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IP fax provider Sagemcom has recently released a white paper studying the issue of fax being carried by a SIP trunking service. Both Sagemcom's XMediusFAX and OpenLine products interoperate within a SIP trunking environment, but there are some intricacies and operational benefits to understand before approaching your SIP trunk service provider.

Organizations continue to migrate to VoIP based network infrastructures for two understandable reasons - to increase their productivity and lower operating costs. The emergence and availability of new IP services, such as SIP trunking, help both of these goals.

Seen as a viable alternative to PSTN based trunking, SIP trunking helps tie an organization's internal and external communication requirements together and help pave the way to unified communications, the paper finds, adding that fax is seen as a single component within the range of real-time UC options:

"Like voice, instant messaging, and presence, fax can also be carried by a SIP trunking service… Note that OpenLine, a smart document management platform includes key elements of XMediusFAX as part of its integrated IP fax server offering."

One deployment part of the emerging trend towards SIP trunking implementations is a model where an Internet connection provided by the ITSP connects the fax server tools on the customer premises directly to the Carrier / ITSP managed network on the other side of the demarcation point.

The role of the SBC is to control the data flow of sessions - fax calls, what have you. The SBC controls the signaling as well as the processes for setting up, conducting, and tearing down the fax call or any other form of interactive media communications. SIP trunking alleviates the requirement for an enterprise to have a local connection to the PSTN. Connectivity to the PSTN, as well as all associated hardware maintenance, rests with the carrier/ITSP who manages the PSTN gateway functionality inherently contained within a SIP capable PBX (News - Alert).

Interoperability with SIP trunks is not without its challenges or issues and implementations of SIP and FoIP are wide and broad ranging. The following are some key networking considerations when choosing a SIP service provider for XMediusFAX and OpenLine FoIP applications.

• SIP compliance does not provide the ultimate guarantee of FoIP interoperability with SIP trunks. SIP providers that deploy with SIP standards designed for voice over IP traffic may not be optimized for FoIP operations. Interoperability testing should be used as a gauge to determine the level of FoIP operational readiness.

• Some SIP trunk service providers handle fax traffic via G.711; a voice transport technology adapted to carry fax traffic but not optimized for fax. The only reliable mechanism that is advocated for carrying fax IP traffic is T.38 therefore all network components should support end-to-end FoIP T.38 transmission and termination.

• IP networks are being implemented to address voice traffic requirements. Correspondingly, the network may have been optimized for voice rather than fax traffic. Network QoS parameters (i.e., Latency (delay), Jitter, Packet Loss) must be considered because they impact the real time FoIP application's call success within a SIP enabled network. For high fax quality grade of service, recommended network performance parameters are as follows: [Packet Loss = 0% (however, if present, a transport method using redundancy should be implemented), Jitter < 300 ms, and One-Way Delay < 150 ms]1

For enterprises focused on better use of their installed IP-PBXs to communicate over IP within, as well as, outside the enterprise, a SIP trunking service provided by an ITSP is a viable solution. SIP trunks provide organizations a cost effective migration strategy to an all-IP network operation by removing the TDM based infrastructure that many organizations still carry as a requirement for PSTN connection, according to Sagemcom.

With SIP trunking, fax traffic is routed through the ITSP's SIP enabled network, eliminating the need for purchasing more expensive BRI or PRI lines, as well as, on-premises PSTN gateway equipment. SIP trunking enables IP based fax solutions to seamlessly integrate within emerging Unified Communications (News - Alert) platforms. XMediusFAX and OpenLine employ field hardened IP fax optimized technologies. This Solution Brief highlighted some of the key SIP trunking considerations that need to be effectively addressed to ensure that these technologies are robustly and seamlessly deployed in live networks.