This article originally appeared in the December 2010 issue of Unified Communications Magazine

Last month, Avaya (News - Alert) and Skype announced a strategic partnership around Skype Connect. Skype Connect allows enterprises to connect their enterprise PBX products to Skype services, including PSTN calling and connectivity to the Skype (News - Alert) user base. After the announcement, I got many questions asking the same thing – what is the difference between this and a SIP trunk? The answer, of course, depends on what a SIP trunk is.

And what is a SIP trunk? There are really many different answers to this question.

Definition 1: There is No Such Thing

If you read the SIP specification (RFC3261), you will not find the term SIP trunk anywhere.

Of course, a trunk has meaning in the telecommunications industry, and refers to a circuit between switching systems (such as a PBX (News - Alert) and central office) that can carry a specific number of calls at a time. Trunks come in specific capacities, such as a T1, which can handle 24 concurrent calls.

Based on that definition, SIP trunk is a poor term. There is no circuit, and a SIP connection between hosts on an IP network can carry as much traffic as the intervening IP network can carry. Circuits refer to physical connections, while a SIP relationship is fundamentally a logical one. For this reason, purists will sometimes say that the term is fundamentally meaningless.

Definition 2: A PRI Replacement

However, most people think of a SIP trunk as a replacement for their PRI circuits from a telecommunications carrier.

Today, a large enterprise will purchase many local PRI connections in each branch office. With a SIP trunk, the enterprise can consolidate these into a single SIP trunk that runs out of a small number of data centers.

The SIP trunk provides exactly the same services as the PRI – including QoS guarantees – and is often carried over dedicated MPLS connections. A SIP trunk, like its circuit counterpart, is provisioned with a certain capacity in terms of simultaneous calls. In the case of a SIP trunk, this limit is typically a provisioned constraint and not a physical characteristic of the connection. However, the cosmetic differences aside, a SIP trunk is very much like a traditional circuit-based trunk.

 Definition 3: A SIP Server-to-Server Connection

Other people use the term SIP trunk to refer to any SIP association between servers.

Unlike the connection between a user endpoint and a SIP server (sometimes called a SIP line), a SIP trunk in this definition connects two hosts that service multiple users and will handle calls from different users. A SIP trunk can occur when a caller in one domain dials the SIP URI of a user in another domain, causing a connection to be made inter-domain, on-demand, for delivery of the SIP signaling. Or, a session border controller can send calls to a PSTN gateway in the same domain, and in such a case, the SIP relationship between them is called a SIP trunk. This is the broadest definition of SIP trunk, as it says nothing about what the SIP connection is being used for.

 Definition 4: Inter-Domain Connectivity

The final definition of SIP trunk is a SIP association that is set up between different administrative domains, usually pre-provisioned based on a business relationship.

The SIP connection facilitates a specific set of services that link users in one domain with another. In its simplest incarnation, a SIP trunk can provide users in one domain with PSTN connectivity services, offered by the other domain (as is the case in definition 2 above). However, in this definition, SIP trunks can do much more than just PSTN connectivity. One such example is a SIP trunk set up between two different enterprises, providing voice and video calling between users in respective domains.

Another example of a SIP trunk in this definition is a peering service offered from one provider – the peering hub – with other domains that connect to the peering hub. The peering hub receives SIP requests from member domains, and routes them to the appropriate terminating domain. The services offered through the peering arrangement can include voice and video, but could also be used for presence, chat, and other SIP-enabled features.

And so, to make a long story short, Skype Connect (formerly known as Skype for SIP) is a SIP trunk based on definition 4. Today, it provides connectivity to the PSTN, and the ability to receive inbound calls from Skype users to a specific set of SkypeIDs that are linked with the SIP trunk. However, in the future, it will provide presence, chat, voice and video connectivity services between an enterprise and Skype users.

Jonathan Rosenberg is chief technology strategist at Skype (www.skype.com )