McDewey

Due to the fact that IP Service Providers typically have a mixed environment of Cisco and non- Cisco equipment, it is essential that you use an industry-standard method in order to switch from a voice call to a fax call. This means that the Named Signaling Event (NSE) cannot be used, since NSEs are Cisco-proprietary. There are exceptions to this rule, but they are extremely infrequent. Note: The inability to use a protocol-based switchover means that Skinny Call Control Protocol (SCCP) is only used in fax call flows to IP Service Providers with G711ulaw and is a "best effort." FoIP Transport Methods This document discusses two FoIP transport methods, Fax Pass-Through and T.38 Fax Relay. Fax Pass-Through Fax Pass-Through is a fax transport method where the T30 signals and page data are transported through the IP network as Pulse-Code Modulation (PCM)-encoded data, wrapped in Real-time Transport Protocol (RTP) frames. The Fax Pass-Through switchover is triggered by the detection of the V.21 Preamble on the TGW. The resultant INVITE (for SIP) or Request Mode (for H323) is sent through the CUBE and the rest of the call signaling path to the OGW. The Fax Pass-Through switchover switches over from any voice codec to the codec defined under the Fax Pass-Through configuration (this process is outlined later in this document). Note: An MGCP gateway cannot be configured in order to initiate upspeed to G.711 for Fax Pass-Through. Therefore, any fax that uses pass-through on the CUBE that terminates to an MGCP gateway must be routed with G.711 codec. Note: Fax Pass-Through should not be configured with H.323 if the initial codec is G.711. This causes a H.245 request mode to be sent to switch to G.711 when G.711 is already negotiated. CUCM responds with a H.245 request mode rejection.