mrn-en-7-gsm part 2

Politecnico di MilanoFacoltà di Ingegneria dell’Informazione

MRN – 6 – GSM part 2

Mobile Radio NetworksProf. Antonio Capone

Signaling

A. Capone: Mobile Radio Network 2

Telephone signaling

o In classic telephone service, signaling is in charge of routing and setting up circuits for the service

o In IP telephony, signaling is used for setting up media sessions

o The basic service provided by signaling is the: Basic call

o There are two main components of signaling:n User signaling: for the communication

between user terminals and the networkn Network signaling: for the communication

among network elements


Signaling

o User signalingn It allows user:

o To ask for a specific serviceo To indicate the called party

n And the network to provide information to user on the call statuso Network availableo Call in progresso Destination reachedo etc.


Telephone Network

Signaling

o Network signalingn It allows the exchange of

information among switching stations foro Routing calls along available paths

in the networkn It is also used for providing

supplementary services in addition to the basis one like:o Special numbers (800, 900, etc.)o Calling party notificationo Mobility managemento Call deviationo Etc.


User signaling

o The interface between user and network can be analogic or digital (ISDN)

o The type of user signaling depends on the interface

o In case of analog interface the signaling is also analogic


User signaling:analog interface


user telephone network destinationPhone pick up

Line in use signal

Busy toneBusy tone sound

Number selection

Multi-frequency tonesEcho of number tones

Ringing currentRinging signalRinging tone

Hello?Hello?Hello?

Hi, this is Antonio … Hi, this is Antonio … Hi, this is Antonio …

User signaling:ISDN

o The case of digital interface is rather different o In this case all signaling is based on digital

messages transmitted on layer 2 frames of the D channel of the ISDN interfacen Out band signaling

o The set of protocols for user signaling defines the DSS1 signaling system (ITU-T Q.930, Q.931 e Q.932)


L1 L1 L1LAPDDSS1

TE NT1 LE

LAPDDSS1o DSS1 messages are

transported directly into LAPD frames

User signaling:ISDN


Information Elements for Supplementary services

Messages forBasic call

Packet connection on D channelMultiplexing – error control

Digital interface (physical layer)I.430/I.4311

2

5 6 7

C PlaneU Plane

Q.931

Q.920LAPD

I.430/I.431

Supplementaryservices

I.251 - I.257Messages for

Supplementary servicesQ.932

Non symmetric protocol

User signaling:ISDN

o Messages for the basic call (Q.931) includes a header common to all message types and a sequence of information elements depending on the type


Protocol discriminator

Protocol discriminator CRV Length

CRF CRV

CRV (only for PRI)

0 Message type

Other information elements

byte

1

2

3

4

5

.

.

.

CRV: Call Referece ValuesCRF: Call Reference Flag

User signaling:ISDN


Message type DescriptionALERTING terminal ringing

CALL PROCEDING call in progress

CONNECT set up completed

CONNECT ACKNOWLEDGE acknowledgment of CONNECT message

PROGRESS call in progress

SETUP call setup request

SETUP ACKNOWLEDGE acknowledgment of SETUP message

DISCONNECT call disconnect request

RELEASE call release

RELEASE COMPLETE acknowledgment of RELEASE message

RESTART restart request of signaling procedure

RESTART ACKNOWLEDGE acknowledgment of RESTART messageINFORMATION information for call setup

Basic call


user telephone network destinationPhone pickup

SETUP

SETUP ACKNOWLEDGEBusy tone sound

Number selection

INFORMATIONEcho of number tones

SETUP

ALERTINGRinging tone

Hello?Hello?Hello?

