SIP vs. PRI: What’s the Difference?

sip vs. PRIThere are many ways to deploy a modern business phone system. The PBX hardware and software that provide call control can be physically located at your location or hosted in an offsite data center. You can use handsets on desks or virtual softphones that run on mobile devices or PCs. You also have a choice about how to connect your phone system to the public telephone network.  You can use traditional PRI lines or choose SIP trunking.  PRI and SIP trunking essentially serve the same purpose (they enable voice calls) but they are entirely different technologies with different features and characteristics. Let’s take a closer look at each.

PRI (Primary Rate Interface)

A PRI is a dedicated physical line, usually a T1 connection. Each T1 has 23 voice channels which can all be used at the same time, thus supporting 23 simultaneous calls. PRI lines use a circuit-switch approach to voice connections. Because the PRI is a physical thing, it must be directly connected to the company’s telephone system (PBX). This requires a PRI interface card.

The capacity of each PRI (T1) cannot be expanded beyond 23 concurrent calls. This means that a business expecting up to 30 calls at a given time, for example, will need 2 PRI lines and 2 PRI interface cards, putting them in the position of paying for more capacity than they require.  Should the business expand to the point that more than 46 calls must be supported, they will need to purchase and install yet another PRI.

SIP Trunking (Session Initiation Protocol)

Unlike the physical lines used by PRI, SIP trunking creates a virtual connection to the public switched telephone network (PSTN). This is most often done over an existing connection to the internet. SIP trunks deliver voice as packets in just the same way that other data is transmitted over networks.

Unlike PRI lines, the capacity of a SIP trunk is limited only by the service provider and the amount of bandwidth available to support voice traffic. In the case of SIP, the need for the 24th concurrent call does not require additional hardware or installation.  It is simply a matter of licensing an additional SIP channel from the trunking provider.

Businesses are increasingly turning to SIP trunking for this increased flexibility along with several other advantages, including:

  • In addition to eliminating PRI lines and hardware, SIP trunking also eliminates most long distance calling charges
  • SIP trunks are easy to deploy and can be managed using a simple online control panel
  • In contrast to the long term contracts favored by PRI providers, many SIP trunking providers offer month to month or short term contracts

Given these benefits, it is not surprising that the number of businesses turning to SIP is rapidly growing. In fact, some projections have the number of businesses using SIP for 100% of their toll voice traffic at 42% by 2018. Improvements in the quality and speed of internet connections will only increase adoption of this cost saving technology. While SIP trunking might not be the right choice for every business (yet), it should certainly be considered by most as an affordable, easy to use alternative to PRI.