Page Type: REFERENCE
A feed site number is associated with a group of scanners at a single location that share the same programming, and operate in a coordinated way to efficiently scan the channels designated in that programming. However, if scanners have different characteristics (e.g. radio model, antenna, filter, etc) it is recommended that the scanners be divided into separate groups according to their characteristics, and each group given different programming (and a different feed site number.)
In the field, each scanner is associated with a single feed site number. In the programmer, a feed site number is associated with a set of channels designated for reception from scanners operating under that feed site number. Multiple scanners can operate under the same feed site number, and in this case, the scanners will collaborate to efficiently receive those channels (one scanner will receive each transmission, leaving the other scanners free to receive new traffic on other channels.) Scanners operating under a single feed site number must operate from the same location, since the collaboration system depends on low-latency communication between the PCs controlling the scanners.
When scanners at a single location differ in characteristics that affect their performance at receiving signals, in most cases it is recommended that the scanners be grouped according to their characteristics, and each group given a different feed site number (and therefore different programming.)
In other words, all scanners operating under a single feed site number should have identical characteristics.
There are several reasons for this recommendation.
1 - SURVEY. When an automatic signal survey is performed, one scanner from each feed site number is selected to run the survey. If the scanners at a feed site have different characteristics, the survey results will be erratic (different runs can randomly produce different results). The results may not representative of the group -- if a more capable scanner runs the survey, the results may favorably indicate reception of channels that can only be received by a minority of the scanners on site; if a less capable scanner runs the survey, the results may indicate that certain channels cannot be received there, despite it being the best place to assign those channels. For multiple reasons, interpretation of a survey becomes far more complicated if a feed site contains a mix of scanners with different reception characteristics.
2 - MEASURING PERFORMANCE. The Rangecast system tracks whether feed sites have sufficient receiving capacity, by estimating the percentage of transmissions that may be missed because all receivers are simultaneously occupied receiving signals. The mathematics for this analysis assumes that all receivers have identical receiving characteristics, so any new signal may be received from any scanner. If this is not the case, the Rangecast system may falsely report that a feed site has adequate capacity, when in fact signals that can only be received from a minority of scanners are not being reliably received due to insufficient resources. This can result in an undetected failure to appropriately allocate receiver resources, and an unnecessary loss of transmissions that could have been received with the existing equipment.
3 - CONSISTENT AUDIO QUALITY. In some cases, all scanners at a location can receive a signal, but due to differences in antenna or filter characteristics, the quality of the signal differs between the scanners. If these scanners are combined into a single feed site number, a new transmission will be randomly assigned to one of the scanners. When a transmission is assigned to a scanner that receives that particular signal less clearly, the result will be unnecessarily poor audio quality (on that specific transmission, although the channel will be well received at other times.)
To avoid this problem, scanners with different reception characteristics should be assigned to different feed site numbers. If some signals are better received on one set of radios, and other channels on another set of radios, it is easy to divide out the channels to the scanners where they are best received. If one set of radios is simply superior to another set (better antenna, better model of scanner, etc) then stronger signals can be assigned to the inferior scanners, where the performance difference is not significant, and weaker signals assigned to the superior scanners, where they can be reliably received with better quality.
4 - RECEIVER AVAILABILITY. Some signals can only be received on more advanced models of scanner (or scanners with a special firmware enhancement.)
Sometimes the scanners at a location comprise a mixed fleet, with all scanners are capable of receiving most signals, but only a few of the scanners have the special capabilities needed to detect certain signals. In this situation, there may be an instinct to combine all the scanners into a single feed site, on the basis that this will provide the best performance for receiving the bulk of the signals. (For example, N scanners are available to receive transmissions, and 1 of them is a special model capable of receiving these particular channels.)
Although it is true that combining the scanners would maximize performance on the bulk of the channels, this is generally not the best way to configure the scanners. For two reasons, combining scanners in this way can sharply degrade performance on the particular channels that can only be received on the enhanced scanners.
First, if the superior scanner(s) happen to land on normal channels that could be received by any of the scanners, then Rangecast will be unable to detect or record any transmission on the particular channels that can only be received on these radios. (If the scanners are divided into separate feed site numbers, this cannot happen.)
Second, since the superior scanner(s) include all the ordinary channels in their scan list, the "cycle time" (how long it takes the scanner to complete a scan of all programmed channels) may be relatively long -- meaning that even if the enhanced scanner is not captured by traffic on an ordinary channel, merely the background process of scanning all those ordinary channels may create a delay in how quickly a new transmission is detected on channels that can only be received by particular scanners. (In contrast, for ordinary channels, a long cycle time is not so important because there are more scanners available for detecting a new signal, so the speed that a new transmission is detected will be much faster on ordinary channels than the particular channels that can only be detected by certain scanners.)
For these reasons, it is advantageous to split up scanners so that all the scanners in a feed site number have identical characteristics.
There are exceptions. In particular, if the performance on ordinary channels is not adequate without the addition of the enhanced scanners into the same pool, then it may be necessary to take this step. But it should never be assumed that this is the case, such a decision should be based on actual data. In particular, Rangecast tracks the percentage of transmissions that are likely to be missed due to all receivers being simultaneously occupied; if this metric is low, then there is no problem with a smaller fleet of scanners servicing the ordinary channels. Scanners of different types should only be combined if this metric is high, indicating inadequate performance on the ordinary channels, and the need to add scanners there.