6 Elementary procedures for Group Call Control

04.683GPPGroup Call Control (GCC) protocolTS

6.1 Overview

6.1.1 General

The elementary procedures may be grouped into the following classes:

– group call establishment procedures;

– group call termination procedures;

– call information phase procedures;

– miscellaneous procedures.

Figure 6.1 gives an overview of the main states and transitions on the MS side.

* if MS assumes to be the originator of the group call.

** if not in RR connected mode.

Figure 6.1: Overview group call control protocol/MS side

6.1.2 Group call control states

6.1.2.1 Group call control states at the MS side of the interface

The GCC entity of the MS is described as an extended finite state machine. It performs transitions between (main) states, and in main state GROUP CALL ACTIVE (U3) it performs transitions between sub-states. It has certain parameters and attributes, e.g. configuration parameters and behaviour parameters, which it sets and changes based on interaction with higher and lower (sub-)layers and on message exchange with its peer entity. If a configuration parameter is set to a certain value, the MS shall also adapt the configuration accordingly. Behaviour parameters decide on (part of) the behaviour of the GCC entity. When the GCC entity in the MS receives a message, it shall first analyse whether it shall ignore the message, see clauses 5 and 7.

6.1.2.1.1 Attributes and parameters of GCC in the MS

For the following behaviour parameters, the description is informative.

Parameter

Description

ORIG

Depending on the context, the MS assumes to be the originator of the call (ORIG = T) or not to be the originator of the call (ORIG = F).

COMM

Depending on the context, the MS assumes that communication with its peer entity is enabled in both directions (COMM = T) or not (COMM = F).

For the following configuration parameters the MS shall adapt its configuration according to the parameter value and parameter definition.

Parameter

Definition

D-ATT

D-ATT = T means that the MS attaches the user connection for the group call in the downlink.

D-ATT = F means that the MS does not attach the user connection for the group call in the downlink.

U-ATT

U-ATT = T means that the MS attaches the user connection for the group call in the uplink.

U-ATT = F means that the MS does not attach the user connection for the group call in the uplink.

6.1.2.1.2 NULL (U0)

No group call exists for the GCC entity. When entering the state, parameters shall be set to the following values, and configuration shall be adapted to the new values of configuration parameters: ORIG = F, COMM = F, D-ATT = F, U-ATT = F.

6.1.2.1.3 MM CONNECTION PENDING (U0.p)

The GCC entity has requested the explicit establishment of an MM connection. When entering the state, parameters shall be set to the following values, and configuration shall be adapted to the new values of configuration parameters: ORIG = T, COMM = F, D-ATT = F, U-ATT = F.

6.1.2.1.4 GROUP CALL INITIATED (U1)

The GCC entity has requested the peer entity in the network to establish a group call. When entering the state, parameters shall be set to the following values, and configuration shall be adapted to the new values of configuration parameters: ORIG = T, COMM = T, D-ATT = F, U-ATT = F.

6.1.2.1.5 GROUP CALL ACTIVE (U2)

The group call is established at least in one cell; depending on the sub-state, the MS has attached the user connection in the uplink and/or downlink. The following sub-states exist:

– separate link (U2sl);

– wait for receive mode (U2wr);

– receive mode (U2r);

– wait for send and receive mode (U2ws);

– send and receive mode (U2sr);

– no channel (U2nc).

6.1.2.1.6 GROUP CALL PRESENT (U3)

The MS has received a notification about an ongoing group call. Higher layers are requested to accept or reject the call. When entering the state, parameters shall be set to the following values, and configuration shall be adapted to the new values of configuration parameters: ORIG = F, COMM = F, D‑ATT = F, U-ATT = F.

6.1.2.1.7 GROUP CALL CONNECTION REQUESTED (U4)

The MS has received a notification about an ongoing group call. Higher layers have decided to accept the call. When entering the state, parameters shall be set to the following values, and configuration shall be adapted to the new values of configuration parameters: ORIG = F, COMM = F, D‑ATT = F, U-ATT = F.

