Interview Questions

Landline Calls

India is having the largest growing telecom market with half of its population served by GSM/CDMA technology.

But still people have land line telephones. Either for home/office purposes. And above all, state run BSNL/MTNL and private firm Airtel provide broadband services over land line through DSL technology.

Before we understand how a wireless system works, lets see how a wire line system works?

Essentially land line phone connection has 2 components. A wired line coming to your house and a junction box in streets where the connection to individual homes are split/connected towards the exchange.

All the way from home to a telephone exchange (a place full of electronic components, designed to provide you facilities of making calls, various facilities, connectivity to the network and other subscribers) a two wired copper wire runs. This is what makes the land line system work.

All voice calls are circuit switched, meaning throughout the duration of the call, a circuit/physical voice path/time slot is available. Upon termination of the call, circuit/voice path/time slot is freed up.

When you lift the telephone instrument, you get a dial tone. Exchange senses this and feeds you the dial tone. You can start dialing digits. Exchange receives the digits and starts processing the digits for deciding where to route the call. Based on the digits dialed, it understands the type of the call being made and selects the circuit accordingly. For eg., if you dial a digit starting with 9xxxx xxxxx it knows that it is a mobile number and selects the trunk/circuits leading to that number.

India currently follows 10 digit numbering plan. (exclude 0 prefix in STD) That means any phone number in India can be identified by stating the city code and phone number.
For eg., 44-xxxx xxxx means a number in Chennai for sure.

So while the exchange selects a circuit, it talks to multiple exchanges and over signaling protocols meant for this purpose. One robust and underlying protocol in modern telecommunication networks is Signaling System No.7(SS7).

Finally the terminating exchange identifies the called party number (dialed digits) and routes the call to the land line subscriber. The telephone rings. Upon answer, both the parties can talk to each other and the circuit is connected. Once either of the subscriber places the receiver back, call is terminated and circuit is released.

Just in case, if the called subscriber is busy, terminating exchange communicates back to the originating exchange through the signaling protocol so no circuit is reserved.
Similar conditions include lines in trunk are busy, called subscriber is not in a position to answer the call (bill not paid, do not disturb, out of service).

This is as simple as it gets. But the technology behind the system is robust, and has undergone generations of technology improvements over few decades of the last century.

Phone call continues...