This document discusses mobile communication and call detail records (CDR). It begins by providing background on the increasing use of mobile phones and their advanced capabilities. It then explains the basics of mobile communication, including cellular networks, GSM standards, and full-duplex communication systems. The document delves into technical aspects like network components, addresses/identifiers used, and multiplexing. It defines CDRs and their importance for investigations by providing location data. CDR analysis is illustrated as a tool to trace criminals based on calls made near crime scenes.

1. VIVEK PARIPUDI
Mobile
Communication
& its Relevance
to CDR

2. 2 Mobile Communication & its relevance to CDR
With increasing number of mobile phones, the world is covered with the
network, these mobile phones rely upon. Results say that the ratio of the number of
mobile phones is increasing rapidly with respect to the population of the world. At the
same time, it is also apparent that not only the number of handsets is increasing but
also the technology used behind making of these handsets is also developing at an
enormous rate. Now, mobile phones are no less than a full fledged computer with all
and even more features than the computer could possibly have. A mobile phone can
now make a document, edit a presentation, send emails, download attachments, record
movies, snap pictures, record location details etc. It is this convincing era of mobile
phones that has made them the most popular gadgets of all times.
Even though the technology used in mobile phones are developing and advancing in
an exuberant way, yet the main purpose of a mobile phone lies in its ability to connect
to people by making calls, sending messages etc. After all this is what the mobile
phones are known for and will always be known for.
This paper illustrates the modus operandi of mobile communication and the
advent of a not so known but promising concept of CDR. This paper will solve all the
doubts and questions regarding mobile communication and the useful concept of
CDR.
MOBILE COMMUNICATIONS:
The word “Mobile” when attached to a device makes the device functional
irrespective of the geographical location being changed regularly. Hence, this takes us
to a conclusion that a mobile phone is a device that is used to communicate
incorporating radio waves as links while moving around a geographical area. The
communication is established by connecting to a Cellular Network* of a service
provider. A service provider here is the one who provides the services of
communication through a cellular network. Also known as a mobile phone operator, a
service provider allows the mobile phone, the access to Public Telephone Network**.
Once nicknamed as the brick due to its weight (around 2 pounds), a mobile
phone evolved from being as heavy as a brick to as light as a pebble. The first mobile
telephone calls were made from cars in the year 1946. They were the Bell System’s
Mobile Telephone Service composed of vacuum tubes, relays which carried a weight
of around 80 pounds (36 KG). NTT Japan launched the first commercial automated

3. 3 Mobile Communication & its relevance to CDR
cellular network at Tokyo in the year 1979. It was followed by the subsequent
launches of cellular network systems in countries like Denmark, Norway and Sweden
etc.
GSM: GSM stands for Global System for Mobile Communications is a standard set
developed by the ETSI (The European Telecommunication Standards Institute) in
order to set protocols for digital cellular networks used by the mobile phones.
The developed GSM standards replaced the existing first generation of the
cellular networks which were Analog*** and originally described a circuit switched
network optimized for full duplex voice telephony.
TYPES OF COMMUNICATION SYSTEMS:-
In the previous statements, we witnessed the occurrence of a
term “full duplex”. Now let’s get into the actual concept of the
mentioned term. There are following types or modes of a
Communication System:-
a) Simplex Communication Systems.
b) Half-duplex Communication Systems.
c) Full-duplex Communication Systems.
a) Simplex Communication Systems:- A Simplex Communication
System denotes one way communication. It works on the concept of a sender
and a receiver. Both the sender and the receiver play their own specific roles
i.e., a sender sends and the receiver receives. An example of a Simplex
Communication System is the FM, AM transmissions by a radio station to the
public, pager etc.
b) Half-duplex Communication Systems:- A Half-duplex
Communication System is an advanced version of the Simplex type of
communication in which the terminals play roles of both sender and receiver
but only one direction at a time, i.e., once a terminal begins receiving a
message, it has to wait for the transmitter to stop transmitting, only then the
terminal can begin transmitting message. Antennas incorporated are of types:
trans-receivers that can receive as well as transmit at the same time. Example
for a half-duplex communication system is a walkie-talkie.
c) Full-duplex Communication Systems:- A full-duplex
communication system is the most improved and advanced form of mobile
communication which allows communication in both sides at the same time in

