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Introduction to mobile telephony
Some cellular telephony concepts
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Guintech Informatique
Tana Guindeba
Jr Engineer
May 2014

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Mobile Telephony
Definition:
GSM: Global System for Mobile Communication, is a standard
developed in the 80s in order to define the parameters for a digital
cellular communication network. The GSM standard is universally
used for wireless communication networks. In Canada, United
States and in Japan, there is also the CDMA (Code division multiple
access) standard .
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Mobile Telephony
Origin:
Since the early 70s, Bell Laboratories in the USA did have the
idea of developing a cell structure based mobile radio system.
But, the marketing of cellular phone networks has been
materialized later in the 80s.
Before that time, there was only theoretical studies about mobile
telephony.
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Why to use a cell phone?
For subscribers:
Mobility: Used anywhere where there is authorized signal
Practical : In contact with everyone and at all times
 Useful during emergencies (eg. on road)
Secondary features: music, photos, videos, games,..
Possibility of internet, info, news, applications,… etc.
Professional use (email, calendar, ...). ….etc.
For the operator:
No need to dig the groung to install wiring.
Less laborious maintenance
Increased number of customers, VS a single line by family
Sale of value-added services (more $$$), ….etc.
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Some terminologies
SIM: Subscriber Identity Module, contains information on the
Subscriber, also datas and applications.
MS: mobile station, Equipment used by the subscriber to access
the services offered by the operator (mobile + SIM).
IMEI: International Mobile Equipment Identity, single code
identifying the mobile, verified at every utilization. Also detect
lost or stolen phon. Can be obtained by digiting *#06#
IMSI: International Mobile Subscriber Identity, unique number
identifying a GSM subscriber. (usually 15 digits, associated
with mobile phone users)
BTS: Base Transceiver Station, communicate with mobiles, form
the cells (we can see them on the towers, mountains,…)
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Some terminologies
BSC: Base Station Controller, connects several BTS, manages
handoff between BTS.
MSC: Mobile Switching Center, control center, actions
coordination, many functions run here.
EIR: Equipment Identity Register, identity of mobile device ( list
of authorized IMEI)
HLR: Home Location Register, information about subscribers
(position, status, services,..).
VLR: Visitor Location Register, concern a region, current area.
HLR and VLR communicate for update.
AUC: Authentication Centre, manage security policies, calls and
network protection
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GSM network architecture
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Here is what a
basic mobile
network looks
like. The
GMSC routes
calls coming
from or going
outside of the
network
Image Source :
www.efort.com

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Mobile network operator
Company that delivers mobile services and other telephony
services, such as internet service. The user accesses the cellular
network of the operator through a SIM card inserted into a mobile
device (like phone, tablet or pager, ...).
There are mainly two categories of mobile operators: Those who
have their own infrastructure, called MNO (Mobile Network
Operators).
And those who rent and sell portions of networks from MNO,
under their own brand. They are called MVNO (mobile virtual
network Operators)
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Mobile network operator
In Canada, as MNO we have: Bell, Rogers, Telus, ...
As MVNO in Quebec we have, for example:
•Virgin Mobile, Solo Mobile : using the Bell network
•Fido, ChatR : using the network of Rogers
•Koodo, Public Mobile : using the network of Telus
The usable frequency band to becom an MNO is limited.
I think that in Canada the whole spectrum is allocated.
Getting a License is necessary in order to be mobile operator.
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Telephony cells
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The area covered by a cellular system (signal available) is
divided into cells. Each cell contains one or more transceivers
(BTS) for forming a frequency band. there are several types of
cells, depending on the size of the area, the number of
subscribers, the presence of obstacles, the power of BTS,…
1. Femtocells: (some meters)
2. Picocells: (dozen meters)
3. Microcells: (urban areas,
short antennae)
4. Macrocells: (urban areas,
long antennae)
5. Megacells: Satellites
(hundreds kms)

