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How SIP Trunking Rules Over Analog and PRI

SIP, which stands for Session Initiation Protocol, is one kind of Internet protocol, and is associated with VoIP (Voice over Internet Protocol). SIP Trunking is a new and superior way to carry external phone calls on your existing PBX system, compared to the old standard of analog circuit switching through PRI trunks.

There are three main components to SIP Trunking: IP PBX, an Internet Telephony Service Provider (AKA a SIP Trunking Provider), and the router and Ethernet switch. When used together, an existing TDM system can be modified to take advantage of SIP trunking. SIP trunking providers sell a service, and that is different from the hardware of the gateway and the IP PBX, which is itself a separate component. So, it makes sense that there may be SIP Trunking compatibility issues. Fortunately, with the proper amount of planning and the right components, it should not be difficult to deploy SIP trunking.

With PRI, the connections are physical. The phone signal must create a circuit, meaning an end-to-end connection, which is open the entire time that the phone is being used. When someone else wants to use phone on the existing PBX, there must be an open line, which is why PRI trunks typically have about a one-to-three ratio of phones to trunks, since it’s very rare that all lines will be busy at the same time. With SIP trunking, the connections are all in Internet protocol. The bandwidth necessary is much lower because whenever there isn’t an audio signal, no information is being sent over the lines. SIP trunking can save about 30% or more on monthly telephony costs by centralizing and eliminating redundant connections.

PRI connections require a dedicated circuit, so that all calls that are not internal require going through a local and long-distance phone carrier. Calls are typically expensive and international calls even more so, because a dedicated trunk must be opened and stay connected for the duration of the call. By contrast, SIP trunks terminate over the Internet for the majority of the connection. Typically, inbound and local calls are free. SIP providers terminate calls for a much lower rate, and international calls are inexpensive; calls to parts of Europe and South America, for example, are less than 2 cents per minute on many providers. SIP trunking has both local and long-distance calls as part of one package, so billing is one less thing that a company has to worry about.

One big advantage that SIP trunking has over PRI connections is that PRI appliances have 24 channels. If you want to scale, you must scale up in increments of 24, which may be a shock to the budget for, depending on the size of the organization, a small gain. SIP trunks can increase or decrease in increments of only one, and many providers allow what’s called “burstable trunks,” meaning that at peak usage, the number of trunks can temporarily increase, rather than have inadequate service. For SIP trunking, all it takes is a software update and an agreement with your SIP trunking provider and nearly instantly you can increase your capacity.

Another advantage that SIP trunking has over analog PBXs is that SIP trunking can connect seamlessly to other offices within the organization that have the same SIP trunking provider. This is particularly important for disaster preparedness. In case of power outage or other emergency, calls will be automatically rerouted to other offices or call centers for as long as there is an outage. Another advantage of this feature is that SIP trunks give more accurate information to E911 operators, so if the emergency is so great that someone calls 911, the first responders have more information at their disposal. It is possible to connect different offices using a PRI, but it is not very cost effective or easy to do. Because SIP trunking is not location-specific, it is also easy to get DID phone numbers from any area code.

Overall, SIP trunking is superior to PRI for its cost, scalability, and flexibility. With current standards, it’s easier than ever to attach TDM equipment to SIP trunking service. SIP trunking usually favors larger businesses with more than twenty users. Now that it easier than ever to send your phone service through the Internet, SIP trunking is a great way for businesses to save money and get more features.

Learn more about SIP Trunking

Robert Pepper

VoIP Enthusiast & Editor at GetVoIP

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