1. A
TECHNICAL REPORT
ON STUDENTS INDUSTRIAL WORK EXPERIENCE SCHEME (SIWES)
UNDERTAKEN AT
NOKIA SIEMENS NETWORKS, 98/100 APAPA-OSHODI EXPRESSWAY,
LAGOS STATE.
SUBMITTED BY
OLAFUSI MICHAEL O.
EEE/04/2995
TO
THE DEPARTMENT OF ELECTRICAL ELECTRONICS ENGINEERING
FEDERAL UNIVERSITY OF TECHNOLOGY, AKURE,
ONDO STATE
IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE AWARD OF
BACHELOR OF ENGINEERING (B.ENG) DEGREE IN ELECTRICAL AND
ELECTRONICS ENGINEERING
NOVEMBER 2008
2. DEDICATION
In our lives three set of people matter most
First and greatest is God Almighty, who daily re-creates us
Second is our family, the people we love the most
The third is our friends, the bright side of us
To them do I dedicate this report
i
3. ACKNOWLEDGEMENT
First of all, I acknowledge the overwhelming help God gave me throughout the
scheme. He encouraged me not to give up on getting a relevant (to my field of study)
reputable company and when I was getting to the end of my rope, He came to my rescue.
Without His support and miracles I would not have been alive, let alone have a successful
Industrial training attachment.
I appreciate my parents and siblings for their constant help and support, especially my
mother who took my placement over-personal and my father for his constant financial
support even when I could not give a reasonable report of expenditure.
I heartily express my appreciation to Mr. Tope Akinkuowo (Transmission Manager,
Zain Nigeria) and Mr. Zijad Demirovic (Project Manager, Nokia Siemens Networks
Nigeria) for making it possible for me to do my Industrial Training attachment at Nokia
Siemens Networks.
I also appreciate Mr. Kehinde Oke (Senior Engineer, Nokia Siemens Networks) for
sparing some of his precious time to teach me all I could understand about
Telecommunications transmission and radio telephone network system. I also thank my
colleagues Ifekauche Onyeka and Agho Osasu for helping make my six months industrial
training attachment a swell time.
Sincerely, all the technicians at the Pre-installation and Transmission department
taught me so much that, even though they are too many for me list name by name I could
not have had a very successful Industrial Training attachment without them.
ii
4. ABSTRACT
The Student Industrial Work Experience Scheme (SIWES) is designed to give
University undergraduates in Nigeria the relevant practical knowledge and industrial
exposure they need to fully understand the application of the theoretical knowledge they
acquire within the four walls of the lecture halls. I was fortunate to serve my six months
industrial work experience at Nokia Siemens Networks, an international
telecommunications company involved in fixed telephone network system and mobile
telephone network system installation and servicing for telephone network operators and
multinational organizations all over the world.
This report is a comprehensive summary of all that I learnt and was involved in
throughout my industrial attachment at the Radio access business unit of the Lagos,
Nigeria branch of the company. I learnt the fundamentals of telecommunications, the
different type of telecommunications systems, the operational difference between a fixed
telephone line network and a mobile telephone network, and how the GSM network is
implemented. I was involved in a couple of site works, twice at the Twenty-first century
telecommunications company fixed line switching and transmission capacity expansion,
and twice at Zain's Ibadan, Oyo state Base Transceiver Stations upgrade. At the company's
Transmission Pre-installation Office (TPO), I was able to learn how the different
transmission equipments operate, are installed, troubleshooted and remotely monitored. I
was shown the different types of waveguides, coaxial cables, optical fibres and twisted
copper cable, and was made to understand their areas of application.
The chapter one gives a brief introduction to the history and operations of Nokia
Siemens Networks with an organogram of the company. Chapter two discusses the basics
of telecommunications and the media used in signal propagation. Chapter three delves
straight into the core of the GSM network architecture and how the operate together to
make mobile phone calls possible. The last major chapter, chapter four talks extensively on
the Base Transceiver Station and its radio access link segment where I worked on at the
sites.
iii
5. LIST OF FIGURES
Figure 1.1: Nokia Siemens Networks Organizational chart..................................................4
Figure 2.1: Communication link between two telephones.....................................................5
Figure 2.2: A four-pair copper cable......................................................................................7
Figure 2.3: A typical coaxial cable.........................................................................................8
Figure 2.4: A typical optic fibre..............................................................................................9
Figure 2.5: Wireless communication links............................................................................10
Figure 3.1: Frequency allocation in the GSM 900 and GSM 1800 band..............................12
Figure 3.2: A mobile station..................................................................................................13
Figure 3.3: Time Division Multiple Access principle............................................................15
Figure 3.4: The Network Switching Subsystem (NSS)..........................................................17
Figure 3.5: The Base Station Subsystem (BSS).....................................................................20
Figure 3.6: A diagramatic representation of the management function of the NMS.............23
Figure 3.7: Obanla trying to call Ajegunle.............................................................................23
Figure 3.8: The frequency reuse chart....................................................................................25
Figure 3.9: Synchronization of the mobile station with the network.....................................26
Figure 3.10: Channel request and allocation..........................................................................27
Figure 3.11: A summary of the GSM architecture.................................................................28
Figure 4.1: The SRA 4 unit.....................................................................................................31
Figure 4.2: Local Craft Terminal software.............................................................................31
Figure 4.3: The NetBuilder software......................................................................................32
Figure 4.4: On site testing of connectivity between two BTSs using the SRA 4 handset......32
Figure 4.5: The SRA 4 unit fully connected to the other network units.................................33
Figure 4.6: The coaxial cables connecting the indoor units to the outdoor units...................34
Figure 4.7: The coaxial cables entering into the BTS shelter.................................................34
Figure 4.8: An ODU...............................................................................................................35
Figure 4.9: An antenna with two ODUs closely attached......................................................35
Figure 4.10: The 6 – 13 GHz ODU........................................................................................36
Figure 4.11: The 15 – 38 GHz ODU.....................................................................................36
iv
6. Figure 4.12: The different polarizations.................................................................................37
Figure 4.13: Directional high performance shielded antenna already installed......................38
Figure 4.14: Mutual impedance between parallel /2 dipoles not staggered. Curves Re and
