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GSM Call Flow

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GSM Call Flow Call processing means all steps which set up,

maintain, and then end a call. According to the Telecom Glossary put out by the

American National Standard for Telecommunications,call processing means: 1. The sequence of operations performed by a

switching system from the acceptance of an incomingcall through the final disposition of the call.

2. The end-to-end sequence of operations performedby a network from the instant a call attempt isinitiated until the instant the call release iscompleted. . . .

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GSM Call Flow contd.., The first part to mobile call processing is

initialization. It's what happens when you first

turn on your phone. You get a connection to a nearby cell site,

then the cellular network checks youraccount.

If you have a valid telephone number andyour account is good then your call proceeds.Let's take this step by step.

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GSM Call Flow contd.., You turn on your phone. Assume that you're

in your home location.

First of all a connection is to be establishedwith near by BTS. It is not possible to makea call unless your mobile has a link to a cellsite.

So a connection to cellular system is to beestablished which means that a frequency isneeded to transmit on.

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GSM Call Flow contd.., So the mobile tries to find out broadcast channels.

Again, the BCCH is not a dedicated radio frequency.It is rather a channel within the bit stream carried by

any of the frequencies in a cell. A base station's Broadcast Control Channel

continuously sends out identifying information aboutits cell site and helps the mobile to get some

information about the cell site. For the initial period mobile acts as a receiver

checking for a signal from any base station with inthe range.

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GSM Call Flow contd.., The mobile scans the available frequencies

and measures the received level on each

channel. Finally the GSM system decides which cell has

to handle the mobile station which is usuallythe cell site delivering the highest signal

strength to the mobile. As a next step, the mobile receives

corresponding bursts on FCCH and SCH andsynchronizes with the cell site.

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Location Management Involves two basic operations

Paging

Search by system to track the mobile

MSC broadcasts message

Target replies in the uplink channel

Update Mobile sends update message on the uplink

channel when it changes its location

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Location management in GSM ishandled by Mobility Management layer

in the GSM protocol architecture. A powered-on mobile is informed of an

incoming call by a paging message.

If the paging is done exactly for onecell, then network requires updatingeach time it changes the cell whichincrease the burden on the system

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Because of that a compromise solutionused in GSM is to group cells into

Location areas Updating messages are required when

the mobile station moves betweenlocation areas.

Also mobile stations are paged in thecells of their current location area.

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MS StatesAn MS can be in one of the following states:

Detached: MS is powered off

Attached: MS power is on An attached MS can be:

Idle: MS has no dedicated channel allocated and listens toBCCH and PCH

Active: MS has a dedicated connection to the network

Changing from idle to active mode can be a result of locationupdating, call setup, short message transfer etc.,

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Location updating When mobile powered ON

Performs update indicating its IMSI(

International Mobile Subscription Id ) The above procedure called IMSI Attach

Procedure When moves to new Location Area or a Different

PLMN Update message sent to new MSC/VLR If MS authorized in the new MSC/VLR

then subscribers HLR updates the current location sends a message to the old MSC/VLRto cancel itsVLR entry

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When a mobile station is power off itperforms an IMSI detach procedure in order

to tell the network that it is no longeravailable.

Registration:

This is the process in which an MS informs anetwork that it is attached.

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IMSI Attach When an MS is switched on, the IMSI attach

procedure is executed. This involves thefollowing steps:

1. The MS sends an IMSI attach message tothe network indicating that it has changedstate to idle.

2. The VLR determines whether there is arecord for the subscriber already present. Ifnot, the VLR contacts the subscribers HLR fora copy of the subscription information.

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IMSI Attach contd.., 3. The VLR updates the MS status to

idle.

