Inter-Site Call Set-up

Individual calls between two mobiles registered at different sites are handled by the two sites without intervention of external central equipment.

The main operational steps are as follows:

● The CMCE-L at the originating site receives a U-SETUP from its BLE and signals the call to its local CMCE-U.

● The CMCE-U consults the GDDB, detects that the call is an inter-site call and signals the call, via the inter-site communication platform, to its peer on the destination site.

● The CMCE-U at the destination site signals the call to its local CMCE-L.

● The CMCE-L then sends a D-SETUP to its BLE.

Further call-related signalling is routed along the same path between the CMCE- Ls.

The CMCE-Ls consult their AIRMs, for allocation of air interface resources, at through-connect.

7.16.1.

Calls Originating at the PABX/PSTN Gateway

The PABX-GW receives a SETUP request from either the PABX or PSTN and signals the call to its local CMCE-U, which handles the distribution of the call as per the call set-up procedure.

PABX-GW is responsible for the inter working of the signalling between the ISDN and the internal signalling protocol.

On through-connect the switching layer at the Gateway PC allocates a Tetra codec to transcode the voice.

If a mobile modifies a call to be half-duplex, the PABX-GW will clear down the call in both directions.

7.16.2.

Calls Terminating at the PABX/PSTN Gateway

When the PABX-GW receives a call from its local CMCE-U, it signals the call to the ISDN.

The PABX-GW is responsible for the inter working of signalling between the ISDN and the internal signalling protocol.

At through connect, the switching layer at the Gateway PC allocates a Tetra Codec to transcode the voice and connects the inputs/outputs of the Codec to the appropriate channels on the ISDN.

If a mobile modifies a call to be half-duplex, the PABX-GW will reject the call attempt .

7.16.3.

Calls Originating at a Dispatcher

When the DISPSERV CSCI receives a call from a dispatcher, it signals the call to its local CMCE-U, which then handles the call set up.

The DISPSERV CSCI is responsible for the inter working of the signalling between the Dispatcher access protocol and the internal signalling protocol. On through connect, the switching layer at the Gateway PC allocates a Tetra Codec to transcode the voice and sets up a VoIP connection between the dispatcher and the Gateway.

7.16.4.

Calls Terminating at a Dispatcher

The DISPSERV CSCI receives a call from its local CMCE-U and signals the call to the Dispatcher.

The DISPSERV CSCI is responsible for the inter working of the signalling between the Dispatcher access protocol and the internal signalling protocol. On through-connect the switching layer at the Gateway PC allocates a Tetra Codec to transcode the voice and sets up a VoIP connection between the dispatcher and the Gateway.

Also the system allocates only one Tetra Decoder for all dispatchers monitoring a group call.

7.16.5.

Call Restoration Multi Site

Call restoration is handled by the CMCE-U application at each sites. Up to three sites may be involved:

● The old site where the moving subscriber previously held the call.

● The new site where the call will be restored.

● The peer site which hosts the other party in an individual call.

The main operational steps for restoration of an individual call are as follows:- At the new site:

The CMCE-L receives a U-CALL RESTORE indication from LLC and signals this to CMCE-U. To reduce the length of the speech break, the AI resource allocation may proceed in parallel with inter-site signalling.

The CMCE-U uses the call identifier to locate the “old” site and signals the call restoration request to the old site.

At the old site:

The CMCE-U looks up the context related to the call and sends the call

parameters, which includes information about the used E1 sub-channels and the peer site, back to the “new” site.

It then checks to see if the call still has local participants, if not, it clears the call locally.

At the new site:

The CMCE-U signals the call restoration to the “peer” site, including information about the new E1 sub-channel bearing the speech of the moved subscriber. If the restored call is queuing for resources at this point, then this signalling will be delayed until resources become available.

At the peer site:

The CMCE-U updates the information and switches the downlink speech channel to the new E1 sub-channel.

In the above, the “old” and “peer” may be the same site, as may the “new” and “peer”.

7.16.6.

Call Restoration – Group Call

The restoration of a group call is as follows:

The CMCE-L receives a U-CALL RESTORE indication from LLC and signals this to CMCE-U. To reduce the length of the speech break, the AI resource allocation may proceed in parallel with inter-site signalling.

The CMCE-U uses the call identifier to locate and signal the call restoration request to the old site.

At the old site:

The CMCE-U looks up the context related to the call and sends the call

parameters, which includes information about the used E1 sub-channels and the peer site used by the talking party back to the new site.

At the new site:

If the moving party is currently transmitting, the CMCE-U signals the information about the new E1 sub-channel bearing the speech of the moved subscriber to all peer entities participating in the group call.

If the restored call is queuing for resources at this point, then this signal will be delayed until resources are available. Otherwise, if another party is currently transmitting, it switches the downlink speech channel to read the E1 sub-channel of the talking party.