The document describes the key components involved in a mobile phone call: 1) Base Transceiver Stations (BTS) receive radio signals from phones and pass them to the Base Station Controller (BSC). The BSC manages radio resources and handles handovers between BTS. 2) The Mobile Switching Center (MSC) is the "heart" of the network and handles routing calls between mobile phones, including during handovers between BSCs. 3) The Home Location Register (HLR) is a central database that stores location and profile data for each mobile phone to enable call routing and handovers as phones move between MSCs.

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
BTS
1
BSC BTS
1
ME2 ME1
BTS
2
BTS
2
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
BTS
1
BSC BTS
1
ME2 ME1 ME2
BTS
2
BTS
2
BSCMSC

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