Telecom basics

As a recruiter my basic understanding about telecom call flows ,how it functions .

1. Contents Introduction to call flow Base station Controller Mobile Switching Centre Home Location Register

2. Introduction Here we will see how the call happens between a sender and receiver . Imagine calling a friend on the other side of town. As you chat away, your phone converts your voice into an electrical signal, which is then transmitted as radio waves. This radio waves are captured by Base transceiver station ,we can all it as cell tower or cell site . A cell tower houses the electronic communications equipment along with an antenna to support cellular communication in a network. A cell tower is usually an elevated structure with the antenna, transmitters and receivers located at the top. A cell site have 32 channels. Out of which 1 channel is used for signal processing ,another for Synchronization and other 30 channels for traffic i.e 30 people can talk under one cell site All the BTS/Cell sites are connected through a BSC (Base Station Controller) to the network. BSC acts as an interface between network and Moblie Equipment (ME)

3. Basic Diagram Sender BTS BTS Receiver BSC

4. Base Station Receiver (BSC) When we dial a number and press the call button ,in background BTS request for a channel from BSC. Once the user finishes the call ,the channel will be released back. If the user moves from one cell site to another how the connection happens ? ME1 BTS1 BSC BTS1 ME2 ME1 BTS2 BTS2 ME2

5. Mobile Switching Center (MSC) MSC alias Telecom Exchange ,comes into picture when user moves from one cell site to another . MSC is the heart of Telecom network . ME1 BTS1 BSC BTS1 ME2 ME1 ME2 BTS2 BTS2 BSC MSC

6. MSC Continued……………… The MSC is mostly associated with communications switching functions, such as call set-up, release, and routing. However, it also performs a host of other duties, including routing SMS messages, conference calls, fax, and service billing as well as interfacing with other networks, such as the public switched telephone network (PSTN). The MSC plays a significant role in handovers, particularly handovers involving multiple base station controllers - known as inter-BSC or intra-MSC handovers - as well as those involving multiple MSCs, known as inter-MSC handovers. In an inter-BSC handover and upon detecting that a mobile device is approaching the edge of its cell, a BSC requests handover assistance from its MSC. The MSC then scans a list of adjacent cells and their corresponding BSCs and facilitates the handover to the appropriate BSC.

7. Home Location Register(HLR) As mobile phones move, it is important for the MSC to determine each phone’s location to effectively facilitate routing communications between them. For this task, the MSC works with a large database known as the home location register (HLR), which stores relevant location and other information for each mobile phone. Usually there will be one HLR for each operator. Accesing one HLR by multiple MSC takes time . So MSC maintains small subset of HLR .This is called VLR