GSM Call Procedures
A GSM call is a sequence of radio-resource, mobility-management, call-control, and switching procedures. The mobile station first obtains a signaling connection. The network can then authenticate the subscription, protect the radio link, select a traffic channel, and connect the call through the MSC.
The main procedures are:
- Location update: The MS tells the network which location area serves it.
- Authentication and ciphering: The network verifies the subscription and can activate radio-link ciphering.
- Mobile-originated call setup: The MS requests a call and supplies the called-party address.
- Mobile-terminated call setup: The network routes an incoming call, pages the MS, and delivers the call.
- Handover: The network changes the serving radio resource while an active call continues.
Procedure 1: Location Update
A location update records the MS in the location area where it is currently available. The tells the network that the phone is reachable and identifies the area in which the MS should be paged. After power-on or a location-area change, the MS reads broadcast channels, compares the area identity, and reports the change. The procedure is separate from a call, although the same signaling channels and security procedures can be used when a call is requested.
A typical sequence is:
- Cell selection and camping.
The MS searches for suitable broadcast carriers and reads the FCCH and SCH for frequency and frame synchronization. It then reads the , which identifies the PLMN, cell, location area, access parameters, and neighboring-cell information. The MS compares the broadcast location-area identity with the value stored on the SIM. It camps on a cell that satisfies the GSM selection criteria rather than simply choosing a radio level in isolation.
- Random access.
When an update is required, the MS sends a Channel Request in an access burst on the shared . The request includes an establishment cause and a random reference. It is the first uplink message in the procedure.
- Initial assignment.
The BSS handles the initial access and returns an Immediate Assignment on the downlink . The assignment specifies the frequency, time slot, and timing information for an or, when the network chooses very early assignment, a traffic channel.
- Location update request.
On the dedicated signaling connection, the MS sends LOCATION UPDATING REQUEST. It identifies itself with a when the serving network recognizes that value. The IMSI can be requested or supplied when the temporary identity is unavailable or unknown. The request also carries the old location area and the update type.
- Authentication and ciphering.
The can request authentication data from the HLR and AuC. The network sends an AUTHENTICATION REQUEST containing a challenge, and the SIM returns AUTHENTICATION RESPONSE. If the operator enables ciphering, the network sends CIPHERING MODE COMMAND and the MS confirms with CIPHERING MODE COMPLETE. The long-term key is never sent over Um.
- Authentication: The VLR and HLR, working with the AuC and SIM, verify a cryptographic response to the network challenge.
- Ciphering: After successful authentication, the network and MS activate radio-link ciphering for subsequent signaling and speech.
- Database update.
The serving VLR records the subscriber and informs the using the MAP location-update procedure on the D interface. The HLR stores the new VLR address and asks the old VLR to cancel the former registration. The signaling path can also transfer the subscription data required by the new VLR.
- Acceptance and temporary identity.
The network sends LOCATION UPDATING ACCEPT. It may include a TMSI reallocation command so that later radio messages use a temporary identity. The MS confirms the reallocation and keeps the new value for subsequent procedures.
- Release.
After the signaling transaction, the SDCCH is released. In idle mode the MS continues to read broadcast and paging information at the intervals defined by its GSM paging configuration.
Procedure 2: Mobile-Originated Call Setup
In a mobile-originated call, the user supplies the destination number and the MS asks the network for a circuit-switched connection. GSM separates the initial access and mobility-management exchange from the call-control messages that carry the call information.
- Requesting radio access.
The MS sends CHANNEL REQUEST on RACH with a mobile-originating establishment cause. The BSS returns IMMEDIATE ASSIGNMENT on AGCH. The assignment usually gives an SDCCH for signaling. A network may choose early or very early traffic-channel assignment according to its resource policy.
- Requesting the service.
The MS sends CM SERVICE REQUEST with the mobile identity, classmark, ciphering-key sequence, and requested service type. The network may run identification, authentication, and ciphering procedures. It accepts the MM connection with CM SERVICE ACCEPT or rejects it with CM SERVICE REJECT.
