Hello What is up guys?
It is again Dr. Moazzam Islam Tiwana here.
And in today's class we are going to talk about the Architecture Of GSM system.
Talking about architecture of GSM System. There are two main parts of a GSM System:
First one is known as the Base Station Subsystem or BSS or also know as the access network.
And the other main part is the Network Subsystem or NSS or the Core Network
Now, why Base Station Subsystem (BSS) is called as the Access Network?
BSS is called as the Access Network as Mobile connects to Access Network, to communicate with the GSM Network.
Now, whenever a mobile user (or a mobile subscriber) wants to make a call or avail any of the services of the network....
He needs a SIM to use the mobile.
Now, here it is important to differentiate between two identities.
One is the identity of the mobile station itself,  which is its IMEI (International Mobile Equipment Identity) number.
And the second one is the identity of the SIM, or in other words of the subscriber who is using the mobile.
And this identity i characterized by IMSi (International Mobile Subscriber Identity)
So, when a mobile user wants to use the network, he needs a SIM with a valid IMSI burned into it.
And, then this user can use this SIM in a mobile having a valid IMEI number, to make a call etc.
Now when mobile user wants to connect to a network, or make a call
Then he will use his mobile Station to connect to the Base Station (also known as BTS).
And he makes this connection over the Radio Interface, which is also known as Air Interface.
And it is denoted as Um. So, Radio or Air Interface is denoted as Um
Now, talking about Base Station Subsystem (BSS), you can see that here we have BTSs.
