In 1960s people started thinking of potential value in allowing
computers to share information on research and development in
scientific and military fields. Internet was the result. Leaving what
happened in between, the next milestone was in 1965 when
Lawrence Roberts connected a computer at Massachusetts with a
computer at California over dial-up telephone lines. The feasibility of
wide area networking was proved. The Internet matured in the 70's
as a result of the TCP/IP architecture replacing the earlier Network
Control Protocol (NCP)and universally adopted by 1983.
The National Science Foundation funded NSFNet as a cross-
country 56 Kbps backbone for the Internet in 1986. As the commands
for e-mail, FTP, and telnet were standardized, it became a lot easier
for non-technical people to learn to use the nets.
So people were able to make good use of the nets - to
communicate with people around the world and to share files and
resources. An archiver for ftp sites was created in 1989. The
commands to search Archie were UNIX commands, and it required
some knowledge of UNIX to use it to its full capability. Followed were
1. Wide Area Information Server (WAIS), which would index the full text of files
in a database and allow searches of the files
2. Gopher, which needed no knowledge of UNIX or computer architecture to
3. VERONICA searchable index of Gopher menus
Definition of Broadband
The definition of broadband has changed over time to time. In the 1980s and early
1990s, broadband referred to rates greater than 45 megabits per second (Mbps),
and "wideband" referred to rates between 1.5 and 45 Mbps. In 1995, broadband
commonly referred to anything 1.5 Mbps and higher. In 2000, it was defined as a
service that is at least 200 kbps in each direction.(Reference: BROADBAND BRINGING
HOME THE BITS - NATIONAL ACADEMY PRESS, Washington, D.C.)In India, TRAI
defines Broadband as:
“An ‘always-on’ data connection that is able to support interactive services including
Internet access and has the capability of the minimum download speed of 256 kilo
bits per second (kbps) to an individual subscriber from the Point Of Presence (POP)
of the service provider intending to provide Broadband service where multiple such
individual Broadband connections are aggregated and the subscriber is able to
access these interactive services including the Internet through this POP. The
interactive services will exclude any services for which a separate licence is
specifically required, for example, real-time voice transmission, except to the extent
that it is presently permitted under ISP licence with Internet Telephony.”
A question to be necessarily considered is whether "broadband" refers exclusively to
Internet service or is a more inclusive term that refers to a set of data
communications services. Whether broadband is used to bring the Internet to the
home or small business at much higher speed and with characteristics such as
always-on, or is it really about delivering to the home a bundle of digital services.
Definitely, it is going to offer many services such as Video on demand, live telecast or
Interactive games in addition to a fast web browsing service.
What is GSM
The Global System for Mobile communications (GSM: originally from Groupe
Spécial Mobile) is the most popular standard for mobile phones in the world. GSM
service is used by over 2 billion people across more than 212 countries and
territories. Its ubiquity makes international roaming very common between mobile
phone operators, enabling subscribers to use their phones in many parts of the
GSM differs significantly from its predecessors in that both signaling and speech
channels are digital call quality, and so is considered a second generation (2G)
mobile phone system. This has also meant that data communication was built into
the system from the 3rd Generation Partnership Project (3GPP).
The main differentiator to previous mobile telephone systems, retrospectively
dubbed 1G, is that the radio signals that 1G networks use are analog, while 2G
networks are digital. Note that both systems use digital signaling to connect the
radio towers (which listen to the handsets) to the rest of the telephone system.
History of GSM
GSM standard is an European standard which has addressed many problems directly
related to compatibility, especially with the development of digital radio technology.
From 1982 to 1985 discussions were held to decide between building an analog or
digital system. After multiple field tests, a digital system was adopted for GSM.The
next task was to decide between a narrow or broadband solution. In May 1987, the
narrowband time division multiple access (TDMA) solution was chosen. Although
standardized in Europe, GSM is not only a European standard. GSM networks are
operational or planned in Europe, the Middle East, the Far East, Africa, North and
South America, and Australia. The acronym GSM now aptly stands for Global System
for Mobile communications
WHAT IS ANTENNA
Antennas transform wire-propagated
waves into space-propagated waves.
