ALTTC TRAINING REPORT
TELECOMMUNICATION NETWORK AND
AMBRISH KUMAR SHUKLA (1002931012)
(ELECTRONICS AND COMMUNICATION ENGINEERING)
Krishna Institute of Engineering & Technology
Ghaziabad – Meerut Highway (NH – 58)
Uttar Pradesh, INDIA
“It is not possible to prepare a training report without the assistance &
encouragement of other people. This one is certainly no exception.”
On the very outset of this report, I would like to extend my sincere and
heartfelt obligation towards all the personages who have helped me in this
endeavor. Without their active guidance, help, cooperation and
encouragement, I would not have made head way in the report. First and
foremost, I would like to express my sincere gratitude to my training guide,
I was privileged to experience a sustained enthusiastic and involved interest
from her side. This fuelled my enthusiasm even further and encouraged me to
boldly step into what was a totally dark and unexplored expanse before
me.She always fuelled my thoughts to think broad and out of the box.
I would also like to thank all employee of ALTTC for organizing and
permitting the Vocational training program for us.
Ambrish Kumar Shukla
Objectives and Scope of Training
The objective of training is to fetch the information about various technical
areas concerning Telecommunication fundamental, Digital communication,
Broadband, GSM, CDMA, Wi-MAX, Wi-Fi,
Microwave communication, Digital Switching, Advance Optical Network,
Radio Communication and Mobile Communication.
The training made me learn about the transmitting and receiving the signal and
finally establishing communication path. The practical use of electronics as well
as communication can be understood with this training.
TABLE OF CONTENTS:
(1) About ALTTC
(4) Power Line Communication (PLC)
(5) Free Space Optics (FSO)
Advanced Level Telecom Training Centre (ALTTC), Ghaziabad is the apex
training institute of BSNL.ALTTC was set up as a joint venture of
International Telecommunication Union, Geneva, UNDP and the Government
of India in 1975. ALTTC functions on the frontiers of telecom technology,
finance and management and imparts training to the leaders in the business. The
strength of ALTTC lies in the state of art labs, massive infrastructure and
trained, talented and qualified human resource pool.
The Centre's Mission statement is:
"To Deliver Excellence Through Training"
The training areas cover vast spectrum of topics such as Digital Switching and
IN; Mobile Communication: GSM, 3G, CDMA; Data communication and
Information Technology: MPLS, VPN, Broadband, IPv6, Database
Administration, Server administartion, IT Security; Optical Networks: SDH,
DWDM, NGSDH, NGN, Access Networks, Management, Telecom Finance,
Building Science (Civil and Electrical) and Telecom Network Planning.
The term BROADBAND refers to high speed internet access. It is non-specific
term. In fact there is no specific international definition for broadband.
As the Internet market continues to grow, demand for greater BW and faster
connection speed have led to broadband access to all consumer.
The rapid growth of distributed business application, e-commerce and BW
intensive application (such as multimedia, video conferencing and video-ondemand generate the demand for BW and access network.
NEED OF BROADBAND
PROFESSIONAL ACTIVITIES:1. Telecommuting
2. Video conferencing
3. Home based business
4. Home office
ENTERTAINMENT ACTIVITIES:1. Web surfing
2. Video on-demand
3. Video games
CONSUMER ACTIVITIES:1. Telemedicine
2. Distance learning
3. Information gathering
5. Video conferencing among friends and family
In India DoT has issued a broadband policy in 2004, keeping in mind,
Broadband connectivity is defined as“A data connection which has capability of minimum download speed of 256
kbps is said to be broadband.”
In 2012 new NTP was announced and Broadband speed was revised to 2 mbps
in place of 256 kbps.
Practically obtained speed is 56 kbps.
Types of Broadband Services:
DIGITAL SUBSCRIBER LINE (DSL)
DSL is a family of technology that provides high speed Internet access by
transmitting digital data over the wires of a local telephone network.
DSL service is delivered simultaneously with wired telephone service on the
same telephone line.
DSL uses higher frequency band for data transmission.
The bit rate of consumer DSL service typically ranges from 256 kbps-40mbps
in downstream direction depending on DSL technology used, line condition and
service level implementation.
WHY USE DSL?
Traditional MODEM can provide data rate up to 56 kbps, to achieve high speed
internet access another techniques named DSL was used.
Sampling rate of telephone company =8000 samples /sec. Each sample is
represented by 8 bits. One bit is used for control purpose. Hence each sample is
effectively represented by 7 bits.
Data rate =8000*7=56000 bits/sec i.e 56 kbps
WHERE DSL IS USED
DSL service is used on a local telephone line. As telephone line is twisted pair
cable capable of handling BW upto 1.1 MHz. But voice utilizes only 4 KHz
BW. So to enhance the efficiency of cable pair , a portion of large BW is
utilized for data communication.
