CÉLULA DE TRABAJO

This is the most common type of call. Home zones are generally assigned to match up to geographic areas where radios are used most frequently, such as a patrol district or management area. When possible, talkgroups and radio users should be configured so that the majority of the calls take place within this geographic area, thus reducing the need for interzone resources.

5.4.1.1.1 Call Request

A talkgroup call begins with a call request. The call request resolution determines whether the call is set up or not. A talkgroup call request is initiated when the caller selects the appropriate mode on the radio and then presses the PTT button.

• When the caller presses the PTT button, the call request, in the form of an Inbound signaling Packet (ISP) is sent over the control channel to the current site. The information is passed to the Site Controller for processing and routing to the Zone Controller through the site router.

• The Zone Controller, for the zone where the request originates, determines if this is a valid call request by checking the access configuration information stored in the VLR. If it is a valid request, the Zone Controller checks its talkgroup-to-home zone map table to see which zone is the home zone of the talkgroup. For group calls, the home zone of the group becomes the controlling zone for the call, regardless of which zone the caller is in when the request is made.

When the call request originates from a BTS site, the request is encapsulated as 10Base-T Ethernet packets by the base station and is sent to the Site Controller. The controller then encapsulates the traffic in Frame Relay packets and transmits the traffic to the MSO.

5.4.1.1.2 Call Setup

Figure 5-4 Intrazone Talkgroup Request from a BTS Site

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

5.4.1.1.2 Call Setup

Once a valid call request is received, the Zone Controller starts to set up the call:

• The Zone Controller checks the VLR to determine talkgroup affiliations and radio location. This information indicates which sites need to participate in the call.

• The Zone Controller checks that all needed resources, such as channels at sites and consoles are available to establish the call. See5.8 Busy Call Handling, page 5-34for details.

5.4.1.1.3 Call Grant

When and where to use:

Use the below process to see how to grant talkgroup calls. The figure illustrates the process.

Figure 5-5 Talkgroup Call Grant

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

Process Steps

1 Routing information is sent to the appropriate MSO and remote site routing equipment.

2 The Home Zone for the call request sets up a core router as the distribution point for the audio information. This audio focal point is known as the Rendezvous Point (RP) and its router becomes the Rendezvous Point router. The Rendezvous Point for intrazone audio is the core router.

3 Channel assignments are sent to the needed sites.

4 The Site Controllers in the BTS sites broadcast the channel assignment, through the main control channel (MCCH), to the radios and send the activation message to the assigned traffic channels. The assigned traffic channels send a join message back to the MSO.

5.4.1.1.5 Talkgroup Call Continuation and Teardown

Process Steps

1 When the radio user speaks into the microphone of the radio, the radio converts the speaker's analog audio into ACELP and transmits the signal to the assigned traffic channel.

2 The audio signal is transmitted by the radio over the assigned frequency to the caller's site and received by the assigned traffic channel.

3 The traffic channel places the audio into the site's Ethernet LAN as IP packets and routes the audio signal through the site router to the assigned rendezvous point router (core router) at the MSO.

4 The rendezvous point router forwards the audio to any device that responded with a join message to the Zone Controller's call grant. The BTS base radios respond with a join message.

5 The talkgroup members already locked on to the traffic channel receive the audio.

5.4.1.1.5 Talkgroup Call Continuation and Teardown

When and where to use:

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 Zone Controller.

Control information flows continually during a call: over the control channel during call setup and embedded in the digital audio signal during the active call phases.

Process Steps

1 When the speaker releases the PTT button, a message is sent to the controlling Zone Controller. If the call is message trunked, a message hang time timer starts when the message is received. All system resources, previously assigned to the call, are held available during the timer's hang time period.

2 If a person responds to the initial caller, by pressing the PTT button within the hang timer period, the call continues. The message hang timer is reset and the new speaker's audio is routed as the source audio using the traffic channels and router assignments already allotted for this call.