They receive electromagnetic waves
and pass them onto a receiver or
they transmit electromagnetic waves,
which have been produced by a
transmitter.All the features of passive
antennas can be applied for
reception and transmission alike
(reciprocality). On one side RF cable
is connected and the other side it is
the environment, therefore the
surroundings of the antenna have a
strong influence on the antenna's
Principle of an antenna
A) A transmitter sends a high frequency wave into a co-axial cable. A pulsing
electrical field is created between the wires, which cannot free itself from the cable
b) The end of the cable is bent open. The field lines become longer and are
orthogonal to the wires.
c) The cable is bent open at right angles. The field lines have now reached a length,
which allows the wave to free itself from the cable. The apparatus radiates an
electromagnetic wave, whereby the length of the two bent pieces of wire
corresponds to half of the wavelength. This is the basic principle of λ/2-dipole
Polarization can be defined as the direction of oscillation of the electrical field vector
and has been discussed earlier.
For Mobile communications generally vertical polarization is used.
For Broadcast systems horizontal polarization is used.
Omni directional Antennas:
The classical omni directional lamda/2 antennas are of two types
1. Ground Plane
2. λ/4-skirt Antenna
The names indicate how the antenna is decoupled from the mast.In both cases the
horizontal radiation pattern covers 360 ° and vertical half power beamwidth is 78
°.Hence there will be lot of waste of energy both upwards and downwards in the
desired horizontal plane.
Ground Plane Omni Directional Antenna
In this case, a conductive plane is achieved via 3 counterweighted poles. The ground
plane antenna can cover the complete frequency range because it is a wideband
Directional antennas are provided with reflectors behind the radiating element. This
focuses the energy in a desired direction avoiding transmission in the rear side of the
antenna. The directional antennas are classified into the following types:
1.Grid Parabolic Reflector antennas
2. Parabolic Reflector antennas.
4. Array antennas.
The first two types of antennas are mainly used in
fixed point-to-point radio links and the grid types are
employed up to 2GHz whereas the solid parabolic
reflector antennas are used for higher frequencies.
The connectivity between the antennas to the
equipments is by coaxial cable up to 2GHz and for
higher frequencies it is by hollow copper tube called
wave-guide. The beam width of these antennas
depends on the diameter of the antenna and
frequency of operation. They produce very narrow beams.
Cassegrain antennas are associated with Satellite communication are comparatively
larger which makes them to be fixed on the ground or roof tops and orient
themselves towards the satellite by operating gear arrangement either manually or
Array antennas are more predominantly used in broadcasting and mobile
communications. There are two types (i)End Fire Arrays,(ii)Panel Antennas
End-fire Arrays:Yagi Antennas
Yagi antennas are very common due to their simple and cheap method of
construction. The gain and bandwidth of Yagi antennas are electrically coupled with
one other which is an electrical disadvantage, ie. one criterion is weighed off the
other. The mechanical concept is not suitable for extreme climatic conditions.
Disadvantages of Copper Based Access Networks
Eventhough there is a vast and extensive copper based access network,there are
1.Copper is costly and its resources are diminishing.
2.Installation of copper network is time consuming and costly affair
Advantages of wireless
1. Provisioning of connectivity is faster.
2. Since there is no physical medium, fault liability is reduced.
3. Mobility is inherent.
4. Reconfiguration is simple.
5. Cost of installation is independent of distance.
6. Connections can be at longer distances than copper based network.
Cellular Concepts - Introduction
Even though multiple access techniques allowed multiple users to share the medium
simultaneously, due to constraints in providing resources, an amount of blocking will
exist. The amount of blocking is called “Grade Of Services”(GOS).