TYPES OF DSL SERVICE:
SDSL- Symmetric Digital Subscriber Line
ADSL- Asymmetric Digital Subscriber Line
HDSL- High Bit Rate Digital Subscriber Line
VDSL- Very High Bit Rate Digital Subscriber Line
Symmetric DSL (SDSL)
UPLOAD SPEED = DOWNLOAD SPEED
Upstream= 768 kbps
Downstream= 768 kbps
Ex. - Suitable for residential subscriber who need equal speed in both
HDSL AND VDSL
Downstream = 1.544 - 2 Mbps
Upstream = 1.544 – 2 Mbps
22 – 55 Mbps
Downstream = Upstream = 34 Mbps (If Symmetric)
Asymmetrical DSL (ADSL)
1. ADSL is an asymmetric communication technology designed for
residential users; it is not suitable for businesses.
2. Downstream data rate is greater than upstream data rate.
3. Downstream = 1.5 – 8 Mbps
Upstream = 16 – 640 Kbps
MODULATION TECHNIQUE FOR ADSL
The modulation technique that is used for ADSL is Discrete Multi-tone
Modulation (DMT). It combines FDM and QAM.
In DMT an available BW of 1.104 MHz is divided into 256 parallel stream.
Each parallel stream is known as sub channel or tone or bin or bucket.
Each channel uses a BW of 4.312 KHz and can carry maximum 15 bits.
Voice – Channel 0
Upstream = Channel 6 to 30 (25 channels)
Downstream = Channel 31 to 255 (255 channels)
DMT FREQUENCY SPECTRUM
Digital Subscriber line Access Multiplexer
A DSLAM is a Network Device, usually placed at a Telephone Company
Central Office, That Receives Signals from multiple customer of DSL
Connections and puts the signals on a high-speed backbone line using
DSLAM enables a phone company to offer business or home users the
fastest Phone Line Technology (DSL) with the fastest backbone Network
WiMAX is a standards-based technology enabling the delivery of last mile
wireless broadband access as an alternative to wired broadband like cable and
DSL. Wi-MAX provides fixed, nomadic, portable and, soon, mobile wireless
broadband connectivity without the need for direct line-of-sight with a base
station. In a typical cell radius deployment of three to ten kilometers, Wi-MAX
Forum Certified systems can be expected to deliver capacity of up to 40 Mbps
per channel, for fixed and portable access applications.
This is enough bandwidth to simultaneously support hundreds of businesses
with T-1 speed connectivity and thousands of residences with DSL speed
connectivity. Mobile network deployments are expected to provide up to 15
Mbps of capacity within a typical cell radius deployment of up to three
kilometers. It is expected that WiMAX technology will be incorporated in
notebook computers and PDAs by 2007, allowing for urban areas and cities to
become "metro zones" for portable outdoor broadband wireless access.
The bandwidth and range of Wi-MAX make it suitable for the following
• Connecting Wi-Fi hotspots with other parts of the house hold.
• Providing a wireless alternative to cable and DSL for "last mile” broadband
• Providing data and telecommunications services.
• Providing a source of Internet connectivity as part of a business continuity
plan. That is, if a business has a fixed and a wireless Internet connection,
especially from unrelated providers, they are unlikely to be affected by the same
Standards Associated With Wimax
IEEE 802 refers to a family of IEEE standards dealing with local area networks
and metropolitan area networks. More specifically, the IEEE 802 standards are
restricted to networks carrying variable-size packets. (By contrast, in cell-based
networks data is transmitted in short, uniformly sized units called cells.
Isochronous networks, where data is transmitted as a steady stream of octets, or
groups of octets, at regular time intervals, are also out of the scope of this
standard.) The number 802 was simply the next free number IEEE could assign,
though “802” is sometimes associated with the date the first meeting was held
— February 1980.
IEEE 802.16 : The IEEE 802.16 Working Group on Broadband Wireless
Access Standards, which was established by IEEE Standards Board in 1999,
aims to prepare formal specifications for the global deployment of broadband
Wireless Metropolitan Area Networks. The Workgroup is a unit of the IEEE
802 LAN/MAN Standards Committee. A related future technology Mobile
Broadband Wireless Access (MBWA) is under development in IEEE 802.20.
Although the 802.16 family of standards is officially called Wireless MAN, it
has been dubbed “Wi-MAX” (from "Worldwide Interoperability for Microwave
Access") by an industry group called the Wi-MAX Forum. The mission of the
Forum is to promote and certify compatibility and interoperability of broadband
In January 2003, the IEEE approved 802.16a as an amendment to IEEE 802.162001, defining (Near) Line-Of- Sight capability.
• In July 2004, IEEE 802.16REVd, now published under the name IEEE
802.16-2004,introduces support for indoor CPE (NLOS) through additional
radio capabilities such as antenna beam forming and OFDM sub-channeling.
• Early 2005, an IEEE 802.16e variant will introduce support for mobility.
