Chap01 intro

Introduction

Chapter 1

This chapter is designed to provide the student with an overviewof the system nodes and signaling. It addresses systemcomponents, their functions, features, and requiredspecifications. It provides the basic definitions for signaling.

OBJECTIVES:Upon completion of this chapter the student will be able to:

• Describe the network elements and their main functionality

• List the nodes realized in the AXE-10 technology

• Describe some of the capabilities and features of OSS

• List the protocols used on different GSM interfaces.

GSM Advanced System Technique

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1 Introduction

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1 Introduction

Table of Contents

Topic Page

SYSTEM ARCHITECTURE IN ERICSSON’S GSM SYSTEM............... 1

GENERAL.................................................................................................................... 1

SWITCHING SYSTEM................................................................................................. 3

BASE STATION SYSTEM ........................................................................................... 8

OPERATION AND SUPPORT SYSTEM ................................................................... 12

ADDITIONAL NODES................................................................................................ 13

GSM PROTOCOLS AND INTERFACES............................................. 17

DEFINITION .............................................................................................................. 17

INTERFACES ............................................................................................................ 17

LAYERED STRUCTURE OF PROTOCOLS.............................................................. 17

PROTOCOLS IN GSM............................................................................................... 18


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SYSTEM ARCHITECTURE IN ERICSSON’S GSM SYSTEM

GENERAL

In Figure 1-1 the system model for Ericsson’s GSM system isshown.

SS

MSC/VLR

OSS

Switching System

Signaling

Traffic and Signaling

EIR

ILR

Base Station SystemBSS

BSC

RBS

MS

GMSC

IWU

AUC

HLR

MXE

MIN

SOG

BGWSMS-GMSCSMS-IWMSC

TRC

FNR

Figure 1-1 Architecture of Ericsson’s GSM system.

Abbreviations:

AUC AUthentication Center

BGW Billing GateWay

BSC Base Station Controller

EIR Equipment Identity Register

FNR Flexible Numbering Register

GMSC Gateway MSC

HLR Home Location Register


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ILR Interworking Location Register

IWU InterWorking Unit

MIN Mobile Intelligent Node

MS Mobile Station

MSC Mobile services Switching Center

MXE Message Center

OSS Operation and Support System

RBS Radio Base Station

SMS-GMSC Short Message Service, Gateway MSC

SMS-IWMSC Short Message Service InterWorking MSC

SOG Service Order Gateway

TRC Transcoder Controller

VLR Visitor Location Register

Ericsson’s GSM systems are cellular mobile telephony systems.They are implemented using Ericsson's latest AXE switchingsystem technology. This means that all features and servicesimplemented in AXE over the years are standard. In addition,Ericsson’s GSM systems will utilize future AXE developments.

Software development is carried out in stages, creating aplatform where functionality improves with time. The fact thatthe AXE system is modular makes it possible to makemodifications, additions and deletions without affecting existingsystem operations.

Hardware development keeps pace with increasing performancerequirements and state-of-the-art technology. Using the open-ended structure of AXE architecture, new hardware (forexample, processors and line terminals) is being introducedcontinuously.

The network is divided into three major systems:

• Switching System (SS)

• Base Station System (BSS)

• Operation and Support System (OSS)

Each of these contains a number of functional units, which makeup the entire system. The functional units are parts in the varioushardware units.

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SS is responsible for performing call processing and subscriberrelated functions. It includes the following functional units:

• Mobile services Switching Center (MSC)

• Home Location Register (HLR)

• Visitor Location Register (VLR)

• AUthentication Center (AUC)

• Equipment Identity Register (EIR)

• Flexible Numbering Register (FNR)

Most radio related functions are performed in the Base StationSystem (BSS). BSS includes:

• Base Station Controller (BSC)

• Transcoder Controller (TRC)

• Radio Base Station (RBS)

The Operation and Support System (OSS) handles errormessages originating from the network. OSS has access to boththe switching system and the base station system (via BSC).

SWITCHING SYSTEM

Mobile Services Switching Center

The Mobile services Switching Center (MSC) performs thesystem telephony switching functions. It also controls calls toand from other telephony and data systems, such as the PublicSwitched Telephone Network (PSTN) and Public Land MobileNetwork (PLMN). In Ericsson’s GSM system, the VLR (seenext section) is always integrated with the MSC to form aMSC/VLR.