Hi, this is Antonio … Hi, this is Antonio … Hi, this is Antonio …

CALL PROCEEDING

CALL PROCEEDING

ALERTING

CONNECTCONNECT ACKNOWLEDGECONNECT

CONNECT ACKNOWLEDGE

ChannelD

ChannelB

Basic call


Terminal 1 network Terminal 2

DISCONNECT

RELEASE

RELEASE COMPLETE

DISCONNECT

RELEASE

RELEASE COMPLETE

Supplementary services


Supplementary services with number identification

DDI: Direct Dial InMSN: Multiple Subscriber NumberCLIP: Calling Line Identification PresentationCLIR: Calling Line Identification RestrictionCOLP: Connected Line Identification PresentationCOLR: Connected Line Identification RestrictionMCI: Malicious Call IdentificationSubaddressing



Supplementary services with call offer and completionCT: Call TransferCFB: Call Forwarding on BusyCFNR: Call Forwarding on No ReplayCFU: Call Forwarding on UnconditionalCD: Call deflectionCW: Call WaitingCH: Call HoldCall Park and Pickup CCBS: Completion of Calls to Busy Subscribers



Other supplementary servicesCONF: Conference Calling3PTY: Three Party Service

CUG: Closed User Group (Reti Private Virtuali)PNP: Private Numbering Plan OCB: Outgoing Call Barring

CRED: Credit Card CallingAOC: Advice of ChargeRC: Reverse Charging

Supplementary serviceso Supplementary serives are accessible through

specific user procedures and a functional protocol

o The functional protocol makes use of Q.932 Facility messages that includes the Facility and User-to-User information elements, used in command-response mode:

o requesto replyo erroro reject


Facility Hold Hold AcknowledgeHold Reject RetrieveRetrieve Acknowledge Retrieve Reject

Q.932 Messages

Network signaling

o Modern network signaling is called out of band signaling, because it makes use of separate communication channels

o It is also called Common Channel Signaling (CCS) since it is based on a packet switching network

o CCS isn A datagram network n With network nodes separate from those of

the circuit switching networko SS7 (signaling system no. 7) is the protocol

stack widely used for telephony


SS7


ServiceSwitching

Point

SignalingTransfer

Point

Service Control

Point

SSP

SCP SCP

SSP

STP STP STP STP

SS7


ServiceSwitching

Point

SignalingTransfer Point

SSP

STP

SSP

Physical channel

STPSEP

Transport trunk

SS7Protocols

SignalingTransfer Point

/Signaling End Point

SS7o The lower part of the SS7 protocol stack

includes the physical, link and network layers for the datagram packet switched network

o This part of the stack is called MTP (Message Tranfer Part)

o It is rather similar to IP stack


SP

SP

SP

MTP-1

MTP-2

MTP-3MTP-1

MTP-2

MTP-3

MTP-1

MTP-2

MTP-3

SS7


1

2

3

MTP - 2

MTP - 1

MTP - 3

SCCP

TCAP

OMAP ASEISDN-UP

(ISUP)7

Datagram routing

Transport and connection

ISUP: ISDN User partTCAP: Transaction Capabilities Application PartSCCP: Signaling Connection Control PartMTP: Message Transfer Part

OMAP: Operation, Maintenance and Administration Part

ASE: Application Service Element

Basic call andSupplementary services

ISUP


In ISUP messages map one to one DSS1 messages

SETUP

ALERTING

CONNECT

DISCONNECT

Initial Address Message (IAM)

Address Complete Message (ACM)

Answer Message (ANM)

Release (REL)

….. ….

Basic call in SS7


TE LESETUP

INFORMATION

SETUP

ALERTING

CALL PROCEEDING

ALERTING

CONNECT

CONNECT ACKNOWLEDGE

CONNECT ACKNOWLEDGE

TELE Transit station

IAMIAM

ACMACM

CONNECT

ANMANM

Channel B Channel BEnd to end circuit

DISCONNECTREL

RLCREL

RLCDISCONNECT

RELEASE

RELEASE COMPL

RELEASE

RELEASE COMPL

Intelligent Network (IN) application services


SSP

STP

Service Control

PointSCPSTP

SS7Network

It provides IN services (mapping of 800 numbers, call screening, private numbering plan, televoting, …)

It processes supplementary service requests from users and application messages from other IN nodes

IPIntelligent Peripheral

Intelligent Network (IN) application services


IN provides information services that are then used for enriching the basic call service

STP STP STP STP

SCPSSP IP

TCAP


Messages for a service execution are grouped into transactions

ASEApplication

Service Element

ASEApplication

Service Element

transaction

BeginContinue

EndAbort

UnidirectionalOriginating TIDDestination TIDP-abort-causeU-abort-cause

Component Portion

TCAP


Messages are used for remote function calls according to a client-server paradigm, however the protocol is symmetric Invoke

