Wednesday, October 20, 2010

How does a call flow in a GSM network?

The first time I read this off Wikipedia, I was petrified, thinking my career in Telecom has ended before it began. Before 2007, all I knew was a call was passed on from one cellsite to another to reach my handset. Little did I know, there were multiple entities involved in the process.

Lets start by switching your phone on.

When a mobile phone or in telecom parlance mobile station is switched on, it is necessary to read its BCCH(broadcast Control Channel) to determine its orientation within the network.

The mobile has to synchronize in frequency and time with its network. The mobile station will scan around the available frequencies and picks the strongest.

So do you ever wonder how your telecom operator knows where your phone is at all times? Your phone performs a Location Update to search for the strongest frequency and keeps contacting your telecom operator.

Let me just take you through the flow very simply.

Your phone is registered with your HLR (Home location Register). Imagine this as a register that records all customer information. The customers services, plans, products, history . ie if the customer has activated Caller or dialer tunes, his HLR will have it recorded. Even if the customer is suspended/ terminated, it will show.

So when the phones turned on, it first contacts the HLR, the HLR allows the connection and depending on whether you are in the same circle or in a different one, it will transfer your details to the VLR (Visited location Register) you have latched on to.

How does the call flow?

The user will enter the phone number he wants to call and press send/ dial. The mobile phone send s a call setup request to the users mobile network via the nearest BTS(Base transmission station)

The BTS transfers the call to a BS C( base station controller). One BSC handles multiple BTS.

The call setup request message is handled next by the Mobile Switching Center, which checks the subscriber's record held in the Visitor/ Home Location Register to see if the outgoing call is allowed.

If so, the MSC checks the NDC (National destination code) Eg: 919820 for Vodafone Mumbai and sends this request to the B party operators MSC.

The B party MSC will check if incoming calls are allowed to the called number and transfer this request to the BSC this number is currently latched on to. The BSC then transfers the call to the nearest BTS of the called number. Thus completing the call flow.


  1. Hi Kim,
    I would have gone through atleast more than ten demo illustrations of how a mobile call originates. Believe me, no one could have made it look so easier to understand with all the necessary information yet simple and clear to understand.
    Though I wasn't as petrified :-) as you were when I went through all the diagrams, I was really taken aback to see the innumerable abbreviations I had to remember to make a write-up if someone asks me to.
    Yours is too good and I felt it was GOD'S Grace that I cam across your note. Thank you very much!

  2. superb, a little summarized, but excellent

  3. superb, a little summarized, but excellent