Im are the resistive and reactive parts of the impedance.....................................39
Figure 4.15: The 21 pair twisted cables being made into E1 transmission lines....................41
Figure 4.16: The specialized crimper and clamp for fixing the E1 DB-32 connectors..........42
Figure 4.17: A general tool box..............................................................................................43
Figure 4.18: The Digital Distribution Frame (DDF)..............................................................44
Figure 4.19: The Multiplexer (Surpass HiT 7070, under testing)..........................................45
Figure 4.20: The Synchronous Radio Access XL (SRA XL)................................................46
v
7. LIST OF TABLES
Table 3.1: GSM 900 frequency channels............................................................................13
Table 3.2: GSM 1800 frequency channels..........................................................................14
Table 4.1: The standard frequency allocation table............................................................29
vi
8. TABLE OF CONTENTS
DEDICATION............................................................................................................................i
ACKNOWLEDGEMENT..........................................................................................................ii
ABSTRACT...............................................................................................................................iii
LIST OF FIGURES...................................................................................................................iv
LIST OF TABLES.....................................................................................................................vi
TABLE OF CONTENTS..........................................................................................................vii
1.0 INTRODUCTION.................................................................................................................1
1.1 NOKIA SIEMENS NETWORKS …........................................................................2
1.1.1 BRIEF HISTORY AND ORGANIZATIONAL STRUCTURE............2
1.1.2 ORGANIZATIONAL CHART..............................................................3
2.0 TELECOMMUNICATIONS BASICS.................................................................................5
2.1 TELECOMMUNICATIONS TRANSMISSION MEDIA.......................................6
2.1.1 COPPER.................................................................................................7
2.1.2 COAXIAL CABLES..............................................................................8
2.1.3 OPTIC FIBRES......................................................................................8
2.1.4 WIRELESS (ELECTROMAGNETIC WAVES)...................................9
3.0 GLOBAL SYSTEM FOR MOBILE COMMUNICATIONS (GSM).................................11
3.1 GSM NETWORK ARCHITECTURE....................................................................16
3.2 NETWORK SWITCHING SUBSYSTEM (NSS)..................................................16
3.2.1 MOBILE SERVICES SWITCHING CENTRE (MSC)........................17
3.2.2 VISITOR LOCATION REGISTER (VLR)..........................................18
3.2.3 HOME LOCATION REGISTER (HLR)..............................................18
3.2.4 AUTHENTICATION CENTRE (AU).................................................18
3.2.5 EQUIPMENT IDENTITY REGISTER (EIR)......................................19
3.3 BASE STATION SUBSYSTEM (BSS).................................................................19
3.3.1 BASE STATION CONTROLLER (BSC)............................................20
3.3.2 BASE TRANSCEIVER STATION (BTS)...........................................20
3.3.3 TRANSCODER (TC)...........................................................................21
vii
9. 3.4 NETWORK MANAGEMENT SUBSYSTEM (NMS)........................................22
3.5 PRACTICAL ILLUSTRATION...........................................................................23
4.0 BASE TRANSCEIVER STATION RADIO ACCESS LINK..........................................29
4.1 THE SYNCHRONOUS RADIO ACCESS STM-4 (SRA 4) UNIT....................30
4.2 THE COAXIAL CABLE.....................................................................................33
4.3 THE OUTDOOR FREQUENCY CONVERTER AND SIGNAL AMPLIFIER
UNIT (ODU)........................................................................................................35
4.4 DIRECTIONAL ANTENNA...............................................................................37
4.5 TWISTED PAIR COPPER CABLE....................................................................41
5.0 CONCLUSION AND RECOMMENDATION................................................................47
REFERENCES........................................................................................................................48
viii
10. CHAPTER ONE
1.0 INTRODUCTION
The Student Industrial Work Experience Scheme (SIWES) was established in 1973 to enable
undergraduates in various Nigerian universities to acquire relevant practical and industrial experience
in their various fields of study. This is to help the students better understand what they are being
taught in the universities and to practically apply them.
At Nokia Siemens Networks, where I observed the SIWES, I was exposed to many major
telecommunications equipment like the Nokia Siemens Networks Synchronous Radio Access STM-4
(SRA 4) unit, Synchronous Radio Access Trunk (SRT) unit, Surpass HiT 7070 Multiplexer, Digital
Distribution Frame (DDF), EWSD high capacity switch, various optic fibres, waveguides, twisted
pair copper cables and antennae.
I was involved in the following projects,
1. The expansion of the fixed line telephone network of the Twenty-first century
telecommunications company branch at Ikeja, Lagos state.
2. Another expansion of the fixed line telephone network of the Twenty-first century
telecommunications company branch at Victoria Island, Lagos state.
3. The powering of Zain's Base Transceiver Station Synchronous Radio Access STM-4
(SRA 4) system in Ibadan, Oyo state.
4. The configuration of another Zain's Base Transceiver Station Radio Access link at
Ibadan, and testing of connectivity between the station, another nearby station and the
nearest Mobile services Switching Centre.
5. The coupling of shielded twisted seven-pair copper cables into a standard E1 jack for use
with the Surpass HiT 7070 multiplexer and the Digital Distribution Frame (DDF).
6. The installation of Very Small Arperture Terminal (VSAT) for data communications at
the Nokia Siemens Networks Lagos branch.
I was able through the company and personal efforts to learn the following,
1
11. 1. The use of AutoCAD and even used it on some occasions to reproduce in softcopy
some company project site diagrams.
2. The installation, multi-user capability and administration of the linux operating
system. I tried my hands on Ubuntu, Kubuntu and OpenSuse linux distributions.
3. Microsoft windows XP operating software management and administration, use of
system restore, registry edit, and password reset through a bootable XP installation
CD and a password breaker floppy disk.
4. Use of Microsoft Excel to prepare stock lists and faulty equipments record.
5. Oracle 10g R2 database mangement software and SQL relational query language.
6. Medium size computer network ( less than a thousand computers involved) setup and
management using cisco switches, conventional routers and wireless routers.
1.1 NOKIA SIEMENS NETWORKS
Nokia Siemens Networks started operations on the 1st
April, 2007 as a result of a merger
between the former Network Business Group department of Nokia and the Carrier-related
operations department of Siemens International.