GOS is a measure of the probability that a percentage of the offered traffic will be
blocked or delayed. It is commonly expressed as the fraction of calls or demands that
fail to receive immediate service. The aim is to achieve the GOS equal to 0
Based on GOS and resource availability (no. of carriers/no. of timeslots/both) the
traffic handling capacity of the system is calculated. If this total traffic is divided by
traffic per subscriber, we get number of subscribers supported by the system. For
these purposes Erlang B table (Blocking calls cleared )is useful.
Cellular Concepts - What is a cell?
Cell is the basic geographic unit.They are base stations transmitting over that small
area.Cells are usually represented on paper as hexagon.In reality the shape is not so
because of the landscape and man-made structures.The base stations can be
employing omni directional or directional antenna.
Cell size depends on sub density and demand in that given area. To start with cell
can be of maximum size 30Km radius and subsequently can be split into smaller cells.
Usually in rural areas the cells are big and in urban will be smaller.
The figure represents a GSM reference model for a PLMN (Public Land Mobile
The GSM network is divided into four major systems
1. Switching system (SS)
2. Base station system (BSS)
3. Mobile station (MS)
4. Operation and maintenance centre(OMC)
The switching system (SS) also called as Network and Switching System(NSS) is
responsible for performing call processing and Subscriber-related functions. The
switching system includes the following functional units
1. Mobile Switching Centre
2. Home Location Register
3. Visitor Location Register
4. Equipment Identity Register
5. Authentication Centre
6. GSM Architecture - The Functions Of MSC
1. Call handling that copes with the mobile nature of subscribers considering
Location Registration,. Authentication of subscribers and equipment,
Handover and Prepaid service.
Home location register contains
1. The identity of mobile subscriber called IMSI (International Mobile Sub
2. ISDN directory number of mobile station.
3. Subscription information on services.
4. Service restrictions.
5. Location Information for call routing
One HLR per GSM network is recommended and it may be a distributed database.
Permanent data in HLR changed by man-machine interface. Temporary data like
location information changes
It refers to the terminal equipment used
by the wireless subscriber. It consists of
1. SIM -Subscriber Identity Module
2. Mobile Equipment.
SIM is removable and with appropriate SIM, the network can
be accessed using various mobile equipments.
The equipment identity is not linked to the subscriber.
The equipment is validated separately with IMEI and EIR.
The SIM contains an integrated circuit chip with a
microprocessor, random access memory (RAM)and read only memory
SIM should be valid and authenticate the validity of MS while
accessing the network.
SIM also stores subscriber related information like IMSI ,cell
location identity etc.
GSM RADIO LINK
BTS and MS are connected through radio link this air interface is
called Um.A radio wave is subject to attenuation,reflection,Doppler shift and
interference from other transmitter.These effects causes loss of signal
strength and distortion which will impact the quality of voice or data. To
cope with the harsh conditions,GSM make use of an efficient and protective
signal processing. Proper cellular design must ensure that sufficient radio
coverage is provided in the area.
Special Features of GSM
3. Time Slot Staggering
4. Timing Advance
5. Discontinuous transmission
6. Power Control
7. Adoptive equalization
8. Slow Freq Hopping
Since the air interface is vulnerable to frauduland access,it is
necessary to employ authentication before extending the services to this
subscriber.Authentication is built around the following notions.
1. Authentication Key(Ki) resides only in two places, SIM card and Authentication
2. Authentication Key (Ki) is never transmitted over air.
It is virtually impossible for unauthorised individuals to obtain this
key to impersonate a given mobile subscriber.
The MS is authenticated by the VLR with a process that uses three
1. RAND which is completely random number
2. SRES which is an authentication signed response.It is generated by applying an
authentication algorithm (A3) to RAND and Ki
3. Kc which is cipher key.The Kc parameter generated by applying the cipher key
generation algorithm (A8) to RAND and Ki.
Data is encrypted at the transmitter side in blocks of 114 bits by taking 114-bit plain text
data bursts and performing an EXOR(Exclusive OR)logical function operation with a 114-bit
The decryption function at the receiver side is performed by taking the encrypted data block
of 114 bits and going through the same"exclusive OR"operation using the same 114-bit
cipher block that was used at the transmitter.
