The ability to send modem and fax calls over SIP trunks is an important consideration for both the service provider and enterprise. Enterprises that cannot achieve adequate fax and modem call success over SIP trunks should maintain these services over their tradi- tional TDM PSTN trunks. Fax calls terminated by a SIP trunk service provider might have slightly lower speeds than TDM trunks and might support fewer native fax protocols. For example, Super Group 3 (SG3) fax and color faxing cannot be completed over SIP trunks at present. Cisco voice gateways provide mechanisms to transmit SG3 fax calls over SIP trunks using G3. There are two common ways of transmitting fax over IP: Fax Relay (T.38) Fax passthrough or G.711 Fax T.38 as a Fax Method for SIP Trunks T.38 is a standard for implementing faxing over an IP network. For T.38 fax calls to work over a SIP trunk, both the originating and terminating endpoints must support T.38 faxing. T.38 fax is more resilient to delay, jitter, and loss on an IP network than a fax pass- through method. The T.38 configuration can replicate both the low-speed V.21 and the high-speed T4 traffic generated with a fax. The replication of packets can aid in providing a higher fax success rate when using unified communications (UC) SIP trunks. A T.38 capability is indicated in the SDP, and the negotiation can occur through a REINVITE request or by using a named signaling event (NSE) in the RTP stream. The NSE option is supported by Cisco voice gateways and the CUBE. Cisco products implement the User Datagram Protocol (UDP) as the transport layer for T.38 fax. Fax Pass-Through as a Fax Method for SIP Trunks Fax pass-through involves the originating and terminating gateways changing their codec to G.711 and fixing the jitter buffer (to a recommended 200 ms) to send fax information in-band in the speech path. Fax pass-through consumes more bandwidth than T.38 Fax Relay and is generally used only to ease interoperability to non-T.38 capable endpoints. After the fax tone is detected, the ups