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What is SIP (Session-Initiation-Protocol) and how does it work?

Are you in telecommunications management? Would you like to restructure your company’s PBX (Private Branch Exchange) or invest in a SIP-telephone system? Perhaps you are looking for knowledge on Internet telephony for other reasons.

In any case you will certainly encounter technical terms – the most prominent being VoIP and SIP. Sipwise will gladly explain!

What is VoIP?

To comprehend the architecture and purpose of SIP, we need to address the very foundation first – VoIP-Telephony.

In the past, organizations and households used analog or ISDN (Integrated Services Digital Network) connections, employing different means of setting up a call and connecting to the PSTN (Public Switched Telephone Network).

Today, All-IP-Telephony is common, ISDN is quickly becoming a legacy technology. Communicating over an IP-Network however, that’s when VoIP (Voice-over-Internet-Protocol) comes into play. The Internet transmits data packets, hence for voice transmission to work at all with Internet telephony, we require this protocol to execute the data transfer.

To initiate a call over the Internet, necessary hardware (e.g. IP Telephone) or software (“Softphone” , i.e. a program that enables telephony)-appliances need to be available, then VoIP uses a broadband Internet connection, computer networks or the VoIP provider’s servers to convert and transmit voice data. For connecting devices within an IP-Network, SIP is required.

What about SIP?

SIP is an abbreviation for “Session Initiation Protocol”. As the name suggests, it is a protocol for establishing, terminating and controlling communication connections between two or more participants – most likely in an IP-Network. To put it differently, SIP serves the same purpose like a handshake to greet people before a conversation
commences and ends.

In short, this means that the connection request – the dialing, so to speak – is done. Responsible for this is a SIP trunk, which acts as kind of a central interface. You will learn more on SIP Trunks (or SIP Trunking) in our next article.

How does SIP work?

To maintain our human example, the tasks the SIP has to carry out are similar to those of a telephone operator at a switchboard – way back in the early days of telephony. His or her main responsibility was to connect or disconnect a call between two participants and nothing else. The operator did not control any other aspects of the conversation.

SIP controls the establishment of call connections, it is not responsible for the actual voice transmission. Has a connection been initiated, communication takes place directly between the two participants. Other protocols in the SIP environment are employed to determine the modalities of communication between the participants. For instance – Details of the connections are managed by the Session Description Protocol (SDP).

SDP can be used to negotiate which codes or network addresses are to be used – among other tasks. The actual transmission of data is carried out via transport protocols such as RTP (Real-Time Transport Protocol). Due to strict separation between connection establishment and modality adjustment, a large degree of flexibility is achieved.

Why should I use SIP-Telephony?

SIP’s technological importance for real-time communication is comparable to „HTTP“ for the web. It is the main force in rendering the use of IP telephony not only an alternative to the use of a traditional telephone system, but a new key technology in its own right. Classic, hardware-based telephone systems are thus largely obsolete today.

SIP telephone systems enable features that increase mobility and productivity of its users and generate enormous cost advantages at the enterprise level.

About Sipwise

Sipwise’s unified communication platforms are targeted at fixed, converged and wireless service providers supporting a variety of access technologies like mobile radio, Cable, xDSL, FTTx, WiFi and WiMAX.

With years of expertise in VoIP and UC solutions, Sipwise works with clients all over the world and is supporting over 100 commercial deployments by MNOs, MVNOs, Telcos, cable and network providers, utilities and “Over-the-Top”- operators.

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