The cipher block used by both ends of transmission path for a given transmission direction is
produced at the BSS and MS by an encryption algorithm called A5.The A5 algorithm uses a
64-bit cipher key(Kc),produced during the authentication process during call setup and the
22-bit TDMA frame number(COUNT) which takes decimal values from 0 through 2715647
and has a repetition time 3.48 hours (hyperframe interval).The A5 algorithm actually
produce two cipher blocks during each TDMA period.One for the uplink path and the other
for the downlink path.
The mobile station already has to be frequency agile, meaning it can move between a
transmit, receive, and monitor time slot within one TDMA frame, which normally are on
different frequencies. GSM makes use of this inherent frequency agility to implement slow
frequency hopping, where the mobile and BTS transmit each TDMA frame on a different
Network attachment is a process of selecting an appropriate
cell(radio frequency)by the mobile station to provide the available
services,and making its location known to the network
The process starts when the mobile station is switched on,and ends
when the mobile station enters the idle mode.In idle mode the mobile
station does not have a traffic channel allocated to make or receive a
call,but the Public Land Mobile Network(PLMN) is aware of the existence of
the mobile station within the chosen cell.
When a mobile station is switched on it attempts to make contact
with a GSM PLMN by performing the following action
1. Measures the BCCH channels.
2. Search for a suitable cell.
The mobile station measures the signal strength of the
BCCH(Broadcast Control Channel)channels received.It stores in a list of
information of about 30 of these BCCH channels,such as the signal strength
and the frequency corresponding to these BCCH channels.
Call to an active Mobile Station
As an active Mobile Station(MS) moves around in the coverage area
of a Public Land Mobile Network(PLMN),it reports its movements so that it
can be located when required using the Locations Update procedure.
When a Mobile Services Switching Center(MSC) in the network needs
to establish a call to an MS operating in its area the following Happens:
To eliminate the need for network-wide paging broadcasts,the PLMN
needs to know the approximate positions of the MSs that are active within
its coverage area.To enable the approximate positions of any MS to be
represented by a single parameter,the total area covered by the network is
divided into location areas.
A Location area(LA) is a cluster of one or more radio cells.The cell
cluster fulfills the following requirements:
1. The BTSs in a location area are controlled by one or more BSCs
2. BSCs that serve the same location area are always connected to the same MSC
3. Radio cells with BTSs controlled by a common BSC can lie in different location areas.
Location area Identity
Every radio transmitter in the PLMN broadcast,via a control channel,a Location Area
Identity(LAI) code to identify the location area that it serves.
When an MS is not engaged in a call,it automatically scans the BCCH transmitted by the base
stations in the locality and selects the channel that is delivering the strongest signal.The LAI
code broadcast by the selected channel identifies the location area in which the MS is
currently situated.This LAI code is stored in the Subscriber Identity Module(SIM) of the
Types of Identification Numbers
During the performance of the location update procedure and the
processing of a mobile call different types of numbers are used:
1. Mobile station ISDN Number(MSISDN)
2. Mobile Subscriber Roaming Number(MSRN)
3. International Mobile Subscriber Identity(IMSI)
4. Temporary Mobile Subscriber Identity(TMSI)
5. Local Mobile Station Identity(LMSI)
Each number is stored in the HLR and/or VLR.
Mobile Station ISDN Number
The MSISDN is the directory number allocated to the mobile subscriber.It is dialed to make a
telephone call to the mobile subscriber.
