T38 Fax Relay Cisco Modem Relay and Cisco Text Relay Configuration for H323 and SIP
Different modulation types can each be configured with their own relay protocol. A fax modulation may use Cisco fax relay or T.38, modems can use Cisco modem relay or modem relay for secure communications, and text communications can use Cisco text relay. Each traffic type has its own unique relay transport methods.
Example 10-3 illustrates a configuration where fax, modem, and text traffic are each assigned an appropriate relay transport method. T.38 fax relay is chosen for the fax traffic, Cisco modem relay for modems, and Cisco text relay for text communication.
Example 10-3 T.38 Fax Relay, Cisco Modem Relay, and Cisco Text Relay Configuration for H.323 and SIP
Example 10-3 T.38 Fax Relay, Cisco Modem Relay, and Cisco Text Relay Configuration for H.323 and SIP
In Example 10-3, H.323 fax calls matching dial-peer 100 will use the T.38 fax relay protocol because of the command fax protocol t38. In addition, because this command line does not contain the nse keyword, the H.323 protocol stack will be used for the T.38 switchover rather than NSE packets. Whenever a switchover from voice mode to T.38 occurs using the voice signaling protocol itself, this is referred to as protocol-based T.38 fax relay.
Within this same T.38 command line, you see that both the low-speed (ls-redundancy) and high-speed (hs-redundancy) redundancy options for T.38 fax relay are set to zero. This effectively disables redundancy for T.38 fax relay.
The last option in the fax protocol t38 command line under dial-peer 100 in Example 10-3 is fallback. This option allows the gateway to try an alternative transport method for the fax call should the switchover to T.38 fax relay fail. In this case, the fallback option is set to cisco, which means that a switchover to Cisco fax relay will be attempted by the voice gateway if the T.38 fax relay switchover is unsuccessful.
Another fax relay configuration command found under dial-peer 100 in Example 10-3 is fax-relay ecm disable. This is an optional fax relay configuration command that prevents fax machines from using the ECM feature. Optional fax relay commands such as this one are not required, but they allow the user to customize certain aspects of the fax relay configuration.
Dial-peer 100 in Example 10-3 also contains the command modem relay nse codec g711ulaw gw-controlled. This command activates gateway-controlled Cisco modem relay using an NSE-based switchover. Any V.34 modulated modem call should trigger cisco modem relay when this command is present.
Cisco text relay is not explicitly enabled under dial-peer 100. However, the combination of the default dial-peer setting of text relay protocol system and text relay protocol cisco under voice service voip enables Cisco text relay for this dial-peer.
Dial-peer 200 in Example 10-3 is a SIP dial-peer configured for T.38 fax relay and gateway-controlled Cisco modem relay like dial-peer 100. However, Cisco text relay is configured explicitly under dial-peer 200 even though Cisco text relay is already enabled globally under voice service voip.
The reason for the explicit configuration of Cisco text relay under dial-peer 200 is to access additional text relay options. In this case, the user wanted to configure text relay rtp redundancy 1 for this SIP dial-peer. However, this optional text relay command cannot be accessed at the dial-peer level unless text relay protocol cisco is explicitly defined at the dial-peer level first.
Post a comment