5.4 Call clearing

24.0083GPPCore network protocolsMobile radio interface Layer 3 specificationRelease 18Stage 3TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

5.4.1 Terminology

The following terms are used in the present document in the description of clearing procedures:

– A traffic channel (see 3GPP TS 44.003 [16]) is "connected" when the channel is part of a circuit-switched connection established according to the present document.

– A traffic channel is "disconnected" when the channel is no longer part of a circuit-switched connection, but is not yet available for use in a new connection.

5.4.2 Exception conditions

Under normal conditions, the call control entity of the mobile station or of the network initiates call clearing by sending a DISCONNECT message to its peer entity; then both entities follow the procedures defined in subclauses 5.4.3 and 5.4.4 respectively.

As an exception to the above rule, the call control entity of the mobile station or of the network, in response to a SETUP or START CC or CC-ESTABLISHMENT CC-ESTABLISHMENT CONFIRMED or RECALL message, can reject a call by stopping all running call control timers, responding with a RELEASE COMPLETE message, releasing the MM connection, and returning to the "null" state, provided no other response has previously been sent.

As a further exception, the call control entity of the network may initiate call clearing by stopping all running call control timers, sending a RELEASE message, starting timer T308, and entering the "release request" state.

NOTE: This way to initiate call clearing by sending a RELEASE message should not be used by the network:

– if in-band tones/announcements are provided and the network decides to use the procedure described in subclause 5.4.4.1.1.1 or 5.4.4.2.1;

– if the network wants to have the opportunity to respond to information sent by the mobile station during call clearing, e.g. when the network indicates that "CCBS activation is possible".

A call control entity shall accept an incoming RELEASE COMPLETE message used to initiate the call clearing even though the cause information element is not included.

A control entity shall accept an incoming RELEASE message used to initiate the call clearing even though the cause information element is not included.

Furthermore, a call control entity shall regard an incoming RELEASE COMPLETE message as consistent with any of its states; a call control entity shall regard an incoming RELEASE message as consistent with any of its states except the null state: a call control entity of the mobile station shall regard an incoming DISCONNECT message as consistent with any of its call control states except the "null" state, the "release request" state, and the "disconnect indication" state; a call control entity of the network shall regard an incoming DISCONNECT message as consistent with any of its call control states except the "null" state and the "release request" state.

NOTE: This allows the introduction of shorter call clearing procedures in the future.

5.4.3 Clearing initiated by the mobile station

5.4.3.1 Initiation of call clearing

Apart from the exceptions identified in subclause 5.4.2, the call control entity of the mobile station shall initiate clearing by: stopping all running call control timers, sending a DISCONNECT message; starting timer T305; and entering the "disconnect request" state.

5.4.3.2 Receipt of a DISCONNECT message from the mobile station.

The call control entity in the network in any state except the "null" state and the "release request" state shall, upon receipt of a DISCONNECT message:

– Stop all running call control timers;

– initiate procedures to clear the network connection and the call to the remote user;

– send a RELEASE message to its peer entity;

– start timer T308; and

– enter the "release request" state.

NOTE: The RELEASE message has only local significance and does not imply an acknowledgement of clearing from the remote user.

5.4.3.3 Receipt of a RELEASE message from the network

The call control entity of the mobile station in any state except the "null" state and the "release request" state, shall, upon receipt of a RELEASE message: stop all running call control timers; send a RELEASE COMPLETE message; release the MM connection; and return to the "null" state.

5.4.3.4 Receipt of a RELEASE COMPLETE message from the mobile station

A call control entity of the network in any call control state shall, upon receipt of a RELEASE COMPLETE message from its peer entity in the mobile station: stop all running call control timers; release the MM connection; and return to the "null" state.

5.4.3.5 Abnormal cases

The call control entity of the mobile station in the "disconnect request" state, shall upon expiry of timer T305: send a RELEASE message to the network with the cause number originally contained in the DISCONNECT message and optionally, a second cause information element with cause #102 "recovery on timer expiry", start timer T308, and enter the "release request" state.