4. Acknowledgement is sent to the MS.

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Mobile Originated Call (MOC)

1. The MS uses RACH to ask for a signaling channel.

2. The BSC allocates a signaling channel, using AGCH.

3. When the channel is allocated, it sends its IMSI andVLR is signed as busy.

4. The MS sends a call set-up request via SDCCH to theMSC/VLR. Over SDCCH all signaling preceding a calltakes place. This includes:

Marking the MS as active in the VLR The authentication procedure

Start ciphering

Equipment identification

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Sending the called partys number to the network Checking if the subscriber has the service Barring

of outgoing calls activated

5. The MSC/VLR instructs the BSC to allocate anidle TCH. The BTS and MS are told to tune tothe TCH.

6. The MSC/VLR forwards the called party

number to an exchange in the PSTN, whichestablishes a connection to the subscriber.7. If the called subscriber answers, the

connection is established.

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MOC (Call set-up MS to PSTN)

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Mobile Terminated Call (MTC)

The major difference between an MTC andMOC is that in a call to an MS the exact

location of the mobile subscriber is unknown. Therefore, the MS must be located using

paging before a connection can beestablished.

Below is the description of the call set-upprocedure for a call from a PSTN subscriberto a mobile subscriber.

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MTC contd..,

1. The PSTN subscriber keys in the MSs telephonenumber (MSISDN).

The MSISDN is analyzed in the PSTN, which identifies

that this is a call to a mobile network subscriber. A connection is established to the MSs home GMSC.

Now the call has entered the dialled partys network

2. The GMSC analyzes the MSISDN and queries the

HLR for information about how to route the call tothe serving MSC/VLR.

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MTC contd..,

3. The HLR translates MSISDN into IMSI, anddetermines which MSC/VLR is currently

serving the MS. The HLR also checks if the service, Call

forwarding to Cnumber is activated, if so,the call is rerouted by the GMSC to that

number.

4. The HLR requests an MSRN from theserving MSC/VLR.

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MTC contd..,

5. The MSC/VLR returns an MSRN via HLR tothe GMSC.

6. The GMSC analyses the MSRN and routesthe call to the MSC/VLR.

7. The MSC/VLR knows which LA the MS islocated in. A paging message is sent to thecorresponding BSC.

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MTC contd..,

8. The BSCs distribute the paging message tothe BTS in the desired LA.

9. When the MS detects the paging message,it sends a request on RACH for a SDCCH.

10. The BSC provides a SDCCH, using AGCH.

11. SDCCH is used for the call set-upprocedures.

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MTC contd..,

Over SDCCH all signaling preceding a calltakes place. This includes:

Marking the MS as active in the VLR The authentication procedure

Start ciphering

Equipment identification 12. The MSC/VLR instructs the BSC to

allocate a TCH.

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MTC contd..,

The BTS and MS are told to tune to the TCH.The mobile phone rings. If the subscriber

answers, the connection is established.

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Call to MS from PSTN (MTC)

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Mobile Originated Call (MOC)

PSTN GMSC

VLR

BSS

MSC

MS1

2

6 5

3 4

9

10

7 8

1, 2: connectionrequest

3, 4: security check 5-8: check resources

(free circuit)

9-10: set up call

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Mobile Terminated Call (MTC)

PSTNcalling

stationGMSC

HLR VLR

BSSBSSBSS

MSC

MS

1 2

3

4

5

6

7

8 9

10

11 12

13

1610 10

11 11 11

14 15

17

1: calling a GSM subscriber

2: forwarding call to GMSC

3: signal call setup to HLR

4, 5: request MSRN from VLR6: forward responsible MSC to

GMSC

7: forward call to current MSC

8, 9: get current status of MS

10, 11: paging of MS

12, 13: MS answers

14, 15: security checks

16, 17: set up connection

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MTC/MOC

BTSMS

paging request

channel request

immediate assignment

paging response

authentication requestauthentication response

ciphering command

ciphering complete

setup

call confirmed

assignment command

assignment complete

alerting

connect

connect acknowledge

data/speech exchange

BTSMS

channel request

immediate assignment

service request

authentication requestauthentication response

ciphering command

ciphering complete

setup

call confirmed

assignment command

assignment complete

alerting

connect

connect acknowledge

data/speech exchange

MTC MOC

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Call flow when MS is inroaming

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Billing Procedures

GSM uses Time Division Multiple Access(TDMA) technology to divide all channels toeight time slots for carrying data. 8 time slots

provides 8 calls simultaneously. But one slotis dedicated to controlling. At the end of each call MSC produces a CDR

and they are stored in a file and this file is

sent to Billing Gateway (BGW). BGW converts the data into standartized

format and route them to the BSCS. BSCs produces a bill in billcycle.

