Unified Communications for the Enterprise

One of the frequent questions that we get from our customers is "I have heard about SIP, but what is a SIP Trunk?" Let's look into this interesting subject.

With traditional carrier-based telecom services, a business would generally order a trunk circuit from the local exchange provider, such as a T-1 or ISDN PRI (Primary Rate Interface) circuit, as an alternative to ordering multiple single lines. A PRI circuit, the most commonly deployed voice circuit for IP-PBX systems, can handle 23 channels (or conversations) simultaneously, and is less costly than 23 individual lines due to economies of scale. The 23 individual channels in the trunk are delineated by a process which is called Time Division Multiplexing, or TDM.

Now let's look at the SIP part.

You know that SIP represents the Session Initiation Protocol, a signaling protocol developed to establish telephone calls over an Internet Protocol-based network. SIP enables telephone and multimedia calls (or sessions) to be established (or initiated), and is frequently found in premises switching systems, known as IP-PBXs. However, SIP is a packet switching protocol, not a circuit switching protocol, like TDM, so it is has some fundamentally different technical characteristics. In brief, if you use a SIP-based switching system (such as an IP-PBX) with a TDM trunk circuit to gain access to the outside world, you must provide a protocol conversion from SIP to/from TDM. This conversion introduces signal distortions, delays, and other undesirable results, not to mention the cost factor associated with having to buy trunk capacity in chunks of 23.

But there is an easy solution to this challenge. With SIP Trunking (packet-switched) instead of TDM service (circuit-switched), no signal format conversion is required, yielding a simpler system and superior voice quality. SIP Trunking provides a more efficient way to both place and receive telephone service from a carrier.

So there you have it, a definition for a SIP Trunk, and just one of the many services that Smoothstone provides our customers.

Some of the many additional benefits of SIP Trunking will be discussed in future posts.

John Wesselman