This is Sangram Desai, working as a Developer in mobile domain for last 3/4 years.
Recently I got a chance to work on a project in Windows Mobile in VOIP technology. I went through some documents which have explained this topic but from technical perspective.
So here in this article I tried to explain its overview in more simple terms so that it can be understood by everyone. This is my first article so your comments and sugestions are most welcome. :)
VOIP is a buzzword nowadays & why not. As the rates of the internet and its speed are quickly moving in opposite directions, everyone wants to talk instead of just chat on the internet & that is exactly why VOIP is for.
Technically speaking, VOIP is a protocol that sets the standard about the transmission of voice over the internet. The best examples of VOIP are SKYPE, Yahoo chat and Google chat.
In this article we will understand how VOIP actually works on mobiles and what are the protocols involved in it.
But first a major question, Why the Cellular operators will allow VOIP calls on their network as it will directly hamper their core business i.e. voice communication?
The cellular operators today are reluctant about this VOIP business model and are trying to neglect it. But slowly and silently VOIP will arrive in their business model for sure.
In India due to core neck to neck competition the call rates are low as compared to other countries but this is not the case outside India i.e. in Europian countries/U.S. The call rates are still very high there and mobile VOIP is an answer to change this picture.
Read the following article to get the VOIP rate planin different countries.
About Indian market, I think there is still long way to go but I heard operators like AIRTEL are slowly taking interest in VOIP on their handsets.
Now Let us understand how this VOIP works on actual handsets.
VOIP is basically comprised of two more protocols, SIP and RTP. SIP stands for Session Initiation Protocol and RTP is for Real Time Protocol. SIP is basically responsible for establishing the connection between two handsets and initiation of session between them whereas RTP is the protocol that is responsible for transmission of voice data. So in short whenever a VOIP call is made from handset A to handset B then first SIP comes into picture till a session is established between the handsets & then RTP comes for voice data transmission.
Now Let us understand how SIP works. In between these two handsets there exists a server, we call it as SIP server. it performs the following two functionalities in the initial process.
1) It maintains a list of "registered" handsets.(I will come to the registration process in next point.) &
2)whenever any handset wants to make a call then it "verifies" the request.
The SIP server maintains a list of registered devices. Registration is a process of letting know the server that I am active now and I can send/receive the calls. This is exactly similar with the login process of any messanger like yahoo or google chat. Whenever you login with your ID and password, you become active and then anybody can ping you and viceversa.
So when you open any VOIP application, it asks for your number(same as username) and password. Once verified, you are registered with the server.
There are many free and commercials SIP servers in the market. They are Asterisk, Freeswitch, Icewrap etc..
In the Next article, we will understand what is the role of RTP and how does it transfer the voice packet.