1. BHARTI AIRTEL Ltd.
INTERNSHIP REPORT
2015
SIDDHANT BAJPAI
SRM UNIVERSITY, ECE-3rd yr
11-Dec-15

2. 2 | P a g e
INDEX
1.) Introduction ………….3
2.) Acknowledgement …………..4
3.) Chapter-1
1.) Optical Fiber Communication …………..5
2.) Cellular Generations …………..6
3.) Global System For Mobiles …………..7
a. GSM PLMN Organization …………..7
b. Mobile Station …………..8
c. Base Transceiver Station …………..9
d. Base Station Controller …………..10
e. Mobile switching Center …………..11
4.) Chapter-2
1.) OSI Model …………..14
2.) SS7 Layers ………….16
5.) Chapter-3
1.) Call Flow in GSM ………….19
6.) Conclusion …………. 23

3. 3 | P a g e
INTRODUCTION
The role of telecom has evolved from being voice service to data and now with IoT.
Initially the main focus of cellular technologies was to provide voice services. Then with the technology
evolution data services are also incorporated.
Mobile technologies have evolved from voice tech. to data tech, with speeds reaching up to 170 Mbps.
The cellular technology starts with 1G (Analog) which provides very less data rate & no Roaming services also.
Then came the new cellular technology, 2G (Digital) services which enhanced with better data rates and voice
services. In 2G various cellular technologies were developed- GSM, CDMA, GPRS & EDGE. 2G does support
only CS (Circuited Switched) technique. GPRS & EDGE are 2.5G & 2.75G & support both PS (Packet
switched) & CS.
With further needs & improvement 3G technology started, which came with much better data rates up to 2Mbps
& voice services also. Various technologies were developed in 3G, WCDMA (UMTS) & HSDPA. It supports
CS technique.
Now it’s the age of 4G (Digital) in which up to 170Mbps data rate can be achieved. LTE technology was
introduced in same & 4G supports only PS (Packet switched) technique for all voice services also. It’s only IP
based.
Various entities function in sync to provide mobile communication. Mobile communication architecture involves
BSS, NSS, and OSS.
Authorities which govern the telecom needs in India- TRAI, DoT. They frame regulations pertaining to mobile
communication services and ensure that standards set by ITU are followed.

4. 4 | P a g e
ACKNOWLEDGEMENT
I take this opportunity to express my profound gratitude and deep regard to my guide S.S. Saravanan,
DGM-Design & Planning, AP & Telengana Circle for his exemplary guidance, monitoring and constant
encouragement throughout the course of this project. The blessing, help and guidance given by him
time to time shall carry me a long way in the journey of life on which I am about to embark.
I also take this opportunity to express a deep sense of gratitude to Kaustubh Singh Ranjan & Saumya
Khanna, Sr. Executive, NSG Team Airtel, for their cordial support, valuable information and guidance,
which helped me in completing various tasks throughout my Internship.
I am obliged to NSG Team of Bharti Airtel Ltd., A.P. for the valuable information provided by them in
their respective fields. I am grateful for their cooperation during the period of my Internship.

5. 5 | P a g e
Chapter-1
OPTICAL FIBER COMMUNICATION:
 Transmitting information from one place to another by sending
pulses of light through an Optical Fiber using Light Intensity
Modulation.
 It uses Laser as the light source.
 Advantages:
o Less Attenuation(0.2dB/km)
o Very high i/f carrying capacity.
o Greater safety & immune to EMI &RFI, moisture & corrosion.
 An Optical fiber consists of core, cladding & buffer (protective outer
coating)
 The cladding guides the light along the core by using the method of TIR
(Total Internal Reflection).
 TIR-For all angles of incidence greater than the critical angle, the incident
ray get reflected back into the denser medium itself.
 Fiber Optics with glass fiber, it uses light wavelengths around 850,1310, 1550 nm(IR).
 Transmission Losses or Attenuation of the signal in an optical fiber is measured in db/km.
o Losses: Material Absorption, Rayleigh scattering, radiative losses.
 Basic elements to determine transmission system performance:
o Fiber loss factor
o Type of fiber
o Transmitter
o Receiver Sensitivity
o No. of splice
o Types of splice
o Margin: Min. sensitivity of signal, Receiver can intake.
 Fiber Optic Loss Budget Calculation:
o Fiber Loss:14.5km*0.35dB=5.075dBm
o Fusion splice loss:0.2dB
o Terminating connectors loss:5*1dB=5dB
o Margin:5dB
o Total Loss:5.075+0.2+5+5=15.275dB
o Transmit Power:-3dBm
o Budget Loss(Rx level):-3-15.275=-18.275dBm=total loss+transmit power
o Receiver sensitivity: -20dBm