Possible services provided by Wi-MAX are widespread over various data
communication services including entertainment, information and commerce
services. The first round of Wi-MAX technology is expected to be nomadic,
meaning that CPEs will be portable, but not truly mobile. But with Samsung’s
new developments on hand-over, the technology may become truly mobile,
offering the 20 Mb/s to 30 Mb/s at speeds up to 120 km/h Wi-MAX enthusiasts
are touting. For entertainment services, Wi-MAX will provide high quality
VoD/MoD/AoD, real- time streaming broadcasting, 3G network games and
MMS. Web Browsing, file downloading and interactive information services
will be provided as information services by Wi-MAX.
Power line communication (PLC)
Power line communication (PLC) carries data on a conductor that is also used
simultaneously for AC electric power transmission or electric power
distribution to consumers. It is also known as power line carrier, power line
digital subscriber line (PDSL), mains communication, power line
telecommunications, or power line networking (PLN).
A wide range of power line communication technologies are needed for
different applications, ranging from home automation to Internet access which
is often called broadband over power lines (BPL). Most PLC technologies limit
themselves to one type of wires (such as premises wiring within a single
building), but some can cross between two levels (for example, both the
distribution network and premises wiring). Typically transformers prevent
propagating the signal, which requires multiple technologies to form very large
networks. Various data rates and frequencies are used in different situations.
A number of difficult technical problems are common between wireless and
power line communication, notably those of spread spectrum radio signals
operating in a crowded environment. Radio interference, for example, has long
been a concern of amateur radio groups.
FREE SPACE OPTICS (FSO)
Free-space optical communication (FSO) is an optical
communication technology that uses light propagating in free space to transmit
data for telecommunications or computer networking. "Free space" means air,
outer space, vacuum, or something similar. This contrasts with using solids such
as optical fiber cable or an optical transmission line. The technology is useful
where the physical connections are impractical due to high costs.
OPTICAL COMMUNICATION SYSTEM
Fundamentals of Optical Fiber Systems:
Light plays a vital role in our daily lives. It is used in compact disc (CD)
players, in which a laser reflecting off a CD transforms the returning signal into
music. It is used in grocery store checkout lines, where laser beams read bar
codes for prices. It is used by laser printers to record images on paper. It is used
in digital cameras that capture our world and allow pictures to be displayed on
the Internet. It is the basis of the technology that allows computers and
telephones to be connected to one another over fiber-optic cables. And light is
used in medicine, to produce images used in hospitals and in lasers that perform
BASIC FIBER OPTIC COMMUNICATION SYSTEM
Fiber optics is a medium for carrying information from one point to another in
the form of light. Unlike the copper form of transmission, fiber optics is not
electrical in nature. A basic fiber optic system consists of a transmitting device
that converts an electrical signal into a light signal, an optical fiber cable that
carries the light, and a receiver that accepts the light signal and converts it back
into an electrical signal. The complexity of a fiber optic system can range from
very simple (i.e., local area network) to extremely sophisticated and expensive
(i.e., long- distance telephone or cable television trunking). For example, the
system shown in Figure 1 could be built very inexpensively using a visible
LED, plastic fiber, a silicon photo-detector, and some simple electronic
circuitry. On the other hand, a typical system used for long-distance, highbandwidth telecommunication could cost tens or even hundreds of thousands of
Basic fiber optic communication system
The basic question is, "How much information is to be sent and how far does it
have to go?" With this in mind we will examine the various components that
make up a fiber optic communication system and the considerations that must
be taken into account in the design of such systems.
Figure : Typical Fiber Optic Cable
Advantage of Optical Fiber Communication:
Enormous potential bandwidth: The optical carrier frequency has a far greater
potential transmission BW than metallic cable systems.
Small size and weight: Optical fiber has small diameters. Hence, even when
such fibers are covered with protective coating they are far smaller and lighter
than corresponding copper cables.
Electrical Isolation: Optical fibers which are fabricated from glass or
sometimes a plastic polymer are electrical insulators and unlike their metallic
counterpart, they do not exhibit earth loop or interface problems. This property
makes optical fiber transmission ideally suited for communication in electrically
hazardous environments as fiber created no arcing or spark hazard at abrasion or
Signal security: The light from optical fiber does not radiate significantly and
therefore they provide a high degree of signal security. This feature is attractive
for military, banking and general data transmission i.e. computer networks
Low transmission loss: The technological developments in optical fiber over
last twenty years has resulted in optical cables which exhibits very low
attenuation or transmission loss in comparison with best copper conductors.
Potential low cost: The glass which provides the optical fiber transmission
medium is made from sand. So, in comparison to copper conductors, optical
fiber offers the potential for low cost line communication.
Disadvantage of Optical Fiber Communication:
1. It requires a higher initial cost in installation
2. Although the fiber cost is low, the connector and interfacing between the
fiber optic costs a lot.
3. Fiber optic requires specialized and sophisticated tools for maintenance
Basic Law of Optics
Total Internal Reflection:
Optical fibers work on the principle of total internal reflection
The angle of refraction at the interface between two media is governed by
Refraction & Total Internal Reflection
Multimode optical fiber will only propagate light that enters the fiber within a
certain cone, known as the acceptance cone of the fiber. The half-angle of this
cone is called the acceptance angle, θmax ,
Numerical Aperture :
Numerical Aperture is the measurement of the acceptance angle of an optical
fiber, which is the maximum angle at which the core of the fiber will take in
light that will be contained within the core. Taken from the fiber core axis
(center of core), the measurement is the square root of the squared refractive
index of the core minus the squared refractive index of the cladding.