The call control entity of the network in the "release request" state, shall, at first expiry of timer T308, retransmit the RELEASE message, start timer T308, and stay in the "release request" state. At second expiry of timer T308, the call control entity of the network shall: release the MM connection; and return to the "null" state.

5.4.4 Clearing initiated by the network

Apart from the exception conditions identified in subclause 5.4.2, the call control entity of the network shall initiate clearing by: sending a DISCONNECT message; and entering the "disconnect indication" state. The DISCONNECT message is a local invitation to clear the call.

NOTE: When the network initiates clearing by sending a RELEASE message, the procedures described in subclauses 5.4.3., 5.4.3.4 and 5.4.3.5 are followed.

A mobile station that does not support the "Prolonged Clearing Procedure" shall comply with the requirements of subclause 5.4.4.1 and shall ignore subclause 5.4.4.2. A mobile station that supports the "Prolonged Clearing Procedure" shall comply with the requirements of subclauses 5.4.4.2 and shall ignore subclause 5.4.4.1.

5.4.4.1 Clearing initiated by the network: mobile does not support "Prolonged Clearing Procedure"

Sublause 5.4.4.1 only applies to mobile stations that do not support the "Prolonged Clearing Procedure" option.

5.4.4.1.1 Clearing when tones/announcements provided

When in-band tones/announcements are provided (see subclause 5.5.1), the call control entity of the network may initiate clearing by sending a DISCONNECT message containing progress indicator #8 "in-band information or appropriate pattern now available", starting timer T306, and entering the "disconnect indication" state.

5.4.4.1.1.1 Receipt of a DISCONNECT message with progress indicator #8 from the network

i) if an appropriate speech traffic channel is not connected, continue clearing as defined in subclause 5.4.4.1.2.1 without connecting to the in-band tone/announcement;

ii) if an appropriate speech traffic channel is connected, attach the user connection for speech if it is not yet attached and enter the "disconnect indication" state. In that state, if upper layers request the clearing of the call, the call control entity of the MS shall proceed as defined in subclause 5.4.4.1.2.1.

5.4.4.1.1.2 Expiry of timer T306

The call control entity of the network, having entered the "disconnect indication" state after sending a disconnect message with the progress indicator #8, shall, upon expiry of timer T306, continue clearing by sending a RELEASE message with the cause number originally contained in the DISCONNECT message; starting timer T308; and entering the "release request" state.

5.4.4.1.2 Clearing when tones/announcements not provided

When in-band tones and announcements are not provided, the call control entity of the network shall initiate call clearing by stopping all running call control timers, sending a DISCONNECT message without progress indicator, starting timer T305 and entering the "disconnect indication" state.

5.4.4.1.2.1 Receipt of a DISCONNECT message without progress indicator or with progress indicator different from #8 from the network

The call control entity of the mobile station in any state except the "null" state, the "disconnect indication" state, and the "release request" state, shall, upon the receipt of a DISCONNECT message without progress indicator information element or with progress indicator different from #8:

– stop all running call control timers;

– send a RELEASE message;

– start timer T308; and

– enter the "release request" state.

5.4.4.1.2.2 Receipt of a RELEASE message from the mobile station

The call control entity of the network in any state except the "null" state and the "release request" state, shall, upon receipt of a RELEASE message: stop all running call control timers; send a RELEASE COMPLETE message; release the MM connection; and return to the "null" state.

5.4.4.1.2.3 Abnormal cases

The call control entity of the network, having entered the "disconnect indication" state after sending a DISCONNECT message without progress indicator or with progress indicator different from #8, shall upon expiry of timer T305: send a RELEASE message to the mobile station with the cause number originally contained in the DISCONNECT message; start timer T308; and enter the "release request" state. In addition to the original clearing cause, the RELEASE message may contain a second cause information element with cause #102 "recovery on timer expiry".

