SIP Fax & The Session Initiation Protocol (SIP)
SIP is a VoIP signaling protocol used for creating sessions in an IP network. A SIP Fax session can be an ordinary telephone call or T.38 fax call or it could be a multi-media conference call. A SIP fax, having been developed as a mechanism to establish sessions, does not know about the details of a session, it just initiates, terminates and modifies sessions. This simplicity means that a SIP fax scales, is extensible, and fits reasonably well in various architectures and deployment scenarios. SIP dialogues for guaranteeing reliability, ordering, or data integrity. thus, udp provides anacts a request-response protocol and resembles two other Internet protocols, Hypertext Transfer Protocol (HTTP) and the Simple Mail Transfer Protocol (SMTP), protocols that drive the world wide web and email. SIP fax messages are encoded in ASCII text format, suitable for humans to read.
Although other VoIP signaling protocols exist, SIP fax methods have been standardized and governed primarily by the IETF (Internet Engineering Task Force), while other protocols, such as H.323, have been set by the International Telecommunication Union (ITU), a telephone industry group. So SIP is distinguished by its proponents having roots in the IP community rather than the telecommunications industry. SIP fax methodologies have strong momentum in the marketplace, compared with H.323. Nevertheless, a SIP fax offers a loose concept of a call and the integration of sometimes disparate standards is largely left up to each provider. As an outcome, the SIP fax method has become a protocol carrying quite a few interoperability problems – and it such problems with fax.
T.38, the ITU standard for Fax-over-IP (FoIP), is gaining strong adoption in the enterprise space, as it helps companies lower their communications costs, improve customer service, and adhere to regulatory compliance rules. However, in the last year, the market situation for FoIP has evolved. Previously, enterprises would create an IP LAN based on SIP fax standards or H.323 and connect it to the PSTN via a gateway. This made it easy to implement IP fax because all corporations had to do was install a gateway at the edge of the IP network and have the IP fax server make IP fax calls to the gateway, where the call would be converted to TDM streams for conventional faxing to the destination. So all that mattered was the performance on the local network and the gateway conversion. Today, however, enterprises and carriers are making capital expenditures for telecom largely in terms of SIP fax infrastructure, creating demand for SIP trunking and reducing the need for gateways. The absence of gateways and substitution of SIP trunking is causing problems with effective support of fax in access-provider and backbone IP networks.
This is not to say that SIP fax methods are at fault. In fact, SIP fax problems are arising because of the success of SIP. SIP compliance does not guarantee FoIP will work well over SIP trunks. This is because SIP fax providers sometimes implement disparate standards which can creates real problems.
Some SIP providers handle fax traffic via G.711 which is a voice transport technology that has been adapted to carry fax traffic. But this scheme is not optimal for faxing. T.38 is the protocol for carrying fax over IP networks and it has safeguards in it that include spoofing other nodes so they don’t realize that some packets arrive out of order (Conventional faxing, like voice communications, is a real-time, synchronous process, which makes timing the arrival of fax signals a straightforward situation. The IP process is asynchronous. However, data in the communication stream is broken into packets and the packets then sent over different network routes. Faxing over an asynchronous network – where delay, jitter and packet loss commonly occur – can increase transmission failures. T.38 makes it seem to fax nodes that a communication is coming in a real-time synchronous manner.)
Also, many IP networks have not have been optimized for fax, but rather for voice traffic. The User Datagram Protocol (UDP) is an IP protocol that is typically used within SIP fax networks. UDP uses a simple transmission model without implicit hand-shaking dialogues for, ordering, data integrity, or guaranteeing reliability. Thus, UDP provides an unreliable service and packets may appear duplicated, arrive out of order, or go missing entirely. But such network characteristics must be considered because they impact the FoIP system’s call success within a SIP fax enabled network. For excellent fax quality service, recommended network performance characteristics include:
- Jitter should be less than 300 ms
- Packet loss should equal zero
- One-way delays should be less than 150 ms
Network testing should be used as a gauge to determine the level of FoIP operational readiness in SIP fax networks.