Modem Relay
Cisco modem relay provides support for modem connections across traditional timedivision
multiplexing (TDM) networks. Modem relay demodulates a modem signal at one
voice gateway and passes it as packet data to another voice gateway, where the signal is
remodulated and sent to a receiving modem. On detection of the modem answer tone,
the gateways switch into modem pass-through mode and then, if the call menu (CM) signal
is detected, the two gateways switch into modem relay mode.
There are two ways to transport modem traffic over VoIP networks:
■ Modem pass-through: The modem traffic is carried between the two gateways in
RTP packets, using an uncompressed voice codec, G.711 mu-law or a-law. Although
modem pass-through remains susceptible to packet loss, jitter, and latency in the IP
network, packet redundancy can be used to mitigate the effects of packet loss in the
IP network.
■ Modem relay: The modem signals are demodulated at one gateway, converted to
digital form, and carried in the Simple Packet Relay Transport (SPRT) protocol. SPRT
is a protocol running over UDP packets to the other gateway, where the modem signal
is re-created, remodulated, and passed to the receiving modem.
In this implementation, the call starts out as a voice call, switches into modem passthrough
mode, and then into modem relay mode.
Modem relay significantly reduces the effects that dropped packets, latency, and jitter
have on the modem session. Compared to modem pass-through, it also reduces the
amount of bandwidth used.
Modem relay includes these features:
■ Modem tone detection and signaling
■ Relay switchover
■ Controlled redundancy
■ Packet size
■ Clock slip buffer management
Consider the modem relay characteristics in the following sections.