G.711 Packetization Periods
Overhead for
Packetization G.711 Payload Layers 3 and 4 Packet Size Bit Rate
10 ms 80 byte 40 byte 120 byte 96 kbps
20 ms 160 byte 40 byte 200 byte 80 kbps
Packet redundancy might be used to mitigate the effects of packet loss in the IP network.
Even so, fax pass-through remains susceptible to packet loss, jitter, and latency in the IP
network. The two endpoints must be clocked synchronously for this type of transport to
work predictably.
Performance might become an issue. To attempt to mitigate packet loss in the network,
redundant encoding (1X or one repeat of the original packet) is used, which doubles the
amount of data transferred in each packet. The doubling of packets imposes a limitation
on the total number of ports that can run fax pass-through at one time. One fax passthrough
session with redundancy needs as much bandwidth as two G.711 calls without
VAD.
Fax pass-through does not support the switch from G.Clear to G.711. If fax pass-through
and the G.Clear codec are both configured, the gateway cannot detect the fax tone.
Fax pass-through is supported under these call-control protocols:
■ H.323
■ SIP
■ Media Gateway Control Protocol (MGCP)