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Billing Procedures (Contd)

BSCs produces the bill related to the informationstored in CDR. These informations are;

If the target number is a constant or a mobile phone

If it is a mobile phone, its zone location The duration of the call

If there is a special campaign applied to targetnumber

By considering all these things, BSCs prepares a bill.

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Roaming in the Network

Changing Cells within an LA:

MSs are constantly moving around in the cellular

network. The MS location information stored in the VLR is the

LA.

If an MS changes cells within a LA, the network is not

updated.

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The MS knows that the new cell belongs tothe same LA by listening to the BCCH in thenew cell.

The BCCH broadcasts the cells LAI.

The MS compares the last LAI received withthe new LAI.

If they are the same, it means that the MShas not changed LA's and does not need toinform the network.

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Location Updating (Same MSC/VLR)

If an MS detects a change in LAI on theBCCH, it informs the network.

When the MS sends the LocationUpdating message, the MSC/VLRdetermines whether it is an MS, which

is already registered, or if it is an MSvisiting from another MSC/VLR.

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Location Updating (sameMSC/VLR)

MS BTS BSC MSC/VLR

Location Update Request

Authentication

Ciphering

Location Update Accept

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Location Updating contd..,

1. The MS listens to BCCH in the new cell todetermine the LAI. The received LAI information iscompared to the old one. If they differ, a location

update is necessary. 2. The MS establishes a connection with the network

via SDCCH. Authentication is performed.

3. If authentication is successful, the MS sends aLocation Updating Request to the system.

4. The system acknowledges Location Updating andrequests BTS and MS to release the signalingchannel.

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Location Updating (New MSC/VLR)

When an MS roams into a new LA, locationupdating is performed.

Also the LA may belong to a new MSC/VLR. If so the Location Update Request is received

by the new VLR and it executes theprocedure below.

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Location Updating (New MSC/VLR)contd..,

1. Authentication is performed when it is successful, the VLRchecks its database to determine whether or not it has a recordfor this MS-subscription.

2. When the VLR finds no record for the MS, it sends a request

to the subscribers HLR for a copy of the MS-subscription. 3. The HLR passes the information to the VLR and updates its

location information for the subscriber. The HLR instructs theold VLR to delete the information it has about the MSsubscription.

4. The VLR stores its subscription information for the MS

including the latest location and status (idle). The VLR sendsacknowledgement to the MS.

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Location Update

MS NewMSC

NewVLR

HLROldVLR

Location Update

Request Update Location

Area Update Location

Cancel Location

Cancel Location

AckInsert Sunscriber

Data

Insert Sunscriber

Data Ack

Update LocationAck

Update Location

Area Ack

Location Update

Accept

Location Update Operation

Figure 5

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Location Updating, type PeriodicRegistration

Periodic registration is a feature which forcesMSs to send a registration message to the

network at predefined intervals. If an MS should miss such a registration, the

network will mark the MS as detached.

This may occur if an MS is out of the area of

coverage and ensures that needless paging isnot performed.

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Location Updating, type PeriodicRegistration contd..,

If the network uses periodic registration, theMS will be informed, on the BCCH of howoften periodic registration must beperformed.

Periodic registration has an acknowledgmentmessage.

The MS tries to register until it receives thismessage.

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Detaching from the Network

IMSI Detach:

IMSI detach enables the MS to indicate to the

network that it is switched off. At power off, the MS sends an IMSI detach

message to the network.

On reception, the HLR marks the

corresponding IMSI as detached. No acknowledgement is sent to the MS.

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Implicit Detach

If the MS sends an IMSI detach message to thesystem and the radio link quality is poor, the systemmight not be able to decode the information.