6. 6 | P a g e
Cellular Generations:
1G  2G  2.5G+/2.75G  3G  3.5G  4G
1G:
 Analogue Modulation
 Voice Traffic only
 Roaming services were rare across PLMN’s.(Public Land Mobile Network).
2G:
 GSM, CDMA
 GSM uses 900MHz & 1800MHz frequency bands.
 Digital System
 Support Circuit Switched (CS) technology.
2.5G+/2.75G:
 2.5G : GPRS
 2.5G+/2.75G: EDGE
 GPRS & EDGE support both Packet (PS) & Circuit (CS) switched technique.
3G:
 WCDMA(UMTS), GPRS
 It uses 900MHz & 2100MHz frequency bands.
 Mobile Access to Internet/E-mails
3.5G:
 HSDPA,HUPA
4G:
 LTE/EPC
 It uses 1800MHz (FDM) & 2300MHz (TDM) frequency bands.
 Speed up to 300Mbps.
 Only Packet Switched (PS) technique for all services including voice.
 Only IP based.

7. 7 | P a g e
Spectrum Bands available in India:

8. 8 | P a g e
GSM: Global System For Mobiles
GSM PLMN(Public land Mobile N/W) Organization:
 BSS: Base Station subsystem
o Radio Resource Management.
o Radio Link Management
 NSS: Network Station Subsystem
o Switching
o Mobility & Connection Management
o Charging
 NMS: Network Management Subsystem
o Fault management
o Configuration & performance management
GSM – 2G Network Architecture:
MS  BTS  BSC  MSCHLR, VLR, IN, EIR

9. 9 | P a g e
GSM specifications define 2 open interfaces within GSM network:-
 Abis interface (Open air interface): Between MS (Mobile station) & Base Station or BTS
(Antennas). Abis carries Traffic information of all the mobiles in the cells controlled by the BTS.
 A-Interface: Between BSC (Base Station Controller) & MSC (Mobile Services Switching
Center).
Circuited Switched Core:
 Technologies like GSM, CDMA & GPRS use CS core network.
 It uses circuited Technology for data transfer.
Packet Switched Core:
 Technologies like GPRS & EDGE use PS core network which includes primarily the SGSN &
GGSN.
 PS core sends data in form of packets & each packet having unique ID no.
 Both SGSN & GGSN process user data & signaling.
 SGSN: Serving GPRS Support Node: It is like the MSC of CS Core.
 GGSN: Gateway GPRS Support Node: It is like the GMSC of CS core.
IN/VAS: Intelligent Network/Value Added Services
 It checks for Prepaid or Postpaid charging.
MS: Mobile Station
o Mobile station provides user access to GSM network for Voice & Data.
o Subscriber data is read from a SIM card that plugs into MS.
o Each MS has a unique number called as IMEI number, which is stored in EIR for authentication
purposes.
o Mobile camps on to the GSM network through a BTS serving the cell.
o Mobile also scans neighboring cells and reports signal strength.
o Mobile Transmit and Receive voice at 13 KB/s over air interface.
Mobile Station Identities:
o MSISDN : Human Identity used to call a Mobile Station.
o IMEI: (International Mobile Station Equipment Identity)-Serial number unique to every Mobile
Station

10. 10 | P a g e
SIM-Subscriber Identity Module
o Removable Module inserted when the subscriber wants to use the MS.
o SIM no. mapped to IMSI.
o SIM features and contents are personalized by the Service Activator.
o MS also stores some temporary data on SIM during operation.
IMSI- : Network Identity unique to a SIM.
Base Station Cell Site Architecture
BTS - Base Transceiver Station
o BTS has a set of Transceivers to communicate with mobile's in its area.
o One BTS covers one or more than one cell
o The capacity of a cell depends upon number of transceivers in a cell.
o BTS is connected to the BSC through Abis Interface, which is a 2Mb/s.
o BTS transmit and receive voice at 13 kbps over air interface to the mobiles.

