How GSM Works – Part 1(Making and Receiving Calls)

How GSM Works – Part 1
What happens when you enter a friend’s number or your hubby’s number and press the green button. And bang! the phone rings and she picks and says “Hello, Hello…Are you there?”
Even though you may not be a Tech Pro like me, it would still be ok for you to have an idea of what happens behind the scenes. One thing I promise you is that the my explanation would be very clear, not technical.
This would even help you appreciate what we tech professionals the the telecoms world have put in place for you to ensure that your calls are seamless. You may also see how easy it is for your communication and other data to be used for purposes you may not have thought of.

I would break down this lesson into 3 short subtitles

  • 1. Initializing a Call
  • 2. Making a Call
  • 3. Receiving a Call


1. Initializing a Call – you switch your phone and it searches and finds a network
This is the stage between when the phone is turned on to when the phone has signal indication and you are ready to dial a number.

When the phone is turned on, it does not have a frequency yet to latch on to so it needs to search for signal. (Sometimes you see the indication – “Searching….” immediately the phone is turned on
(Remember that in transistor radio, you need to manually search for frequencies but in GSM, this is done automatically when the phone is turned on).

So the handset scans the area searching for the nearest mast (BTS – Base Transceiver Station) and then picks the signal broadcast by the antenna mounted in this communication mast. This is because the mast(BTS) continually broadcasts signals to all mobile phones in the area using the BCCH(forget about this! but if you are curious, you can read read it up on GSM Channels). This broadcast is called Paging.

Once the handset finds a frequency, it then sends a message up to the mast the tell it about its presence (like saying “I’m here o!). The mobile phone sends this message using RACH(read up or forget!). The message you mobile phone sends to the mast includes the following:

your phone number
Identification number or IMEI(read up GMS terms explained).
Once the mast receives this message, it then sends an acknowledgement to the phone saying “oh yes, i can see you, you are free to register with me!”

At this point, the phone registers with the BTS and the informs it of its exact location. This is where the network signal now appears to replace the “Searching…” indicator.


After the phone has successfully registered with the BTS, the BTS assigns a dedicated channel to the phone. The dedicated channels(DCCH) includes: SDCCH, ACCH, SACCH or FACCH (read or forget!)

At this point, the phone is ready to make and receive a call

Not that all the process mentioned above takes just a few seconds

Making a Call – You enter a number an press send
This a about what happens when you dial a number and press the Send button
When you dial and sent a number, the phone sends the number across to the BTS

The BTS assigns a specific voice channel and a time slot to the mobile phone that would be used for the call

The BTS sends this number together with your own number, your location and its own ID (that is the BTS ID) across to the BSC, also called RNC(take this as a more powerful mast, though is much more!)

The BSC forwards this details to the MSC (Mobile Switching Center)

Using this information, the MSC does a lookup in the Mobile Databases called the VLR and HLR to get more information about this user. Your data is stored in these databases.( I would discuss this databases in a different article)

The MSC would be able to find the location of the receiving number in this database.

The MSC routes the call request to the receiver’s BTS via the receivers BSC

Receiving a Call – Phone rings and you pick
This has to do with what happens when your phone starts ringing to when you picks and starts talking.
Note that when you are not using your phone, it is not really idle. It is listening to any information that may be sent by a BTS it is connected to.

If there is an incoming call, the BSC, would know the location of the receiver, and send the call request to the cell where the receiver is located

Since this incoming call request is a broadcast, the phone receives it and compares the number with its own number to see if it matches.

If the number matches, then the phone sends an acknowledgement back to the BTS(don’t tell me you don’t know what a BTS is by now!)

Authentication is carried out to ensure that the caller is authorized to make the call

After a successful authentication, the phone starts ringing

When you pick, a connection is established between the caller and the receiver and you can now start talking.

The Steps are summarized in the four steps below.

This is the much we can take here. Am I too technical? Do you want it to be a bit simpler. Let me know in the comments. Thanks though for your time.

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