Thursday, February 17, 2011

What happens when I press a calling key on my cell phone?

The phenomena of you hitting your call button  to place a call is known as "Call Flow Process" in technical term. So the better way of asking this question would be What is the Basic call flow in a wireless network?
Now to understand the very basic Call flow you need to have an idea of the network architecture of wireless network which I have explained in my previous post. But for a quick overview, wireless network  consists of BTS, BSC and MSC.
Now take an example of Number A calling a Number B:
The moment Number A places a call  "Call Initiate signal" reaches MSC (Mobile Switching Center) informing it about your intention to call number B.
MSC is a like brain of the telecommunication network which has its nerves connected to different other network entity.
Now MSC checks if Number A is authorize or in other words whether it has a permission to make call or not. If it does it analysis the Number B and finds out where the current location of Number B in the network.
It finds the location of Number B with the help of HLR (Home Location Register).
HLR is like a cerebrum cortex in the brain which stores the information  of all the users who are currently active on the network.
Once MSC gets the location information of Number B it instructs BSC ( Base Station Controller) to sent a signal to Number B intimating it about the incoming call from Number A.
Now BSC is like a Spinal Cord which helps to transfer signals to and fro from MSC.
BSC sends the signal to the correct Number B with the help of BTS ( Base Transceiver station).
BTS is like the endpoint of your nerve from the spinal cord. All the Mobile Station ( i.e nothing but your cell phone) which are active on the network are connected to BTS via Air interface and thats where the name comes from ' Wireless Telecommunication'.
Once BTS send the signal to Number B its cell phone starts ringing. As soon as Number B hits green key to answer the call exactly opposite signal flow takes places from BTS to BSC and then to MSC and finally call gets connected. All this signal exchanges takes place in a fraction of a sec and the time from you placing the call and hearing a ring back tone is usually between 5 to 6 sec.

Have a look interesting stuff:


  1. helllo sir,
    i have a problem.I just wanted to know that how network(MSC) will get to know a perticular number which is preesed in mobile ?. How it differenciate one nummber from other(i.e how it vl come to know whether 4 is pressed or 6 or 7 ). What is technology it apply ?

  2. I'm guessing it is encoded in the request, like how automated phone support systems know you are pressing "1" for more information or pressing "2" to speak to a sales advisor etc...

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  4. Hello sir.. can u giv me plz brief info. info about MSC..

  5. msc will get the info through ss7 technology in that service switching point will do the same..