11. 11 | P a g e
BSC - Base Station Controller
o Several BTS's are connected to one BSC
o BSC manages channel allocation, handovers and release of channels at connected BTS's.
o BSC connects to each BTS on an Abis interface & to the MSC on A interface.
o BSC has the entire database for all cell parameters associated with the BTS's.
 TRAU - Transcoder / Rate Adaptation Unit
o The MSC is based on ISDN switching.
o ISDN has speech rate of 64kbps. Mobile communicates at 13 Kbps.
o TRAU converts the data rates between 13 KB/s GSM rate to 64 Kbits /s Standard ISDN
rate
o TRAU can be collocated with the BTS, BSC or MSC or it can be a separate unit.
o Collocated with MSC, BSC, BTS.
o Separate Unit

12. 12 | P a g e
MSC - Mobile Switching Centre
o Exchange where calls are established, maintained and released.
o Database for all subscribers and their associated features.
o Communicates with BSC's on MS side and with PSTN on fixed line side.
o MSC is weighted on the number of subscribers it can support.
Multiple MSC's:
o HLR - Home Location Register
o MSC has all subscriber database stored in HLR.
o HLR has all permanent subscriber databases.
o MSC communicates with HLR to get data for subscribers on call.
o HLR will have the series of all subscriber numbers, which may not be activated or issued.
o VLR - Visiting Location Register
o A subscription when activated is registered in VLR
o VLR has all the subscriber no's. which are activated.
o It also has temporary database of all activated subscribers ( on/off, location )
o MSC communicates with HLR for subscribers coming from different MSC's
and if found valid and then registers them in its VLR.

13. 13 | P a g e
o AUC - Authentication Centre
o Authentification is a process by which a SIM is verified.
o Secret data and the verification process algorithm are stored at AUC.
o AUC is the element which carries out the verification of SIM.
o It is associated with the HLR.
o EIR : Equipment Identity Register
o EIR is the Mobile Equipment Database which has a series of IMEI's
o MSC asks the Mobile to send its IMEI
o MSC then checks the validity of IMEI with the EIR.
o All IMEI are stored in EIR with relevant classifications.
o Classifications of IMEI( Mobile Stations):
o White List (Mobile is allowed on the Network)
o Grey List (Mobile needs to be checked)
o Black List (Mobile is not allowed on the Network)
o BC - Billing Centre
o BC Generates the Billing Statement for each Subscriber
o BC may be directly connected to the MSC
o MSC sents the billing information ( duration of call ) to BC
o BC then produces the billing amount based on the units set.
SMSC - Short Message Service Centre
o Message is sent to a particular mobile.
o Message transfer takes place through SMSC
o Messages are be sent through a Manual Terminal connected to SMSC
o Short Message Service: SMS
o Short Message is sent to a particular Mobile Station

14. 14 | P a g e
Channels in GSM Air Interface .
o BCH - Broadcast CHannel
o Allows Mobiles to tune to BTS freq. - FCCH-This channel carries a 142 bit zero sequence and
repeats once in every 10 frames on BCH
o Allows Mobiles to identify Network - BCCH Used to broadcast Cell and Network identity.
BCCH occupies 4 frames on BCH and repeats once every Multiframe.
o TCH - Traffic Channel
o Traffic Channel carries the Voice data.
o Two blocks of 57 bits contain voice data .
o One TCH is allocated for every active call. While call is in progress if there is degradation in
quality of current channel, BTS may shift the communication to another TCH on a different
Carrier and/or Time slot.
o A Full rate TCH carries 13 KB/s voice data, and Half rate TCH carries a 6.5 KB/s voice data.
Location Update
o Process of informing MSC about new Location Area is Location Update.
o MSC should know the Location of the Mobile for paging.
o Mobile is continuously changing Location Area.
o Mobile when changes Location Area informs the MSC about its new L.A
Handover:
o Handover is a process by which the control/communication of a Mobile is transferred from one
cell to another.

15. 15 | P a g e
Chapter-2
OSI Model:
Layer 7: Application
o The application layer is the OSI layer that is closest to the user.
o It provides network services to the user’s applications.
o It differs from the other layers in that it does not provide services to any other OSI layer, but
rather, only to applications outside the OSI model.
o Examples of such applications are spreadsheet programs, word processing programs.
Layer 6:Presentation
o The presentation layer ensures that the information that the application layer of one system
sends out is readable by the application layer of another system.
o If necessary, the presentation layer translates between multiple data formats by using a
common format.

