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Processing Media Using the T.38 Protocol
November 2009 240
In endpoint facsimile devices, such as a network fax server, the
T.38 protocol provides the equivalent to the modem in traditional
faxing. In a gateway, the T.38 protocol is used to translate
T.30 protocol and image data from the modems in the gateway to and
from the IP endpoint connection, using the following procedure.
1. With T.38 in an endpoint, the application connects a T.38 fax
server to an IP network and transmits the T.30
protocol and fax
image data to the receiving gateway using T.38
packets over the
IP network.
2. The receiving T.38 gateway, in turn, translates the T.38 packets
and repackages them into T.30
protocol signals and transfers
them to the receiving fax machine using modem modulation.
3. The receiving fax machine has a T.30 protocol engine that
communicates with the T.30
protocol engine in the fax server
through the gateway.
4. With T.38 in a gateway, the sending fax machine sends a fax
using modem modulation to transport T.30
protocol and image
data to a gateway via the PSTN.
5. The gateway demodulates the incoming T.30 fax signals and
image data and repackages them into T.38
packets.
6. The gateway then sends the T.38 packets to a T.38 endpoint,
which then delivers the packets in T.30
protocol so the endpoint
can receive the fax.
Gateway-to-gateway scenarios are also possible where two fax
machines communicate using two gateways. In that case, the
T.30 protocol engines in the two fax machines are transported across
the packet network using T.38.