This content is part of the Essential Guide: The basics of SIP trunking explained
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SIP Trunking Explained: Choosing SIP vs. PRI

Part one of this series, SIP Trunking Explained, looks at SIP vs. PRI and explains the differences between packet-switched SIP trunking for your UC and VoIP needs and circuit-switched PRI.

Editor's note: Part one of our SIP Trunking Explained series looks at the differences between packet-switched SIP...

and circuit-switched PRI. Check out the rest of the series (see box below) for essentials on how to select a SIP trunking provider, enabling SIP trunking in your legacy equipment, calculating how much bandwidth you'll need for SIP trunking, the SIP trunking advantages over circuit-switched option, and the security implications of SIP trunking.

SIP trunking is a Voice over Internet Protocol (VoIP) and streaming media service based on the Session Initiation Protocol (SIP). SIP trunks allow multiple simultaneous streams of VoIP calls to be routed through a carrier's network and delivered to an organization as voice packets.

SIP trunking services are sold by Internet telephony service providers and provide telephone and unified communications (UC) services to customers that have SIP-based PBXs and UC systems.

SIP Trunking Explained

You've got the basics, now read the rest:
Find the right SIP service provider for your telephony system
How much VoIP bandwidth you need for SIP trunking
Highlight reel: SIP trunking advantages
Security checklist for SIP trunking services

In millions of global installations, customers use SIP trunks exclusively, which connect them to digital unified communications options. In many cases, SIP trunking has completely substituted the well-known T1/E1 and Primary Rate Interface (PRI) connections to the public-switched telephone network (PSTN) because it offers easier and more inexpensive ways to connect.

Because SIP trunking is the latest trend in the world of telecommunications IT, it's hard to imagine that a new telephone system wouldn't support SIP trunking. 

SIP vs. PRI: What's the difference?

In many cases, PRI and SIP trunking serve the same purpose: They enable simultaneous calls over a line, but their features and characteristics are very different. Let's take a look at a few of the major differences between PRI and SIP trunking.

PRI is a single physical line, often a T1 connection in North America or an E1 in Europe. A T1 has a total of 23 voice channels, and an E1 has a total of 32 voice channels, all of which can be used simultaneously. In addition, PRI uses a circuit-switched model for its voice connections between endpoints and has guaranteed quality of service (QoS).

Since PRI is an actual physical line, it requires a physical connection to connect to the company's telephony system. This connection commonly comes in the form of an expensive PRI interface card on the telephony system.

Furthermore, companies with PRI lines usually need to purchase a generous amount of phone numbers that are then assigned to their internal departments, call centers and personnel. As a result, a company with 92 phones would probably purchase 92 phone numbers from its telecommunications provider to cover its maximum simultaneous-use needs (one phone number for every physical device).

Since one PRI line (T1) will only allow up to 23 simultaneous calls, if the company needs to have all 92 phone lines available for calls at one time, it needs to purchase four PRI lines, which means an increase in PRI line rentals, plus the hardware needed to connect, or interface, the line with the telephony system. This is often a problem for call centers, where increasing the capacity of simultaneous phone calls translates into big costs.

Telecommunications providers prefer to offer dedicated data lines directly to a customer's premises to ensure the quality and stability of their SIP trunks.

In contrast to PRI, SIP trunking is a virtual connection to the PSTN. This virtual connection runs on top of a data connection (like the Internet) that typically already exists in an organization. This makes SIP trunking easier to install.  SIP trunks use a packet-switched networking model that terminates to the service provider via IP and is typically a best-effort delivery with no QoS guarantees.

While businesses opt for running SIP trunks directly over the Internet, telecommunications providers prefer to offer dedicated data lines directly to a customer's premises to ensure the quality and stability of their SIP trunks.

The capacity of a SIP trunk is related only to the capacity the service provider can handle and the bandwidth available on the data line between the customer and service provider.

Going back to our previous example, if the same company decided to switch its PRI lines to SIP trunks, it would then require only one SIP trunk to handle all 92 simultaneous calls, assuming there is enough bandwidth in the line between the company and service provider.  The savings on the multiple PRI lines and additional hardware (interface cards) needed obviously warrants careful consideration for any organization.

Next: Part two of our SIP Trunking Explained series offers advice on how to find the right SIP trunking provider for your business and how to enable your existing equipment to make SIP trunking possible.

Next Steps

Taking the first step: Migrating from the PSTN to SIP trunking

Lower your mobile network costs with SIP trunking services

VoIP vs. SIP trunking: How well do you understand them and what are their differences?

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