How a
mobile phone
call is made
By Matthew Weller
When you dial and call a number your mobile phone sends out a signal to a
Base Station Transceiver (those huge poles with a small satellite on the top of
them) using microwaves
Mobile Microwaves Signals being sent/recieved
BST
The Base Station Transceiver (BST) then relays this signal to the Mobile
Switching Centre (MSC), which controls all of the calls active on the mobile
network.
MSC
BST
Signals being sent/recieved
MSC
Making the call to a landline
The MSC then checks if the mobile phone has enough credit and permission to
make the call, if it finds that the phone can make the call it transfers it along
the PSTN (Public Switching Telephone Network).
MSC
MSC
PSTN
Signals being sent/received
PSTN
The PSTN then allocates a unique frequency to the MSC, and connects the
call to the desired landline phone
PSTN
PSTN
Signals being sent/received
Landline
So what happens next?
When you make a call to another mobile, your mobile sends a microwave
signal to the BST, which then sends a signal to the MSC. The MSC then finds
the BST the other mobile is connected to using its active database.
MSC
MSC
Signals being sent/recieved
BST
The BST the other mobile is connected then receives the signal from the MSC,
and then sends the signal to the other mobile phone using microwaves (its
basically a reverse process of the first step really), and then your mobile
receives the call.
BST
Signals being sent/recieved Microwaves
Mobile
Making a call to another mobile
The MSC does some super awesome maths, as it can determine which BST
your mobile phone should be connected to based on signal. So if your on the
move the MSC locates your mobile by its active database, and if it starts to
lose signal from one BST, and gains signal from another BST, the MSC tells
your mobile to switch to the strongest BST signal.
Moving from A to B causes
the MSC to tell the phone
to switch BST’s based on
the strongest signal
Moving from B to C causes the MSC
to tell the phone to switch BST’s
based on the strongest signal
B
C
A
But what if I travel while I call?
MSC