret key used for

service requirement authentification and for coding the communication. An AUC is generally

associated to each HLR. The assembly can be integrated in the same equipment, though

functionally they do not come from the same subsystem.




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3.5. Exploitation and maintenance subsystem

The Network Management consists of all the actions that permit the recording and

control of performances and the resource usage in order to maintain a certain quality lever for

the users. The different management functions include:

- Commercial management (declaring subscribers, of the terms invoices andstatistics);

- Security management (declaring intruders, ability level);

- Exploitation and management of performances (observations on traffic and

quality, configuration changing for adapting it to the network`s changes,

supervision of mobile phones,) [GSM 12.07];

- The control of system`s configuration (setting the logistic level, introduction of

new equipment and new functionalities) [GSM 12.06];

- Maintenance (faults detection, equipment testing)

The diversity of the equipments present in the GS network both in the set of types

(emitters-receivers, commuters, databases) and also of the set of providers leads to an

approach partially structured and hierarchical. The Norm [GSM 12.00] presents two levels:

- OMC, Operation and Maintenance Centre,

- NMC, Network Management Centre.

The NMC permits the general administration of a networks subassembly throw a

centralized control when the OMC permits a local supervision of the equipments. Many

OMCS will supervise for example the BSC and BTS assembly on different areas. Others will

supervise MSC and VLR. The minor incidents are transmitted to the OMCs which filter them,

while the major ones are sent to the NMC. The decoupling between OMC and NMC is notdefined in the norm for the assembly of administration functions. In general the NMC

corresponds to the exploration system of the TMN.




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3.6. Interfaces presentation

Name Localization Utility

Um MS-BTS Radio layer

Abis BTS-BSC Divers

A BSC-MSC DiversGMSC-HLR HLR interrogation for incoming callC

SM-GMSC – HLR HLR interrogation for incoming messages

VLR-HLR Administration of subscribes details and

localization

D

VLR-HLR Supplementary services

MSC – SM-GMSC Transport of short messagesE

MSC-MSC Handover execution

G VLR-VLR Administration of subscriber s̀ details

F MSC-EIR Identity verification of the terminal

B MSC-VLR Divers

H HLR-AUC Autentification exchange information

Fig.3.9. List of interfaces in the GSM system

All the interfaces defined by a letter are totally specified in the norm. As specified

before the decoupling between VLR and MSC realized by the constructors is not generally as

in the norm, interface B being slightly respected. The D interface model permits to a

MSC/VLR to talk to HLRs of other networks. It`s conformity with the norm permits makes it

an international one.The A interface separate NSS from the BSS. The conformity of BSC with MSC allows

all operators to have different suppliers for NSS and BSS.

3.7. The layered architecture of a radio subsystem

The GSM recommendations set that a decoupling of functions and a repatriation of

this on different equipments. Its layered structure takes the decoupling respecting the general

philosophy of the OSI layers such that the specification of a mobile radio system leads away

from the classical 7 layers structure.

In the BSS there are found three layers based on OSI: the physical layer, the data link

layer and the network layer, this last one being composed of several sub layers with several

interfaces.

Layer 1 or the physical layer defines the set of information transmission and reception

physical means. On the Abis interface the transmission is numeric, usually on 64kbit/s taking

the recommendations G.703, G.705 and G.732 a UIT. On the radio interface this layer is very




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complicated requiring a lot of special operations: error correcting coding, logical channels

multiplexing, radio measurements recording.

Layer two or the data link layer monitors the viability of transmission between two

equipments from a protocol. The adopted protocols fallow the mechanism called ARQ

Automatic Repeat reQuest and are pretty similar to the HDLC protocol. The connection

between BTS and BSC is administered throw LAPD used in RNIS. Between MS and BTS the

specification of a physical layer requires an adapted LAPD protocol, LAPDm (m for mobile).

Layer three has as objectives the setting, maintaining and freeing (of words or data) of

the common circuits with a fix network subscriber. This layer is divided in 3 sub layers.

The set of aspects strictly connected to radio is integrated in the Radio Resource (RR)

sub layer, while all the other aspects are split between the sub layers MM and CM. The RR

sub layer administrates the instantiation, maintenance and liberation of different logical

channels. Inside the MS, its role is to select cells and to monitor the specified path for

determining the effective measurements throw the physical layer. The entity RR is presented

in MS and BSC. Most of the messages that are reported are sent throw BTS withoutinterpretation.