6.1.2.1.8 TERMINATION REQUESTED (U5)

The MS (which assumes to be the originator of the group call) has been in state U1 or U2 and has sent a TERMINATION REQUEST message to the network. When entering the state, parameters shall be set to the following values, and configuration shall be adapted to the new values of configuration parameters: ORIG = T, COMM = T, D‑ATT = T, U-ATT = T.

6.1.2.1.9 Sub-states of the Group call active state

NOTE: Control of the user connection attachment is seen as a group call control function. This is a reason why the sub-states are distinguished.

6.1.2.1.9.1 separate link (U2sl)

A dedicated one to one RR connection exists between MS and network. When entering the sub-state, parameters shall be set to the following values, and configuration shall be adapted to the new values of configuration parameters: COMM = T, D-ATT = T, U-ATT = T.

6.1.2.1.9.2 wait for receive mode (U2wr)

The MS has asked the network to enter receive mode but a dedicated one-to-one channel is still assigned. When entering the sub-state, parameters shall be set to the following values, and configuration shall be adapted to the new values of configuration parameters: COMM = T, D-ATT = T, U‑ATT = F.

6.1.2.1.9.3 receive mode (U2r)

The MS uses a group call channel. When entering the sub-state, parameters shall be set to the following values, and configuration shall be adapted to the new values of configuration parameters: COMM = F, D-ATT = T, U-ATT = F.

6.1.2.1.9.4 wait for send and receive mode (U2ws)

The MS (which was in U2r or U2wr) has asked for the uplink. When entering the sub-state, parameters shall be set to the following values, and configuration shall be adapted to the new values of configuration parameters: COMM = F, D‑ATT = T, U-ATT = T.

6.1.2.1.9.5 send and receive mode (U2sr)

The MS has been informed by its peer entity that the uplink is granted. When entering the sub-state, parameters shall be set to the following values, and configuration shall be adapted to the new values of configuration parameters: D-ATT = T, U-ATT = T.

6.1.2.1.9.6 no channel (U2nc)

The GCC entity has been informed that no channel is currently available. Timer Tno channell is running. When entering the sub-state, parameters shall be set to the following values, and configuration shall be adapted to the new values of configuration parameters: COMM = F, D-ATT = T, U-ATT = T.

6.1.2.1.10 GCC Timers in the MS

Table 6.1 specifies the timers used in GCC. The denotation of columns is defined as follows:

timer ::= name of the timer;

set ::= under which conditions the timer is set (i.e., started);

stopped ::= under which conditions the timer is stopped;

running in state(s) ::= in which state(s) the timer may be running;

action at expiry ::= which actions the GCC entity shall perform at expiry;

value ::= the duration between setting the timer and expiry of the timer ("s" denotes

"second(s)" "xx – yy" means that any value between xx and yy is permitted).

Table 6.1: Specification of timers used in GCC

timer

set

stopped

running in state(s)

action at expiry

value

Tno channel

when entering U2nc

when leaving U2nc

U2nc

see subclause 6.3.1

3 s

TMM-est

when entering U0.p using the set-up procedure

when entering U1 using the immediate set-up procedure

when leaving U0.p or U1

U0.p, U1

see subclause 6.2.1

5 s

Tterm

when sending a TERMINATION REQUEST

when receiving a TERMINATION or TERMINATION REJECT

U5

abort group call

10 s

Tconn req

when entering state U4

when leaving state U4

U4

abort group call

10‑30 s

6.1.2.1.11 Consistency of parameters and states

The MS shall consider the following parameter values as inconsistent with the state or sub-state:

ORIG = T is inconsistent with states U3 and U4.

COMM = T is inconsistent with states U0, U3, U4, and with sub-states U2nc and U2r of state U2.

All other values of parameters ORIG, COMM, D-ATT, and U-ATT shall not be considered by the MS as inconsistent with a state or sub-state.

6.1.2.2 Group call control states at the network side of the interface

6.1.2.2.1 NULL (State N0)

No group call exists for the GCC entity.

6.1.2.2.2 GROUP CALL INITIATED (N1)

The GCC entity has received the indication that a peer entity in a MS wants to establish a group call for a certain group identity.