5.4.4.1.3 Completion of clearing

A call control entity of the mobile station in any call control state shall, upon receipt of a RELEASE COMPLETE message from its peer entity in the network: stop all running call control timers; release the MM connection; and return to the "null" state.

5.4.4.1.3.1 Abnormal cases

The call control entity of the mobile station in the "release request" state shall at first expiry of timer T308 retransmit the RELEASE message and restart timer T308. At second expiry of timer T308, the call control entity of the mobile station shall: release the MM connection; and return to the "null" state.

5.4.4.2 Clearing initiated by the network: mobile supports "Prolonged Clearing Procedure"

Sublause 5.4.4.2 only applies to mobile stations that support the "Prolonged Clearing Procedure" option.

5.4.4.2.1 Clearing when tones/announcements provided and the network does not indicate that "CCBS activation is possible"

When in-band tones/announcements are provided (see subclause 5.5.1) and CCBS is not applicable, the call control entity of the network may initiate clearing by sending a DISCONNECT message containing progress indicator #8 "in-band information or appropriate pattern now available", either not containing an Allowed Actions IE or containing an Allowed Actions IE indicating "CCBS activation is not possible", starting timer T306, and entering the "disconnect indication" state.

5.4.4.2.1.1 Receipt of a DISCONNECT message

The call control entity of the MS in any state except the "null" state, the "disconnect indication" state, and the "release request" state, shall, upon receipt of a DISCONNECT message with progress indicator #8 and, either not containing an Allowed Actions IE or containing an Allowed Actions IE indicating "CCBS activation is not possible":

i) if an appropriate speech traffic channel is not connected,

– stop all running call control timers;

– send a RELEASE message;

– start timer T308;

– enter the "release request" state; and

– not connect to the in-band tone/announcement.

– stop all running call control timers;

– send a RELEASE message;

– start timer T308; and

– enter the "release request" state.

5.4.4.2.1.2 Expiry of timer T306

The call control entity of the network, having entered the "disconnect indication, shall, upon expiry of timer T306, continue clearing by sending a RELEASE message with the cause number originally contained in the DISCONNECT message; starting timer T308; and entering the "release request" state.

5.4.4.2.2 Clearing when the network indicates that "CCBS activation is possible"

When Activation of CCBS is possible, the call control entity of the network may initiate clearing by sending a DISCONNECT message containing the Allowed Actions IE with an indication that "Activation of CCBS is possible" and starting T338. Optionally, progress indicator #8 "in-band information or appropriate pattern now available" may also be contained in the DISCONNECT message (in which case, T338 shall not be greater than T306).

5.4.4.2.2.1 Receipt of a DISCONNECT

Relative to the current state the following procedures apply:

– The call control entity of the MS in the "null" state, the "disconnect indication" state and the "release request" state, shall, upon receipt of a DISCONNECT message react as described in clause 8.

– The call control entity of the MS in the "disconnect request" state, shall, upon receipt of a DISCONNECT message:

– stop all running call control timers;

– send a RELEASE message;

– start timer T308; and

– enter the "release request" state.

– The call control entity of the MS in any other states, shall, upon receipt of a DISCONNECT message with an Allowed Actions IE indicating "Activation of CCBS is possible" pass the "Activation of CCBS is possible" indication to the upper layer, enter the "disconnect indication" state, stop all running call control timers and await a response from the upper layers.

If the DISCONNECT message contained the progress indicator #8 "in-band information or appropriate pattern now available" and an appropriate speech traffic channel is connected, then the MS shall attach the user connection for speech if it is not yet attached. If the DISCONNECT message did not contain the progress indicator #8 "in-band information or appropriate pattern now available" any connected speech traffic channel shall be disconnected.

Response from the upper layers:

i) If the upper layers request the clearing of the call, the call control entity of the MS shall:

– stop all running call control timers;

– send a RELEASE message;

– start timer T308; and

– enter the "release request" state.

ii) If the upper layers request that the "CCBS activation is to be attempted" then the MS shall

– send a RELEASE message containing a Facility IE including an

– Invoke=CCBSRequest to the network;

– stop all running call control timers;

– start timer T308; and

– enter the "release request" state.

