VoIP SIP Communication

Session Initiation Protocol (SIP) is a signalling protocol used for establishing communication sessions within an IP based network (VoIP). A session could be just as simple as a two-way phone call or it could be a more advanced multi-media based conference session. The ability to establish these sessions allows additional innovative services become possible, such as voice-enriched e-commerce, web page click-to-dial, Instant Messaging with contact lists, and IP type centrex solutions.

In recent years Voice over IP innovators (VoIP) have adopted SIP as its preferred protocol of choice for data signalling. SIP communication complies to RFC standards (RFC 3261), established by the Internet Engineering Task Force (IETF), the body responsible for administering and developing the mechanisms that operate over the Internet. SIP is still an evolving technology which is being extended further as it starts to mature.

SIP protocol is a request-response based protocol that closely resembles two other better know internet protocols, HTTP and SMTP (the protocols that power the world wide web and email); consequently, SIP sits comfortably alongside Internet based applications. Using the protocol, SIP VoIP services become another web type application and integrates easily into other internet services. SIP protocol can be viewed as a simple toolkit that service providers can use to build converged voice and multimedia based services.

The IETF work to straightforward guidelines: specify only what you need to specify. SIP protocol is very much of this mold; having been developed purely as a way to establish sessions, it does not know about the details of a session itself, it just initiates, terminates and modifies sessions. This simplicity means that SIP communication scales, it is extensible, and it sits very comfortably in different architectures and deployment scenarios.

When providing SIP phone services over VoIP there is a need for a number of different standards and protocols to be joined together - specifically to ensure transport (RTP), to authenticate users (RADIUS, DIAMETER), to provide directories (typically known as LDAP), to be able to guarantee voice quality (QoS, RSVP, YESSIR) and to interconect with today's "traditional" based telephone network (PSTN).

SIP Session Initiation Protocol (SIP protocol background)

VoIP Explained

Put simply VoIP (Voice over IP) or IP telephony is the transmission of a telephone call over IP (computer) based networks instead of traditional telephone networks or PSTN.

The Internet Protocol (IP) was originally designed for data networking, however due to the success of IP becoming a world standard for data networking it has led to it being adapted to carry telephone calls (Voice over IP or VoIP).

VoIP Definition:

Voice over IP, or VoIP, is the technology used to transmit voice conversations over an internal or external data network using IP packets (digital form); without loss in functionality, reliability or quality; and in compliance with the International Telecommunications Union specifications. The term is also used to refer to the hardware and software used to carry such calls over the network.

How is a Voice over IP call made?

A voice signal from a Voice over IP phone (or an older phone connected through a suitable VoIP adapter) is passed through a VoIP device that converts the regular telephone voice signal to a digital one so it can use a broadband internet connection where it travels to the destination equipment. The digital signal is then converted back to the original voice call.

In other words, when the originator calls a number the Voice over IP (VoIP) phone logs on to the routing server - which looks up the destination IP number that's associated with the dialled phone number - and it makes the connection. If the destination number isn't using VoIP, and doesn't have the phone number tied in with an IP number, then it is recognised that the destination number is a Public Switched Telephone Network (PSTN) phone and the call is routed through the PSTN.

SIP Protocol