Communications in Norfolk, Communications in Suffolk, Business Telephone Systems in Norfolk

Standards


There are several 'standards' for communicating with Voice over IP. These can be split into 'open standards′ that are available for anyone to use, and proprietary systems. H.323 and SIP fall into the former category, while Skype uses its own proprietary system.

H.323 is a standard for teleconferencing that was developed by the International Telecommunications Union (ITU). It supports full multimedia audio, video and data transmission between groups of two or more participants, and it is designed to support large networks. H.323 is network-independent: it can be used over networks using transport protocols other than TCP/IP. H.323 is still a very important protocol, but it has fallen out of use for consumer VOIP products due to the fact that it is difficult to make it work through firewalls that are designed to protect computers running many different applications. It is a system best suited to large organizations that possess the technical skills to overcome these problems.

SIP (Session Initiation Protocol) is an Internet Engineering Task Force (IETF) standard signalling protocol for teleconferencing, telephony, presence and event notification and instant messaging. It provides mechanisms for setting up, managing, and tearing down connections, but not for transporting the audio or video data. It is probably now the most widely used protocol for managing Internet telephony. Like all IETF protocols, SIP is defined in a number of RFCs (Request For Comments), principally RFC 3261.

A SIP-based VOIP implementation may send the encoded voice data over the network in a number of ways. Most implementations use Real-time Transport Protocol (RTP), which is defined in RFC 3550. Both SIP and RTP are implemented on UDP, which, as a connectionless protocol, can cause problems with certain types of routers and firewalls. Usable SIP phones therefore also need to use STUN (for Simple Traversal of UDP over NAT), a protocol defined in RFC 3489 that allows a client behind a NAT router to find out its external IP address and the type of NAT device. Thanks to STUN, setting up SIP-based VOIP hardware or software behind a home or small office firewall should be a simple affair, but in practise it can still be troublesome.