The numerical aperture of the fiber is closely related to the critical angle and
is often used in the specification for optical fiber and the components that work
The numerical aperture is given by the formula:
Types of Optical Fiber
Optical fibers come in two main types. Single-mode fiber has a small core that
forces the light waves to stay in the same path, or mode. This keeps the light
signals going farther before they need to be beefed up, or amplified. Most longdistance, or long-haul, fiber optic telephone lines use single-mode fiber.
The second type, called multimode fiber, has a much larger core than singlemode fiber. This gives light waves more room to bounce around inside as they
travel down the path. The extra movement eventually causes the pulses to
smear, and lose information. That means multimode fiber signals can’t travel as
far before they need to be cleaned up and re- amplified. Multimode fibers can
carry only a third or less the
Information carrying capacity—or bandwidth—than single-mode fiber and they
won't work for long distances.
Network engineers prefer multimode fiber for shorter-distance communication,
such as in an office building or a local area network (LAN), because the
technology is less expensive. However, with the growing demand for more
bandwidth between computers and over the Internet, single-mode fiber is
becoming more popular for smaller networks, too.
Therefore, we can categorize the fiber optic communication in two categories:
1. Step Index
a) Single Mode
2. Graded Index
These types of fibers have sharp boundaries between the core and cladding,
with clearly defined indices of refraction. The entire core uses single index of
Single Mode: Single mode fiber has a core diameter of 8 to 9 Step Index
microns, which only allows one light path or mode.
Multimode Step-Index Fiber: Multimode fiber has a core diameter of 50 or
62.5 microns (sometimes even larger). It allows several light paths or modes .
This causes modal dispersion – some modes take longer to pass through the
fiber than others because they travel a longer distance
Multimode Graded-Index Fiber
Graded-index refers to the fact that the refractive index of the core gradually
decreases farther from the center of the core. The increased refraction in the
center of the core slows the speed of some light rays, allowing all the light rays
to reach the receiving end at approximately the same time, reducing dispersion.
The light rays no longer follow straight lines; they follow a serpentine path
being gradually bent back toward the center by the continuously declining
refractive index. This reduces the arrival time disparity because all modes arrive
at about the same time. The modes traveling in a straight line are in a higher
refractive index, so they travel slower than the serpentine modes. These travel
farther but move faster in the lower refractive index of the outer core region.
Attenuation and pulse dispersion represent the two most important
characteristics of an optical fiber that determine the information-carrying
capacity of a fiber optic communication system. The decrease in signal strength
along a fiber optic waveguide caused by absorption and scattering is known as
attenuation. Attenuation is usually expressed in dB/km.
Dispersion, expressed in terms of the symbol ∆t, is defined as pulse spreading in
an optical fiber. As a pulse of light propagates through a fiber, elements such as
numerical aperture, core diameter, refractive index profile, wavelength, and
laser line width cause the pulse to broaden. This poses a limitation on the
overall bandwidth of the fiber .
GSM (Global System for Mobile Communications, originally Groupe
Spécial Mobile), is a standard set developed by the European
Telecommunications Standards Institute (ETSI) to describe protocols for second
generation (2G) digital cellular networks used by mobile phones. It became the
de facto global standard for mobile communications with over 80% market
The GSM standard was developed as a replacement for first generation (1G)
analog cellular networks, and originally described a digital, circuit-switched
network optimized for full duplex voice telephony. This was expanded over
time to include data communications, first by circuit-switched transport, then
packet data transport via GPRS (General Packet Radio Services)
and EDGE (Enhanced Data rates for GSM Evolution or EGPRS).
Further improvements were made when the 3GPP developed third generation
(3G) UMTS standards followed by fourth generation (4G) LTE Advanced
"GSM" is a trademark owned by the GSM Association. It may also refer to the
initially most common voice codec used, Full Rate
The network is structured into a number of discrete sections:
Base Station Subsystem – the base stations and
Network and Switching Subsystem – the part of
the network most similar to a fixed network, sometimes just called the "core
GPRS Core Network – the optional part which
allows packet-based Internet connections.
Operations support system (OSS) – network
Base Station Subsystem
GSM is a cellular network, which means that cell phones connect to it by
searching for cells in the immediate vicinity. There are five different cell sizes
in a GSM network—macro, micro, pico, femto, and umbrella cells. The
coverage area of each cell varies according to the implementation environment.