6.1.2.2.3 GROUP CALL ACTIVE (N2)

The group call is established in at least one cell; there may be a MS which has seized the uplink or not; there may be talking dispatchers or not.

6.1.2.2.4 GROUP CALL ESTABLISHMENT PROCEEDING (N3)

The GCC entity wants to accept the group call, has initiated establishment of corresponding group call channels, and, if there is a calling user. has sent a CONNECT message to the calling user (if there is a calling user).

6.1.2.2.5 TERMINATION REQUESTED (N4)

The GCC entity has asked lower sub-layers to terminate the group call in all cells and waits for a confirmation that the group call has been terminated in all cells.

6.2 Procedures for establishment of a group call

6.2.1 Activation of a group call by the network

The GCC entity in the network may initiate the activation of a group call with a certain group call reference and priority in a list of cells by asking lower layers to establish the group call with that group call reference and priority in those cells. It then waits until it is informed by lower (sub-)layers that resource activation was sufficiently successful, and enters state N2, GC ACTIVE.

6.2.2 Mobile originated establishment

Higher layers in the MS may ask the GCC entity in state U0, NULL, to establish a group call, either using the immediate set-up procedure or using the set-up procedure. The request contains a group-id and may contain a priority indication.

On request of higher layers to establish a group call using the set-up procedure, the GCC entity of the MS builds an appropriate SETUP message and asks lower (sub-)layers to establish an MM connection explicitly (i.e. by use of a CM SERVICE REQUEST message) and to transmit the SETUP message. It then enters state U0.p, MM CONNECTION PENDING. In state U0.p, when informed by lower sub-layers that an MM connection has been established, the GCC entity in the MS shall stop timer TMM-est and enter state U1, GC INITIATED.

On request of higher layers to establish a group call using the immediate set-up procedure, the GCC entity of the MS builds an appropriate IMMEDIATE SETUP message and asks lower (sub‑)layers to establish an MM connection implicitly (see GSM 04.08) and to transmit the IMMEDIATE SETUP message. It sets timer TMM-est and then enters state U1, GC INITIATED.

The network GCC entity in state NULL may receive a set-up message from its peer entity in the originating MS. This set-up message is either a SETUP message or an IMMEDIATE SETUP message. The network enters state N1, GC INITIATED.

In state N1, the network decides whether:

(a) the establishment is accepted; or

(b) the establishment rejected; or

(c) the MS is passed to an existing group call.

In case (a), the GCC entity in the network considers the peer entity in the MS having sent the set-up message to be the calling user and asks lower layers to activate the appropriate resources. It then:

1) waits until it is informed by lower (sub-)layers that resource activation was sufficiently successful, then sends a CONNECT message to the calling user, and enters state N2, GC ACTIVE; or

2) sends a CONNECT message to the calling user and enters N3, GC ESTABLISHMENT PROCEEDING. In state N3, the GCC entity is informed by lower layers whenever the status of resources for the group call is changed. When informed that activation of resources was sufficiently successful, the GCC entity in the network enters state N2, ACTIVE.

The CONNECT message specifies the group call reference of the group call and indicates that the MS is the originator of the group call.

In case b), the further proceeding is as defined in subclause 6.2.2.1.

In case c), the GCC entity in the network considers the peer entity in the MS having sent the SETUP message not to be the calling user; it may ask lower layers about the status of appropriate resources; it may ask lower layers to modify the resources for the group call. It then:

1) waits until it is informed by lower (sub-)layers that resource modification was sufficiently successful, then sends a CONNECT message to the calling user, and enters state N2, GC ACTIVE; or

2) sends a CONNECT message to the calling user and enters N3, GC ESTABLISHMENT PROCEEDING. In state N3, the GCC entity is informed by lower layers whenever the status of resources for the group call is changed. When informed that activation of resources was sufficiently successful, the GCC entity in the network enters state N2, ACTIVE.

The CONNECT message specifies the group call reference of the group call and indicates that the MS is not the originator of the group call.