The number consists of Country Code(CC) of the country in which the mobile station is
registered(for example India 91) followed by national mobile number which consists of
Network Destination Code(NDC) and Subscriber Number(SN).A NDC is allocated to each GSM
The composition of the MSISDN is such that it can be used as a global title address in the
Signaling Connection Control Part(SCCP) for routing message to the HLR of the mobile
Mobile Station Roaming Number
The MSRN is the number required by tye gateway MSC to route an incoming call to a MS
that is not currently under the gateway's control
Using the MSISDN a mobile terminated call is routed to the gateway MSC.Based on this
MSISDN the gateway MSC requests for a MSRN to route the call to the current visited MSC
International Mobile Subscriber Identity
A MS is identified by its IMSI.The IMSI is embodied in the SIM of the mobile equipment.It is
provided by the MS anytime it accesses the network
Mobile Country Code(MCC) : The MCC component of the IMSI is a 3-digit code that uniquely
identifies the country of the domicile of the subscriber.It is assigned by the ITU-T
Mobile Network Code(MNC) : The MNC component is a 2-digit code that identifies the home
GSM PLMN of the mobile subscriber.It is assigned by the government of each country.For
GSM-1900 a 3-digit MNC is used.
Mobile Subscriber Identification Number(MSIN) : The MSIN is a code that identifies the
subscriber within a GSM PLMN.It is assigned by the operator.
The process of automatically switching a call in progress from one
traffic channel to another to neutralise the adverse effects of the user
movements. Hand over process will be started only if power control is not
The Hand Over process is MAHO(Mobile Assisted Hand Over).It starts
with the Down Link Measurements by the MS(Strength of the signal from
BTS,Quality of the signal from BTS).MS can measure the Signal Strength of
the 6 best neighboring BTS down link(candidate list)
Mobile To Land Call Scenario (Mobile Origination)
Phases of Mobile To Land Call . The following table lists the phases of
a Mobile To Land Call
1. Request for services;the MS requests to setup a call
2. Authentication : the MSC/VLR requests the AUC for authentication parameters,Using
these parameters the MS is authenticated.
3. Ciphering : using the parameters, which were made available earlier during the
authentication, the uplink and the downlink are ciphered
4. Equipment Validation :the MSC/VLR requests the EIR to check the IMEI for validity
5. Call setup :the MSC establishes a connection to the MS.
7. Call release;the speech path is released
Mobile To Land Call Scenario-Phases of Mobile To Land Call
The user enters the digits of the telephone with STD code incase of
land line or without STD code incase of mobile and presses the "send" key
after all digits have been entered
1. MS transmits a channel request message over the Random Access Channel(RACH)
2. Once the BSS receives the Channel Request message,it allocates a Stand-alone
Dedicated Control Channel(SDCCH) and forwards this channel assignment
information to the MS over the access Grant Channel(AGCH).It is over the SDCCH
that the MS will communicate with the BSS and MSC until a traffic channel is
3. The MS transmits a service request message to the BSS over the SDCCH.Included in
this message is the MS TMSI and Location Area Identification(LAI).The BSS forwards
the service request message to the MSC/VLR.
Mobile To Land Call Scenario-Phases of Mobile To Land Call
The IMEI code is secure and physically protected against
The Equipment Identity Register(EIR) is responsible for storing the
IMEI codes that identify the mobile-equipment deployed in the GSM system.
History Of Wireless
The idea of cell-based mobile radio systems appeared at Bell Laboratories (in USA) in the
early 1970s. However, mobile cellular systems were not introduced for commercial use until
the 1980s. During the early 1980s, analog cellular telephone systems experienced a very
rapid growth in Europe, particularly in Scandinavia and the United Kingdom. Today cellular
systems still represent one of the fastest growing telecommunications systems.
But in the beginnings of cellular systems, each country developed its own system, which was
an undesirable situation for the following reasons:
1. The equipment was limited to operate only within the boundaries of each
2. The market for each mobile equipment was limited.
In order to overcome these problems, the Conference of European Posts and
Telecommunications (CEPT) formed, in 1982, the Group Special Mobile (GSM) in order to
develop a pan-European mobile cellular radio system (the GSM acronym became later the
acronym for Global System for Mobile communications). The standardized system had to
meet certain criteria:
The cellular structure
In a cellular system, the covering area of an operator is divided into cells. A cell corresponds
to the covering area of one transmitter or a small collection of transmitters. The size of a cell
is determined by the transmitter's power.
The concept of cellular systems is the use of low power transmitters in order to enable the
efficient reuse of the frequencies. In fact, if the transmitters used are very powerful, the
frequencies can not be reused for hundred of kilometers as they are limited to the covering
area of the transmitter.