Macro cells can be regarded as cells where the base station antenna is installed
on a mast or a building above average rooftop level. Micro cells are cells whose
antenna height is under average rooftop level; they are typically used in urban
areas. Pico cells are small cells whose coverage diameter is a few dozen metres;
they are mainly used indoors. Fem to cells are cells designed for use in
residential or small business environments and connect to the service provider’s
network via a broadband internet connection. Umbrella cells are used to cover
shadowed regions of smaller cells and fill in gaps in coverage between those
Cell horizontal radius varies depending on antenna height, antenna gain, and
propagation conditions from a couple of hundred metres to several tens of
kilometres. The longest distance the GSM specification supports in practical use
is 35 kilometres (22 mi). There are also several implementations of the concept
of an extended cell, where the cell radius could be double or even more,
depending on the antenna system, the type of terrain, and the timing advance.
Indoor coverage is also supported by GSM and may be achieved by using an
indoor pico cell base station, or an indoor repeater with distributed indoor
antennas fed through power splitters, to deliver the radio signals from an
antenna outdoors to the separate indoor distributed antenna system. These are
typically deployed when significant call capacity is needed indoors, like in
shopping centres or airports. However, this is not a prerequisite, since indoor
coverage is also provided by in-building penetration of the radio signals from
any nearby cell.
GSM carrier frequency
GSM networks operate in a number of different carrier frequency ranges
(separated into GSM frequency ranges for 2G and UMTS frequency bands for
3G), with most 2G GSM networks operating in the 900 MHz or 1800 MHz
bands. Where these bands were already allocated, the 850 MHz and 1900 MHz
bands were used instead (for example in Canada and the United States). In rare
cases the 400 and 450 MHz frequency bands are assigned in some countries
because they were previously used for first-generation systems.
Most 3G networks in Europe operate in the 2100 MHz frequency band. For
more information on worldwide GSM frequency usage, see GSM frequency
Regardless of the frequency selected by an operator, it is divided
into timeslots for individual phones. This allows eight full-rate or sixteen halfrate speech channels per radio frequency. These eight radio timeslots
(or burst periods) are grouped into a TDMA frame. Half-rate channels use
alternate frames in the same timeslot. The channel data rate for all 8
channels is 270.833 kb it/s, and the frame duration is 4.615 msec.
The transmission power in the handset is limited to a maximum of 2 watts
in GSM 850/900 and 1 watt in GSM 1800/1900.
GSM has used a variety of voice codecs to squeeze 3.1 kHz audio into between
6.5 and 13 kbit/s. Originally, two codecs, named after the types of data channel
they were allocated, were used, called Half Rate (6.5 kbit/s) and Full
Rate (13 kbit/s). These used a system based on linear predictive coding (LPC).
In addition to being efficient with bitrates, these codecs also made it easier to
identify more important parts of the audio, allowing the air interface layer to
prioritize and better protect these parts of the signal.
As GSM was further enhanced in 1997 with the Enhanced Full Rate (EFR)
codec, a 12.2 kb it/s codec that uses a full-rate channel. Finally, with the
development of UMTS, EFR was refactored into a variable-rate codec
called AMR-Narrowband, which is high quality and robust against interference
when used on full-rate channels, or less robust but still relatively high quality
when used in good radio conditions on half-rate channels.
Subscriber Identity Module (SIM)
One of the key features of GSM is the Subscriber Identity Module, commonly
known as a SIM card. The SIM is a detachable smart card containing the user's
subscription information and phone book. This allows the user to retain his or
her information after switching handsets. Alternatively, the user can also change
operators while retaining the handset simply by changing the SIM. Some
operators will block this by allowing the phone to use only a single SIM, or only
a SIM issued by them; this practice is known as SIM locking
Sometimes mobile network operators restrict handsets that they sell for use with
their own network. This is called locking and is implemented by a software
feature of the phone. A subscriber may usually contact the provider to remove
the lock for a fee, utilize private services to remove the lock, or use software
and websites to unlock the handset themselves.
In some countries (e.g., Bangladesh, Brazil, Chile, Hong
Kong, India, Lebanon, Malaysia, Nepal, Pakistan, Singapore) all phones are
sold unlocked. In others (e.g., Singapore) it is unlawful for operators to offer
any form of subsidy on a phone's price.
The Switching System
The switching system (SS) is responsible for performing call processing and
subscriber-related functions. The switching system includes the following
• Home location register (HLR)—The HLR is a database used for storage and
management of subscriptions. The HLR is considered the most important
database, as it stores permanent data about subscribers, including a subscriber's
service profile, location information, and activity status. When an individual
buys a subscription from one of the PCS operators, he or she is registered in the
HLR of that operator.
• Mobile services switching centre (MSC)—The MSC performs the telephony
switching functions of the system. It controls calls to and from other telephone
and data systems. It also performs such functions as toll ticketing, network
interfacing, common channel signaling, and others.
• Visitor location register (VLR)—The VLR is a database that contains
temporary information about subscribers that is needed by the MSC in order to
service visiting subscribers. The VLR is always integrated with the MSC. When
a mobile station roams into a new MSC area, the VLR connected to that MSC
will request data about the mobile station from the HLR. Later, if the mobile
station makes a call, the VLR will have the information needed for call setup
without having to interrogate the HLR each time.