Return-Result-LastReturn-Result-Not-Last

Return-ErrorReject

Invoke-IDLinked-ID

Operation-CodeError-Code

General-problemReturn-Result-ProblemReturn-Error-Problem

Parameter

Invoke

Return Result

Return Error

Reject

IN service example: 800 mapping


IP(access station)

SCP(network database)

BEGIN - invoke ID # 1(request for number 800 - xxxxx)

CONTINUE - invoke ID # 2(automatic responder: ask additional digits)

CONTINUE - RR ID # 2(additional digits)

END - RR ID # 1(number mapping)

GSM signaling


GSM signaling

o SS7 signaling used in PSTN is not sufficient for GSM

o Additional functionalities are added for mobile radio networks

o For the network part the SS7 network is extended with additional application layer services (IN services)

o For the access part new protocols are added for allowing:n The signaling exchange between MS and MSCn The communication between BTS, BSC and

MSC


GSM SS7


MTP=Message Transfer PartISUP=ISDN User PartMAP=Mobile Application Part

SCCP=Signalling Connection and Control PartBSSAP=Base Station Sub-system Application PartTCAP=Transaction Capabilities Application Part

SCCP

MTP level 2

MTP level 1

MTP level 3

BSSAP TCAP

MAP

ISUP

Access network signaling


MTP

MS BTS BSC MSC

MTPfisicofisicofisicofisico

LAPDLAPDLAPDmLAPDm

RRBTSM BTSM

RR

SCCP SCCP

BSSMAP BSSMAPDTAPBSSAP

CM

RR

BTSM=BTS ManagementBSSMAP=BSS Mobile Appl. PartDTAP=Direct Transfer Appl. Part

RR=Radio Resource managementMM=Mobility ManagementCM=Connection Management

MM

CM

MM

BSSMAP

o BSSMAP is the protocol for the exchange of signaling between BSC and MSCn all BSSMAP messages sent from MSC

requires an interpretation by the BSCn For example:

o Messages for starting paging procedure for a new incoming call

o Messages for the authentication procedureo Etc.


DTAP

o DTAP is in charge of creating a direct communication channel between MSC and MS

o It is a kind of tunnel of signaling messages

o DTAP is actually used only between MSC and BSC, since then at the other interfaces signaling messages are simply forwarded by lower layer protocols

o DTAP are BSSMAP are part of the BSSAP


BTSM

o BTSM is the protocol for signaling exchange between BSC and BTSn all BTSM messages sent by the BSC

require an interpretation by the BTSn It is mainly used by the BSC for

configuration commands n And by the BTS for sending radio

measurements


Radio Resource management (RR)

o It’s the level that manages radio resources which involves MS, BTS e BSC

o The RR:n Manages initial access messages on RACH

and AGCHn Handles measurements on the SACCHn Manages exchange of messages for resource

reassignment during handovern Manages timing advance and power controln etc.


Connection Management (CM)

o CM manages the exchange of messages for call setup

o It is mainly based on the same user signaling of PSTN

o CM messages trigger other actions (like RR or BTS configuration) performed through other protocols


Mobility Management (MM)

o MM manages the signaling exchange for mobility management (location update, paging) and authentication

o Examples of MM proceduresn Location updatingn Authenticationn IMSI attach and detach


Protocol Stack in HLR


SCCP

MTP level 2

MTP level 1

MTP level 3

TCAP

MAP

Signaling Protocols


MAPTCAP

SCCP

MTP

BSSAP MAPTCAP

SCCP

MTP

SCCP

MTP

BSSAP

SCCP

MTP

MSC

HLR

PSTNExchange

BSC

TUP /ISUPTUP /

ISUP

Numbers and identities

o Telephone numbers:n MSISDN (Mobile Station ISDN Number)n MSRN (Mobile Station Roaming Number)n Handover Number

o User and terminal identities:n IMSI (International Mobile Subscriber

Identity)n TMSI (Temporary Mobile Subscriber

Identity)n IMEI (International Mobile Equipment

Identity)


Numbers and identities

o Network IDs:n LAI (Location Area Identity)n CGI (Cell Global Identity)n RSZI (Regional Subscription Zone

Identity)n BSIC (Base Station Identity Code)


