SIP Trunking versus PRI: the facts.

When compared to traditional PRI, SIP Trunking involves many benefits to companies. SIP trunk allows a company to replace
traditional fixed PSTN lines with PSTN connectivity via a SIP trunking service provider on the Internet. SIP trunks can offer
significant cost-savings for enterprises, eliminating the need for local PSTN gateways, costly ISDN BRIs or PRIs.

The following table summarizes the main advantages of SIP Trunking over traditional PSTN circuits.

SIP Trunks
Circuits / Hardware

Connections are physical:

Each circuit requires physical connection and costly termination hardware.

Connections are virtual:

Number of available trunks is a function of available bandwidth, not physical termination hardware or circuits.

Depending on your requirements and the type of service offering, Session Border Controllers (SBC) may be required for security. SBCs can be a significant cost consideration.

Scale / Growth

Scaling up requires the installation of new circuits and additional termination hardware at specific increments. In the case of PRI the increment is 23 voice channels.

Scales up or down easily and quickly (a software configuration change) and can offer automatic and on-demand burst capabilities.

While the technology accommodates these advantages not all service offerings will provide them.

Backup / Redundancy

Providing sufficient backup circuits to remote sites in an IPT-distributed architecture can negatively impact the ROI.

Only way to accommodate loss of hardware or facility where PRI’s terminate is to build-in excess capacity with associated cost impact.

Automatic IP re-routing capabilities allow practical geographic distribution of PSTN connectivity to sites with limited or network redundancy.

Can be designed to retain PSTN reachability and capacity in the event of the loss of terminating hardware (or even an entire office location) without the need to build in excess capacity.


Cost is usually per circuit per month.

If you require one, or a few, more voice channels than the fixed increment, the cost model for PRI is inefficient.

A variety of pricing models (i.e. usage based) are likely to emerge, including on-demand capacity. Relative to PRI circuits and the associated supporting hardware, IP Trunking costs are likely to be significantly lower.

Unfortunately, in Canada the pricing models have not been compelling to date. However, competition is increasing so hopefully that will begin to change soon.

Capacity Planning

Additional capacity must be planned well in advance since considerable lead time may be required for the ordering and installation of new circuits and termination hardware.

While capacity planning is still important, adding additional capacity can be as simple as a software change. Additionally, providers are likely to offer burst capabilities to accommodate brief periods of higher than anticipated utilization.

Disaster Recovery / Business Continuity

While possible, diverting calls to alternate locations can be complex and expensive.

Diversity across service providers is usually cost prohibitive.

The technology allows for automatic call rerouting to pre-defined locations should the location go offline (a huge business continuity benefit).

Can accommodate diversity across service providers much like is done today with Internet access via BGP.

Summarize by Rick McCharles.