4. 4 Mobile Communication & its relevance to CDR
both directions. Examples of full-duplex communication systems are landline
telephones, mobile phones etc.
Current generation mobile phones are examples of Full-duplex Communication
systems.
MULTIPLEXING:-
It’s a technique in which multiple signals are combined into one signal and
transmitted over a telecommunication channel.
GSM: Network Components:-
Now that we get to know the introduction of GSM and basic concepts of how
does a communication takes place, let’s get to study some of the technical aspects of a
GSM standard mobile phone. Let’s first see how a GSM network is designed.
The structure of a GSM communication standard contains of the following
parts:-
1) The Base Station Subsystem.
2) Network and Switching Subsystem.
3) GPRS core network.
4) Maintenance of the network [done through the OSS (Operations Support System)].
1) Base Station Subsystem:- The Base Station Subsystem contains the base station
antenna along with their specific cell configurations. A GSM communication system
provides the slot for five different sizes of cells. These are macro, micro, pico, femto
and umbrella cells. They have their coverage based on the vicinity. The Base Station
antenna for a macro cell is installed in the mast or on the top of a building above the
average roof top level. For micro cells, they are below the roof top level; used mainly
in urban areas. Picocells are mainly used indoors due to their low coverage diameter
that ranges up to few dozen meters. Femtocells are used for household or office
networks like for a broadband internet connections. Umbrella cells play the role of
covering the gaps between smaller cells.
2) Network and Switching Subsystem: - A network and switching subsystem is the
core network of any GSM communication system. This subsystem deals with the
establishing and switching of connection requests from the base station subsystems.
3) GPRS core network: - GPRS stands for General Packet Radio Service. This is an
enhancement for the GSM communication system and allows packet based internet
connections. It is due to the GPRS, the GSM standard phones can be made internet
enabled. One more service for packet based internet connection is the EDGE service.
EDGE stands for Enhanced Data rates for GSM Evolution.

5. 5 Mobile Communication & its relevance to CDR
4) Maintenance of the network: - Maintenance is done through OSS (Operational
Support System).
GSM: Addresses and Identifiers:-
GSM maintains the segregation between the user and the device by providing them
with different set of unique codes called the addresses and identifiers. The most
important addresses and identifiers are presented as follows:-
i) International Mobile Equipment Identity (IMEI) number.
ii) International Mobile Subscriber Identity (IMSI) number.
iii) Mobile Subscriber ISDN (MSISDN) number.
iv) Mobile Station Roaming (MSRN) number.
v) Local Area Identity (LAI).
vi) Temporary Mobile Subscriber Identity (TMSI) number.
vii) Local Mobile Subscriber Identity (LMSI) number.
viii) Cell Identifier (CI).
i) International Mobile Equipment Identity (IMEI) Number:-
The IMEI number is a unique identifier of a mobile station internationally. It is
allotted by the manufacturer of the equipment and is taken into the process of
registration by the network operator. The database of the IMEI numbers is stored in
the EIR****. A IMEI number is divided into following parts: TAC, SNR and SP.
TAC stands for Type Approval/Allocation Code. A TAC is an eight digit code that is
centrally assigned uniquely for a handset. SNR stands for Serial Number is a six digit
code that is unique for a particular unit of a phone model. SP denotes Spare. So, the
format of an IMEI number is given as:
IMEI = FAC + SNR + SP.
However, before the year 2002, the TAC used to be a six digit code followed
by another code termed as FAC which indicated the location where the particular
phone was manufactured.
ii) International Mobile Subscriber Identity:-
The IMSI or the International Mobile Subscriber Identity is a unique
identification associated with the Subscriber Identity Module (SIM) cards. A mobile
station can be operated perfectly only if a SIM with a valid IMSI is inserted in the
mobile station with a valid IMEI. The usual length is 15 digit but can vary from
service providers to service providers. IMSI consists of the following subs:
a) MCC (Mobile Country Code).
b) MNC (Mobile Network Code).
c) MSIN (Mobile Subscriber Identification Number).