Nokia Siemens Networks operates in 150 countries located in all the major continents of the
world, ranking second in both Wireless networks infrastructure and Operator services, and third
in Wireline networks infrastructure. They are headquartered in Espoo, Finland with over 60,000
highly skilled professionals worldwide providing infrastructure and services to about 1,400
corporate customers and infrastructural setup connecting over 1 billion people all over the world.
1.1.1 BRIEF HISTORY AND ORGANIZATIONAL STRUCTURE
Nokia Siemens Networks came out of two industry giants – Nokia and Siemens. Nokia is a
world leader in mobile telecommunications, connecting people to each other and the information
that matters to them with easy-to-use and innovative products like mobile phones, devices and
solutions for imaging, gaming, media and businesses. Nokia has been in existence since 1865,
2
12. though first as a furniture company which later evolved into a multinational telecommunications
company and a pioneer in mobile communications development.
On the other hand, Siemens has been a global powerhouse in electrical engineering and
electronics since 1847, presently with over 461,000 employees in over 190 countries working to
develop and manufacture products, design and install complex systems. The company focuses on
the areas of Information and Communications, Automation and Control, Power, Transportation,
Medical, and Lighting.
On June 19, 2006, Nokia and Siemens announced that they intend to merge the Networks
Business Group of Nokia and the carrier-related operations of Siemens into a new company, to be
called Nokia Siemens Networks. This 50-50 joint venture eventually on April 1, 2007, created a
global leader with strong positions in important growth segments of fixed and mobile network
infrastructure and services.
Nokia Siemens Networks has its operations grouped into five different business units,
namely:
1. Converged core business unit,
2. IP Transport business unit,
3. Radio Access business unit,
4. Broadband Access business unit, and
5. Operations and business software business unit.
I worked in the Radio Access business unit which is concerned with the setting up of radio
links between different network stations/nodes and configuration of the radio access equipments.
The network nodes are usually branches of a bank or base stations of a mobile telephone
network.
1.1.2 ORGANIZATIONAL CHART
Nokia Siemens Networks' organizational chart is as shown below.
3
14. CHAPTER TWO
2.0 THE BASICS OF TELECOMMUNICATIONS
Telecommunications is the assisted transmission of signals over a distance for the purpose of
communication. A telecommunication system consists of three basic elements, namely:
1. A transmitter that takes information and converts it to an easily transmittable signal,
2. A transmission medium that carries the signal , and
3. A receiver that receives the signal and converts it back to a useable information.
Oftentimes, a single equipment can act as both a transmitter and a receiver and it is referred
to as transceiver.
Telecommunication that involves one transmitter and one receiver over a dedicated line of
transmission is called a point-to-point communication. While telecommunication that involves
one powerful transmitter and several receivers is called broadcast communication. An example of
a point-to-point communication is communication over a telephone line (phone call), even
though there may be many transmitters and receivers along the communication path, only one
transmitter and receiver is actively used, others are simply serving as repeaters, to amplify and re-
propagate the signal. Also, an example of a broadcast communication is the conventional free-to-
air radio broadcast where a radio station uses one powerful transmitter to send signals to
numerous transistor radios.
A simple illustration of telecommunications would be a Plain Old Telephone (POT) system
Figure 2.1: Communication link between two telephones
5
copper wire line
Telephone A Telephone B
15. The transmitter is the mouthpiece of each of the two telephones, the receiver is the earpiece
of each of the two telephones and the transmission medium is the copper wire between the two
telephones. This is a point-to-point communication because the transmitter of telephone A is
using a dedicated link over the copper wire to communicate with the receiver of telephone B, and
same with the transmitter of telephone B and the receiver of telephone A.
When you speak through the mouthpiece of telephone A, your voice which is in an analogue
form and of low frequency (hence cannot of itself reach the other party of telephone B) is made
to alter the electrical properties of the mouthpiece in a predictable way. These electrical
alterations (electrical signals) are transmitted through the copper wires to the receiver of the other
telephone which then regenerates the audio speech. This shaping of a signal to convey
information is called modulation.
If we want to setup a plain old telephone network system for a town or large community, we
will probably need to run a copper wire from each telephone to every other telephone in the
network. This will be very cumbersome and uneconomical, so usually there are some copper
wires that are made to carry communications signals for more than one point-to-point
communication. This will require a special device called a multiplexer to combine several point-
to-point communication signals to be transmitted on one copper wire. There will also be a
demultiplexer at the other end to separate the different communications signals. A modem is
usually used to perform the operations of both the multiplexer and demultiplexer at both
communication ends. The combination of several communications signal to be transmitted over
one transmission line is called multiplexing.
A collection of several transmitters, receivers and/or transceivers that can communicate with
one another is known as a network.
2.1 TELECOMMUNICATION TRANSMISSION MEDIA
There are four basic types of transmission media used for transmission of signals in
telecommunications, namely,
1. Copper cable,
2. Coaxial cable,
6
16. 3. Optical fibre, and
4. Wireless.
2.1.1 COPPER CABLE
Copper cable is the most extensively used transmission media and often in conjunction with
other media. It is very cheap to implement and in form of a twisted pair cable, it is quite
satisfactory for Public Switched Telephone Network (PSTN) lines and voice communications.
But as data communications were been implemented in most telephone networks including the
PSTN, copper became unsuitable due to the high degenerative effect it has on high frequency
data signals. Also, the load coils that are frequently added to copper loops longer than than
18,000 feet to block frequencies higher than the standard 64kbits/s voice modulated signals
frequency, are low-pass filters which greatly attenuate higher frequencies that characterize data
signals. Data signals require higher frequencies compared with voice modulated signals in other
to achieve a very high bandwidth.
Copper cable is still much in use as a transmission medium, but it is not used for very high-
traffic data communication. Since, all telecommunications networks now provide both voice and
data communication over the same set of infrastructure, copper cable as a transmission medium
is now limited to low traffic network areas and cover a relatively short distances.