- Sending call information.
Once the MM connection is active, the MS sends SETUP. The message contains the called-party address and the bearer capability needed for the requested speech service. The network acknowledges that it has received the call information with CALL PROCEEDING.
- Assigning the traffic channel.
The MSC analyzes the call and sends an ASSIGNMENT REQUEST to the BSS using BSSMAP on the A interface. The BSS selects a compatible TCH and sends the corresponding RR ASSIGNMENT COMMAND to the MS. The MS returns ASSIGNMENT COMPLETE, and the BSS reports the result to the MSC. The network may perform this assignment before remote alerting or later, after the far end answers.
- Connecting the far end.
For a call leaving the PLMN, the MSC uses the appropriate interworking and signaling path, commonly ISUP, to send an IAM toward the destination. When the called side starts ringing, the MS receives ALERTING. After the remote party answers, the network sends CONNECT and the MS returns CONNECT ACKNOWLEDGE. The speech connection is then attached to the assigned traffic channel.
Procedure 3: Handover
Handover transfers an active call to a new radio channel or cell. It is a radio-resource procedure coordinated with call control, so the speech path can remain connected while the serving cell changes.
During a call, the MS measures the received level and quality of its serving channel and the broadcast carriers of neighboring cells. It sends the results in MEASUREMENT REPORT messages on the associated with the traffic channel. The phone can report several neighboring carriers whose frequencies were supplied by the serving cell's BCCH.
The BSC evaluates the reports and radio conditions. An intra-BSS handover can be handled within the BSS. When the target belongs to another BSC or MSC area, the MSCs coordinate the inter-BSS or inter-MSC procedure. Load, quality, level, and operator thresholds can all influence the decision.
The serving and target network entities reserve a compatible channel, including a frequency and time slot in the target cell. The MSC and BSS exchange BSSMAP handover messages when the target crosses a BSC or MSC boundary. The serving side then sends HANDOVER COMMAND to the MS on the , which can temporarily steal traffic bursts for urgent signaling.
The MS tunes to the target channel and transmits HANDOVER ACCESS. The target BTS detects the access, the MS completes the handover procedure, and the network releases the old radio and terrestrial resources. The duration depends on the handover type, synchronization, load, and network implementation. GSM specifies the sequence and supervision timers, but it does not establish one guaranteed speech-interruption value. The design goal is to keep the interruption short. A failed target access causes the network to follow the defined handover-failure and call-retention procedures.
Procedure 4: Mobile-Terminated Call Setup
A mobile-terminated call starts outside the radio access network. The GMSC receives the MSISDN, queries the HLR, and resolves the subscriber's current serving MSC. The network then uses paging to locate the MS inside its registered location area.
- Call arrival: An external network delivers the MSISDN to a GMSC or another gateway that can interrogate the home network.
- Routing lookup: The GMSC sends MAP SEND ROUTING INFORMATION to the HLR. The HLR asks the serving VLR for a temporary Mobile Station Roaming Number with PROVIDE ROAMING NUMBER. The VLR allocates an MSRN from the serving MSC's current pool.
- Routing to the serving MSC: The HLR returns the MSRN. The GMSC routes the call to the MSC currently serving the subscriber and releases the temporary routing context when appropriate.
- Paging: The serving MSC asks the BSS to page the MS. The BSS broadcasts PAGING REQUEST on the PCH throughout the registered location area, using the TMSI when available.
- Radio response: The MS recognizes its identity and sends CHANNEL REQUEST on RACH with a paging-response cause. The BSS returns IMMEDIATE ASSIGNMENT for an SDCCH, and the MS establishes the signaling connection.
- Call-control exchange: After the required MM and security procedures, the network sends SETUP. The MS answers with CALL CONFIRMED, starts user alerting, and sends ALERTING. The network can assign the TCH early or late according to the selected call-establishment option.
- Answer and speech: When the called user answers, the MS sends CONNECT. The network returns CONNECT ACKNOWLEDGE and attaches the speech connection to the assigned traffic channel.