Rangecast FAQ
[FAQ-1313] Interpreting talkgroup survey results

Page Type: REFERENCE

OVERVIEW

A talkgroup survey is one portion of the Rangecast signal survey, measuring the consistency of reception of each talkgroup as received through each possible radio configuration of receive site and broadcast tower.

BACKGROUND

Example of a talkgroup survey. Columns are identified at both the top and bottom of the table, the example here shows the end of the table (there is more information presented in the column footer at the end of the table.)



REFERENCE

TRUNKED RADIO RECEPTION

Trunked radio systems may include multiple broadcast towers. (Note: if multiple physical towers multicast, always simultaneously broadcasting the exact same content on the same radio frequencies, for programming purposes this is considered a single tower.)

In Rangecast, a tower (like any single signal) is received from a single receiver site, selected as the optimal choice based on signal quality and overall receiver loads.

Each talkgroup is assigned to be received from a specific combination of receive site and broadcast tower. Therefore an essential part of the programming process is choosing which combination of receiver/tower to use for each talkgroup. In some trunked systems, the answer is simple - all traffic should be received from a particular receiver/tower combination. But in many situations, talkgroups need to be divided among several configurations. The talkgroup survey is designed as a way to use the radios to measure and compare actual performance to guide these decisions.


TALKGROUP SURVEY REPORT

In the talkgroup survey report, each row corresponds to a single talkgroup, and each column corresponds to a specific option for how that talkgroup may be received (a receiver/tower combination.) The currently selected option (what is currently programmed into the scanners) is highlighted with a blue background.

If a cell has a number, that is a count of the number of sample periods (usually one hour) included at least one transmission detected on that talkgroup using that column's reception option (a specific radio monitoring a specific tower.)

Rangecast uses a count of the number of hours with signal, instead of the number of transmissions, because this is a good measure of consistency. Some trunked systems carry a talkgroup on a tower when a mobile unit from the agency is within range of that tower, but then drop the talkgroup from that tower if no mobile unit is in range. A high number on the hourly count indicates that the tower consistently carries the talkgroup, and that the receiver consistently is able to monitor the tower - what is desired.

The color dot beside the number is an approximate indication of signal quality. The color represents the grade of the worst factor on the most recent occasion when the control channel signal quality was measured from that receiver location. (The same color is shown in the column header/footer.)

If a cell is blank (no number shown), this means the talkgroup was not detected from that receiver/tower combination during the period of the survey.

If a cell is blank but the background is beige, that means the talkgroup was detected on this tower from some other radio - indicating that the talkgroup can likely be received with this option, even if it was not detected during the survey. (This is often helpful when assigning talkgroups that have a very low use, with the highest counts shown around 1 or 2.)

The actual counts shown include a significant amount of random noise. For example, if a talkgroup is used for only a few transmissions in an hour, it's quite possible for a radio to miss all the transmissions because at those exact moments it was monitoring a different tower. So when comparing columns, although higher numbers are generally better, small differences in the count should not be considered important.


USING THE SURVEY RESULTS

It is generally recommended that you select the minimal number of receiver/tower combinations, because this makes the most efficient use of the radio receivers. (A scanner steps through each tower represented in its scan list, and if the number of towers is large, the scanner may take several seconds to loop around and test all signals, delaying detection of new transmissions.)

If you intend to receive two talkgroups from a tower, and one has a higher count from site A while the other has a higher count from site B, do NOT try to chase this apparent difference by thinking you should use both receiver/tower combinations. The tower is broadcasting the same signal to all receivers, so any difference like this must be due to statistical randomness. Only select a single receive site for the tower, and do not worry about the fact that some talkgroups might show a higher number for some other receiver monitoring the same tower.

When comparing receive sites for a specific tower, pay attention to the bold number in the column footer. This is the average number of talkgroups detected during each sample period, which is a very good indication of overall signal quality (this measure is usually more sensitive and meaningful than the indicated color.)

When deciding where to receive talkgroups, it is important to consider the loading on the scanners (the fraction of time they are in use), so you do not overload a small number of radios instead of spreading out the loads more evenly. It is sometimes better to receive a tower from a second or third choice site by quality, if this distributes the loads more evenly, or places the tower on a receive site with more scanners in service.


COLOR QUALITY INDICATIONS

In the color header/footer/dots associated with a column, the usual colors are (in declining order of quality) green, yellow, orange, red. You may also see white - this means the control channel was not detected during the control channel survey, and white can be interpreted as the lowest grade (worse than red). Since these colors are based on a single quality measure of the control channel, it is common for the colors to shift slightly if a new control channel survey is conducted.

