This content is part of the Essential Guide: The basics of SIP trunking explained

Choosing SIP vs. PRI: What are the differences?

SIP trunking and PRI both facilitate voice communications, but through very different capabilities. Learn the pros and cons of SIP vs. PRI, from costs to deployment models.

SIP trunking is a voice over IP and streaming media service based on the Session Initiation Protocol. 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 services to customers that have SIP-based PBXs and UC systems.

In millions of global installations, customers use SIP trunks exclusively, which connect them to digital UC 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. To determine which technology is right for your organization, compare SIP vs. PRI to understand their costs, capabilities and limitations.

Pros and cons of SIP trunking

SIP trunks operate using the Internet Protocol. If the phone system uses VoIP and operates over the data network, then SIP trunks are a native IP extension of their phone system to the PSTN. In many cases, the entire phone platform is also running the SIP protocol for internal setup and signaling. Thus, the SIP trunk is simply the same protocol used as a bridge to connect the local PBX to the outside world.

SIP trunking

Because SIP trunks are software-centric compared to PRI, they are far more elastic and scalable. Adding or reducing the number of calls a SIP trunk handles usually only takes a change in configuration on both sides of the trunk. The real limitation in the case of a SIP trunk is the bandwidth between trunk endpoints.

That leads us to some drawbacks of SIP trunking. For one, many SIP trunk architectures allow a SIP trunk to ride across the same internet link that employees use to surf the internet, stream video and perform other internet-based tasks. This creates a situation where voice traffic riding across the SIP trunk can be negatively affected if there is insufficient bandwidth to handle both the calls traversing the SIP trunk and standard internet traffic. Thus, it's important to watch internet throughput closely so bottlenecks don't occur.

While businesses can 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. This does, however, come with an added cost.

Pros and cons of PRI trunks

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.

The benefits of PRI trunk technology over SIP trunks are dwindling by the day. That said, organizations may encounter certain situations where PRI trunks are a better -- and perhaps only -- choice. First, if the phone system is very old and resides on its own digital network as opposed to sharing the corporate LAN, it's possible this system won't even have SIP trunking as a PSTN connectivity option. Thus, you're left with PRI trunking as your only choice.

Second, the location where you need PSTN connectivity may not have SIP trunking as an option. Surprisingly enough, large sections of the United States can get a PRI connection installed, but not a SIP trunk. This largely depends on the PSTN carriers available and whether their equipment can handle SIP or not.

PRI circuits are also fully dedicated connections compared to SIP trunks. Each voice call has its own dedicated space on the trunk. Therefore, issues inherent in overuse are nonexistent.

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 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.

Because one PRI line, T1, will only allow up to 23 simultaneous voice 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.

SIP trunking explained

Now you know the pros and cons of SIP vs. PRI. Read the rest of our series to make the most of your SIP trunking deployment.

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 contrast to PRI, SIP trunking is a virtual connection to the PSTN. This virtual connection runs on top of a data connection, such as 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.

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 voice 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 needed obviously warrants careful consideration for any organization.

Next Steps

Taking the 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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