16. 16 | P a g e
Layer 5:Session
o The session layer establishes, manages, and terminates sessions between two communicating
hosts.
o The session layer provides its services to the presentation layer.
o It also synchronizes dialogue between the two hosts' presentation layers and manages their
data exchange.
Layer 4:Transport
o The transport layer segments data from the sending host's system and reassembles the data
into a data stream on the receiving host's system.
o Layer 4 is the boundary between media-layer protocols and host-layer protocols.
o It also deals with Flow Control (Windowing), and Reliability of Communication
(Acknowledgement).
Layer 3:Network
o The network layer is a complex layer that provides connectivity and path selection between two
host systems that may be located on geographically separated networks.
o Examples: path selection, routing, and addressing.
Layer 2:Data Link
o The data link layer provides reliable transit of data across a physical link.
o The data link layer is concerned with physical addressing, network topology, network access,
and error notification, ordered delivery of frames.
o Example: frames and media access control.
o Subdivided Data Link Layer:
 LLC – Logical Link Control(logical address-IP address)
 IPv4-32 bit & IPv6-128bit
 Allows multiple layer three protocols, such as IP and IPX, to be simultaneously
supported along with multiple frame types.
 MAC- Media Access Control(physical address-48 bits)
 Lower sub layer
 Appends physical address of destination computer onto the frame.
Layer 1:Physical
o The physical layer defines the electrical, mechanical, procedural, and functional specifications
for activating, maintaining, and deactivating the physical link between end systems.
o Such characteristics as voltage levels, timing of voltage changes, physical data rates,
maximum transmission distances, physical connectors, and other, similar, attributes are defined
by physical layer specifications.

17. 17 | P a g e
SS7: Signaling System 7
o SS7, signaling system number 7, supports the PSTN (public switched telephone network) in:
 Call setup,
 Call management, and
 Call release
-between telephone offices and CPE (customer premises equipment) to simply transport
voice traffic & data/video traffic.
o SS7 uses –
 Inband Signalling mechanism:
 The signaling information and the user data (e.g. voice) are carried on the same
channel.
 out-of-band signaling Mechanism:
 Some applications require a high quality transfer of the signaling information so
this band is used.
 In the “out-of-band” signaling, the signaling information is carried in a separate
and dedicated physical channel, and the user data is carried in other channels.
o SS7 Protocol layers:

18. 18 | P a g e
MTP1: Message Transfer Part Level 1
o The MTP1 (message transfer part level 1) is the physical layer.
o It provides a transport for an individual signaling data link.
o It defines the physical, electrical, and functional characteristics of a signaling data link
and the means to access it.
MTP2: Message Transfer Part Level 2
o The MTP2 (message transfer part level 2) is the data link layer.
o It provides the functions and procedures to transfer signaling messages over an
individual signaling data link.
o It is primarily responsible for error free transmission of data for some basic
communication functions.
o Once information enters the network, it must be transferred in the proper sequence and
without error between network nodes over each segment of the transmission path.
MTP3: Message Transfer Part Level 3
o The MTP3 (message transfer part level 3) is the network layer.
o MTP3 also provides the means to establish, maintain, and terminate connections
between systems.
o It provides services that transmit the data through the network from originator to its final
destination.
SCCP: Signaling Connection Control Part
o SCCP (signaling connection control part), which is a combination of parts of the network
layer and parts of the transport layer.
o It relies on the MTP level 1, 2, 3 to provide additional functions for both connectionless
and connection- oriented network services to transfer circuit-related and non-circuit-
related signaling information between switches .
o When coupled with the MTP layers, SCCP provides specialized routing and
management and control functions for the transfer of higher level messages between the
MTP layers and the SCCP users.
TCAP: Transaction Capabilities Application Part
o The TCAP is transaction capabilities application part.
o It provides application level functions for special SS7 services.
o Service information exchanged between the SSP and the network database would
typically be defined within the TCAP.
o The TCAP are supported by SCCP.
o TCAP messages are encapsulated into the SCCP header part.

19. 19 | P a g e
ASE: Application Service Element
o ASE (application service element) at a SSP represents an application uses the TCAP to
initiate a transaction by sending a query message to a remote SCP.
o While an ASE at the SCP uses the TCAP to end the transaction by returning a TCAP
response message to the originating SSP.

