How is SIP Trunking Delivered?

Hosted/cloud PBX and SIP trunks are the two most common ways of deploying VoIP services for enterprises. Both options require Internet connections to function, but the actual implementation differs broadly.

What are SIP trunks?

The word ‘trunk’ is a legacy term left over from when analog business lines were required for voice communication. The trunk was nothing more than a bundle of circuits or lines delivered from the phone carrier to an organization enabling them to make calls.

It is a misnomer to refer to SIP connections as ‘trunks’ since it is not a physical bundle of circuits at all. Instead it is a logical connection delivered over the Internet by the ITSP. In other words it is a virtual connection. However since the basic concept is the same, the term has stuck around. It is now widely used and accepted in the VoIP industry.

Requirements for SIP trunking

As with any type of VoIP deployment, the basic requirement for SIP trunks is Internet access. The bandwidth and speed requirements will be determined by the number of concurrent calls that the organization expects to make as well as enough overhead to allow access for other web applications such as email. Sometimes the organization’s existing connections will be sufficient but quite often, businesses will have to upgrade their speeds to handle the extra bandwidth necessary for VoIP calls.

Other than Internet access, an IP PBX that is compatible with the SIP protocol is required. Most modern PBX systems can handle SIP and if not, they may require the installation of an SIP card or license. On the other hand, many businesses have older PBX boxes that cannot handle VoIP. In that case, VoIP gateways are needed. Setting up these gateways can be expensive and so many businesses opt for upgrading the PBX box entirely in one go.

SIP trunk delivery/configuration options

There are a number of different ways in which enterprises can get SIP trunks to their premises. Since it is a logical connection, nothing needs to be delivered physically. An SIP trunk is simply an access link between a service provider’s network and the enterprise communication system. SIP channels can be purchased in multiples of one rather than the traditional multiplier of 23 for analog lines. Hence the SIP access link can be whatever size as required by the company.

SIP trunks can be used as a one-on-one replacement for existing PRI lines. If a company has offices in 6-7 different locations, individual SIP trunks can be connected to each office. The advantage is that it is simple to understand and deploy while also providing resiliency. If one SIP trunk is experiencing problems, it does not affect the other locations. However this decentralization also means that equipment has to be duplicated at multiple places and can be expensive.

Another option is to centralize the SIP trunks in a single data center. Each location would be connected to the data center via an Internet backbone. Internal calls can be connected directly while external calls will be routed through the consolidated data center. While it has the advantage of consolidation and saves resources, it also introduces a single point of failure i.e. if the data center goes down for any reason whatsoever, the entire organization may be left without phone access.

Some organizations prefer to use a hybrid approach combining both the above options. The SIP trunks can be connected to two or more locations – one of which may include an emergency recovery site. This option offers redundancy, resiliency and allows flexible call routing. If one of the trunks is experiencing problems, calls can be routed through the other one regardless of where it originated.

SIP trunks – Quality concerns

When VoIP first started gaining traction, call quality was a significant concern. However with improved Internet access, calls made through SIP trunks can easily match toll quality. Some organizations have also chosen to use MPL S – Multiprotocol Label Switching which improves call quality. MPL S is a more expensive IP connection that typically comes with service level agreements that guarantee quality levels and specify worst-case parameters for jitter, packet loss etc.

Apart from MPLS, organizations may also want to use Session Border Controllers and SIP compatible firewalls. SBCs provide a number of functions including security-related ones and hence are deployed even when compatibility is not an issue. QoS settings on the various network equipment such as routers should also be configured to flag VoIP traffic as high priority. It ensures that voice calls do not have to wait in line when being routed around the network.

As with most technology implementations, the devil is in the details when it comes to SIP trunks. There are numerous configurations, options and pricing plans available in the organizations need to make an informed choice about which alternative is the best one for them.