Figure 2.2: A four-pair copper cable
7
17. 2.1.2 COAXIAL CABLE
Coaxial cable is a special adaptation of copper. It consists of a single strand of copper
shielded by a foam-like insulator or air dielectric and an electromagnetic shield of a conductive
foil, with interwoven strands of wire between the outermost insulator and the foil. Coaxial cable is
more like an antenna than a regular cable because it carries an electromagnetic wave between the
inner core and the shielding. It has superior signal quality because the shielding mostly prevents
interference from reaching the signal. Coaxial connectors are designed to have the same impedance as
the cable and to maintain its shielding. The main connector types are the BNC connector used for
computer networking, and the F connector used for cable television. Cable terminators are closed
connectors that are placed on all open ends of a coaxial cable network to minimize signal loss and
interference. Because of its construction, the coaxial cable can conveniently transmit high
frequency signals for a longer distance and lower attenuation than the conventional copper cable
would. Usually, dozens of television channels each 6MHz wide can be multiplexed on a single
coaxial cable for satellite television broadcast reception.
But still, the coaxial cable still has the limitation of attenuating very high frequency signals,
and is not usually used for very long distances.
Figure 2.3: A typical coaxial cable
8
18. 2.1.3 OPTICAL FIBRE
Optical fibre is simply a very thin strand of specially treated glass (about a few micrometers
in diameter), padded with a flexible insulator material (cladding) with an outer jacket to prevent
breakage. It transmits signals in form of refracted light rays. It is an ideal transmission medium
with practically no attenuation for thousands of miles of very high frequency signal transmission.
It can transmit bandwidths of 110 Gb/s on a single strand as tiny as 10 micrometer diameter.
(Coring, 2006).
Since most telecommunications signals are in electrical form, an electrical-to-optical signal
converter chip is used at each end of the optic fibre transmission line. Most telephone network
operators use optic fibre backbone installation. Though, it is quite expensive to implement, but on
a per-bandwidth basis, it is the cheapest form of telecommunication medium. It's only limitation
is that it not economical for local network loops of low traffic load and short distances.
Figure 2.4: A typical optic fibre
2.1.4 WIRELESS
Wireless transmission involves the use of electromagnetic waves of various frequencies for
telecommunication transmission. In reality, a wireless transmission medium can be anything
ranging from the atmosphere to even water body, as long as the electromagnetic waves is not
reflected or absorbed completely. Hence, a wireless transmission medium goes beyond just air
and free space media, it means any material that the signal can be propagated through. Wireless
transmission can be implemented through several equipments like microwave transmitters,
synchronous satellites, low-earth orbit satellites, cellular transmitters and personal
9
19. communication services (PCS) devices. In fact, the GSM (Global System for Mobile
communications) that I will discuss extensively in the following chapter, uses wireless.
Wireless has the advantage of the fact that it could be implemented in remote or
mountainous locations where wired connections will be too expensive or impossible to
implement. But wireless is the most expensive transmission mode per-bandwidth basis.
Figure 2.5: Wireless communication links.
10
20. CHAPTER THREE
3.0 GLOBAL SYSTEM FOR MOBILE COMMUNICATIONS (GSM)
The major application of wireless communication is for speech or voice communications.
Though, radio telephony has been in use for many decades, but in a very limited way, usually for
communications between different military bases and research institutes.
The GSM is a radio telephony standard set up to allow commercial internationally
standardized cellular (use of cells in a network with frequency channels that can reused) radio
telephone networks in the world. More than 80 percent of the mobile telephone network systems
in the world uses the GSM standard. This makes it possible for companies to produce phones that
comply with the standard and can work with any mobile telephone network operator that uses the
GSM standard. For example, you can buy any of Nokia, Samsung or Siemens GSM phone and
use it with either Zain, MTN or Glo network operators in Nigeria.
GSM operates in four standardized frequency ranges/bands namely,
GSM 850,
GSM 900,
GSM 1800, and
GSM 1900.
In Nigeria, we use both the GSM 900 and 1800 bands.
11
21. Figure 3.1: Frequency allocation in the GSM 900 and GSM 1800 band
The uplink refers to a signal flow from the mobile station (MS) to the Base Transceiver
Station (BTS), while the downlink refers to the signal flow from the Base Transceiver station to
the mobile station.
The mobile station is a combination of a terminal equipment (usually a mobile phone) and a
subscriber data usually stored on a subscriber Identity Module chip (SIM). Hence, mobile phone
+ SIM = Mobile station.
Figure 3.2: A mobile station
12
22. The simultaneous use of separate uplink and downlink frequencies enables communication
in both the transmit (TX) and the receive (RX) directions. The radio carrier frequencies are
arranged in pairs and the difference between these uplink and downlink frequencies is called the
duplex frequency. Each of these uplink and downlink frequency ranges are divided into carrier
frequencies spaced at 200kHz.
Table 3.1: GSM 900 frequency channels.
CHANNEL UPLINK SIGNAL
(MHz)
DOWNLINK SIGNAL
(MHz)
1 890.1 – 890.3 935.1 – 935.3
2 890.3 – 890.5 935.3 – 935.5
3 890.5 – 890.7 935.5 – 935.7
4 890.7 – 890.9 935.7 – 935.9
5 890.9 – 891.1 935.9 – 936.1
6 891.1 – 891.3 936.1 – 936.3
7 891.3 – 891.5 936.3 – 936.5
8 891.5 – 891.7 936.5 – 936.7
9 891.7 – 891.9 936.7 – 936.9
10 891.9 – 892.1 936.9 – 937.1
11 892.1 – 892.3 937.1 – 937.3
12 892.3 – 892.5 937.3 – 937.5
13 892.5 – 892.7 937.5 – 937.7
14 892.7 – 892.9 937.7 – 937.9
... ... ...
13
24. In GSM 900, the duplex frequency is 45MHz and in GSM 1800, it is 95MHz. The lowest
and highest channels are not used in both GSM 900 and 1800 bands to avoid interference with
services using neighbouring frequencies.
The radio transmission in GSM networks is based on digital technology and is implemented
using the Frequency Division Multiple Acess (FDMA) for cell allocation to a Base Transceiver
Station (BTS) and the Time Division Multiple Access (TDMA) for resource share among several
mobile stations in a cell. As for the FDMA, each BTS (covering a cell) is allocated different radio
frequency channels to avoid interference in adjacent cells. While in TDMA, each Mobile Station
is allocated a time slot to send and receive data.