• Authentication centre (AUC)—A unit called the AUC provides
authentication and encryption parameters that verify the user's identity and
ensure the confidentiality of each call. The AUC protects network operators
from different types of fraud found in today's cellular world.
• Equipment identity register (EIR)—The EIR is a database that contains
information about the identity of mobile equipment that prevents calls from
stolen, unauthorized, or defective mobile stations. The AUC and EIR are
implemented as stand-alone nodes or as a combined AUC/EIR node.
The Base Station System (BSS)
All radio-related functions are performed in the BSS, which consists of base
station controllers (BSCs) and the base transceiver stations (BTSs).
BSC—The BSC provides all the control functions and physical links between
the MSC and BTS. It is a high-capacity switch that provides functions such as
handover, cell configuration data, and control of radio frequency (RF) power
levels in base transceiver stations. A number of BSCs are served by an MSC.
BTS—The BTS handles the radio interface to the mobile station. The BTS is
the radio equipment (transceivers and antennas) needed to service each cell in
the network. A group of BTSs are controlled by a BSC.
The Operation and Support System
The operations and maintenance centre (OMC) is connected to all equipment in
the switching system and to the BSC. The implementation of OMC is called the
operation and support system (OSS). The OSS is the functional entity from
which the network operator monitors and controls the system. The purpose of
OSS is to offer the customer cost-effective support for centralized, regional, and
local operational and maintenance activities that are required for a GSM
network. An important function of OSS is to provide a network overview and
support the maintenance activities of different operation and maintenance
Additional Functional Elements
Message centre (MXE)—The MXE is a node that provides integrated voice,
fax, and data messaging. Specifically, the MXE handles short message service,
cell broadcast, voice mail, fax mail, email, and notification.
Mobile service node (MSN)—The MSN is the node that handles the mobile
intelligent network (IN) services.
Gateway mobile services switching centre (GMSC)—A gateway is a node
used to interconnect two networks. The gateway is often implemented in an
MSC. The MSC is then referred to as the GMSC.
GSM interworking unit (GIWU)—The GIWU consists of both
Hard ware and software that provides an interface to various networks for data
communications. Through the GIWU, users can alternate between speech and
data during the same call. The GIWU hardware equipment is physically located
at the MSC/VLR.
GSM Network Areas
The GSM network is made up of geographic areas. As shown in Figure 3, these
areas include cells, location areas (LAs), MSC/VLR service areas, and public
land mobile network (PLMN) areas.
The cell is the area given radio coverage by one base transceiver station. The
GSM network identifies each cell via the cell global identity (CGI) number
assigned to each cell. The location area is a group of cells. It is the area in which
the subscriber is paged. Each LA is served by one or more base station
controllers, yet only by a single MSC. Each LA is assigned a location area
identity (LAI) number.
An MSC/VLR service area represents the part of the GSM network that is
covered by one MSC and which is reachable, as it is registered in the VLR of
MSC/VLR Service Areas
The PLMN service area is an area served by one network operator.
PLMN Network Areas
Before looking at the GSM specifications, it is important to understand the
following basic terms:
Bandwidth—the range of a channel's limits; the broader the bandwidth, the
faster data can be sent
Bits per second (bps)—a single on-off pulse of data; eight bits are equivalent
to one byte
Frequency—the number of cycles per unit of time; frequency is measured in
Kilo (k)—kilo is the designation for 1,000; the abbreviation kbps represents
1,000 bits per second
Megahertz (MHz)—1,000,000 hertz (cycles per second)
Milliseconds (msec)—one-thousandth of a second
Watt (W)—a measure of power of a transmitter.
Specifications for different personal communication services (PCS) systems
vary among the different PCS networks. Listed below is a description of the
specifications and characteristics for GSM.
Frequency band—the frequency range specified for GSM is 1,850 to 1,990
MHz (mobile station to base station).
Duplex distance—the duplex distance is 80 MHz. Duplex distance is the
distance between the uplink and downlink frequencies. A channel has two
frequencies, 80 MHz apart.
Channel separation—the separation between adjacent carrier frequencies. In
GSM, this is 200 kHz.
Modulation—Modulation is the process of sending a signal by changing the
characteristics of a carrier frequency. This is done in GSM via Gaussian
minimum shift keying (GMSK).
Transmission rate—GSM is a digital system with an over-the-air bit rate of
Access method—GSM utilizes the time division multiple access (TDMA)
concept. TDMA is a technique in which several different calls may share the
same carrier. Each call is assigned a particular time slot.
Speech coder—GSM uses linear predictive coding (LPC). The purpose of LPC
is to reduce the bit rate. The LPC provides parameters for a filter that mimics
the vocal tract. The signal passes through this filter, leaving behind a residual
signal. Speech is encoded at 13 kbps.
GSM Subscriber Services
There are two basic types of services offered through GSM: telephony (also
referred to as Tele-services) and data (also referred to as bearer services).
