Working out how many talkpaths are needed for logging voice calls.

In Capacity Max, if voice calls need to exit the system, they will do so via the VRC Gateway. A voice call will exit the system if the (or one of the) endpoint(s) is a dispatch console or a voice logger monitoring the call.  A voice can also exit the system if the radio user makes a telephone call. 

The VRC Gateway is there to serve as a single endpoint (both physical and in the IP world) for all voice traffic entering or exiting the radio infrastructure. This allows the application to source voice and prescence information from a single location.

For every voice call exiting or entering the system, one talkpath is consumed. The talkpath is not linked to the number of users or talkgroups. Rather, a talkpath should be seen as a trunk between the radio network and non-radio network. 

In the case of a voice logger, the circumstances are somewhat unique: when calculating the number of talkpaths needed for a voice logging application, one has to assume that all voice calls must go to the application. So the number of required talkpaths will be higher when compared to the number requried for a dispatcher.

The maximum (theoretical) number of talkpaths for logging talkgroup calls can be calculated using the following formula:

T = 2R - S

Where:

T is the maximum number of talkpaths.

R is the number of trunked repeaters in the system.

S is the number of RF sites.

The probability that T talkpaths would be needed is very low since the above assumes that each and every call only goes out on one RF site and that all voice calls are group calls. In practice, this is seldom true and so we can assume the following:

1) More than M% of voice calls on a system will be talkgroup calls.

2) On average, a talkgroup call will go out on D sites.

From experience, the value of M and D are 90% and 3 - that is, 90% of voice calls on any radio system are talkgroup based and a talkgroup call will go out across three sites on average.

Therefore we can say:

P = (2R - S) / D

Where:

P is the average number of talkpaths required for talkgroup calls  (see above).

R is the number of trunked repeaters in the system.

S is the number of RF sites.

D is the average number of sites occupied by a talkgroup call.

The above calculation does not take M into account nor does it include any provision for private calls. To account for both of these variables, we need the following:

N = (2R - S) * 1,5 * (1-(M/100))

Where:

N is the maximum number of talkpaths required for private calls.

M is the percentage of talkgroup calls. So if M is 90% then (1-(M/100)) will be 0,1.

1,5 is the average number of sites required for a logged voice call.

To get the number of needed talkpaths add P and N then round up to the next whole number. Optionally, one can add a safety margin of a few extra talkpaths but I generally reccomend going with P+N talkpaths then adding a few extra if the voice logger indicates that a few calls got missed/queued becuase of insufficient free talkpaths.

Capacity Max can either use Fast Start or All Start for talkgroup calls. This means that, depending on how the system is configured, initiating a talkgroup call could either result in the call being queued or busied if there are no free talkpaths at that point in time.

PS. I am German so use a comma as a decimal seperator. A higher resolution version of the above diagram can be found here.