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May 2010 | Volume 13 / Number 5
Ask the SIP Trunk Expert

SRO at SIP Trunking Sessions

By Bud Walder

At a recent conference on enterprise communications, the SIP trunking panel sessions were described as β€œSRO” – standing room only. This could mean either the rooms were too small, or there is a lot of interest in this topic. (It was probably both.)

It was also noted that many of the major PSTN carrier brands were well represented on the panels, as opposed to the smaller, competitive ITSPs that have frequently carried the banner in SIP trunking sessions at past conferences. You could conclude that this service has hit the mainstream, is ready for prime time, and consistent deployment and provisioning methodologies have been well established.

Much of that is true, but there are still some outstanding issues that may need to be addressed – such as how to handle fax transmissions over SIP trunking (implementing T.38 FoIP) and how to ensure a smooth cutoveror SIP trunk turn up experience on the customer premises.

On the fax issue, it is well documented that the circuit-switched fax protocol, T.30, does not fare well on an IP packet network where SIP trunks live. Packet loss and jitter are unkind to fax image data when it is treated as an in-band pass through audio transmission using SIP and RTP. Failure rates grow at an increasing rate as individual fax document lengths increase, and those failure rates are generally deemed unacceptable for any production fax environment where images are part of an automated workflow. Yet ensuring support for the T.38 protocol, which enables reliable FoIP, is frequently a lower priority in the race to offer SIP services to the enterprise.

As for delivering a consistent deployment and provisioning process, there are mixed reports from the front lines. Interoperability, security, network address translation and firewall traversal, legacy switch integration and demarcation are all areas that should be resolved prior to deploying new services. If a comprehensive connectivity plan is left unresolved by the service provider going into deployment, it can result in a hit or miss experience for the enterprise customer. These are legitimate concerns for enterprise customers. Fortunately, many industry players are working hard to address them to accelerate adoption and increase satisfaction rates. IT

Bud Walder is enterprise marketing director at Dialogic (News - Alert) Corp. (

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