Hello,
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 accurate?: 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.
Hi,
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.
Ralph.
From: op25-dev@yahoogroups.com [mailto:op25-dev@yahoogroups.com] Sent: Monday, May 8, 2017 5:44 AM To: op25-dev@yahoogroups.com Subject: [op25-dev] Control channel questions
Hello,
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 accurate?: 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.
Don't assume anything about the voice frequencies. In a correctly configured P25 system you really don't need to know where they are because the control channel will tell your receiver where to tune.
Once you have identified what you think is a control channel, turn off the -T (trunking) feature of OP25 and park it on the control channel. When it is accurately tuned, the system NAC will be scrolling in the terminal window. Note this hex value and set up the appropriate trunk.tsv file so that you can turn trunking back on (-T trunk.tsv). If you got the configuration correct, you should see some lines of data start to appear (including the voice frequencies) in the Traffic tab. If the stars align, you might even get good audio decode :)
Operationally, when a user keys the PTT button on a P25 radio, a series of messages are exchanged that either grants or denys access to the requested TGID. Typically the user hears a "beep" or a "bong" and at that point they can start speaking and this is transmitted over the uplink freq. The repeater then turns the data stream back around and retransmitts it on the downlink for other affiliated radios to receive. As a "fly on the wall" scanner user, OP25 can monitor for these messages and use them to determine where to tune, what tgid and timeslot etc. When the PTT key is released, more messages are passed and a message pops out on the control channel notifying client radios that the call has ended. (Note: these messages are coded as various types of DUID and typically carry additional information such as encryption status, radios ids, tgids etc.)
On 09 May 2017 00:44:46 +0000 "gnorbury@bondcar.com [op25-dev]" op25-dev@yahoogroups.com wrote:
Don't assume anything about the voice frequencies. In a correctly configured P25 system you really don't need to know where they are because the control channel will tell your receiver where to tune.
Hi, thank you for that information. OP25 is able to parse packets that arrive via the downstream control channel. As an exercise to myself, I would like to tune my USRP to the *upstream* control channel and see the various packets being sent to the fixed station. Surely this is possible without major architectural changes to OP25, correct?
At 08:44 PM 08-05-2017, gnorbury@bondcar.com [op25-dev] wrote:
Operationally, when a user keys the PTT button on a P25 radio, a series of messages are exchanged that either grants or denys access to the requested TGID. Typically the user hears a "beep" or a "bong" and at that point they can start speaking and this is transmitted over the uplink freq. The repeater then turns the data stream back around and retransmitts it on the downlink for other affiliated radios to receive.
This is not correct. You suggest that there is "a series of messages" followed by "turns the data stream around and retransmits it"; that's an odd way to describe a single inbound request and an outbound grant.
As a "fly on the wall" scanner user, OP25 can monitor for these messages and use them to determine where to tune, what tgid and timeslot etc. When the PTT key is released, more messages are passed and a message pops out on the control channel notifying client radios that the call has ended.
How would radios sitting on a voice channel see this message on the control channel? And why have we never seen evidence of such a "call is over" message on any P25 control channel?
Suppose it depends at what point in time you start counting messages being exchanged. If the radio is already turned on and affiliated then there will be a single request and grant exchanged, otherwise there will be more messages. That said, from the perspective of someone passively monitoring the control channel for the purposes of listening to the chatter, mostly all they care about seeing is the grant and end messages.
At 07:33 AM 09-05-2017, gnorbury@bondcar.com [op25-dev] wrote:
Suppose it depends at what point in time you start counting messages being exchanged. If the radio is already turned on and affiliated then there will be a single request and grant exchanged, otherwise there will be more messages.
Fairly hard for a turned off radio to PTT. Radios register and affiliate as soon as they're turned on and dialed to a trunk talkgroup. So PTT --> request --> grants.
That said, from the perspective of someone passively monitoring the control channel for the purposes of listening to the chatter, mostly all they care about seeing is the grant and end messages.
The mythical end messages. Again, how would subscribers on a voice channel see an end message on the control channel?
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'Ralph A. Schmid, dk5ras' ralph＠schmid.xxx [op25-dev] -
Alex alex323＠gmail.com [op25-dev]
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Andrew Porrett wap＠ica.net [op25-dev]
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gnorbury＠bondcar.com [op25-dev]