Trunking acknowledgment, TX/RX duplexing question

[PhillyPhoto]

I have a question in regards to the physical operation of a radio when it keys up on a trunked system. When you key up, your radio sends out it's ID to the controller, and then the controller tells you if you're allowed to talk, and what freq to switch to etc. My question is, is this performed in a duplex operation where the radio continues to transmit, or does to radio send the information to the controller, and then stop transmitting to await the reply, and then start transmitting once you get the double beep thereby switching over to the proper frequency etc?

[Hartley]

Hi,

In traditional SMR trunking it does it half-duplex; that is, it transmits and receives on different frequencies, but only one at a time - in pretty much the manner you describe. Without expensive (and bulky) duplexers, full-duplex operation is not possible.
On 800 mHz and up systems, however, it is possible to have full-duplex operation if the equipment supports it, but transmitting on the control channel will not be continuous, as many units will potentially be wanting to access the controller (like on a cell system [or nextel])

73 DE Hartley

[RKG]

Here is the simple version:

Press the PTT.

Your radio sends a data stream on the input side of the control channel it was monitoring. This is called an "Inbound Service Word" or ISW. The ISW identifies your radio and the talkgroup it has selected (methods of coding this vary by system type) and signify that you are requesting a voice channel grant for a group call on that talkgroup.

After sending your ISW, radio unkeys and watches the output side of the control channel for a period of time.

Assuming it received the ISW, the system, zone or site controller decides whether to grant your group call channel grant request. It may or may not vet your UserID for validity; it will check for availability of a voice channel. If a channel is granted, it will send a data stream known as an "Outbound Service Word" or OSW over the output side of the control channel. This effectively directs all listener radios who are selected up on the talkgroup for which you have made the group call ISW to shift to the assigned voice channel and wait there for a period of time.

Upon receipt of an OSW granting the channel it requested, your radio will change to the assigned voice channel and send a short data stream signifying that it made it OK. This is accompanied by a subaudible "connect tone," which is a variant of PL using different frequencies. At this point, your radio issues the "talk permit tone," signifying that you may begin speaking. Upon receipt of the handshake message and connect tone, the voice channel opens up to your transmission just like any conventional repeater, except that it may engage in simulcasting for extending coverage.

What happens when you dekey depends on the system configuration. One possibility is that assigned voice channel will remain available for anyone sending the correct connect tone on the input frequency of that channel for a period of time. A second possibility is that the assigned channel is immediately taken down, and anyone who wishes to respond to you has to go through the entire sequence again. A third variant works like the first design, but each responder's radio, when he keys, will first send an ISW on the input side of the control channel, in order to facilitate such features as PTT-ID display.

As I said at the outset, this is a simplification. The book that actually describes how these systems work is several inches thick.

[PhillyPhoto]

OK, that's what I thought happened. thanks for the help