Figure 3.3: Time Division Multiple Access principle
15
25. 3.1 GSM NETWORK ARCHITECTURE
A connection between two people – the caller and the called person – is the basic service of
all telephone networks. In a GSM network, the establishment of this connection is quite complex
because the users are allowed to move about provided they stay within the overall network
service area of the network operator, unlike fixed telephone networks where each phone location
is fixed.
In practice, the GSM network has to fnd solutions to the following three basic problems
before it can even set up a call,
1. Who is the subscriber?
2. Where is the subscriber?
3. What does the subscriber want?
In other words, the subscriber has to be located, identified and provided with the requested
services.
The GSM network is able to do these and many more through the use of a decentralised
intelligence subsystems, namely
1. Network Switching Subsystem (NSS)
2. Base Station Subsystem (BSS)
3. Network management Subsystem (NMS)
The actual network part needed for establishing call is the NSS and BSS. The NMS is the
operation and maintenance related part of the network and it is needed for the control of the
whole GSM network.
3.2 NETWORK SWITCHING SUBSYSTEM (NSS)
The network switching subsystem is the GSM network subsystem part that handles call
control, charging information, subscriber location information, signalling and subscriber data
storage. It is able to do all these through various component network elements , namely:
16
26. 1. Mobile services Switching Centre (MSC)
2. Home Location Registry (HLR)
3. Visitor Location Registry (VLR)
4. Authentication Centre (AC), and
5. Equipment Identity Register (EIR)
The GMSC stands for Gateway Mobile services Switching Centre and is used to
interconnect with the Public Switched Telephone Network (PSTN).
Figure 3.4: The Network Switching Subsystem (NSS)
3.2.1 MOBILE SERVICES SWITCHING CENTRE (MSC)
The MSC is responsible for controlling calls in the mobile network. It identifies the origin
and destination of a call (mobile station or fixed telephone), as well as the type of call. An MSC
also initiates paging which is the process of locating a particular mobile station to receive a
call.MSC also collects charging information.
17
27. 3.2.2 VISITOR LOCATION REGISTRY (VLR)
In the Nokia Siemens Networks implementation, the Visitor Location Register is integerated
with the MSC. The Visitor Location Register is a database that contains information about
subscribers currently in the service area of the MSC/VLR, such as
1. Identification numbers of subscribers.
2. Security information for authentication of the SIM card and for ciphering.
3. Services that the subscriber can us.
The VLR database is temporary, in the sense that the data is held as long as the subscriber is
within its service area. It also contains the address to every subscriber's Home Location Register,
which I will discus next.
3.2.3 HOME LOCATION REGISTER (HLR)
The Home Location Register maintains a permanent database of the subscribers, their
identification numbers and subscribed services. Also, the HLR keeps track of the current location
of its customers. This makes it possble for the MSC to ask for call routing information from the
HLR to get to the dialled number.
In Nokia Siemens Networks implementation, the Authentication Centre (AC) and the
Equipment Identity Register (EIR) are located in the HLR.
3.2.4 AUTHENTICATION CENTRE (AC)
The authentication centre provides security information to the network, so that SIM cards
can be verified. The AC provides authentication between the mobile station and the VLR. The
AC also issues a so-called authentication triplets upon request and ciphers the information
transmitted between the mobile station and the Base Tranceiver Station.
18
28. 3.2.5 EQUIPMENT IDENTITY REGISTER (EIR)
Just like the Authentication Centre, the Equipment Identity Register is used for security
reasons. But while the AC provides information for verifying the SIM cards, the EIR is
responsible for the IMEI (International Mobile Equipment Identity) number checking to ascertain
the mobile phone's validity on the network.
The EIR contains three lists:
1. White list containing the list of mobile phones allowed to operate normally on the
network,
2. Grey list containing the list of mobile phones whose use will be monitored for security
reasons, and
3. Black list containing the list of mobile phones reported stolen or just not allowed to
operate on the network for security reasons.
3.3 BASE STATION SUBSYSTEM (BSS)
The base station subsystem is responsible for managing the radio network, and it is
controlled by the MSC. Typically, one MSC controls several BSSs. A BSS itself may cover a
considerably large geographical area consisting of many cells.
The BSS consists of the following network elements:
1. Base Station Controller (BSC)
2. Base Transceiver Station (BTS)
3. Transcoder (TC)
19
29. Figure 3.5: The Base Station Subsystem (BSS)
3.3.1 BASE STATION CONTROLLER (BSC)
The Base Station Controller is the central network element in the BSS and it controls the
radio network. All calls to and from the mobile station are connected through the group switch of
the BSC. The BSC is response for initiating the vast majority of all handovers, and it makes the
handover decision based on, among others, measurement reports sent by the mobile station
during a call. Also, information from the Base Transceiver Stations, Transcoders and BSC are
collected in the BSC and forwarded via the Data Communications Network to the Network
management Subsystem (NMS) where they are post-processed into statistical views, from which
the network quality and status is obtained.
It is the BSC that cordinates the operation of several BTSs, and Transcoders. And it is
capable of barring a BTS from the network and collecting alarm information.
3.3.2 BASE TRANSCEIVER STATION (BTS)
The Base Transceiver station is the network element responsible for maintaining
communication with the mobile station. The BTS enables a lot of call and non-call signalling
with the mobile station, in order for the communication system to work well. For example, when
a mobile station is just switched on in a new location area, it will need to send and receive a lot of
20
30. information (as short data bursts) with the network through the BTS before it can begin to receive
and make calls.
The BTS also performs speech processing in order to guarantee an error-free connection
between the mobile station and the network. This includes speech coding, channel coding and
data burst formatting.
3.3.2 TRANSCODER (TC)
For transmission between the mobile station and the Base Transceiver Station, the media
carrying the traffic is a radio frequency. And to enable an efficient transmission of digital speech
information, the digital speech signal is compressed. We must however, also be able to
communicate with and through the fixed network whose speech compression format is different.
So, somewhere between the BTS and the fixed network, we therefore have to convert from one
speech compression format to another, and this is where the Transcoder comes in.