In state U0.p or U1, the GCC entity in the MS shall, on receipt of a CONNECT message, establish the conditions defined for state U2, ACTIVE and the suitable sub-state (see subclause 6.1.2.1), stop timer TMM-est (if running) and enter state U2, ACTIVE. If the immediate set-up procedure has been used, the GCC entity in the MS shall inform lower sub-layers that the MM connection has been implicitly established.

6.2.2.1 Termination during mobile originated establishment

At any time during the mobile originated establishment of a group call, the network may decide to terminate the connection between the two peer entities in the network and MS. In this case the network sends a TERMINATION message to the MS specifying the appropriate cause; it may ask lower (sub-)layers to release associated resources. The further actions are specified in subclause 6.4.

During mobile originated establishment of a group call, the MS may abort the group call, see subclause 6.4.

6.2.2.2 Abnormal cases

At expiry of TMM-est, or radio link failure (see GSM 04.08), the GCC entity in the MS requests lower sub-layers to abort the MM connection establishment and returns to state U0, NULL(this includes clearing of the context related to the group call establishment).

On receipt of an indication of lower sub-layers that the MM connection establishment was unsuccessful, the GCC entity in the MS returns to state U0, NULL (this includes clearing of the context related to the group call establishment).

6.2.3 Mobile terminating group call establishment in the MS

The GCC entity in the MS, being in state U0, NULL, may receive an indication of lower layers that a group call exists. This indication specifies the group-id and a priority. It shall then inform higher layers and enter state U3, GC present. This state may be supervised by a timer at expiry of which the GCC entity clears the context and returns to state U0, NULL.

In state U3, on request of higher layers to join the group call, the GCC entity in the MS stops any running timer, asks lower sub-layers to join the group call, starts timer Tconn req, and enters state U4, GC CONNECTION REQUESTED.

In state U4, on indication of lower sub-layers that the group call has been joint (his indication specifies the mode of the RR connection), the GCC entity in the MS stops any running timer, enters state U2, ACTIVE and appropriate sub-state, establishes the appropriate configurations (see subclause 6.1) and informs higher layers (this includes information about the sub-state). The MS assumes not to be the originator of the group call (ORIG = F).

6.3 Procedures during the active state of a group call

6.3.1 Mobile station procedures in the active state

In the active state, the GCC entity in the MS performs, on receipt of messages from its peer entity, on request of higher layers, and on indication of lower sub-layers, sub-state transitions as defined below together with the necessary related actions.

On request of higher layers, the MS initiates abort or termination of the group call, see subclause 6.4.

If the network initiates group call abortion or termination, the MS reacts as specified in subclause 6.4.

On radio link failure, the MS aborts the group call, see subclause 6.4.

6.3.1.1 Sub-state transitions in the MS

When in sub-state U2sl or U2s of the GC ACTIVE state (U2), the GCC entity in the MS enters on request of higher layers sub-state U2wr and asks RR to enter RR receive mode.

When in sub-state U2r or U2wr of the GC ACTIVE state, the GCC entity in the MS enters on request of higher layers sub-state U2ws and asks RR to enter group transmit mode.

When in the GC ACTIVE state, the GCC entity in the MS may be informed by lower layers that the RR connection mode has changed. It shall then enter the appropriate sub-state and perform appropriate actions as specified below:

– the appropriate sub-state is derived from the RR mode as specified in table 6.2;

– when no RR connection is available, the GCC entity in the MS starts timer Tno channel and enters sub-state NO CHANNEL. In this sub-state, when Tno channel expires, the GCC entity in the MS informs higher layers, asks lower sub-layers to abort resources and enters the idle state;

– further actions in the MS are defined in subclause of 6.1.2.1.

Table 6.2: RR mode

RR mode

U2 sub-state

Idle

U2nc

Group Receive mode

U2r

Group transmit mode

U2sr

Dedicated mode

U2sl

Having entered sub-state U2ws, the MS not considering itself as the originator of the group call (ORIG = F) shall consider the transaction identifier received in the next GCC message from the network to be the TI of the group call.

