What Is the Fax Switchover and Why Is It So Important?

With Cisco voice products (and most others), ALL fax calls start off as a regular VoIP call. This is a simple but critical concept to remember when dealing with faxing using Cisco gear. So, even though you may want to transport the fax call using T.38 fax relay, the fax call will use a voice codec in the beginning. If at any point after this voice call is connected, fax tones are detected, then a fax switchover must occur. This switchover transitions the call from voice mode to the configured fax transport mode (i.e. T.38 fax relay, passthrough, etc.). The reason that the fax switchover is so important is that fax communications rarely work using a voice codec (with G.711 being the main exception). If the call does not switch over to a transport mode designed for the handling of fax communications, then the call will be unsuccessful. For this reason, understanding how a fax switchover works is important for ensuring the success of fax communications in your network. With Cisco voice products, there are three methods used for signaling the fax switchover. - Call Control Protocol - The call control protocol (i.e. H.323 or SIP) that sets up the initial voice call can also be used to handle the fax switchover between the IP endpoints. This is a standards-based method and it is the recommended best practice in most scenarios. A switchover using the call control protocol can be used with the fax transport methods of T.38 fax relay and passthrough. - Named Signaling Event (NSE) Messages - NSEs are Cisco proprietary messages that are sent within the voice media stream between the fax voice gateways. The NSEs allow the Cisco voice gateways to transition to passthrough or T.38 fax relay without needing the call control protocol. In some scenarios, this switchover method may be advantageous but generally it is recommended to use the call control protocol for the fax switchover. - Real-Time Protocol (RTP) Payload Type (PT) Messages - Just like NSE messages, RTP PT messages are sent in the voice media stream. However, the switchover is communicated through simply alternating the RTP PT field in the RTP header. RTP defines dynamic PT values that can be used by vendors and Cisco uses the values of 96 and 97 to handle the switchover for Cisco fax relay. Cisco fax relay is a pre-standard fax relay implementation deployed by Cisco before T.38 was released. This RTP PT switchover is unique to Cisco fax relay. Whenever possible, it is recommended that the fax switchover be handled by the call control protocol. However in the event that this is not possible (because of legacy product issues or lack of support in older Unified CM versions), NSE or RTP PT messages performing the fax switchover are acceptable alternatives. Also, be aware that with the T.38 fax relay and passthrough transport options, you have the ability to choose between a switchover the occurs in the call control protocol or one that uses NSE messages. -David

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