6. 6 Mobile Communication & its relevance to CDR
iii) Mobile Subscriber ISDN Number:- ISDN stands for Integrated Services Digital
Network. It defines standards for communication over voice, video, data or other
network services. The MSISDN is a unique identification of a subscription of a SIM
in a GSM network. In simple words the telephone number associated with a SIM is
nothing but its MSISDN.
The combination of IMEI, IMSI and the MSISDN is useful to identify or track a
particular mobile station or its subscriber.
iv) Location Area Identity:- The Location Area Identity of a PLMN (Public Land
Mobile Network) is the unique identifier of a specific location in the network. It
contains the Country Code (CC), the Mobile Network Code (MNC) and the Location
Area Code (LAC).
v) Temporary Mobile Subscriber Identity:- The VLR (Visitor Location Register) that
stores the current location of the subscriber assigns a TMSI which has its significance
only in the area allotted to the VLR. A TMSI allows the unique identification of a
subscriber.
vi) Local Mobile Subscriber Identity:- It is an additional searching key assigned to
each mobile station within its area to accelerate database access.
vii) Cell Identifier:- The individual cells in a local area are identified by their Cell
Identifiers. It’s a unique number of a GSM cell for a given operator. By knowing this
number, the cell can be known and the position of the cell will give the complete
location based information of a cell phone.
TRIANGULATION:
There are certain device in the market (mobile phones in this case) that can
derive their location based on the mobile networks they come across. Although the
location is not as accurate as that of a GPS enabled phone but the approximate
location of the device can be obtained. This is done when phones apply the concept of
Triangulation. Triangulation basically is a process of determining the location of a
point by measuring incoming angles to it from known points. In mobile
communications, the fixed point is generally the network tower that is fixed at a
specific point in order to provide services to the subscribers in that particular area. By
using triangulation, a network tower can determine the approximate location of where
the subscriber is in its coverage area. The concept of triangulation becomes very
crucial when tracing a moving criminal because the approximate location of the
criminal can be knows due to the mobile station or phone he uses.

7. 7 Mobile Communication & its relevance to CDR
BLACKLISTING:
It gets into help when the mobile equipment gets stolen or is misplaced. The
owner can typically contact the local operator and get the SIM or MSISDN blocked.
This in turn will blacklist the IMEI by entering the device’s IMEI number in to the
Equipment Identity Register. This enables the device to become unusable making
theft of mobile equipment a useless business.
CDR: Concept and importance:-
Mobile Network:- Mobile networks uses a unique Network Switching
Subsystem(NSS) that switches the calls and services of mobile phones when moving
from one base station to another. These base stations also denoted as network tower of
cells give their unique identification known as Cell ID to every mobile station or
mobile phone they serve in their network. By extracting the CDR on the basis if the
Cell ID’s, one can get the database of MSISDN and IMEI numbers of all the
subscribers who are present in the coverage area of that particular network tower or
base station or cell.
Telephone Exchange:- A Telephone Exchange is a system of electronic
settings and components that maintains the call connections as well as their logs.
Whenever a call needs to be connected, it goes through the telephone exchange where
it is routed to the path that will lead it to its destination. A telephone exchange
maintains the logs of all the call that materialized through it. This log which is in the
form of a spread sheet gives all the information related to from where the call is made,
what is the cell id of the terminal the call is materialized from, to whom the call is
made, the IMEI numbers of both the calling and called devices and their unique IMSI
numbers as well. Using these information, the location from where the call was made
can be known. This becomes very useful in the case of tracing criminal activities as
well as stolen or lost mobile phones. The information obtained from the telephone
exchange is termed as CDR.
CDR: CDR stands for Call Data Records. These call data records comprise of
information related to the following heads:
a) The Calling party telephone number.
b) The Called party telephone number.
c) Call date.
d) Call time.
e) Call duration.
f) Cell ID of both parties.
g) IMEI of the calling party.
h) IMSI of the calling party.

