# Trunking Channel Number option

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#### NS9710

##### Member
I notice with the RS PRO-106 I have the option to display the Trunk Control Channel Number, and I do use this option, and it displays a 5 digit code where "TSYS" usually shows, I am just curious what exactly is this and how does it fit in with a trunking system?

Feel free to move this post if needs, I feel this is more of a trunking topic than a Radioshack topic. but others may feel otherwise.

#### talkpair

##### Member
As far as P25 systems go, whenever a voice grant occurs, the control channel communicates frequency information to your scanner using a channel number instead of the frequency itself.

The scanner calculates the receive frequency using a table which is usually downloaded from the control channel.

Tables vary, but here is what one might look like:
Code:
``````  LOW    HIGH             BASE
CHAN    CHAN   OFFSET  FREQUENCY       STEP*
--------------------------------------------
0    2877       0   851.00625       6.25
4415    8191    4415   764.00000       6.25
8192   12287    8192   136.00000       2.50
12288   16383   12288   146.00000       2.50
16384   20479   16384   156.00000       2.50
20480   21760   20480   166.00000       6.25
24576   28671   24576   136.00000       6.25
28672   30657   28669   161.57500       6.25``````
*Step value is in KHz

The table works by taking the received channel number and finding the row in which the channel value falls within the Low/High range.

The offset value from that row is then subtracted from the channel number, then multiplied by the step, then added to the base frequency.

((CHANNEL - OFFSET) * STEP) + BASE FREQUENCY = CHANNEL FREQUENCY

As an example, lets say the channel is 17767.
Because 17767 falls between 16384 and 20479, the offset value 16384 is subtracted from 17767, resulting in a value of 1383.

1383 is then multiplied by step value of .0025 (2.5 KHz) = 3.4575

The value 3.4575 is added to the Base Frequency 156.0, resulting in a channel frequency of 159.4575.

Displaying the channel number on your scanner is probably of little use, unless you're attempting to trouble shoot a manually entered table. In this case, you would want to note the channel number and frequency from the display.

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