The MSC/VLR is based on AXE technology. In Ericsson’sGSM system the AXE in SS is structured in a new wayaccording to a concept called Application Modularity (AM). Fordetailed information about this new concept, refer to AM coursematerial.

The MSC/VLR is responsible for:

• Functions for setting up and controlling calls, includingsupplementary services.

• Functions for handling speech path continuity for movingsubscribers (handover).


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• Functions for updating mobile subscribers’ location (locationupdating and location canceling) in the different locationregisters.

• Functions for updating mobile subscriber data.

• provision of functions for signaling to and from:

– The BSCs and the MSs (using BSSAP, see chapter 10).

– Other GSM entities (using MAP, TUP or ISUP).

– Other networks such as PSTN or ISDN (using TUP orISUP).

• Administrative functions for defining data and handling ofthe mobile subscribers.

• Security related functions that perform authentication orselective authentication, ciphering, (re)allocation andanalysis of the TMSI.

• Functions for IMEI check.

• Functions for receiving and delivering short messages to andfrom the MS.

• Charging and accounting.

Visitor Location Register

The Visitor Location Register (VLR) is a database containinginformation about all MSs that currently are located in the MSCservice area. The VLR contains temporary subscriberinformation needed by the MSC to provide service for visitingsubscribers. The VLR can be seen as a distributed HLR. When aMobile Station (MS) roams into a new MSC service area, theVLR connected to that MSC requests data about the MS fromthe HLR and stores it. When the MS makes a call, the VLRalready has the information needed for call set-up. In Ericsson’sGSM system, the VLR is always integrated with the MSC sothat internal signaling can be used. This setup eliminatessignaling between the two nodes over the network unnecessarythus decreasing the network signaling load.

Gateway MSC

The Gateway MSC (GMSC) is the point in the PLMN wherecalls to mobile subscribers enter the GSM network. Thereforeeach mobile terminating call must be routed via a GMSC in thehome PLMN of the called MS.

The GMSC contains the interrogation facility. That is, theGMSC contains signaling functions for retrieving informationfrom the concerned HLR, which tells how to proceed with call

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set-up. Depending on the interrogation result, the call is eitherre-routed by GMSC to the MSC where the mobile subscriber islocated or forwarded according to the forward-to number.Charging and accounting functions are also implemented in theGMSC.

The GMSC is constructed using the AXE technology and isnormally integrated in the same node as MSC/VLR.

Interworking Unit

The Interworking Unit (IWU) provides an interface to variousnetworks for data communication. At present Ericssonimplements this functionality in the Data TransmissionInterworking unit (DTI) through which users can alternatebetween speech and data during the same call. Its mainfunctions include modem and fax adapter pool plus the node hasthe ability to perform rate adaptation. This was earlierimplemented as the GSM InterWorking Unit (GIWU). Both DTIand GIWU consist of software and hardware.

Short Message Service - Gateway MSC

A Short Message Service Gateway MSC (SMS-GMSC) iscapable of receiving a short message from a Service Center(SC), interrogating an HLR for routing information and messagewaiting data, and delivering the short message to the MSC ofthe receiving MS. In Ericsson’s GSM system, the SMS-GMSCfunctionality is normally integrated in the MSC/VLR node.

Short Message Service - Interworking MSC

A Short Message Service InterWorking MSC (SMS-IWMSC) iscapable of receiving a mobile originated short message from theMSC or an Alert message from the HLR and submitting themessage to the sender's SC. The SMS-IWMSC functionality isnormally integrated in the MSC/VLR node.

Home Location Register

The Home Location Register (HLR) is a database that stores andmanages subscriptions. In a PLMN there is one or severalHLRs. For each “home” subscriber, the HLR containspermanent subscriber data such as:

• The associated numbers - MSISDN and IMSI (seeAppendix).


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• A list of services - teleservices, bearer services andsupplementary services, which the subscriber is authorizedto use.

The HLR also stores and updates dynamic data about each“home” subscriber including subscriber location (VLR-address),services registered to/activated by the subscriber or the operatorsuch as call forwarded numbers and call barring for certain typesof calls.

The HLR can be integrated in the same node as the MSC/VLR,or can be implemented as a separate node. The AXE technologyis used.

The HLR provides functions defined in the GSM specifications.For example:

• Connection of mobile subscribers and definition ofcorresponding subscriber data.