Telephony services are mainly voice services that provide subscribers with the
complete capability (including necessary terminal equipment) to communicate
with other subscribers. Data services provide the capacity necessary to transmit
appropriate data signals between two access points creating an interface to the
network. In addition to normal telephony and emergency calling, the following
subscriber services are supported by GSM:
Dual-tone multi frequency (DTMF)—DTMF is a tone signalling scheme
often used for various control purposes via the telephone network, such as
remote control of an answering machine. GSM supports full-originating DTMF.
Facsimile group III—GSM supports CCITT Group 3 facsimile. As standard
fax machines are designed to be connected to a telephone using analog signals,
a special fax converter connected to the exchange is used in the GSM system.
This enables a GSM–connected fax to communicate with any analog fax in the
Short message services— A convenient facility of the GSM network is the
short message service. A message consisting of a maximum of 160
alphanumeric characters can be sent to or from a mobile station. This service
can be viewed as an advanced form of alphanumeric paging with a number of
advantages. If the subscriber's mobile unit is powered off or has left the
coverage area, the message is stored and offered back to the subscriber when the
mobile is powered on or has reentered the coverage area of the network. This
function ensures that the message will be received.
Cell broadcast—A variation of the short message service is the cell broadcast
facility. A message of a maximum of 93 characters can be broadcast to all
mobile subscribers in a certain geographic area. Typical applications include
traffic congestion warnings and reports on accidents.
Voice mail—This service is actually an answering machine within the network,
which is controlled by the subscriber. Calls can be forwarded to the subscriber's
voice-mail box and the subscriber checks for messages via a personal security
Fax mail—With this service, the subscriber can receive fax messages at any fax
machine. The messages are stored in a service center from which they can be
retrieved by the subscriber via a personal security code to the desired fax
GSM supports a comprehensive set of supplementary services that can
complement and support both telephony and data services. Supplementary
services are defined by GSM and are characterized as revenue-generating
features. A partial listing of supplementary services follows.
Call forwarding—This service gives the subscriber the ability to forward
incoming calls to another number if the called mobile unit is not reachable, if it
is busy, if there is no reply, or if call forwarding is allowed unconditionally.
Barring of outgoing calls—This service makes it possible for a mobile
subscriber to prevent all outgoing calls.
Barring of incoming calls—This function allows the subscriber to prevent
incoming calls. The following two conditions for incoming call barring exist:
baring of all incoming calls and barring of incoming calls when roaming outside
the home PLMN.
Advice of charge (AoC)—The AoC service provides the mobile subscriber
with an estimate of the call charges. There are two types of AoC information:
one that provides the subscriber with an estimate of the bill and one that can be
used for immediate charging purposes. AoC for data calls is provided on the
basis of time measurements.
Call hold—This service enables the subscriber to interrupt an ongoing call and
then subsequently re-establish the call. The call hold service is only applicable
to normal telephony.
Call waiting—This service enables the mobile subscriber to be notified of an
incoming call during a conversation. The subscriber can answer, reject, or
ignore the incoming call. Call waiting is applicable to all GSM
telecommunications services using a circuit-switched connection.
Multiparty service—The multiparty service enables a mobile subscriber to
establish a multiparty conversation—that is, a simultaneous conversation
between three and six subscribers. This service is only applicable to normal
Calling line identification presentation/restriction—These services supply
the called party with the integrated services digital network (ISDN) number of
the calling party. The restriction service enables the calling party to restrict the
presentation. The restriction overrides the presentation.
Closed user groups (CUGs)—CUGs are generally comparable to a PBX. They
are a group of subscribers who are capable of only calling themselves and
Code division multiple access (CDMA) is a channel access method used by
various radio communication technologies.
CDMA is an example of multiple access, which is where several transmitters
can send information simultaneously over a single communication channel. This
allows several users to share a band of frequencies (see bandwidth). To permit
this to be achieved without undue interference between the users CDMA
employs spread-spectrum technology and a special coding scheme (where each
transmitter is assigned a code).
CDMA is used as the access method in many mobile phone standards such
as cdmaOne, CDMA2000 (the 3G evolution of cdmaOne), and WCDMA(the
3G standard used by GSM carriers), which are often referred to as
Steps in CDMA modulation
CDMA is a spread spectrum multiple access technique. A spread spectrum
technique spreads the bandwidth of the data uniformly for the same transmitted
power. A spreading code is a pseudo-random code that has a narrow ambiguity
function, unlike other narrow pulse codes. In CDMA a locally generated code
runs at a much higher rate than the data to be transmitted. Data for transmission
is combined via bitwise XOR (exclusive OR) with the faster code. The figure
shows how a spread spectrum signal is generated. The data signal with pulse
duration of (symbol period) is XOR’ed with the code signal with pulse
(chip period). (Note: bandwidth is proportional to
= bit time) Therefore, the bandwidth of the data signal is
bandwidth of the spread spectrum signal is
is much smaller than
, the bandwidth of the spread spectrum signal is much larger than the
bandwidth of the original signal. The ratio
is called the spreading factor
or processing gain and determines to a certain extent the upper limit of the total
number of users supported simultaneously by a base station.
