H.323 T.38 Fax Relay
Figure 2-11 illustrates an H.323 T.38 relay operation. The T.38 fax relay feature provides
an ITU-T standards-based method and protocols for fax relay.
Data is packetized and encapsulated according to the T.38 standard. The encoding of the
packet headers and the mechanism to switch from VoIP mode to fax relay mode are clearly
defined in the specification. Annexes to the basic specification include details for operation
under SIP and H.323 call control protocols.
Figure 2-11 H.323 Fax Relay Operation
Figure 2-11 shows the H.245 message flow:
1. Initially, a VoIP call is established as if it were a normal speech call. Call control procedures
are followed, and the DSP is put into voice mode, after which human speech
is expected to be received and processed.
2. At anytime during the life of the call, if a fax answer or calling tone (ANSam or
CED) is heard, the DSP does not interfere with the speech processing. The ANSam
or CED tone causes a switch to modem pass-through, if enabled, to allow the tone to
pass cleanly to the remote fax.
3. A normal fax machine, after generating a CED or hearing a CNG, sends a DIS message
with the capabilities of the fax machine. The DSP in the Cisco IOS gateway
attached to the fax machine that generated the DIS message (normally the TGW)
detects the HDLC flag sequence at the start of the DIS message and initiates fax
relay switchover. The DSP also triggers an internal event to notify the call control
stack that fax switchover is required. The call control stack then instructs the DSP to
change the RTP payload type to 96 and to send this payload type to the OGW.
4. The detecting TGW sends a ModeRequest message to the OGW, and the OGW
responds with a ModeRequestAck.
5. The OGW sends a closeLogicalChannel message to close its VoIP UDP port, and the
TGW responds with a closeLogicalChannelAck message while it closes the VoIP
port.
6. The OGW sends an openLogicalChannel message that indicates to which port to
send the T.38 UDP information on the OGW, and the TGW responds with an
openLogicalChannelAck message.
7. The TGW sends a closeLogicalChannel message to close its VoIP UDP port, and the
OGW responds with a closeLogicalChannelAck message.
8. The TGW sends an openLogicalChannel message that indicates to which port to
send the T.38 UDP stream, and the OGW responds with an openLogicalChannelAck
message.
9. T.38-encoded UDP packets flow back and forth. At the end of the fax transmission,
either gateway can initiate another ModeRequest message to return to VoIP mode.
T.38 fax relay uses data redundancy to accommodate packet loss. During T.38 call establishment,
voice gateways indicate the level of packet redundancy they incorporate in their
transmission of fax UDP transport layer packets. The level of redundancy (the number of
times the packet is repeated) can be configured on Cisco IOS gateways.
The T.38 Annex B standard defines the mechanism that is used to switch over from voice
mode to T.38 fax mode during a call. The capability to support T.38 must be indicated
during the initial VoIP call setup. If the DSP on the gateway is capable of supporting T.38
mode, this information is indicated during the H.245 negotiation procedures as part of
the regular H.323 VoIP call setup.
After the VoIP call setup is completed, the DSP continues to listen for a fax tone. When a
fax tone is heard, the DSP signals the receipt of the fax tone to the call control layer,
which then initiates fax changeover as specified in the T.38 Annex B procedures.