Communication services in GSM

o Basic Services n Teleservices: are application services (from

source coding/decoding to information delivery) to like telephone service, SMS, etc.

n Bearer Services: provides end-to-end transport of information flows (like data services)

o Supplementary services:n Same as in PSTN


Teleservices


Service DescriptionGSM

SpecificationCode

Features

Speech (Telephony) T11 The most important service for mobilesystems, normal speech service,including emergency calls

Speech (Emergency Calls) T12 Emergency calls are possibleautomatically

Short Message Service (Mobileterminated)

T21 For the reception of Short messages

Short Message Service (Mobileoriginated)

T22 For sending a short message toanother GSM subscriber

Short Message Service (cellbroadcast)

T23 For sending short messages to morethan one receiver simultaneously withina given cell

Short Message Service


Air A

VLR

MSC

SMS

TCBSCBTS

Mobile TerminatedSMS

Mobile OriginateSMS

6Alfaskopm346Alfaskopm346 Alfaskopm346

Short Message Service (Cell Broadcast)


a k o 3

l o 6

lf s p 3 6

l 6

k 3

A f s 4

A f s m 4

A a k o m 4

p m

a p

Air A

BTS

BTS

BTS

BTS

BTS

BSC

SMS,Cell Broadcast

O & M

Signaling procedures


Examples of signaling procedures

o Network accesso Mobilityo Originated Call Setupo Handovero Terminated Call Setup


Registrations andLocation Update


Turning on a MS

o When a MS is turned on the following procedures are performed:n Cell Selection: the MS selects the BTS

for the connectionn Registration: the MS informs the MSC

that it is active and updates information on the Location Area


Cell Selectiono The MS scans the radio carriers

n The carriers on which the BCCH is transmitted are called c0

n These carries are at constant power (dummy bursts are used) and frequency hopping is disabled

o Through the FCCH the MS synchronizes with the BTS carrier

o Through the SCH the MS synchronizes slot and frame and receives the BSIC

o The MS can now decode the BCCH that includeso LAC (Location Area Code)o CGI (Cell Glocal Identity)o MCC (Mobile Country Code)o MNC (Mobile Network Code)

o The MS selects the BTS with the strongest c0 carrierA. Capone: Mobile Radio Network 52

Registration

o Two possible cases:n The LAI received is the same stored in the

SIM (phone turned off and then turned on in the same LA). An IMSI attach procedure is initiated and the IMSI is marked as active in the VLR.

n No LAI stored or LAI received different from the stored one (phone turned off and then on in a different LA). A Location Update procedure is initiated


Location Update


o Two types of Location Update:n Two LAs of the same MSC/VLR

LA 1 LA 2

BSC

MSC

BSC

VLR

BTS 2BTS 1

Location Update

o Two LAs of different MSC/VLR


LA 1

BSC

MSC

BSC

VLR

BTS 1BTS 2

MSCVLR

2

1

LA 2

Location Update - Intra MSC


BSCMSC

VLR

BTS

IMSI

Location Update Ack + new TMSI

Location Update Req +TMSI old

+ LAI old

Location Update Req

Location Update Ack + new TMSI

New TMSI

New LAC

Location Update - Intra MSC


Messages

CHAN REQIMMEDIATE ASSIGNMENT

AUTH REQAUTH RES

CIPH MOD CMDCIPH MOD COM

LOC UPD ACK

TMSI REALL CMD

CHAN REL

LOC UPD REQ

Channels

RACHAGCH

SDCCHSDCCHSDCCH

SDCCH

SDCCH

SDCCH

SDCCH

SDCCH

+ TMSI new

MS

Network

Location Update - Inter MSC


MSC (old)

VLR

MSC (new)

BTS (new)

VLR

HLR

Location Update - Inter MSC


6. provide subscriber data5. request subscriber data

4. provide subscriber identity (IMSI)

11. cancel old location

MS BSS MSC VLRnew VLRold HLR

2. location update request (TMSI+LAI)

7. security procedures

3. request subscriber identity (TMSI)

9. HLR update

10. acknowledgement update

12. location cancelling accepted

8. Location update

1. channel assignment

Call Setup


Mobile terminated call(originated from PSTN)

o For setting up a call terminated on a mobile user some additional steps w.r.t. the case of fixed lines are required


