SIP VoIP PBX System

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What is a SIP VoIP PBX System apart from an impressive collection of acronyms? We'll try and make sense of it one word at a time.

SIP VoIP PBX System Deconstructed

(1) SIP: stands for Session Initiation Protocol. It's an Internet signaling protocol. Other Internet protocols include: H.232, MGCP and Inter-Asterisk eXchange. SIP operates as a carrier working in conjunction with other protocols including the Session Description Protocol (SDP) and Real Time Transport Protocol (RTP).

See: What is SIP?

SIP has become the standard protocol for VoIP. It was designed to initiate not just voice communication but all types of realtime communication over the Internet including: Instant Messaging (IM), file transfer, application sharing, Presence, white boarding, video conferencing, machine to machine realtime communication and the distribution of alarms. It enables far richer communication than possible via PSTN.

See also: What are SIP Trunks?

SIP is a signaling protocol used to support intelligent end points, whereas the PSTN was intended to support centralized networks with dumb end points. The purpose of SIP is to set up media sessions between clients or end points. It is capable of dynamically opening media ports to permit media to flow during a call and closing them on completion.

SIP has now achieved greater acceptance than it's closest rival H.323 basically because it is a neater and less ambitious package. There are significant similarities between SIP and HTTP syntax.

See also: SIP Softphones - the standards based option

(2) VoIP: relates to a technology that carries voice conversations over IP networks in the same way as data i.e. in packets.

(3) PBX: stands for Private Branch Exchange. Originally the PBX was used to connect telephone extensions within an organization without having to incur the cost of an external phone call. A PBX also permits the sharing of trunk lines between extensions.

Strictly speaking a VoIP PBX doesn't connect extensions. It switches packets of data across networks using the Internet Protocol so that they arrive at an intended destination or end point. These call routing duties can be performed by a circuit based PBX or a soft switch.

VoIP business phone systems have now superseded TDM technology and SIP support has become a standard feature of the VoIP PBX system.

VoIP phone systems that employ the SIP standard offer protection against proprietary lock-ins. While there maybe some additional security in using a proprietary solution, there is invariably a premium payable. On the other hand, SIP compliant components can be mixed and matched, at least in theory. In practice different vendors have interpreted SIP standards differently and there are incompatibilities between brands.

SIP VoIP PBX System and the SIP Trunk

SIP trunking is now extensively used in conjunction with VoIP PBX's to reduce the cost of calls to destinations beyond the reach of corporate LANs. Whenever VoIP business phones or a softphone are used to make calls to off-LAN locations the calls are routed via a local PSTN gateway and converted to traditional TDM telephony. SIP is now widely used as a protocol that can IP-enable a trunk interface and negate the requirement for PSTN gateways.

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