If an appropriate speech traffic channel is connected, transmission of this RELEASE message shall not cause it to be disconnected.

5.4.4.2.2.2 Expiry of timer T338

The call control entity of the network, having entered the "disconnect indication" state after sending a DISCONNECT message with an Allowed Actions IE indicating "Activation of CCBS is possible" shall, upon expiry of timer T338, continue clearing by sending a RELEASE message with the cause number originally contained in the DISCONNECT message; starting timer T308; and entering the "release request" state.

5.4.4.2.3 Clearing when tones/announcements are not provided and the network does not indicate that "CCBS activation is possible"

When in-band tones and announcements are not provided, and, the network does not wish to indicate in the Allowed Actions IE that "CCBS is possible", the call control entity of the network shall initiate call clearing by stopping all running call control timers, sending a DISCONNECT message without progress indicator, either without the Allowed Actions IE or with the Allowed Actions IE indicating that "CCBS is not possible", starting timer T305 and entering the "disconnect indication" state.

5.4.4.2.3.1 Receipt of a DISCONNECT message

The call control entity of the mobile station in any state except the "null" state, the "disconnect indication" state, and the "release request" state, shall, upon the receipt of a DISCONNECT message either without progress indicator information element or with progress indicator different from #8, and, either without the Allowed Actions IE or with the Allowed Actions IE indicating that "CCBS is not possible":

– stop all running call control timers;

– send a RELEASE message;

– start timer T308; and

– enter the "release request" state.

5.4.4.2.3.2 Abnormal cases

The call control entity of the network, having entered the "disconnect indication", shall upon expiry of timer T305: send a RELEASE message to the mobile station with the cause number originally contained in the DISCONNECT message; start timer T308; and enter the "release request" state.

5.4.4.2.4 Receipt of a RELEASE message from the mobile station

5.4.4.2.4.1 Release, CCBS not requested

For a network that does not support the "CCBS activation" option:

For a network that does support the "CCBS activation" option:

The call control entity of the network in any state except the "null" state and the "release request" state, shall, upon receipt of a RELEASE message without a Facility IE including an Invoke=CCBSRequest: stop all running call control timers; send a RELEASE COMPLETE message; release the MM connection; and return to the "null" state.

5.4.4.2.4.2 Release, CCBS Requested

For a network that does not support the "CCBS activation" option:

For a network that does support the "CCBS activation" option:

The call control entity of the network in any state except the "null" state and the "release request" state, shall, upon receipt of a RELEASE message containing a Facility IE including an Invoke=CCBSRequest: stop all running call control timers; then attempt to activate the recall; then send a RELEASE COMPLETE message indicating the success or failure of the recall activation attempt; release the MM connection; and return to the "null" state.

5.4.4.2.5 Completion of clearing

5.4.4.2.5.1 Abnormal cases

The retransmitted RELEASE message need not contain the Facility IE including an Invoke=CCBSRequest, even if the original RELEASE message did contain this IE.5.4.5 Clear collision

Clear collision occurs when both the mobile station and the network simultaneously transfer DISCONNECT messages specifying the same call.

The behaviour of the network call control entity receiving a DISCONNECT message whilst in the "disconnect indication" state is specified in subclause 5.4.3. The behaviour of the MS call control entity receiving a DISCONNECT message whilst in the "disconnect request" state is defined in subclause 5.4.4.

Clear collision can also occur when both sides simultaneously transfer RELEASE messages related to the same call. The entity receiving such a RELEASE message whilst within the "release request" state shall: stop timer T308; release the MM connection; and enter the "null" state (without sending a RELEASE COMPLETE message).

5.4.5 Call clearing for SRVCC from CS to PS

If the MS receives the request from upper layers to release the call locally, the call control entity of the MS shall detach the user connection, if connected, and enter the NULL state.