You may also see purple. This is an indication that Rangecast finds it likely (but not proven) that this receiver/tower combination is actually receiving a DIFFERENT tower. This can happen if the same broadcast frequency is used by two towers, and the signal survey programming includes both towers. Unless you are quite sure you understand the situation, do not select a receiver/tower combination marked in purple. The observed counts there actually belong to a different tower, which is likely represented in some other column.


EXAMPLE

In the talkgroup survey shown above (and again here for reference):




Consider talkgroup 43010 (La Porte Police):

* This channel appears to be in use (some receiver/tower combinations show activity)

* This channel will currently not be received because it is currently set for reception from 51/Santa Fe, but the channel does not appear to be carried on this tower. (Signal strength there is yellow, which is not perfect, but if the channel were carried there it is likely that it would have been seen at least once.)

* The channel would be well received from 21/Southeast (highest count on the row, and signal quality is green)

* The Southeast tower can also be received from 11 and 51, but the lower signal quality results in fewer detections. All receive sites are listening to the same broadcast signal (whatever is sent out from Southeast) so the differences between 11/Southeast, 21/Southeast and 51/Southeast are due to signal quality, not any intrinsic difference in what is being broadcast.

* Although the channel is also received from 11/Southwest with good signal quality (green like 21/Southeast), the counts are much lower, indicating this talkgroup is likely not broadcast consistently from the Southwest tower (only occasionally when a participating mobile unit is in range.)

Next, look at talkgroup 40100 (AC2 Medical Operator).

* This is currently set for reception from 21/Northeast, and this column has the highest count (12) so it would appear to be a good choice.

* However, the channel has nearly equal counts (11) from two receiver/tower combinations with green signal quality -- 21/Southeast and 11/Southwest.

* The small difference in counts may be statistical noise, and the survey should not be read as indicating 21/Northeast is necessarily the best choice. Since green suggests significantly better signal quality, it may be worth checking the control channel survey reports to see why 21/Northeast indicates orange, and whether this is consistent (the most recent quality measurement may have happened to catch a single poor reading, with prior readings showing green -- or 21/Northeast may be showing consistent orange due to some consistent signal issue.) If 21/Northeast is showing consistently marginal signal quality, it may be worth shifting the talkgroup to a green configuration for the improved signal quality.

Next, consider talkgroup 43040 (University of Houston Clear Lake police):

* The currently selected receiver/tower combination, 21/Southeast, shows no counts.

* The channel was detected from 11/Southeast and 51/Southeast. It might appear that the programming should be changed to use one of these combinations instead of 21/Southeast.

* However, we know that each of sites 11, 21 and 51 were receiving the same broadcast signal (Southeast) so there could be no difference on what is transmitted -- and 21 appears to have the best signal quality for reception of Southeast. Therefore the difference (21/Southeast not showing detections, while 11/Southeast and 51/Southeast each have one count) must be due to statistical noise, and is not meaningful. The current programming configuration is optimal for this talkgroup.

Finally, look at talkgroup 43300 (Manvel Police.)

* Observe that some cells show a tan background, and that some of these columns show counts while others do not. The tan background indicates that some receive site detected transmissions from this talkgroup via this tower.

* Therefore, the channel should be received from any other site monitoring the same tower. The absence of counts (at 21/Southeast and 21/Southwest) could be random statistical noise, or be a result of poor signal quality -- but it is not due to the tower never carrying the talkgroup. (At 21/Southeast it is likely statistical noise, since signal quality is green; but for 21/Southeast, with red signal quality, the non-detection is likely due to poor signal.)

* The tan highlighting is useful when placing low-use talkgroups that have very few detections, because it is easier to see receiver/tower combinations likely to work.

The blue highlighting (indicating the currently selected receiver/tower combination for a talkgroup) comes in two shades. This usually shows in the lighter shade, which is an overlay of blue and tan. If the talkgroup is currently set to be received from a tower from which no site has detected any transmissions (on that talkgroup), the blue highlighting will be in a dark blue shade (as for talkgroup 43010.) Therefore the absence of any counts in a dark blue cell is far more important than the absence of counts in a light blue cell.


REPROGRAMMING

When you reprogram the system, the blue highlighting will instantly move to show the newly selected combinations. Therefore you can save changes, and then see what you have missed for a second round of fine-tuning the programming.

CITATIONS

INDEX

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