Feature Overview
Some Media Gateway Control Protocol (MGCP) call agents do not properly pass those portions of Session Description Protocol (SDP) messages that advertise T.38 and named service event (NSE) capabilities. As a result, gateways that are controlled by these call agents are unable to use NSEs to signal T.38 fax relay to other gateways that use NSEs. The new syntax for the mgcp fax t38 command provides a way to enable gateway-controlled T.38 fax relay between an MGCP gateway and another gateway even if the capability to use T.38 and NSEs cannot be negotiated by the MGCP call agent at call setup time. The other gateway can be H.323, Session Initiation Protocol (SIP), or MGCP.
NSEs are the Cisco-proprietary version of named telephony events (NTEs), which are defined in IETF RFC 2833. NSEs and NTEs are used to communicate telephony signaling events that are normally indicated by the presence of tones, such as dual-tone multifrequency (DTMF) or fax transmissions. NSEs and NTEs do not transmit audible signaling tones across the network, but instead work by sending a binary code that is later used to recreate the tone. NSEs use different values to represent events and tones than NTEs use.
NSEs and NTEs are passed in the media stream. They consist of Real-Time Transport Protocol (RTP) packets that have the same source and destination IP addresses and User Datagram Protocol (UDP) ports as the rest of the media stream. However, NSE and NTE packets use different RTP payload types than the rest of the media stream so that they can stand apart from the audio packets in the stream. NSEs are normally sent with RTP payload type 100.
The use of NSEs to signal T.38 fax relay mode switchovers must be configured on both gateways. On H.323 and SIP gateways, use the fax protocol t38 nse force command to specify the use of NSEs for T.38 fax relay. On MGCP gateways, use the mgcp fax t38 gateway force command.
A key benefit of the forced gateway-controlled T.38 fax relay mode is that it has no dependencies on the call agent. This enables forced gateway-controlled T.38 to be configured to work with any release of the Cisco Unified Call Manager. The drawback of this approach is that it only works with other Cisco gateways that support this gateway-controlled T.38 mode, and it will not work with T.38 fax servers or third party gateways. When third party interoperability is required, the standardized ITU-T T.38 mode must be used. This is configured on the gateway by either the mgcp fax t38 command without the gateway force keyword pair option for MGCP or by the fax protocol t38 command without the nse force options for SIP and H.323.
Gehe ich Recht in der Annahme, dass ich bei "Fax process NSE" yes einschalte (falls er von einem anderem Cisco-Device ein NSE Kommando bekommt, aber bei "FAX Passtrough Method" ReINVITE (anstelle NSE) auswähle?
Ich nehme mal an, dass der LC keine NSE-Kommandos verarbeitet...
ML