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 incoming

call through the final disposition of the call. 2. The end-to-end sequence of operations performed

by a network from the instant a call attempt isinitiated until the instant the call release is

completed. . .

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

initialization. It's what happens when you firstturn on your phone.

You get a connection to a nearby cell site,then the cellular network checks your

account. If you have a valid telephone number and

your account is good then your call proceeds.Let's take this step by step….

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 established

with near by BTS. It is not possible to make a call unless your mobile has a link to a cell site.

So a connection to cellular system is to beestablished which means that a frequency is

needed to transmit on

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 in the range.

GSM Call Flow contd.., The mobile scans the available frequenciesand measures the received level on each

channel. Finally the GSM system decides which cell hasto handle the mobile station which is usually

the cell site delivering the highest signalstrength to the mobile.

As a next step, the mobile receivescorresponding bursts on FCCH and SCH and

synchronizes with the cell site.

Location Management Involves two basic operations

Paging Search by system to track the mobileMSC broadcasts messageTarget replies in the uplink channelUpdate Mobile sends update message on the uplink channel when it changes its location.

Location management in GSM ishandled by Mobility Management layerin 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 one cell, then network requires updating each time it changes the cell which increase the burden on the system

Because of that a compromise solution used in GSM is to group cells into “Location areas” Updating messages are required whenthe mobile station moves betweenlocation areas.

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

MS States An MS can be in one of the following states:

Detached: MS is powered off At tach ed: MS power is on An attached MS can be: Idle: MS has no dedicated channel allocated and listens to

BCCH and PCH Active: MS has a dedicated connection to the network Changing from idle to active mode can be a result of location

updating, call setup, short message transfer etc.

Location updating When mobile powered Performs update indicating it’s international mobile subscription id The above procedure called “

When moves to new Location Area or a Different PLMN Update message sent to new If MS authorized in the new then subscriber’s HLR updates the current location sends a message to the old MSC/VLR to cancel it’s

VLR entry

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 subscriber’s HLR fora copy of the subscription information

3. The VLR updates the MS status to idle.

4. Acknowledgement is sent to the MS

Mobile Originated Call (MOC)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 the

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

Marking the MS as “active” in the VLRThe authentication procedureStart cipheringEquipment identificationSending the called party’s 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 an idle TCH. The BTS

and MS are told to tune to the 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

MOC (Call set-up MS to PSTN)MOC (Call set-up MS to PSTN)

Mobile Terminated Call (MTC) Mobile Terminated Call (MTC)

The major difference between an MTC and MOC 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-up procedure for a call from a PSTN subscriber to a mobile subscriber.

1. The PSTN subscriber keys in the MS’s telephone number (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 MS’s home GMSC. Now the call has entered

the dialled party’s network 2. The GMSC analyzes the MSISDN and queries the HLR for information about

how to route the call to the serving MSC/VLR. 3. The HLR translates MSISDN into IMSI, and determines which MSC/VLR is

currently serving the MS. The HLR also checks if the service, “Call forwarding to C–number” is activated,

if so,the call is rerouted by the GMSC to that number.4. The HLR requests an MSRN from the serving MSC/VLR. 5. The MSC/VLR returns an MSRN via HLR to the GMSC. 6. The GMSC analyses the MSRN and routes the call to the MSC/VLR. 7. The MSC/VLR knows which LA the MS is located in. A paging message is sent

to the corresponding BSC.

MTC contd..,MTC contd.., 8. The BSC’s distribute the paging message to the 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-up procedures

Over SDCCH all signaling preceding a call takes 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. The BTS and MS are told to tune to the TCH.

The mobile phone rings. If the subscribe answers, the connection is established

Call to MS from PSTN (MTC)Call to MS from PSTN (MTC)

Mobile Originated Call (MOC) Mobile Originated Call (MOC)

1, 2: connection request 3, 4: security check 5-8: check resources

(free circuit) 9-10: set up call

1: calling a GSM subscriber2: forwarding call to GMSC3: signal call setup to HLR4, 5: request MSRN from VLR6: forward responsible MSC to GMSC7: forward call to current MSC8, 9: get current status of MS10, 11: paging of MS12, 13: MS answers14, 15: security checks16, 17: set up connection

Mobile Terminated Call (MTC) Mobile Terminated Call (MTC)

MTC/MOC

Sequence of Operations for an

Incoming Call

Call flow when MS is in roaming