For transmission over the air interface, the speech signal is compressed using three formats,
namely:
1. FULL RATE compression of 13kbits/s using compression algorithm "Regular Pulse
Excitation with Long Term Prediction" (RPE-LTP).
2. ENHANCED FULL RATE of also 13kbits/s (but better quality then the full rate) using
compression algorithm "Algorithm Code Excited Linear Prediction" (ACELP).
3. HALF RATE compression of 5.6kbits/s using compression algorithm "Vector Sum
Excited Linear Prediction" (VSELP).
But the standard bit rate for speech in fixed network Public Switched Telephone Network
(PSTN) is 64kbits/s using Pulse Code Modulation (PCM). The transcoder takes care of the
change from one bit rate to another. Also, the Transcoder enables Discontinuous Transmission
(DTX) which is used during a call when there is nothing to transmit (no conversation) in order to
reduce interference and mobile phone's battery usage.
21
31. In Nokia Siemens Networks the submultiplexing and transcoding functions are combined in
one equipment called TCSME (Transcoder-Submultiplexer European version) or TCSMA
(Transcoder-Submultiplexer American version).
3.4 NETWORK MANAGEMENT SUBSYSTEM (NMS)
The Network Management Subsystem is the third subsystem of the GSM network, working
in conjuction with the Network Switching Subsystem (NSS) and Base Station Subsystem (BSS)
which I have already discussed. The purpose of the NMS is to monitor the various functions and
elements of the network. In the Nokia Siemens Networks implementation, these tasks are carried
out by the NMS/2000, which consists of a number of workstations, servers, and a router which
connects to a Data Communications Network (DCN).
The operator workstations are connected to the database and communications servers via a
Local Area Network (LAN). The database stores the management information about
communications between the NMS and the equipments in the GSM network known as "network
elements". These communications are carried over a Data Communication Network (DCN),
which consists to the NMS via a router.
The major functions of the NMS are:
1. Fault management to ensure the smooth operation of the network and rapid correction of
any kind of problems detected.
2. Configuration management to maintain up-to-date information about the operation and
configuration status of network elements.
3. Performance management through collection of measurement data from various
individual network elements.
22
32. Figure 3.7: A diagramatic representation of the management function of the NMS
3.5 PRACTICAL ILLUSTRATION
Lets take an MTN subscriber in Engineering building, Federal University of Technology
Akure, Ondo state and call him Obanla.
Figure 3.8: Obanla trying to call Ajegunle
23
33. Let us take another MTN subscriber located in Ajegunle, Lagos state and call him Ajegunle.
I am going to explain what happens when Obanla switches on his phone and dials
Ajegunle's mobile phone.
Note the following already discussed items,
1. A mobile station is a mobile phone that has a Subscriber Identity Module (SIM) connected
to it.
2. A Base Station Subsystem (BSS) comprises the Base Station Controller (BSC), Base
Transceiver Station (BTS) and Transcoder (TC).
3. The Network Switching Subsystem (NSS) comprises the Mobile services Switching
Centre (MSC), Visitor Location Register (VLR), Home Location Register (HLR),
Authentication Centre (AC) and Equipment Identity Register (EIR).
4. Only the Base Station Subsystem (BSS) and Network Switching Subsystem (NSS) are
needed for establishing calls. The Network Management Subsystem (NMS) is for
operation monitoring and maintenance of the GSM network.
The Base Transceiver Stations are arranged in such a way that one BTS covers an
hexagonal area and the surrounding BTSs must use different frequency channels.
24
34. Figure 3.9: The frequency reuse chart.
When subscriber Obanla switches on his mobile station (mobile phone already with a valid
MTN SIM connected), the mobile station scans all the radio channel frequencies for broadcast
signals from all nearby MTN Base Transceiver Stations, measures the distance of each BTS
through a special algorithm and also detects each's signal strength. The mobile station will now
synchronise with the BTS with the best signal sthrength (usually the nearest BTS) and tunes to
it's radio channel frequency. The radio frequency channel used by the BTS is divided into
consecutive periods of time, each one called a Time Division Multiple Access (TDMA) frame.
Each TDMA frame consists of eight shorter periods of time called timeslots. The radio carrier
signal between the mobile station and the BTS is divided into a continuous stream of timeslots
which in turn are transmitted in a continuous stream of TDMA frames.
With the help of a synchronising signal in a TDMA frame broadcast from the BTS, Obanla's
mobile station synchronises itself with the MTN network.
25
35. Figure 3.10: Synchronization of the mobile station with the network.
The timeslots of the TDMA frame represent the physical channels and their contents are
organized into logical channels which are divided into two types, namely:
1. Dedicated channels, and
2. Common channels
A dedicated channel is a logical channel that is allocated exclusively to one mobile station to
transmit speech and data signals. A dedicated channel is also known as traffic cahnnel. But a
common channel is a logical channel used for broadcasting signals to numerous mobile station at
the same time.
After Obanla's mobile station has synchronised itself with the MTN network, the next
process before he can be able to set up a call is registration. The mobile station will make a
request for a logical channel to establish connection with the VLR to inform it about its new
location and routing information. The network acknowledges the request and allocates a logical
chennel. Then the mobile station moves into the allocated channel for further transmissions to
inform the MTN network about its whereabout and how it can be reached.
26
36. Figure 3.11: Channel request and allocation.
Once the mobile station is registered in the network, Obanla can now dial Ajegunle's phone
number. When this done, Obanla's mobile station requests for a dedicated/traffic channel to
Ajegunle's mobile station. This request is sent through the BTS that provides services for
Obanla's mobile station and the BSC that controls the BTS, to the MSC that controls the BSC,
the MSC checks the information coded into Ajegunle's phone number to determine his mobile
station's HLR which will have the routing address to the MSC/VLR (MSC and VLR is usually
located in the same physical equipment) servicing Ajegunle's mobile station. The MSC/VLR
servicing Ajegunle's mobile station is sent this traffic channel request, this MSC will now signal
the BTS servicing the mobile station, which will in turn page (send a special broadcast) to all the
mobile stations synchronised through it. Only Ajegunle's mobile station will respond to the
paging signal, first by ringing and later sending an 'I am busy' signal or a 'I am available' signal,
the MSC will now create a traffic channel from the mobile station through the other MSC to
Obanla's mobile station (provided the mobile station sent an 'I am available' signal). This traffic
channel is then used to establish an audio conversation between Obanla and Ajegunle.