• Maintenance of a database of mobile subscribers andcorresponding subscriber data.

• Subscription to basic services.

• Registration/deletion of supplementary services.

• Activation/deactivation of supplementary services.

• Interrogation of supplementary services status.

• Functions for analysis of mobile subscriber numbers(MSISDN, IMSI, additional MSISDNs) and other types ofaddresses.

• Handling of authentication and ciphering data for mobilesubscribers including communication with an authenticationcenter.

• Functions for communication with GMSC and VLR usingthe No. 7 signaling system and MAP (see chapter 12).

• Statistical functions for collecting data about systemperformance.

Authentication Center

The AUthentication Center (AUC) is a database that stores thefollowing data:

• a RANDom number (RAND)

• a Signed RESponse (SRES)

• a Ciphering Key (Kc)

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These three pieces of data are called triplets. The AUCgenerates and provides one or several triplets for a certain IMSIat each request from the HLR. The HLR forwards the providedtriplets to the serving VLR. At any MS access attempt, the MSCmay send RAND to the MS to authenticate the subscriber’s SIM.The SRES returned by the MS is then compared to the SRESprovided by the AUC (via HLR) to the VLR. The MSC mayalso facilitate ciphering on the radio path by providing the TRXmanaging the traffic with the Kc. Successful ciphering ispossible only if the Kc, which is calculated by the MS duringthe authentication procedure, is identical to the one provided tothe TRX.

The AUC can be implemented as an Application Module in theMSC/VLR.

Equipment Identity Register

The Equipment Identity Register (EIR) is a database that storesthe International Mobile station Equipment Identity (IMEI) foreach MS equipment. Each IMEI is unique. During any MSaccess (except in the case of IMSI-detach), the MSC/VLR mayverify the IMEI. When necessary, the EIR is requested by theMSC/VLR to check the IMEI. The main objective is to ensurethat the equipment is not stolen or faulty. Equipment can beclassified as:

• white listed (permitted for use)

• gray listed (should be tracked for evaluation)

• black listed (barred)

• unknown equipment

The network rejects any access attempt if the equipment isclassified as “black-listed” or “unknown”.

AUC and EIR are implemented either as stand-alone nodes or asa combined AUC/EIR node. AUC and EIR software aredeveloped by Sema Group Telecom Division in Great Britain ona platform of standard hardware from Digital.

Flexible Numbering Register

The Flexible Numbering Register is an optional node used whenthe Flexible Numbering feature or the Number Portabilityfeature are used in the network.


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With the Flexible Numbering it is possible to have no relationbetween a MSISDN number series and a certain HLR in thenetwork. Messages sent from an GMSC towards an HLRregarding a certain MSISDN are routed through the FNR whichredirects the messages to the HLR storing data about thisparticular subscriber.

The Number Portability enables a subscriber to change an GSMoperator without changing his/her MSISDN number. Callstowards this subscriber are routed to the appropriate networkusing information provided by the FNR.

Interworking Location Register

Interworking Location Register (ILR) is a product that exists inthe CMS 40 network only. ILR makes inter-system roamingpossible. This means that roaming between an AMPS networkand a GSM 1900 network is possible. ILR consists of an AMPSHLR and parts of a GSM 1900 VLR.

BASE STATION SYSTEM

The Base Station System (BSS) consists of the BSC and theBTS.

The BSC in Ericsson’s GSM system is based on AXEtechnology. It can be implemented as a stand-alone node orintegrated with MSC/VLR.

Ericsson’s BTS solution is the Radio Base Station (RBS). GSM900/GSM 1800 has two basic series of base stations. They areRBS 200 and RBS 2000. GSM 1900 uses RBS 2000 only.

Both series offer products for indoor and outdoor installations.Indoor installations include foe example RBS 200 (GSM 900),RBS 205 (GSM 1800) and RBS 2202. Outdoor installationsinclude RBS 203 and 204 (GSM 900), RBS 2101 and RBS2102. The latter two are a mini and a compact version in the2000 series.

Base Station Controller

The Base Station Controller (BSC) handles most radio-relatedfunctions and is the BSS's center point. The BSC manages theentire radio network including:

• Configuration of the network.

• Administration and remote control of the RBSs.

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• Handling connections to MSs including handovers.