20. 20 | P a g e
Chapter-3
Call Flow in GSM
1.) Request Access
The MS sends a Channel Request message on the RACH.
(RACH-used by mobile to get attention from Base station in the uplink.)
The BSS responds with a radio resource assignment.
The MS sends a Service Request message to the BSS.
2.) Authentication
The network requires the MS to authenticate itself. The BSS sends an Authentication
Request message to the MS.
(A 128-bit Random Number (RAND) is sent to the MS. The MS computes the 32-bit Signed
Response (SRES) based on the encryption of the RAND with the authentication algorithm (A3) using
the individual subscriber authentication key (Ki). Upon receiving the SRES from the subscriber, the
GSM network repeats the calculation to verify the identity of the subscriber.)
The MS calculates the proper SRES based on the RAND that was given and sends the SRES from
the subscriber to the BSS in an Authentication Response message.
The BSS verifies the SRES. If the SRES is correct then the MS is authenticated and allowed access to
the network. The BSS will send a Service Accept message letting the MS know that the service
request was received and processed.

21. 21 | P a g e
Once authenticated, the BSS orders the MS to switch to cipher mode.( Encrypted voice and data
Communications between the MS and the network is accomplished by using the
ciphering algorithm A5.)
3.) Initial Call Setup
The MS will immediately switch to cipher mode & then sends a Call Setup message to the BSS. The
message includes the address information (MSISDN) of the called party.
The BSS assigns a TCH (TCH carries the voice data) to the MS by sending an Assignment
Command message. This message includes which Transceiver (TRX) and which Time Slot (TS) to
use.
The BSS does not actually assign a TCH to the MS until the MSC sends a Call Proceeding message
to the BSS indicating that the IAM has been sent.
The MS immediately switches to the assigned TCH. The MS sends an Assignment Complete
message back to the BTS on the FACCH.
(FACCH: is a time slot in TCH itself , that is used for signaling data instead of voice traffic.)
4.) Call Setup
The MSC sends an Initial Address Message (IAM) to the GMSC. The IAM contains the MSISDN of the
called party as the MS dialed it.
The MSC will also send a Call Proceeding message down to the BSS and this is when the BSS would
assign a TCH to the MS,.
Based on the dialed number, the GMSC decides where to route the IAM within the PSTN.
The PSTN will continue to route the IAM until it reaches the correct Switching Center and the call
routing is complete. The PSTN will then establish the call circuit and send an Address Complete
Message (ACM) back to the GMSC.
The GMSC then forwards the ACM back to the responsible MSC indicating that the call circuit has
been established.
5.) Call Establishment
Once the MSC receives the ACM, it sends an ALERT message to the MS indicating that the call is
going through. The BSS sends the ALERT message on the FACCH. Once the MS receives the
ALERT, it will generate the ringing sound in the earpiece. The BSS sends an alerting message the
subscriber will hear the line ringing.
Once the called party answers the phone, the PSTN will send an Answer message to the MSC. The
MSC forwards this to the MS in a Connection message.
Once the MS receives the connection message, it switches over to voice and begins the call. All voice
traffic occurs on the assigned TCH.

22. 22 | P a g e
6.) Call Termination
When either the caller or the called party hangs up, the call will be disconnected. Either party can
initiate the disconnect. MS initiates the disconnect. The MS sends a Disconnect message to the BTS
on the FACCH.
The BSS forwards the Disconnect message to the MSC. Once the MSC receives the disconnect
message, it sends a Release message through the GMSC to the PSTN as well as down through the
BSS to the MS.
The MS responds by sending a Release Complete message to the BSS on the FACCH. The BSS
forwards the message up to the MSC. Once the MSC receives the message the call is considered
ended from the call control perspective.
Although the call has ended, the BSS still has a TCH allocated to the MS. The MSC sends a Channel
Release message to the BSS. The BSS forwards the message to the MS.
The MS responds with a disconnect message and returns to an idle mode. The BSS reallocates the
channel for other call or releases the TRX.

23. 23 | P a g e
Conclusion
In a nutshell, the Internship I had at Bharti Airtel Ltd., A.P. circle was very much helpful in knowing
the concepts of 1)Optical Fiber Communication 2)GSM Technology 3) Call flow in GSM. It was an
opportunity to know about the various techniques which are used in networking. The concepts how
Cellular Generations evolved, how a call is established, how it terminates, all these things were very
much interesting. As an intern in Bharti Airtel Ltd., am grateful to those persons who helped directly
and indirectly in this successful completion of the internship and this will surely be helpful in future.