27
37. The Network Management Subsystem only receives information about the call quality, time
of call, subscribers involved and possible faults, which MTN use for maintenance and upgrading
of their network services.
As for the charging, this is done by a separate network element called Biller.
Figure 3.12: A summary of the GSM architecture.
28
38. CHAPTER FOUR
4.0 BASE TRANSCEIVER STATION RADIO ACCESS LINK
The Base Transceiver Station radio access link is the segment of the BTS that deals with the
transmission of the microwave frequency signals.
The GSM network relies on microwave frequencies (3 – 30GHz) to transmit the digital signals
of large bandwidth between the BTSs, an MSC and a BTS. Only the MS operates on the
standardized GSM 800, 900, 1800 and 1900 frequency bands to communicate with the BTS at a data
rate of 13kbits/s for full rate and enhanced full rate compression, or 5.6kbits/s for half rate
compression. The BTS multiplexes all these signals into a Synchronous Transport Module (STM) of
bandwidths 155.52Mbits/s for STM-1, 311Mbits/s for STM-2 and 622Mbits/s for STM-4, which
will be transmitted to the next BTS, an MSC or a BSC. These high bandwidth signals can only be
effectively transmitted via a very high frequency electromagnetic wave commonly referred to as
microwave.
Table 4.1: The standard frequency allocation table
ELECTROMAGNETIC
WAVE TYPE
FREQUENCY
RANGE
WAVELENGTH
(metres)
CCIR CODE
Very Low Frequency
(VLF)
300 Hz – 30KHz 10,000 – 100,000 4
Low Frequency (LF) 30Khz – 300KHz 1,000 – 10,000 5
Medium Frequency (MF) 300Khz – 3MHz 100 – 1,000 6
High Frequency (HF) 3MHz - 30MHz 10 - 100 7
Very High Frequency
(VHF)
30MHz - 300MHz 1 – 10 8
Ultra High Frequency
( UHF)
300MHz - 3GHz 0.1 - 1 9
Super High Frequency
(SHF)
3GHz – 30GHz 0.01 – 0.1 10
29
39. Extremely High Frequency
(EHF)
30GHz - 300GHz 0.001 – 0.01 11
The Base Transceiver Stations are arranged such that each BTS uses a frequency channel that is
distinct from all adjacent BTSs (to communicate with the mobile stations within its reach). This is to
reduce co-channel interference that will result from two or more adjacent BTSs transmitting on the
same frequency channel, hereby leading to cross-talks and reduction in Quality of Service. Ideally,
BTSs cover a circular area of radius, R, but for physical planning purpose this will give overlapped
cells which will make planning extremely complex. So, hexagonal cells are used to represent the
BTS coverage area because it gives no gap and overlap. The minimum distance which will allow the
same frequency channel to be reused will depend on many factors, such as the type of geographic
terrain, the transmitter power, and the desired customer capacity per BTS. (William,1989).
The Radio access link of the BTS in the latest Nokia Siemens Networks implementation
comprises,
1. The Synchronous Radio Access STM-4 (SRA 4) unit,
2. The coaxial cable,
3. An outdoor frequency converter and signal amplifier unit (ODU), and
4. Directional antenna.
4.1 THE SYNCHRONOUS RADIO ACCESSS STM-4 (SRA 4) UNIT
The SRA 4 is a Nokia Siemens Networks proprietary equipment that multiplexes several low
bandwidth signals into a high bandwidth signal for transmission via microwave frequencies or an
optical fibre.
30
40. Figure 4.1: The SRA 4 unit
The SRA 4 is an intelligent device that does more than multiplexing but also, intelligently chooses
the modulation format to use and the path to use in getting to a specific destination/receiver. It is
configured using the Local Craft terminal and the NetBuilder, and it can be used to monitor the
operation of most directly connected network elements.
Figure 4.2: Local Craft Terminal software
31
41. Figure 4.3: The NetBuilder software.
Figure 4.4: On site testing of connectivity between two BTSs using the SRA 4 handset.
32
42. Figure 4.5: The SRA 4 unit fully connected to the other network units
4.2 THE COAXIAL CABLE
The Coaxial cable is a cable consisting of an inner conductor, surrounded by a tubular insulating
layer typically made from a flexible material with a high dielectric constant, all of which is then
surrounded by another conductive layer (typically of fine woven wire for flexibility, or of a thin
metallic foil), and then finally covered again with a thin insulating layer on the outside. The term
coaxial comes from the inner conductor and the outer shield sharing the same geometric axis.
Coaxial cables are often used as a transmission line for radio frequency signals. In a hypothetical
ideal coaxial cable, the electromagnetic field carrying the signal exists only in the space between the
inner and outer conductors. Practical cables achieve this objective to a high degree. A coaxial cable
provides protection of signals from external electromagnetic interference, and effectively guides
signals with low emission along the length of the cable. (Wikepedia, 2008).
It is the coaxial cable that transmits signals between the SRA 4 and the Outdoor Unit (ODU).
33
43. Figure 4.6: The coaxial cables connecting the indoor units to the outdoor units.
Figure 4.7: The coaxial cables entering into the BTS shelter.
34
44. 4.3 THE OUTDOOR FREQUENCY CONVERTER AND SIGNAL AMPLIFIER UNIT
(ODU)
The ODU is the unit always closely attached to the antenna which converts the signals from the
antenna into a form that can easily transmitted down the coaxial cable by impedance matching, it
converts the high frequency microwave signal into a lower intermediate frequency, and amplifies
the signal received from the antenna.
Figure 4.8: An ODU
Figure 4.9: An antenna with two ODUs closely attached
35
45. In Nokia Siemens Networks implementation, the ODUs are of two major types, namely:
1. The 6 – 13GHz ODU
This is the one used in all the Base Transceiver Stations setup by Nokia Siemens Networks in
Nigeria.