Thought some messages, for example the activation order of an emitter refers directly

to the BTS and is sent between MS and BTs or between BTS and BSC. The BTS supports a

first entity called RR for the dialog with MS and a second entity for BSC`s requests treatment,

called BTS Management (BTSM) its homologation is found in BSC.

The itinerary is administered in the MM sub layer, Mobility Management . It takes the

localization charge, the authentification and the allocation of the TMSI.

The sub layer CM, Connection Management , is split in 3 parts: the CC entity, Call

Control, which manages the circuit connections with the final receiver; The SMS entity, Short Message Service, which ensures the transmission and receiver of short messages; The SS

entity, Supplementary Services, which administers the supplementary services.

In the phase 2+, the new entities are defined inside the CM: the entity for Group Call

Control and for distributed calls, Broadcast Call Control, so it is a data administration entity

using packages on signaling channels.

The sub layers Cm and MM are not administered inside the BSS. The set of CM and

MM messages are sent throw the BSS without being analyzed by the BSS or BSC




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CM

MM

RR

LAPDm

Ph. Layer

RR

LAPDm

Ph. Layer

BTSM

LAPD

Ph.Layer

RR

BTSM

LAPD

Ph.Layer

BSSAP

SSCP

MTP3

MTP2

MTP1

CM

MM

BSSAP

SCCP

MTP3

MTP2

MTP1

MS Radio BTS Abis BSC A MSC

Fig.3.10

3.8. The layered architecture of the fix subsystem

The understanding of the fix subsystem architecture requires basic knowledge

acquisition in the matter of semaphore signalization nr.7 (cf. chapter 3.10).

The signaling change inside NSS is done using SS7. The MTP is thus implanted in the

MSC, VLR and HLR. The initiation administration requires the development of the MAP

protocol, Mobile Application Part , which is particular for GSM.

For offering international itineration and sustain the message format evolution, theprotocols SCCP and TCAP are used in dialogs between MSC/VLR – HLR and MSC/VLR –

MSC/VLR. They are placed inside the structure on the layer between MAP and MTP.

For establishing calls, the classical telephonic messages are switched only once

between MSC/VLR and between MSC/VLR and CAA (PSTN centrals). The protocol for call

administration, SSUTR2 (or ISUP in other countries but France) is integrated in MSC/VLR.

The A interface between NSS and BSS based on. SS& includes MTP and SCCP used

in concrete way. Above the SCCP, the BSSAP (BSS Application part ) permits the

initialization of itineration and connection between MS and MSC.

3.9. Mobile station

The term mobile station describes the terminal equipment with a SIM card which

permits the access to the telecommunication services of a PLMN GSM.

In the case a service of data transmission is required; the terminal means is composed

of a data terminal and an adaptive device. Most of the terminals today have a volume between

150 and 450 cm3

and weight between 150 and 350 grams with length between 130 to 200mm,




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height 30 to 65mm and thickness from 20to 50mm.Their autonomy is between 9 to79hours in

stand by and 4 hours in a call. A message can have from 2 to 5 lines with 8 to 16 characters.

Most portable posts accept SIM cards with SIM Full-Size, only few ports accept only micro-

SIM (SIM Plug-In).

For the determination of the non aggregated terminals or stolen ones, each terminal is

equipped with a particular identity IMEI. This identity allows the determination of the

manufacturer (main ones being AEG, Alcatel, Ericsson, Motorola, Nokia, Orbiter, Panasonic

and Siemens).

For most classes of terminals the norm is defined using the maximum emission power.

For GSM 900 most of the sold terminals are portable, in the fourth power class of 2W. The

stations set on board of vehicles are of class 2, having a power of 8W.For DSC1800; the

terminals are in general ported with a 1W power (class 1).

The terminals reduce the mitted power according to the networks requirements, the

minimal level being of 3mW (5dBm) in GSM 900 and of 1mW (0 dBm) in dcs 1800. The

increase step value is of 2dB with a tolerance which can reach up to ±5dB.The power of the terminals conditions their radius in 900 MHz, a power of 8W permits

a radius of several tenths of kilometers, while a power of 2W limits it at several kilometers.

This numbers give size references because the effective radius depends very much on the

medium and the network.

The minimum sensibility of the terminals is of –100 dBm for DCS 1800 equipments,

-102dBm for portable GSM 900 and of –104 dBm for portables and devices on cars.

gsm architecture v2

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