The frequency band allocated to a cellular mobile radio system is distributed over a group of
cells and this distribution is repeated in all the covering area of an operator. The whole
number of radio channels available can then be used in each group of cells that form the
covering area of an operator. Frequencies used in a cell will be reused several cells away.
The distance between the cells using the same frequency must be sufficient to avoid
interference. The frequency reuse will increase considerably the capacity in number of users.
Compatibility with other systems such as ISDN
The decision of adopting a digital technology for GSM was made in the course of developing
the standard. During the development of GSM, the telecommunications industry converted
to digital methods. The ISDN network is an example of this evolution. In order to make GSM
compatible with the services offered by ISDN, it was decide that the digital technology was
the best option.
Additionally, a digital system allows, easily than an analog one, the implementation of future
improvements and the change of its own characteristics.
Architecture of the GSM network
The GSM technical specifications define the different entities that form the GSM network by
defining their functions and interface requirements.
The GSM network can be divided into four main parts:
The architecture of the GSM network is presented in figure
A Mobile Station consists of two main elements:
There are different types of terminals distinguished principally by their power and
1. The `fixed' terminals are the ones installed in cars. Their maximum allowed
output power is 20 W.
2. The GSM portable terminals can also be installed in vehicles. Their maximum
allowed output power is 8W.
3. The handheld terminals have experienced the biggest success thanks to the
weight and volume, which are continuously decreasing. These terminals can
emit up to 2 W. The evolution of technologies allows decreasing the
maximum allowed power to 0.8 W.
The SIM is a smart card that identifies the terminal. By inserting the SIM card into the
terminal, the user can have access to all the subscribed services. Without the SIM card, the
terminal is not operational. The SIM card is protected by a four-digit Personal Identification
Number (PIN). In order to identify the subscriber to the system, the SIM card contains some
parameters of the user such as its International Mobile Subscriber Identity (IMSI). Another
advantage of the SIM card is the mobility of the users. In fact, the only element that
personalizes a terminal is the SIM card. Therefore, the user can have access to its subscribed
services in any terminal using its SIM card.
The Base Station Subsystem
The BSS connects the Mobile Station and the NSS. It is in charge of the transmission and
reception. The BSS can be divided into two parts:
The Base Transceiver Station
The BTS corresponds to the transceivers and antennas used in each cell of the network. A
BTS is usually placed in the center of a cell. Its transmitting power defines the size of a cell.
Each BTS has between one and sixteen transceivers depending on the density of users in the
The Base Station Controller
The BSC controls a group of BTS and manages their radio resources. A BSC is principally in
charge of handovers, frequency hopping, exchange functions and control of the radio
frequency power levels of the BTSs.
The Network and Switching Subsystem
Its main role is to manage the communications between the mobile users and other users,
such as mobile users, ISDN users, fixed telephony users, etc. It also includes data bases
needed in order to store information about the subscribers and to manage their mobility.
The different components of the NSS are described below.
The Mobile services Switching Center (MSC)
It is the central component of the NSS. The MSC performs the switching functions of the
network. It also provides connection to other networks.
The Gateway Mobile services Switching Center (GMSC)
A gateway is a node interconnecting two networks. The GMSC is the interface between the
mobile cellular network and the PSTN. It is in charge of routing calls from the fixed network
towards a GSM user. The GMSC is often implemented in the same machines as the MSC.
Home Location Register (HLR)
The HLR is considered as a very important database that stores information of the
subscribers belonging to the covering area of a MSC. It also stores the current location of
these subscribers and the services to which they have access. The location of the subscriber
corresponds to the SS7 address of the Visitor Location Register (VLR) associated to the
Visitor Location Register (VLR)
The VLR contains information from a subscriber's HLR necessary in order to provide the
subscribed services to visiting users. When a subscriber enters the covering area of a new
MSC, the VLR associated to this MSC will request information about the new subscriber to its
corresponding HLR. The VLR will then have enough information in order to assure the
subscribed services without needing to ask the HLR each time a communication is
The VLR is always implemented together with a MSC; so the area under control of the MSC is
also the area under control of the VLR.