6.3.2 Network procedures in the active state

In the active state the GCC entity in the network performs supervisory functions, maintenance functions and resource modifications which are not further specified. (This includes through-connection of the application data stream(s), which is defined in GSM 03.68).

The network may initiate abort or termination of the group call, see subclause 6.4.

If the MS initiates group call abortion or termination, the network reacts as specified in subclause 6.4.

The network may send a SET PARAMETER message to the MS in order to ask the MS to set parameters to certain values and take consequential actions.

The GCC entity in the network may be informed by lower (sub-)layers that a MS has requested the uplink. It may accept this request; in this case it should send a SET PARAMETER message to the MS requesting the MS to set parameter COMM to the value T.

6.4 Procedures for release, abortion, and termination of a group call

6.4.1 Termination procedure

The MS assuming to be the originator of the group call shall, on request of higher layers:

– if COMM = T, initiate the termination procedure by sending a TERMINATION REQUEST message to its peer entity in the network and setting timer Tterm;

– otherwise, the request is considered to be pending. If the MS is in sub-state U2r, it shall enter sub-state U2ws and ask RR to enter group transmit mode. While the request is pending, the request may be deleted by higher layers. While the request is pending, as soon as COMM = T, the MS shall send a TERMINATION REQUEST message to its peer entity in the network, set timer Tterm and no more consider the request as pending.

The network either accepts the termination by sending a TERMINATION or rejects termination by sending a TERMINATION REJECT. These messages indicate an appropriate cause.

In state U5, on receipt of a TERMINATION REJECT message, the GCC entity in the MS informs higher layers and stops Tterm.

In state U5, on Tterm expiry, the GCC entity in the MS informs higher layers, asks lower sub-layers to abort the group call, clears the context related to the group call, and returns to state U0, NULL.

In any state, on receipt of a TERMINATION message, the GCC entity in the MS informs higher layers, asks lower sub-layers to release the group call, clears the context related to the group call, and returns to state U0, NULL.

At any time during a group call, the network may decide to terminate the connection between the two peer entities in the network and MS. In this case the network sends a TERMINATION message to the MS specifying the appropriate cause; it may ask lower (sub-)layers to release associated resources. The further actions are specified above in this subclause 6.4.

6.4.2 Abort and release procedures

The network may ask lower sub-layers to abort or release the group call. The MS will detect abort of the group call by detecting a radio link failure, and a group call release by release of the RR resources. The GCC entity in the MS shall then inform higher layers, ask lower sub-layers to abort the group call, clear the context related to the group call, and return to state U0, NULL.

The MS shall, on request of higher layers, initiate the release procedure by asking lower sub-layers to release the group call, clearing the context related to the group call, and returning to state U0, NULL.

The GCC entity in the MS shall when required by the GCC protocol, abort the group call by requesting lower layers to abort the group call, informing higher layers, clearing the context related to the group call, and returning to state U0, NULL.

6.5 Miscellaneous procedures

6.5.1 Status procedures

6.5.1.1 Get status procedure

Upon receipt of a GET STATUS message, the MS shall:

– if COMM = T, respond with a STATUS messagereporting the current call state, the current values of configuration and behaviour parameters and cause value # 30 "Response to GET STATUS";

– if COMM = F, consider the response to be pending. If the MS is in sub-state U2r, it shall enter sub-state U2ws and ask RR to enter group transmit mode. While the request is pending, as soon as COMM = T, the MS shall send a STATUS message, reporting the current call state, the current values of configuration and behaviour parameters and cause value # 30 "Response to GET STATUS" and no more consider the request as pending.

6.5.1.2 Set parameter procedure

Upon receipt of a SET PARAMETER message the MS shall set the parameters to the indicated values and the configuration shall be adapted to the new values of configuration parameters, if they are consistent with the current GCC state and sub-state (see subclause 6.1.2). If they are not:

– if COMM, before the message was received, is equal to T, it shall send a STATUS message specifying error cause "message incompatible with protocol state", the state and, if applicable, sub-state, and the state attributes IE;

– if COMM, before the message was received, is equal to F, it shall ignore the message.