Each user in a CDMA system uses a different code to modulate their signal.
Choosing the codes used to modulate the signal is very important in the
performance of CDMA systems. The best performance will occur when there is
good separation between the signal of a desired user and the signals of other
users. The separation of the signals is made by correlating the received signal
with the locally generated code of the desired user. If the signal matches the
desired user's code then the correlation function will be high and the system can
extract that signal. If the desired user's code has nothing in common with the
signal the correlation should be as close to zero as possible (thus eliminating the
signal); this is referred to as cross correlation. If the code is correlated with the
signal at any time offset other than zero, the correlation should be as close to
zero as possible. This is referred to as auto-correlation and is used to reject
An analogy to the problem of multiple access is a room (channel) in which
people wish to talk to each other simultaneously. To avoid confusion, people
could take turns speaking (time division), speak at different pitches (frequency
division), or speak in different languages (code division). CDMA is analogous
to the last example where people speaking the same language can understand
each other, but other languages are perceived as noise and rejected. Similarly, in
radio CDMA, each group of users is given a shared code. Many codes occupy
the same channel, but only users associated with a particular code can
In general, CDMA belongs to two basic categories: synchronous (orthogonal
codes) and asynchronous (pseudorandom codes)
The Mobile Station (MS)
In a cdma2000 1X network, the mobile station—the subscriber’s handset—
functions as a mobile IP client.
The mobile station interacts with the Access Network to obtain appropriate
radio resources for the exchange of packets, and it keeps track of the status of
radio resources (e.g. active, stand-by, dormant). It accepts buffer packets from
the mobile host when radio resources are not in place or are insufficient to
support the flow to the network.
Upon power-up, the mobile station automatically registers with the Home
Location Register (HLR) in order to:
• Authenticate the mobile for the environment of the accessed network
• Provide the HLR with the mobile’s current location
• Provide the Serving Mobile Switching Centre (MSC-S) with the mobile’s
permitted feature set
After successfully registering with the HLR, the mobile is ready to place voice
and data calls. These may take either of two forms, circuit-switched data (CSD)
or packet-switched data (PSD), depending on the mobile’s own compliance (or
lack thereof) with the IS-2000 standard. This document defines protocols for
several critical CDMA interfaces pertaining to packet transmission, namely A1,
A7, A9, and A11.
Mobile Stations must comply with IS-2000 standards to initiate a packet data
session using the 1xRTT1 network. Mobile stations having only IS-95
capabilities are limited to CSD, while IS-2000 terminals can select either the
PSD or CSD. Parameters forwarded by the terminal over the air link (AL) to the
network will determine the type of service requested. Circuit-switched data has
a maximum rate of 19.2 Kbps and is delivered over traditional TDM circuits.
This service allows users to select the point of attachment into a data network
using ordinary dialled digits.
Packet-switched data service has a maximum data rate of 144 Kbps. For each
data session a Point-to-Point Protocol (PPP) session is created between the
mobile station and the Packet Data Serving Node (PDSN). IP address
assignment for each mobile can be provided by either the PDSN or a Dynamic
Host Configuration Protocol (DHCP) server via a Home Agent (HA).
The Radio Access Network (RAN)
The Radio Access Network is the mobile subscriber’s entry point for
communicating either data or voice content. It consists of:
• The air link
• The cell site tower/antenna and the cable connection to the Base Station
• The Base Station Transceiver Subsystem (BTS)
• The communications path from the Base Station Transceiver Subsystem to the
base station controller
• The Base Station Controller (BSC)
• The Packet Control Function (PCF)
The RAN has a number of responsibilities that impact the network’s delivery of
packet services in particular. The RAN must map the mobile client identifier
reference to a unique link layer identifier used to communicate with the PDSN,
validate the mobile station for access service, and maintain the established
The Base Station Transceiver Subsystem (BTS) controls the activities of the air
link and acts as the interface between the network and the mobile. RF resources
such as frequency assignments, sector separation and transmit power control are
managed at the BTS. In addition, the BTS manages the back-haul from the cell
site to the Base Station Controller (BSC) to minimize any delays between these
two elements. Normally a BTS connects to the BSC through un-channelized T1
facilities or direct cables in co-located equipment. The protocols used within
this facility are proprietary and are based on High-level Data Link Control
The Base Station Controller (BSC) routes voice- and circuit-switched data
messages between the cell sites and the MSC. It also bears responsibility for
mobility management: it controls and directs handoffs from one cell site to
another as needed. It connects to each MTX using channelized T1 lines for
voice and circuit switched data; and to un-channelized T1 lines for signalling
and control messages to the PDSN using the 10BaseT Ethernet protocol.
The Packet Control Function (PCF) routes IP packet data between the mobile
station within the cell sites and the Packet Data Serving Node (PDSN). During
packet data sessions, it will assign available supplemental channels as needed to
comply with the services requested by the mobile and paid for by the