[op25-dev] Control channel questions

'Ralph A. Schmid, dk5ras' ralph@schmid.xxx [op25-dev] op25-dev at yahoogroups.com
Mon May 8 07:26:30 UTC 2017



Usually the uplink frequency is quite a bit away, and very often this is a
fixed spacing. In Germany for example the uplink is, depending on the
frequency band, 4.6, 5 or 10 MHz below the downlink. 




From: op25-dev at yahoogroups.com [mailto:op25-dev at yahoogroups.com] 
Sent: Monday, May 8, 2017 5:44 AM
To: op25-dev at yahoogroups.com
Subject: [op25-dev] Control channel questions




Let's say you're scanning the airwaves in the frequency range of a
known P25 system and come across a spike in power at one particular
band that never shuts off.

* You should think that you've located a control channel, correct?

* If so, is it safe to say that the upstream and downstream channels
are side-by-side (one contiguous frequency band)?

* What is the most accurate way of determining whether what you've
discovered is a Phase I or Phase II system?

* When someone keys up a radio, is the following sequence of events
1. The radio sends a packet via the upstream control channel
indicating that it wishes to speak.
2. The fixed site sends a packet via the downstream control channel
indicating a frequency X on which it wants the speaker to transmit.
3. The fixed site sends a packet via the downstream control channel
instructing all radios in the talk group to tune in to a frequency Y.
4. The speaker transmits on frequency X, and the fixed site acts as a
repeater, re-broadcasting the audio to frequency Y.


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