The Authentication Center (AuC)
The AuC register is used for security purposes. It provides the
parameters needed for authentication and encryption functions. These
parameters help to verify the user's identity.
The Equipment Identity Register (EIR)
The EIR is also used for security purposes. It is a register
containing information about the mobile equipments. More particularly, it
contains a list of all valid terminals. A terminal is identified by its
International Mobile Equipment Identity (IMEI). The EIR allows then to
forbid calls from stolen or unauthorized terminals (e.g., a terminal which
does not respect the specifications concerning the output RF power).
The GSM Inter-working Unit (GIWU)
The GIWU corresponds to an interface to various networks for data
communications. During these communications, the transmission of
speech and data can be alternated.
The GSM functions
In this paragraph, the description of the GSM network is focused on the different functions
to fulfill by the network and not on its physical components. In GSM, five main functions can
The MM function is in charge of all the aspects related with the mobility of the user,
specially the location management and the authentication and security.
The GSM radio interface
The radio interface is the interface between the mobile stations and the fixed infrastructure.
It is one of the most important interfaces of the GSM system.
One of the main objectives of GSM is roaming. Therefore, in order to obtain a complete
compatibility between mobile stations and networks of different manufacturers and
operators, the radio interface must be completely defined.
The spectrum efficiency depends on the radio interface and the transmission, more
particularly in aspects such as the capacity of the system and the techniques used in order to
decrease the interference and to improve the frequency reuse scheme. The specification of
the radio interface has then an important influence on the spectrum efficiency.
Code Division Multiple Access(CDMA) is a access method in
which large number of transmissions are combined on the same
RF channel at the same time but are separated by unique
assigned “codes”. This CDMA Access method is used to provide
mobile telephony and works on the cellular principle.
Overview of CDMA System
Access network, the network between local exchange and subscriber, in the Telecom
Network accounts for a major portion of resources both in terms of capital and manpower.
So far, the subscriber loop has remained in the domain of the copper cable providing cost
effective solution in past. Quick deployment of subscriber loop, coverage of inaccessible and
remote locations coupled with modern technology has led to the emergence of new Access
Technologies. The various technological options available are as follows:
1. Multi Access Radio Relay
2. Wireless In Local Loop
3. Fiber In the Local Loop
Wireless In Local Loop (WILL)
Fixed Wireless telephony in the subscriber access network also known as Wireless in Local
Loop (WLL) is one of the hottest emerging market segments in global telecommunications
today. WLL is generally used as “the last mile solution” to deliver basic phone service
expeditiously where none has existed before. Flexibility and expediency are becoming the
key driving factors behind the deployment of WILL.
WLL shall facilitate cordless telephony for residential as well as commercial complexes
where people are highly mobile. It is also used in remote areas where it is uneconomical to
lay cables and for rapid development of telephone services. The technology employed shall
depend upon various radios accesses techniques, like FDMA, TDMA and CDMA.
Different technologies have been developed by the different countries like CT2 from France,
PHS from Japan, DECT from Europe and DAMPS & CDMA from USA. Let us discuss CDMA
technology in WILL application as it has a potential ability to tolerate a fair amount of
interference as compared to other conventional radios. This leads to a considerable
advantage from a system point of view.
Advantages of CDMA System
CDMA wireless access provides the following unique advantages:
There has been a tremendous growth in wireless communication technology
over the past decade. The significant increase in subscribers and traffic, new
bandwidth consuming applications such as gaming, music down loading and
video streaming will place new demands on capacity. The answer to the
capacity demand is the provision of new spectrum and the development of a
new technology – Wideband CDMA or hereinafter referred to as WCDMA.
WCDMA was developed in order to create a global standard for real time
multimedia services that ensured international roaming.