The large BSCs can handle up to 1020 transceivers (TRXs). Thesmaller combined MSC/BSC is designed for recently startedmobile operating companies.

During call set up, a logical channel is allocated to theconnection based on information about the available channels’characteristics. When the connection has been established,signal strength and speech quality are monitored by the MS andthe TRX or TRU (Transceiver Unit, appears instead of the TRXin RBS 2000) and reports are forwarded to the BSC. Thedecision to initiate a connection handover is then made by theBSC.

Strict Ericsson RBS transceiver orientation leads to a master-slave relationship between the BSC and the transceivers. That is,all major activities in the RBS are orchestrated by the BSC. TheBSC sets TRX/TRU configurations and frequencies for eachcell. The BSC thus receives a set of logical channels that can beallocated to connections with MSs. The TRXs/TRUs aresupervised by internal software tests and loop tests. Detectedfailure results in an automatic TRX/TRU reconfiguration,thereby keeping the logical channel set intact.

Transcoder Controller

The BSC product family consists of a combined BSC/TRC, aremote BSC (without transcoders) and a standalone TranscoderController (TRC). The Transcoder Controller contains a set oftranscoders for speech coding/decoding purposes. It alsoperforms rate adaptation for data connections. The transcodersare pooled, meaning that they can be allocated on demand – FullRate, Half Rate or Enhanced Full Rate.

Base Transceiver Station / Radio Base Station

The Base Transceiver Station (BTS), or as Ericsson calls itRadio Base Station (RBS) handles the radio interface to the MS.The BTS is the radio equipment (transceivers and antennas)needed to serve each cell in the network. One RBS includes allBTSs on a site. A group of BTSs is controlled by a BSC. Twotypes of RBS are available:

• the RBS 200 family

the RBS 2000 family. The main RBS functions are:

• Radio transmission, including frequency hopping.


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• Radio signal reception from MSs, including equalizing anddiversity functions to compensate for fading effects.

• Quality measurements: RBS measures signal strength andquality on the uplink. MS measures signal strength andquality on the downlink as well as signal strength onneighboring RBSs. The results are sent to the RBS. The RBSadds its measurements and reports to the BSC.

• Time alignment measurements.

• Transceiver and MS power control.

• Multiplexing on the radio path, channel coding, interleaving,and ciphering.

• Broadcasting system information and paging messages.

• Receiving channel requests from MSs.

The RBS consists of a cluster of autonomous TRXs/TRUs. Thesoftware stored in TRX/TRU Random Access Memories(RAMs) is controlled, loaded, and upgraded from the BSC, thusreducing the need for on-site RBS visits. This remote controlcapability also includes automatic tuning of the combiners incase frequency planning is changed. Strict transceiverorientation means that faults are isolated per TRX/TRU and donot affect adjacent TRXs/TRUs in the same RBS.

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OPERATION AND SUPPORT SYSTEM

Operation and Support System (OSS) is Ericsson’simplementation of OMC. OSS offers the customer cost effectivesupport for centralized, regional and local operations andmaintenance activities required by a cellular network. OSS is thefunctional entity from which the network operator monitors andcontrols the system.

OSS is based on an application in the product familyTelecommunications Management and Operations Support(TMOS).

OSS is a two-level management function. For centralizedcontrol of a network, the installation of a Network ManagementCenter (NMC) with subordinate Operation and MaintenanceCenters (OMC) is advantageous. NMC staff can concentrate onsystem-wide issues, whereas local personnel at each OMC canconcentrate on regional issues. The OMC and NMCfunctionality can be combined in the same physical installationor implemented at different locations.

The OSS is designed as a coherent management system used tosupport a number of other network elements. See Figure 1-2.

Operation and Support System

NMC

OMC OMC

MIN

MSCBSC

BTS

HLR

AUC /EIR

NMC = Network Management CenterOMC = Operation and Maintenance Center

Figure 1-2 OSS provides for central supervision of all networkelements.


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The BTSs are supported through the BSC. Other Ericsson nodesincluding Message Center (MXE) can be supported.

OSS management areas are based on TelecommunicationManagement Network (TMN), which is a model fortelecommunication networks management. The most importantparts are:

• Configuration management

• Fault management

• Performance management

ADDITIONAL NODES

Message Center

Message center (MXE) is the node used for handling ShortMessage Service (SMS), voice mail and fax mail. Theseservices are optional, therefore the node is optional and does notbelong to the basic system structure.

