Interzone Talkgroup Call

The difference between an interzone call and an intrazone call is the other Zone Controllers that must be included in the call control process. In an interzone call, each zone controls its local resources in a similar manner to the previously described intrazone call. However, because the controlling zone must coordinate with the needed participating zones prior to granting the call, there is much more activity during the call request and setup phases.

5.4.1.2.1 Interzone Talkgroup Call Request

The call request is sent by the radio over the control channel at the local zone and site when the radio user presses the PTT button. This request is relayed through the remote site to the local Zone Controller.

Based on the talkgroup ID information in the call request, the Zone Controller receiving the request checks its VLR and determines if the requester is able to make the call. The Zone Controller then checks the talkgroup-to-home zone map and determines if it is the home zone (and thus the controlling Zone Controller) for the call.

If it is the home zone, the local Zone Controller becomes the controlling Zone Controller and takes responsibility for the call. The call request is acknowledged, and the controlling Zone Controller begins to set up the call. If the receiving zone is not the home zone, the call request is passed on to the appropriate Zone Controller, which accepts control of the call and becomes the controlling Zone Controller for the call (see figure below).

Figure 5-6 Interzone Call Request

Gateway Router BTS Zone 1 Patch Panel Core Router Core Router Exit Router Exit Router Zone Controller LAN Switch Dispatch Console Gateway Router BTS Zone 2 Patch Panel Patch Panel Zone Controller LAN Switch

5.4.1.2.2 Interzone Talkgroup Call Setup

The controlling Zone Controller determines which zones must be included in the call and sends a message to the appropriate Zone Controllers, requesting their participation in the call. All interzone call control messages between any pair of Zone Controllers goes over the Interzone Control Path between those two zones. There is an active Interzone Control Path between any two zones in the system.

5.4.1.2.3 Interzone Talkgroup Call Audio Routing

• Notifies the consoles of the talkgroup call and its audio source, if needed. The Zone Controller relays the audio assignments and the channel assignments to the appropriate remote sites through their site control paths.

• Once the resources are assigned, the rendezvous point router becomes the centre of control for audio distribution.

At each site the Site Controller:

• Activates the site assigned as the traffic channel.

• Sends the traffic channel assignment to the affiliated radios over the control channel.

The assigned traffic channel at RF sites send a join message to the RP after they receive the multicast address. As resources are set up in each zone, the radios in the talkgroup in each zone switch to the assigned traffic channel. The initiating user's radio activates the transmit circuitry and begins sending the audio to the receiver at the assigned traffic channel.

5.4.1.2.3 Interzone Talkgroup Call Audio Routing

When the transmitting user speaks into the microphone, the audio signal is transmitted on the assigned traffic channel frequency and received by the site, which routes the audio stream to the core router at the local MSO. The core router relays the audio signal to the assigned sites through their remote site router, to the consoles and over the assigned interzone resources to the exit routers in the participating zones. The exit routers in the participating zones then route the audio to the core LAN switch , the core router, gateway router, consoles in their respective zones, and the assigned sites. The sites transmit the signal to the members of the target talkgroup.

5.4.1.2.4 Interzone Talkgroup Call Continuation and Teardown

When the original speaker releases the PTT button a control message is sent over the traffic channel. This message is extracted from the audio stream by the remote site and forwarded to the controlling Zone Controller. In transmission trunking mode, the call is ended after the PTT released message is received. In message trunking mode, however, the controlling Zone Controller starts the message hang-time timer upon reception of the PTT released message. If another user in the talkgroup responds to the call within the hang-time period, the controlling Zone Controller receives the new call request (either from a site in its zone or from a participating Zone Controller), sees that it is for a talkgroup that has an active call, and continues the call using the resources currently assigned to the talkgroup. The audio source is the only resource change in this instance.

The call is ended when no one from the talkgroup keys-up within the message hang-time period. The controlling Zone Controller sends a message to each participating zone to tear down the call. Each zone goes through the teardown process, disabling the audio and marking the resources used in the call as available for other call assignment.

5.4.1.2.5 Roaming During a Talkgroup Call

When a receiving radio user in an active talkgroup call roams into a new zone, the call is continued automatically. Depending upon whether or not resources, such as a traffic channel, are available to set up the call at the new site, the roaming user experiences the following conditions:

• If resources are available at the new site and the talkgroup call is already active in the new zone (there are talkgroup members at sites within the zone), the roaming user experiences a short loss of audio while the call is set up at the new site.

• If resources are available in the zone to set up the call but the talkgroup call is not active (there are no affiliated talkgroup members in that zone), the roaming user experiences a longer loss of audio while the interzone call setup process takes place. The access control information needs to be transferred from the

home zone HLR to the HLR in the new zone and from there to its VLR, the call request validated in the new zone, and a channel assigned and activated at the new site.

• If resources are not available at the site or in the zone, the call continuation request to the new zone is placed in its busy queue. When the needed resources become available, the roaming user rejoins the call in process. A longer loss of audio occurs in this case.