PSTNCellular

Network

Mobile terminated call


1) The PSTN/ISDN user digits the Mobile Subscriber International ISDN Number (MSISDN) of the called user

MSISDN

PSTNMSISDN: +39 347 652726839 = Country Code (Italy)347 = National Destination code6527268 = Subscriber Number



2) The PSTN/ISDN network analyzes the number and routes the call (ISUP signaling) to the GMSC of the PLMN making use of the National Destination Code (NDC)

3) The GMSC receive the call setup request through the SS7 network with the MSISDN called

GMSC

GSM Network

MSISDN

PSTN

HLR

VLR



4) The GMSC identifies the HLR of the called user through the MSISDN(the GMSC does not know the MS position!)

5) The GMSC send a “HLR Enquiry” message to the HLR (MAP message)

6) The HLR analyzes the message, identifies in its record the IMSI corresponding to the MSISDN and the VLR currently visited by the MS

GMSC

GSM Network

MSISDN

PSTN

MSC

VLR

VLR

HLR


7) The HLR sends a “Routing information request”to the MSC/VLR (MAP message)

8) The MSC/VLR allocates temporarily a Mobile Station Roaming Number (MSRN) for the call


GMSC

GSM Network

MSC

VLR

VLR

HLR

RoutingInformationRequest

o The MSRN is similar to the MSISDN, and it can be used for routing the call using ISUP

o MSRN = CC + NDC + SNn CC = Country Coden NDC = National

Destination Coden SN = Subscriber Number



9) The MSC forwards the MSRN to the HLR (MAP message)

10)The HLR forwards it to the GMSC (MAP message)

11) The GMSC routes the call using MSRN to the MSC/VLR (ISUP signaling)

GMSC

GSM Network

MSC

VLR

VLR

HLR

MSRN

MSRN


12) The MSC/VLR activates the paging proceduren It identifies the currently visited LAn It sends a paging command to BSCs of the

area13)BSCs activate paging on their BTSs using the

PCH (paging message contains the TMSI of the MS)

14)The MS replies to the paging message starting an access procedure on the Random Access CHannel (RACH) and get a Stand alone Dedicated Control CHannel (SDCCH) assigned



15) The MSC/VLR activates the authentication procedure

16) The MS start the call setup signaling (informing it is a reply to paging) – call setup similar to ISDN

17) A TCH is assigned for the call18) Called phone rings and the MSC/VLR

inform caller that called user terminal is ringing

19)The user answer to the call.A. Capone: Mobile Radio Network 68

70 A. Capone: Mobile Radio Network

Mobile terminated call(international roaming)

Mobile terminated call(international roaming)


Mobile terminated call:Recap


Channel resp. (SDCCH)

PSTN/ISDN GMSC HLR MSC/VLR

FixedCaller

Call Setup (MSISDN)

Analyze NumberCall Setup (MSISDN)

IMSI

MSRN

MSISDN

MSRN

Call Setup (MSRN)Page

BSC+BTS Called MS

Page req. (PCH)

Page resp.Channel ack. (SDCCH)Ack.

Authenticat.,ciphering,TMSI reallocat.

Mobile terminated call:Recap


Connect ack.

PSTN/ISDN GMSC HLR MSC/VLR

FixedCaller

Connection Setup

BSC+BTS Called MS

Connect

Connection establishedAlert

TCH Assign Req.

Connection Confirmation

TCH Assign Command

Ringing notice Ringing

Unhook notice

Mobile originated call

o User digits the number and the MS starts the access procedure

o A call setup signaling procedure is activated on the SDCCH

o If the called number is in the same PLMN, a “HLR enquiry” procedure is initiatedn Same procedure as mobile terminated call

o If the called number is of another network the call is routed to the GMSC (ISUP signaling).




9. call set up done

2. Authenticat., ciphering ,TMSI reallocat.

1. channel assignment

EXC GMSC HLR MSC/VLR BSS MS

3. call setup

4. check services etc.

