Why are the H.323 Statistics in a call different than H.323 Bandwidth Settings?

There are two (2) bandwidth settings accessible to the user:
Incoming call bandwidth. This is set through the Admin tools.
Outgoing call bandwidth. Again, this is set through the Admin tools.

In both of these cases, the setting applies only to the transmit bandwidth. That is, these settings are used as the ceiling when selecting audio and video transmit algorithms to be used during the call. If, for example, an outgoing call is placed at 128Kbps, the H.323 Call Manager will select audio and video transmit algorithms such that the sum of their bandwidths will not exceed 128Kbps. Note that the H.323 Call Manager has actually requested 2x this bandwidth from the GateKeeper, which for this example would be 256Kbps.

When the near end sends its receive capabilities, it must put a maximum bit-rate into all of its audio and video capabilities. This represents the maximum bit rate that the near end can receive for that algorithm. For audio algorithms the maximum bit rate is pretty well determined by the algorithm type: G.711 is 64Kbps, as is G.722; G.728 is 16Kbps, G.722.1-24 is 24Kbps, and so on. For video, however, the maximum bit rate is not fixed by the algorithm, but is instead computed by the H.323 Call Manager at the time the call is placed. It uses the input call bandwidth in its video receive capabilities. For our example of a 128Kbps call, the declared receive video max B/R would be 128Kbps, and this would be for both H.261 and H.263 since both are variable rate algorithms.

This means that at the remote endpoint, the transmit call statistics may actually add up to more than 128Kbps. If the remote endpoint selects, for example, G.722 and H.263, then it would in fact be transmitting 64Kbps (audio) + 128Kbps (video) = 192Kbps. The near end, however, will not transmit more than 128Kbps total even if the remote endpoint declares receive capabilities greater than 128Kbps.

To continue our example, assume that we have stinger system A placing a 128Kbps call to stinger system B, which has been configured to use a maximum one-way bandwidth of 768Kbps for incoming calls.

System A would advertise receive video capabilities of 128Kbps (the call bandwidth). System B would advertise receive video capabilities of 768Kbps (the configured one-way max for incoming calls). System A would attempt to select an audio algorithm that had a maximum bit rate of 24Kbps or less, which is the audio B/W ceiling for calls of 128Kbps. System B, however, would attempt to select an audio algorithm that had a max. bit rate of 64Kbps or less, the audio B/W ceiling for 768Kbps calls. If both systems were configured for auto-selection, then system A would select Siren1424 (24Kbps) and system B would select Siren1448 (48Kbps).

For video, system A would select H.263+. Even though the remote endpoint said it could receive H.263 video at speeds up to 768Kbps, system A would only use 104Kbps (i.e. 128Kbps - 24Kbps). System B would also select H.263+. However, since system A had advertised a maximum video receive B/W of 128Kbps for H.263, system A would use all 128Kbps for video since this would still leave it well under its call bandwidth of 768Kbps. The total transmit bandwidth used by the endpoints would therefore be:
System A total TX B/W = 24Kbps (Siren1424) + 104Kbps (H.263+) = 128Kbps
System B total TX B/W = 48Kbps (Siren1448) + 128Kbps (H.263+) = 176Kbps

Unlike H.320, T.120 doesn't figure into the calculations since it is handled completely out of band as if it were a separate call.