Mobile Intelligent Network Node

Mobile Intelligent Network node (MIN) is the node used forhandling the mobile IN services. MIN can be based on the AXEplatform and therefore encompass the experience gained fromintelligent network in fixed telephony. It can also beimplemented as an Application Module in the MSC/VLR.

Service Order Gateway

Operator’s administrative systems, normally called CustomerAdministration Systems (CAS), are very complex. Theseadministrative systems are often inflexible, and lead to highnetwork adaptation costs.

The Service Order Gateway (SOG) is a product enabling CAS toexchange information with Ericsson’s network elements thatcontain service information.

SOG provides a remote interface to network elements, seeFigure 1-3. It combines functionality for subscriptionmanagement in the HLR and the AUC with equipmentadministration in the EIR.

SOG receives message requests from CAS in the form ofCustomer Service Orders (CSO). SOG interprets the data

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content, manipulates the data and delivers appropriateinformation to the relevant network elements in the form ofNetwork Service Orders (NSO).

MCMC

SOGSOG

HLRHLR

IF

CustomerCustomerAdm.Adm.SystemSystem

OSS/OMCOSS/OMC

AUCAUC EIREIR MINMIN

Figure 1-3 Service Order Gateway (SOG).

Billing Gateway

The Billing GateWay (BGW) collects billing information, calledCall Data Records (CDRs), from the network elements. CDRsare accumulated into files and immediately forwarded to post-processing systems that use the files as input. BGW acts as abilling interface to all network elements in an Ericsson network.The flexible BGW interface supports adaptation to new types ofnetwork elements as well.

Most often, the BGW resides near the customer administrationand billing systems and is handled by the administrativeorganization.

To summarize, an operator uses the BGW to collect billinginformation from the network and the SOG to send service orderinformation to different network elements, see Figure 1-4.


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Collection = Billing Gateway (BGW)

HLRHLR

IF

CustomerAdm.System

GMSCGMSC

SOGSOG

Provisioning = Service Order Gateway (SOG)

MSCMSC

BGWBGW

AUCAUC EIREIR

Figure 1-4 Gateway products for billing information and service

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GSM PROTOCOLS AND INTERFACES

DEFINITION

A protocol is an agreement on how to communicate.

INTERFACES

The interface between MSC and BSS is called the A interface,the interface between MSC and VLR the B interface, and theother interfaces are shown in Figure 1-5. Note that the Binterface is an internal interface in Ericsson’s GSM systembecause the MSC and the VLR are in the same unit. The Ainterface is divided into two parts:

• the A interface between MSC and BSC

• the A-bis interface between BSC and BTS

The air interface between MS and BTS is called the Uminterface.

MS BTS BSC MSC HLR

VLR

MSC

EIR

VLR

Um Abis A

B

C

E

F

D

G

AUCH

Figure 1-5 GSM interfaces.

LAYERED STRUCTURE OF PROTOCOLS

The layered structure of protocols is a model used to implementdifferent functions required in complicated systems, usingspecialization. In computer networks or telecommunicationnetworks certain functions common for different nodes can bepointed out. Then specialized software (or software andhardware) providing these particular services can be defined.


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Logically, the communication between user applicationprocesses is viewed as being partitioned into an ordered set oflayers, organized in a vertical sequence.

A message being sent out travels down in the protocol stack ofthe sending node with the layers on the way adding theirinformation to the original message. After travelling on somechosen transmission medium, the message arrives to a receivingnode. There it shall travel upwards in the protocol stack witheach layer in the receiving node analyzing information added bythe peer protocol. While travelling up the bits added by peerlayers in the sending node are removed by the peer layers in thereceiving node. By analyzing these bits the layer of thereceiving node can offer certain services to the higher layers inthe node (e.g. error correction, flow control, etc.). See Figure 1-6.

010

10010

1100101

010

10010

1100101

Peer layers

Sending node Receiving node

Service

Service

Figure 1-6 Layered structure of protocols

PROTOCOLS IN GSM

In parts of Ericsson’s GSM system the ITU Common ChannelSignaling System No. 7 (SS7) is used. SS7 has a layeredstructure with four levels. Figure 1-7 provides an overview ofthe different protocols used on the various interfaces.

