The Session Initiation Protocol (SIP) is the standards-based way in which telephone connections are made using VoIP. Ringing, connecting and managing the call are controlled by SIP. Think of SIP as the IP version of the old Lily Tomlin character, Ernestine the telephone operator.
This update on the progress that the protocol is making, from Verge1 Consulting Principal Dave Michels, is interesting. Michels makes the very important point that the main advantage of SIP is that it cuts costs. The tension is that there are non-standard-i.e., proprietary-ways of fulfilling the tasks that SIP tackles. The picture is even a bit more muddled, since SIP is a loose and general enough protocol that even equipment that follows it may not work together without extra customization -- which defeats the purpose of having an interoperable standard.
Michels says that SIP is used both in trunking and for end points. The challenges to SIP adoption at the end points -- phones and phone-like devices -- is greater, he says, because it is necessary to support all the features to which end users are accustomed. He says that big PBX vendors-Cisco, Avaya, Mitel and ShoreTel-generally offer only limited SIP functionality.
This post from 3CX offers insight into SIP in a couple of ways. The first part of the link describes the variety of device types that use SIP. There are SIP-based soft phones, which use a computer's speakers and microphone. A USB phone looks like a normal phone device, but plugs into and utilizes the functionality of a computer. A hardware SIP phone, which can look like a traditional phone, connects directly to the data network without the need to piggyback on the computer. Finally, an old-fashioneded (analog) phone can go through an analog terminal adapter (ATA) and hook to a data network. The second half of the post offers about 40 questions (with links to the answers) about IP PBXes, SIP and VoIP.
Naturally, much of the work around SIP is highly complex. The bottom line, however, is that this heavy lifting is being done. Today, for instance, Acme Packet said that it has added capabilities for its Net-Net Session Router, which is a session routing proxy. The new features, the press release says, are designed to increase efficiencies while carriers switch from legacy approaches.
Also today, snom Technology AG and CudaTel, a subsidiary of content security firm Barracuda Networks, agreed to provide end-to-end SIP-based VoIP communications to the small and medium-sized business (SMB) segment. The arrangement will include the CudaTel Communications Server and snom's Linux-based SIP VoIP phones.
SIP is an important candidate to provide the necessary signaling element for VoIP networks. While it has a high profile, Michels points out that it still is unclear if it or another approach-or approaches-eventually will predominate.