IP telephony is gaining popularity for sure and soon it will challenge the contemporary telephony system. VoIP (Voice over IP) is cheaper and has sound quality which is improving with each passing day. It is easy to remember anybody’s VoIP user id than his lengthy phone number and when comes, the popularity and demand the competition begins. SIP and Skype are not only two VoIP service providers but are two different technologies altogether. They are serving the same to the users worldwide but the technological approach of both of these players is entirely different.

SIP is Public and Skype is Private. Yes that’s true – SIP is a technology standard developed by Internet Engineering Task Force. It only handles the user’s location on the internet, users authentication, call handling and call forwarding. All the other backend support is given by same old protocols of IETF like RTP, STUN and SDP. SIP is used by various VoIP service providers and it works behind their software interfaces. On the other hand Skype does everything on its own without using any of the protocols defined by IETF. How is done and what technique is used stays a secret because Skype is private company and has not disclosed its technology yet. Skype bundles everything in one single installer and if it is installed completely you would not require any other VoIP supporting protocol for this service to run.

Both SIP and Skype use a central server for authenticating and connecting its users and all the contacts lists, ports and IP addresses are stored on these central servers but with a little difference. Since SIP is used by many service providers so each service provider has its own SIP server and on the other hand Skype has just one central server. Again because SIP is being used by various service providers so there can be same usernames for different people but on Skype username has to be unique across the globe. SIP users can communicate with each other regardless of their service providers but a Skype user can only communicate with another Skype user and nobody else. This also brings in a difference between the call routing scheme of SIP and Skype. SIP agents (service providers) connect with each other for connecting a call but in case of Skype it is only peer to peer connection.

SIP network looks much the old telephony network where each country or region has its own station and they connect together to forward/route a call. On the other hand Skype establishes a call based on peer on peer networking architecture. There is no mediatory agent involved in connecting two Skype users together over a call.

When it comes to the Voice quality and transmission both SIP and Skype uses different technology and methods. When we speak on our microphone the analogue sound is converted into digital data by codecs, then and transmitted to the other person’s computer where it is again made audible by the codecs. SIP’s media transfer is controlled by RTP and the codecs used for coding and decoding are G.711 and G.729, whereas Skype uses iLBC and iSAC codec for the same. Skype sends all its data, be it text or voice in an encrypted format and SIP does not.

Skype is currently the world leader in VoIP service industry but SIP is an open interface and is quite flexible so it is quite possible that other VoIP agents may compete Skype and take away its users in future.