Because no acknowledgment is sent to the MS, nofurther attempt is made.

In this case, the system still regards the MS asattached.

If periodic registration is in use, the system will soondetermine that the MS is detached.

The VLR then performs an implicit detach, markingthe MS as detached.

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MS Purging

MS purging is used to inform the HLRthat the VLR is about to remove a

subscriber record from the VLR.

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Locating

An MS continuously measures signal strengthand quality on its own cell and signalstrength on the BCCH carriers of the

neighboring cells. The measurements are carried out on the

downlink while MS is in active mode. The measurement results are sent to the BTS

on SACCH at regular intervals. The serving BTS measures signal strength

and quality on the uplink.

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Locating contd..,

The measurements from the BTS and MS aresent to the BSC in the form of measurementreports.

Based on these reports, the BSC decides if ahandover is necessary and to which cell.

This is called locating.

As soon as a neighboring cell is considered tobe better than the serving cell, a handover isattempted.

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Types of Handoff contd..,

There are several types of handover,including:

Intra-BTS handover Handover between cells controlled by the

same BSC (Inter BTS/Intra BSC) Handover between cells controlled by

different BSCs, but the same MSC/VLR (Intra

MSC) Handover between cells controlled by

different MSC/VLRs (Inter MSC)

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Intra-Cell Handover

A special type of handover is the intra-cellhandover.

It is performed when the BSC considers the

quality of the connection too low, butreceives no indication from themeasurements that another cell would bebetter.

In that case the BSC identifies anotherchannel in the same cell which may offer abetter quality, and the MS is ordered toretune to it.

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Handover between Cellscontrolled by Same BSC

When performing a handover between twocells controlled by the same BSC, the

MSC/VLR is not involved. However, the MSC/VLR will be informed when

a handover has taken place.

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Intra BSC Handover

BTSold

1.measurement

result

BSC

4.Link establishment

MS

1.measurement

report

HO decision

BTSnew

1.ch. activation

1. ch. activation ack

2.HO command

2.HO command

5.HO complete

5.HO complete

6.Release TCH

3. HO Access

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Intra BSC Handoff contd..,

1. The BSC orders the new BTS to activate aTCH.

2. The BSC sends a message to the MS, viathe old BTS, containing information about thefrequency and time slot to change to and alsothe output power to use. This information issent to the MS using FACCH.

3. The MS tunes to the new frequency, andtransmits handover access information tonew BTS.

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Intra BSC Handoff contd..,

4. When the new BTS detects the handoverbursts, it sends information about TA. This is

also sent via FACCH. 5. The MS sends a Handover Complete

message to the BSC via the new BTS.

6. The BSC tells the old BTS to release the

old TCH.

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Intra MSC Handover

When another BSC is involved in a handover,the MSC/VLR must also be involved to

establish the connection between the twoBSCs.

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Intra MSC Handover

5. HO access

BTSold BSCnew

1. measurement

result

BSCold

6. Link establishment

MSCMS

1. measurement

report

HO decision

1. HO required

BTSnew

2. HO request

resource allocation

3. ch. activation

3. ch. activation ack4.HO request ack4. HO command4. HO command4. HO command

7. HO complete7. HO complete8. Release TCH9. Release TCH

Release completeRelease complete

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Intra MSC Handover contd..,

1. The serving (old) BSC sends a HandoverRequired message to the MSC containing theidentity of the target cell.

2. The MSC knows which BSC controls thiscell and sends a Handover Request to thisBSC.

3. The new BSC orders the target BTS to

activate a TCH. 4. The new BSC sends a message to the MS

via the MSC and the old BTS.

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Intra MSC Handover contd..,

5. MS tunes to the new frequency andtransmits handover access bursts in thecorrect time slot.

6. The new BTS sends information about TA. 7. MS sends a Handover Complete message

to MSC via the new BSC. 8. MSC sends the old BSC an order to release

the old TCH. 9. The old BSC tells the old BTS to release

the TCH.

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