Modem Pass-Through
Modem pass-through over VoIP provides the transport of modem signals through a packet
network by using PCM-encoded packets. It is based on the same logic as fax passthrough:
An analog voice stream is encoded into G.711, passed through the network, and
decoded back to analog signals at the far end.
The following factors need to be considered when determining whether to use modem
pass-through:
■ Modem pass-through does not support the switch from G.Clear to G.711.
■ VAD and echo cancellation need to be disabled.
■ Modem pass-through over VoIP performs these functions:
■ Represses processing functions like compression, echo cancellation, high-pass
filter, and VAD
■ Issues redundant packets to protect against random packet drops
■ Provides static jitter buffers of 200 ms to protect against clock skew
■ Discriminates modem signals from voice and fax signals, indicating the detection
of the modem signal across the connection, and placing the connection in a state
that transports the signal across the network with the least amount of distortion
■ Reliably maintains a modem connection across the packet network for a long
duration under normal network conditions