8. 8 Mobile Communication & its relevance to CDR
The above validates itself as the checklist of an ideal CDR. However
sometimes, depending upon the intensity of the scenario, special formats of CDR’s
can be requested from the exchange that can contain the IMEI and IMSI of the called
party as well. Using these information, the investigator can actually trace the
approximate location of where the call has been made from and where was it
received. The investigation becomes very easy for investigators and police officials
when only the analysis of the CDR can enable their hands reach the culprit. The
following scenario will try to make the concept of CDR clear:
 Suppose a dead body is lying in the middle of a junction of a city. There are no
traces of any device, any weapon whatsoever found near the dead body. There
dead body’s face is smashed badly and it is completely naked. The only clue you
could find are the marks of tires of a vehicle. To make matters worse, it started
raining cats and dogs. Hence there is no clue left whatsoever. Now, you are from
the investigation team and you are given the task to investigate in to the matter
and come up with a result. How would you investigate? What could possibly be
the clue for you to investigate? On what grounds will you be able to track the
criminal?
The answer is CDR. As mentioned in the beginning, a dead body was
found lying in the middle of a junction with no devices, no clothes, smashed face
etc. Now, since the dead body is lying in the middle of the junction, it is an
obvious fact that more than two roads will lead to the crime spot. The roads are
covered with mobile towers which gives services to the customers around
themselves. As an investigator, you only need to request the telephone exchange
to provide you with the Call Data Records (CDR) of all the providers who have
their towers in the roads leading to the crime spot. The requested CDR will
contain information based on all the above mentioned heads.
Now, all you need to do is to find out unique sets of those numbers whose
services covered the road towers and ended at the crime spot. As a normal human
tendency, every criminal after committing a crime will always try to wipe out the
traces or evidences. In this due course of him wiping out his evidence, he will also
destroy the MSISDN number or the SIM card he was using to communicate with
his counter parts. This is the mistake done by him. As mentioned earlier, the IMSI
number of the SIM is registered along with the IMEI number of the phone with
the service provider, and once the SIM is destroyed or removed, the entire
information gets updated with the IMSI number of the new SIM. Here, the
conclusions can be made that if there are two different IMSI numbers but one
common IMEI number for both of them, that means that the person has changed
the MSISDN. The CDR will give the complete information based on them. Hence,
the culprit is caught.

9. 9 Mobile Communication & its relevance to CDR
 Another scenario will make the concept clearer:
A jewelers shop is robbed at around 3AM in the morning. The close circuit
TV cameras which are the only clue clearly show that a man breaks the shutter
of the shop, enters inside, opens his bag, transfers all the ornaments into his
bag and elopes.
Next morning, the police come to know about the case when the owner of the
shop lodges a complaint saying that his shop has been robbed. The police
arrive at the crime scene along with investigators to investigate. The first thing
they ask for is the footage captured by the closed circuit TV camera. Upon
witnessing the CCTV footage, the investigator finds out that the thief received
a call in his phone. The call time and duration can be easily retrieved from the
information provided by the CCTV footage. It was hence brought to a
conclusion that the call was made at around three in the morning and that it
was made for duration of 15 seconds. The next step taken by the investigator
was to request for the CDR of that particular time interval of that particular
Cell ID (Unique code of a network tower). Upon receiving the CDR, the only
thing the investigator needs to do is to come up with unique MSISDN
numbers that received a call at around 3AM in the morning for 15 seconds.
Since the time is very odd for anyone to make a call, there was only one
MSISDN and that was of the criminal’s. Hence he was traced and was put
before the court to face trial. Even in this case, the CDR played the prime role
in tracing out the culprit.
Hence, CDR can be an extremely useful tool to trace out the criminals.
*Cellular Network: A Cellular network is a radio network distributed over land areas
called cells, each served by a transreciever that can both transmit as well as receive
the mobile signals.
** Public Telephone Network: It is a network of world’s public networks. It consists
of telephone wires, fiber optic cables, microwave transmission links etc connected to
the switching centres responsible for making calls and flow of data packets.
***Analog: A signal that contains information using non quantized variances in
frequency and amplitude.
****EIR: EIR stands for Equipment Identity Register. It is basically a database which
contains the IMEI number of blacklisted handsets.

10. 10 Mobile Communication & its relevance to CDR
References:-
http://www.wikipedia.org/ - Wikipedia.
https://www.google.co.in/- Google.
http://www.tutorialspoint.com- GSM Addressing
http://www.cisco.com- CISCO
http://www.techopedia.com- Techopedia