Cisco Fax Relay

Cisco fax relay is a proprietary fax relay implementation on Cisco voice gateways. Developed before the T.38 fax relay standard, Cisco fax relay provides the same basic functionality as T.38, but there are differences with regard to packet formats and the mechanisms used for switching into fax relay mode.

Just like T.38, Cisco fax relay demodulates all incoming fax signals and passes the fax tones and HDLC frames across the IP network using data packets. At the far end, these data packets are modulated back into analog signals and transmitted to the attached fax devices.

Being a Cisco proprietary protocol, a detailed look at the packet formats and other operational intricacies is not possible with Cisco fax relay. However, a simple high-level overview can be provided. Figure 5-14 illustrates the basic Cisco fax relay packet structure.

Figure 5-14 Cisco Fax Relay Packet Structure

Figure 5-14 Cisco Fax Relay Packet Structure

i Cisco Proprietary Formatting , i i i Cisco Proprietary Formatting ,

Cisco fax relay frames are transported using RTP. What distinguishes Cisco fax relay packets from other real-time traffic is the unique RTP payload type of 122. Looking back to Table 4-1, you will see that this payload type is a dynamic one and suited for proprietary implementations such as Cisco fax relay.

To switch over to Cisco fax relay, Cisco voice gateways use special signaling packets in the RTP stream. However, unlike the previous RTP stream signaling packets that have been discussed, these packets are not NSEs. Instead, specific RTP dynamic payload types are used for the signaling. The switchover is not NSE or protocol-based but rather based on the RTP payload type (PT). Figure 5-15 details the Cisco fax relay switchover process.

The detection of fax flags (V.21 preamble) at the terminating gateway triggers the transition to Cisco fax relay. The TGW notifies the OGW of the impending switchover with a special RTP message using a PT value of 96 (PT-96). Note that this RTP message is different from NSE messages, which use a payload type of 100 followed by a specific event ID.

The PT-96 Cisco fax relay switchover packet from the TGW to the OGW is ACK'd (acknowledged) with a PT-97 message from the OGW back to the TGW. This switchover message and ACK process is then repeated in the direction from the OGW to the TGW before the Cisco fax relay call is established.

You should realize that the PT-96 and PT-97 packets are only seen during the switchover to coordinate the gateway's transition from voice mode to Cisco fax relay. After the switchover has completed, Cisco fax relay packets use a PT of 122.

Figure 5-15 Cisco Fax Relay Switchover

Sending Fax Machine

Sending Fax Machine

fVr-

Fax passthrough call is established if NSE passthrough has been configured on the voice gateways. Otherwise, this step does not occur.

fVr-

Fax Passthrough Call

Answering Fax Machine

Fax Passthrough Call

PT-96 Cisco Fax Relay Switchover

Answering Fax Machine

CED Tone

V.21 Preamble

PT-97 Cisco Fax Relay Switchover ACK

PT-96 Cisco Fax Relay Switchover

PT-97 Cisco Fax Relay Switchover ACK

^^ Cisco Fax Relay Call

NOTE Because of the use of the RTP voice stream for communicating the Cisco fax relay switchover, the call signaling protocol stack is not directly involved with the switchover process. No special provisions are needed within the call signaling protocol itself, so Cisco fax relay can work with H.323, SIP, MGCP, and SCCP gateways.

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