MSC

In the stack on the right hand side of the MSC, (which is theinterface to the VLR, HLR, GMSC and the PSTN), the MTP,SCCP, TCAP, MAP and ISDN–UP/TUP reside. Descriptions of

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these protocols are located in chapters 8, 9, 11, 12. MTP is usedas a transport medium and for addressing and routing. Itcorresponds to layer 1, 2 and parts of layer 3 according to theOSI reference model. SCCP is added to SS7 signaling to keepthe signaling model in line with the OSI model. SCCP addsfunctions for establishing logical connections and also for moreextensive addressing and routing. MTP and SCCP together formthe Network Service Part (NSP) and correspond to layer 1-3 inthe OSI model.

TCAP and MAP are both layer 7 protocols. TCAP providesfunctions to communicate with the far end of the signaling chainand to set up multiple dialogues. MAP is a protocol speciallydesigned for mobile communication. It is used for signalingwith databases (HLR, VLR, EIR, AUC), for short messages, andfor certain signaling messages during an inter MSC handover.

ISDN-UP and TUP correspond to part of OSI layer 3 up to layer7 and are used between the MSC and the ISDN/PSTN for callset-up and call supervision.

Note that in the communication between MSC, BSS and MSonly three layers are used.

A interface

Signaling over the A interface, or, between the switching systemand the base station system, is done according to the BSSAP(see chapter 10) using the network service part of SS7 fortransmission.

Towards the radio interface and the MS, layer 3 is divided intothree sub layers:

• Radio Resource management (RR)

• Mobility Management (MM)

• Connection Management (CM).

These sub layers are explained in chapter 5. At this point it issufficient to note that the CM and MM layers reside in the MSC,and a major part of the RR layer resides in the BSC.

Instead of using ISDN–UP messages towards the MS, thesemessages are converted into CM messages. As these messagesare unique to this environment, they can be more specializedand hence more effectively coded. The conversion from ISDN–UP to CM is performed in the MSC, as well as the conversionbetween MAP messages and MM messages. Call control


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messages (within CM) such as registration of supplementaryservices are also mapped onto a MAP message in the MSC.

The protocol used to transfer the CM and MM messages is theBSSAP. This protocol is also used for direct control of the BSS,for example when the MSC asks for a channel to be allocated bythe BSC. BSSAP uses the MTP and the SCCP protocol.

CM

MM

RR

LAPDm

S.layer 1

RR’

LAPDm

S.layer 1 S.layer 1

LAPD

BTSM

RR

BTSM

LAPD

S.layer 1MTP

SCCP

BSSAP

CM

MM

(RR)

BSSAP

SCCP

MTP MTP

SCCP

TCAP

MAP

ISUP/TUP

Abis

MS BTS BSC MSC

UmRadio

A

CM = Connection ManagementMM = Mobility ManagementRR = Radio Resource ManagementLAPDm = Link Access Procedures for Dm-channelBTSM = BTS ManagementLAPD = Link Access Procedures for D-channel

BSSAP = Base Station System Application PartSCCP = Signaling Connection Control PartMTP = Message Transfer PartMAP = Mobile Application PartTCAP = Transaction Capabilities Application PartTUP = Telephony User PartISUP = ISDN User Part

Figure 1-7 Signaling model.

A-bis interface

A-bis is the interface between the BSC and BTS. The protocolused at layer 2 on A-bis is the LAPD (see chapter 6). This is anISDN protocol and is therefore not described in the GSMrecommendations. LAPD has functions for error detection andcorrection, as well as for frame delimitation. That is, insertion offlags at the beginning and end of a frame.

At layer 3 most messages, including RR messages, pass the BTStransparently. However, some RR messages are closely relatedto the radio equipment and must be handled by the BTS. TheBTS Management (BTSM) entities manage these messages. Aconversion to the relevant RR messages is carried out in theBTS. An example of an RR message is the ciphering message,where the cipher key is only sent to the BTS and not to the MS.

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Um interface

Layer 2 protocol on Um is called LAPDm (see chapter 5) and isa modified LAPD protocol. The difference between LAPD andLAPDm is that the error correction and detection on Um is alayer 1 function, meaning that these functions are removed fromthe LAPDm protocol. Another difference is that the messageframes on LAPD can be much longer than the frames onLAPDm, as the LAPDm frames must fit into the bursts.


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