What are the SIP trunking basics?

What SIP trunking basics should you know before you deploy? SIP trunking guru Chris Partsenidis explains what you need to know about SIP trunking before deploying it in your voice system.

SIP trunking is a popular method for enterprises to move their voice services onto the Internet. But before they...

deploy, enterprises should know the basics of SIP trunking, from the bandwidth issues affecting SIP to how to select the right SIP trunking provider.  In this Q&A, SIP expert Chris Partsenidis explains the SIP trunking basics enterprises should know.

What is the difference between SIP and PRI?

PRI is a single physical line, often a T1 connection in North America or an E1 in Europe. 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.

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.

Learn more about the differences between SIP and PRI.

What should I look for in a SIP provider?

Not all SIP trunking providers are the same. Some have large networks with plenty of resources and the ability to handle thousands of calls per customer, while others have smaller networks with limited capacity, which can also limit the quality of your SIP calls.

Generally, if you are looking at purchasing a SIP trunk that will support only a few voice channels, then Internet-based SIP providers might offer a flexible and cheap solution without a contract.

If your company's requirements include a large number of voice channels, it might be worth looking for a SIP provider that will terminate its end of the SIP trunk on equipment located on your company's premises.

Learn more about the SIP trunking options available to your enterprise.

What bandwidth issues should I be concerned about?

Latency is one of VoIP's biggest enemies. Callers will begin to notice round-trip voice delay of about 250 to 300 ms or greater, which is why latency should be kept under 150 ms one way (source to destination). The further away the SIP provider, the bigger the latency will be.

It's always a good idea to test the latency to your SIP provider by simply pinging their SIP trunk endpoint. This is a very simple and revealing test that will help you get an indication as to whether latency will be an issue with the provider.

Learn how to support SIP trunking bandwidth needs.

What are the advantages of using SIP trunking?

SIP trunking has become the preferred method of connecting SIP-enabled telephony systems with service providers for many reasons.

  • Reduced installation/setup costs. SIP trunks require only a reliable IP network path to the customer's telephony system. The simplicity and flexibility of SIP trunks allow providers to significantly lower their setup costs and pass savings to their customers.
  • Upgrade SIP trunks quickly with no additional equipment. Many telephony systems support more than one SIP trunk, which means customers can implement multiple SIP trunks to serve different departments or services.
  • Back up links over any Internet connection for business continuity.  In case the primary link to the SIP provider fails, almost any stable Internet connection can be used to register the SIP trunk to the provider. This can help limit a communications outage and can provide a backup telephony service for emergency calls.

Read on to learn the other major advantages of SIP trunking.

How can I make sure my SIP trunks are secure?

SIP trunk security encompasses a number of different issues. To address them, most security vendors prefer a layered approach to provide an effective way of isolating and protecting the telephony system and the communications path to the SIP service provider.

Here are some tips to help identify which areas of SIP security need to be changed or redesigned to help avoid unpleasant surprises.

  • Ensure complex passwords for your SIP trunk: SIP trunk providers require authentication in order to allow incoming and outgoing calls from the SIP trunk. Make sure complex passwords are used for the authentication process to your SIP provider.
  • Limit access to the telephony system: Only specific people from specific locations should have access to the telephony system. Always ensure your telephony systems are isolated in a separate VLAN and the correct VLAN security policies are in effect.
  • Accept SIP traffic only from your SIP provider: Block traffic from all external sources except your SIP provider. This will help limit access to your telephony system and minimize chances of unauthorized access.

Get more tips on how to lock down your SIP trunks.

Next Steps

Why SIP trunking is better than PRI and BRI

What is the purpose of a SIP trunking service?

SIP trunking closes mobility cost gaps

This was last published in December 2014

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