5. all ok6. call is proceeding

8.set up the call

10. alert

7. allocate TCH

11. if B answers the call will be connected



CHAN REQIMM ASSIGN

RACHAGCH

SDCCH

SDCCH

SDCCH

SACCH

TCH

SDCCHassignment

ServiceRequest

AuthenticationCiphering

TCH/SACCHAssignment

TCH/SACCH Release

DISCONNECTRELEASE

RELEASE COMPLTE

SET UPASSIGNMENTCONNECT ACK

SABMUAMS

Network

Number portability


Fixed network:a) Call forwarding

Number portability


Fixed Networkb) Query and Release

Number portability


Fixed Networkc) All call query

Number portability


Mobile radio networka) Call forwarding

Number portability


Mobile radio networkb) All-call query (ver. 1)

MSISDN

IRN

Number portability


Mobile radio networkc) All-call query (ver. 2)

MSISDN

Handover


Handover in GSM

o In GSM (like in all other cellular technologies) the handover procedure is initiated by the network, even it is based also on measurements provided by the MS

o Using adjacent BTS list, the MS scans c0 carriers and makes received power measurements

o The measurements are sent to the BSC using the SACCH (a complete set every 480 ms)

o The BSC analizes measurements from MS and BTS and if necessary (algorithms may be different) initiates a handover procedure


Handover parameters

o MS:n Signal strength on the BCCH carrier of

the adjacent cells (RXLEVNCELLn)n Signal strength of the TCH (RXLEV)n Quality (BLER) of the TCH (RXQUAL)

o BTSn Signal strength from MS on the TCH

(RXLEV)n Quality of the TCH (RXQUAL)n Timing Advance


Handover activation

o Low quality (RXLEV and/or RXQUAL below threshold)

o Distance MS/BTS above a threshold (timing advance)

o Better adjacent cello Traffic (cell too loaded)o Control and maintenance


Types of Handover

4 types• Intra Cell - Intra BSC• Inter Cell - Intra BSC• Inter Cell - Inter BSC• Inter MSC


Intra Cell - Intra BSC Handover


Air A

BTS TCBSC

Old radio channel

New radio channel

Inter Cell - Intra BSC Handover


A ir A

TCBTS

BTS

BSC

New cell / BTSOld cell / BTS

Inter Cell - Intra BSCHandover

The procedure is completely controlled by the BSC• the BSC identifies the best BTS and the best

TCH• the BSC activate the TCH in the new BTS• the BSC command the MS to switch to the

new carrier and time slot in the new BTS• The MS switches the call• The old channel is released• The BSC informs the MSC/VLR than an

handover has been executed and if necessary performs a location upddate

• The MS start collecting measurements in the new cell


Inter Cell - Inter BSC Handover


A i rA

New cell / BTS

BTS

BTS

B SC TC

BSC TC

VLRM S C

Old cell / BTS

Inter Cell - Inter BSC Handover

The handover procedure is initiated by the BSC• the BSC identifies the best BTS• the BSC sends a message to the MSC/VLR

with the new BTS identity• The MSC commands the new BSC to prepare

for handover• The new BSC assigns a TCH for the MS and

informs the MSC• The MSC forward TCH information to the

previous BSC• The previous BSC command the MS to switch

to the new TCH• The MS switches• The old channels are released


Inter MSC Handover


Air A

BTS

BTS

B SC TC

BSC TC

VLRM S C

VLRM S C

Old cell / BTS

New cell / BTS

Inter MSC Handover

The procedure is initiated by the BSC• The BSC starts the handover and selects the

BTS

• The BSC informs the MSC/VLR sending a “handover request” to the anchor MSC/VLR

• The anchor MSC/VLR sends a request to new MSC/VLR

• The new MSC/VLR allocates a HandOver Number (HON) and transmits it to the anchor MSC/VLR

• The anchor MSC/VLR used the HON for setting up a circuit with the new MSC/VLR (ISUP)


Inter MSC Handover (2)

• The new MSC/VLR creates a connection with the new BSC

• The new BSC reserves a TCH in the new BTS and informs the new MSC

• The new MSC informs the anchor MSC

• The anchor MSC informs the BSC that commands the MS to switch to the new TCH

• the MS switches and activates the new channel

• The old channels are released


HandOver Number

o Same format of MSRN and MSISDNo HON = CC + NDC + SN

n CC = Country Coden NDC = National Destination Coden SN = Subscriber Number


mrn-en-7-gsm part 2

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