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Mobile: Start up
At the mobile phone startup up, the following operations are
performed:
1. Validation of the SIM card through the secret code (PIN) if
this feature is enabled
2. The GSM receiver scans the channels of the GSM band and
measures the level of the received signal on each channel
3. The phone check the best signal.
4. Mobile retrieves information about the cell and the network.
It transmits identification information to the BTS to update
the location. (VLR / HLR)
5. Startup is completed, the mobile stay in idle mode and
performs some routines.
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Mobile: Routines
1. Frequently read the PCH (paging channel) that indicates a
possible incoming call.
2. Read the signaling channels of neighboring cells.
3. Measures the level of the distribution channels (BCH) of
the neighboring cells to eventually start a handover.
That’s means, even in idle mode, the mobile performs
continuously certain tasks. Some operations consume the
battery, such as signal measurements and the Handover.
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Mobile: receiving Call
When someone dials a mobile subscriber’s number:
1. The call is routed to the nearest MSC which will research
the IMSI in the HLR and the location of the mobile in the
VLR.
2. The nearest MSC broadcast into the location area, a
message for the called mobile.
3. The called mobile is identified by the SIM card
4. Mobile receives the ring control from the BTS
5. The subscriber picks up the phon and communication is
established.
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Mobile: Making call
The mobile sends a call request to the MSC through the BS and
BSC. Let’s see how:
1. The subscriber dials a phone number
2. The request comes to the BTS that cover the current cell
3. Passes through the BSC to reach MSC (network switch)
4. User identification and its rights control are done at this level
5. The call is forwarded to the public network (if the destination
is on a different network)
6. Other routes (MSC - BSC - BTS) to destination, depending on
the distance of the destination cell.
7. Ring and pikup from receiver side, then establishment of the
communication.
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Mobile: In move
When moving, it may happen that we change cell
(handover). That requires reaction from the network devices, in
order to maintain ongoing communication or network connection.
Depending on the magnitude of the move, the BTS, the BSC or
even the MSC may be involved.
Note the possibility of partial or total loss of communication
during a move according to the available channels and the
technique used to change the BTS.
Changing from one MSC to another is an inter – systems transfer.
In that case, we have the so called "long distance" that I
denounced near the CRTC.
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Mobile: In move
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In inter – systems transfer, we may also need to change
network (operator). In this case, we have a roaming. For
example, when we are in areas not covered by our operator,
such as abroad (for eg: A Canadian in visit in Liberia)
Some countries are more advanced than others in managing
roaming (in terms of customer benefit), such as :
•In Mali, a subscriber of some operator can receive calls being
in Senegal, Guinea Bissau, Ivory coast, Niger, .... Without any
extra cost.
•In Italy a subscriber of an operator who travels to Germany,
Ireland, France Great Britain, Spain,… will not be charged for
roaming.

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Equipment: BTS
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wikimedia.org
gsmweb.cz
Some operators pay fees to occupants
of buildings that bear their BTS.
itiltd-india.com

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Equipment: BSC
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privateline.com
telcomstar.com
Images of BSC

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Equipment: MSC
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Most of the network functions run in
the MSC. Such as value added services.
An MSC with its support.
wordpress.com

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Conclusion
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Here ends this trip about mobile telephony concept. It is more
technical than that, but I try to make it easy to understand for the
public. Things we could note:
•At a given moment, we are in an area (cell) served at least by a
transceiver (BTS).
•At startup, the mobile looks for a good and accessible signal,
then authenticate in order to update information in HLR or VLR.
•In motion, a subscriber may change BTS, BSC and even MSC.
This can cause a communication failure, loss of some words or
more work for the network.
•The mobile and the BTS are linked by radio waves.
•Data (SMS, MMS, ..) are routed the same way as voice. Both of
them are encoded at the source and decoded at the destination.

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Conclusion
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•The equipments are provided with micro programs (software).
•They are supported and controlled by servers running operating
systems such as Linux.
•The operators were abusing making us pay additional fees for
national "long distance“ call.
•It is better not to use a stolen and declared mobile phone
•The authorities can fin out the position of the user of a mobile,
and as we know, they can intercept communication.
•New technological concepts such as the use of internet protocol
(IP) in mobile telephony are not covered in this presentation.

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Thank you!
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Guintech Informatique Writing of dogon people, Mali

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