Figure 4.10: The 6 – 13GHz ODU
2. The 15 – 38 GHz ODU (Not currently used by Nokia Siemens Networks in Nigeria)
Figure 4.11: The 15 – 38GHz ODU
36
46. 4.4 DIRECTIONAL ANTENNA
An antenna is a tranducer that generates a radiating electromagnetic field in response to an
applied alternating voltage and the associated alternating electric current, or can be placed in an
electromagnetic field so that the field will induce an alternating current in the antenna and a voltage
between its terminals. (Wikipedia, 2008)
There are two fundamental types of antennae, namely:
1. Omni-directional antenna that radiates or receives electromagnetic wave in all direction
equally.
2. Directional or uni-directional antenna that radiates or receive electromagnetic wave better
in a specific direction than in all other directions.
In the GSM network, high performance shielded directional antennae are used to transmit
signals from one BTS to another BTS or an MSC. The polarization, which is the orientation of the
electric field vector of the electromagnetic wave produced by the antenna, is horizontal.
Figure 4.12: The different polarizations
37
48. Figure 4.14: Mutual impedance between parallel /2 dipoles not staggered. Curves Re and Im
are the resistive and reactive parts of the impedance.
Current circulating in any antenna induces currents in all others. One can postulate a mutual
impedance between two antennas that have the same significance as the in ordinary
coupled inductors. The mutual impedance between two antennas is defined as:
where is the current flowing in antenna 1 and is the voltage that would have to be applied
to antenna 2 – with antenna 1 removed – to produce the current in the antenna 2 that was produced
by antenna 1.
From this definition, the currents and voltages applied in a set of coupled antennas are:
39
49. where:
• is the voltage applied to the antenna i
• is the impedance of antenna i
• is the mutual impedance between antennas i and j
Note that, as is the case for mutual inductances,
Gain (directivity): This is a measure of the degree to which an antenna focuses power in a
given direction, relative to the power radiated by a reference antenna in the same direction. Units of
measure are dBi (isotopic antenna reference) or dBd (half-wave dipole reference). The two gain
measurements can be converted using the following formula:
dBi = dBd + 2.1
If the directivity of the transmitting and receiving antennas is known, it is possible to compute
the power received by the receiving antenna using either of the formulas below:
When using dB:
PRECEIVED = PTRANSMITTER + GT + GR + 20log(λ) – 20log(d) – 21.98
Antenna gain should be expressed in dBi, wavelength and distances in m and powers in dBm or
dBW.
When using gain ratios and powers in W:
PRECEIVED = PTRANSMITTERGTGRλ2
16π2
d2
Where, GT is the gain of the transmitter
GR is the gain of the receiver
d is the distance between the transmitter and the receiver in metres
λ is the wavelength in metres
40
50. 4.5 TWISTED PAIR COPPER CABLE
Sometimes, remote connections are made to monitoring the Network Monitoring Subsystem or
a Base Station Controller through twisted copper cables. In Nokia Siemens Networks
implementation, we use the 21 pair twisted copper cable to form the standard E1 (2.048 Mbits/s)
European standard pleisynchronous transport module line.
Figure 4.13: The 21 pair twisted cables being made into E1 transmission lines
41
51. Figure 4.14: The specialized crimper and clamp for fixing the E1 DB-32 connectors
42
52. Figure 4:15: A general tool box.
Also, copper cables are used to connect between the Digital Distribution Frames, EWSD
Switches, and Multiplexers in the Gateway MSC connecting to the Public Land Mobile Networks.
43
56. CHAPTER FIVE
5.0 CONCLUSION AND RECOMMENDATION
Conclusively, the SIWES programme has given me the priviledge and opportunity of
understanding the fundamentals of telecommunications and how the mobile telecommunications
network operates. I now clearly see how all I have been taught in the lecture hall fit together to be
applied in the designing of telecommunications systems and in particular the GSM network. From
the elementary mathematics used in calculating and mapping the frequency reuse chart to the
advanced mathematics used in designing the compression codes, modulation techniques and cross-
correlation of signals to eliminate interference. In this report, I explained in a much simplified form
the principle and system of operation of the GSM network with much emphasis on the Base
Transceiver Station Radio Access Link.
I was able to train as a Cisco Certified Network Associate and a Oracle 10g Database
Administration Certified Associate. I was also, able to work on the Linux edition of Unix Operating
Software and understand its peculiar use as a operating system for most backbone central computer
or server. The experience and knowledge I have gained during the SIWES will be greatly beneficial
to my career as an Electrical Electronics Engineer.
I will like to make the following recommendations to help make the SIWES programme more
effective:
◦ The University should partner with some companies in order to send them in a specified
pattern, some students from relevant departments to undertake their SIWES at those
companies. This will help lessen the problem of students not getting a company that will
provide experience that is relevant to their field of study.
◦ The monthly stipend promised by the Industrial Training Fund (as stated in the SIWES
placement request letter) should be promptly paid to ease the financial pressure on students
during the SIWES programme.
◦ The SIWES supervisors should be made to visit every student at their company of
placement at least twice.
47
57. REFERENCES
Macario R. C. V. (1997): “Cellular Radio Principles and design” Second editions, Macmillan
Press, London. pg7-12
Nokia Networks (2002): “GSM Air Interface & Network Planning Training Document”. Nokia
Networks Oy, Finland
Nokia Networks (2002): “GSM Architecture Training Document”. Nokia Networks Oy,
Finland
William C. Y. L. (1989): “Mobile Cellular Telecommunications System” McGraw-Hill, New
York. pg26-27
http://en.wikipedia.org/wiki/Antenna_(radio) (2008): “Antenna (radio) - Wikipedia, the free
encyclopedia”
http://en.wikipedia.org/wiki/Base_Transceiver_Station (2008): “Base transceiver station -
Wikipedia, the free encyclopedia”
http://www.iec.org (2006):“Fiber-Optic Technology” Coring
http://www.images.google.com_images_hl=en&q=telephone (2008): “Telephone – Google
Image Search”
http://www.images.google.com_images_hl=en&q=WIRELESS (2008): “WIRELESS - Google
Image Search”
http://www.nokiasiemensnetworks.com/about_us (2008): “Introduction to Nokia Siemens
Networks”
http://www.tech-faq.com/coaxial-cable.shtml (2008): “What is a Coaxial Cable?”
48