With the support of ITU (International Telecommunication Union) a specific
spectrum was allocated – 2GHz for 3G telecom systems. The work was later
taken over by the 3GPP (3rd Generation Partnership Project), which is now the
WCDMA specification body with delegates from all over the world. Ericsson
has for a long time played a very active role in both ITU and 3GPP and is a
major contributor to WCDMA and the fulfillment of the vision of a global
mobile telecommunication system.
Code Division Multiple Access and WCDMA
Code Division Multiple Access (CDMA) is a multiple access technology where the users are
separated by unique codes, which means that all users can use the same frequency and
transmit at the same time. With the fast development in signal processing, it has become
feasible to use the technology for wireless communication, also referred to as WCDMA and
CDMA2000. In cdmaOne and CDMA2000, a 1.25 MHz wide radio signal is multiplied by a
spreading signal (which is a pseudo-noise code sequence) with a higher rate than the data
rate of the message. The resultant signal appears as seemingly random, but if the intended
recipient has the right code, this process is reversed and the original signal is extracted. Use
of unique codes means that the same frequency is repeated in all cells, which is commonly
referred to as a frequency re-use of 1.
WCDMA is a step further in the CDMA technology. It uses a 5 MHz wide radio signal and a
chip rate of 3.84 Mcps, which is about three times higher than the chip rate of CDMA2000
(1.22 Mcps). The main benefits of a wideband carrier with a higher chiprate are:
1. Support for higher bit rates
2. Higher spectrum efficiency thanks to improved trunking efficiency (i.e. a better
3. Higher QoS
Further, experience from second-generation systems like GSM and cdmaOne has enabled
improvements to be incorporated in WCDMA. Focus has also been put on ensuring that as
much as possible of WCDMA operators’ investments in GSM equipment can be reused.
Examples are the re-use and evolution of the core network, the focus on co-sitting and the
support of GSM handover. In order to use GSM handover the subscribers need dual mode
3 G MOBILE (UMTS)
1. UMTS is the convergence of mobile communications, Information Technology
(IT) and multimedia technologies. UMTS creates new opportunities for
network operators, service providers and content providers to generate
revenue and seize market share. The benefit of UMTS is richer, more
powerful communication.UMTS is a suite of radio and network technologies
2. better spectrum efficiency,
3. high data transmission rates (up to 2 Mbit/s), worldwide roaming capability,
4. the capability to offer new multimedia applications and services,
5. interoperability with both fixed and mobile telecommunications services.
UMTS is the natural evolution from GSM and other second generation (2G)mobile systems.
It provides interconnection with 2G networks as well as other terrestrial nd satellite-based
networks.UMTS presents a unique opportunity to cater to the needs of individuals in the
Information Society. As a multi-national, multi-sector system that supports numerous
protocols and transport technologies, UMTS eliminates barriers that oneposed problems for
communications and enables the creation and delivery of fully personalized communication
services to both mass
Limitations of 2G systems
The limitations of 2G mobile systems such as GSM include:
1. congestion, There are more than 300 million wireless subscribers worldwide
and thus a need to increase system capacity.
2. limited mobility around the world,
3. There is a need for global standardization.
4. limited services.
5. There is a need for new multimedia applications and services.
Wideband - Code Division Multiple Access (WCDMA)
WCDMA optimally divides the available radio spectrum on the air interface into a number of
channels and defines how these channels are allocated to the many users accessing the
network. WCDMA allows for variable bit rates and variable Quality of Service (QoS). WCDMA
1. better spectrum efficiency,
2. wider coverage,
3. support for all types of services (circuit, packet and multimedia),
4. enhanced privacy,
IP Multimedia subsystem (IMS)
IMS (IP Multimedia Subsystem) enables and drives efficient converged service offerings. It is
the key to delivering multimedia services with telecom-grade quality of service across fixed
and mobile accesses. It creates new opportunities for operators who want to deliver
attractive, easy-to-use, reliable and profitable multimedia services – including voice,
pictures, text and video, or any combination of these –with existing services. Users benefit
by being able to enjoy attractive converged multiple services regardless of access network
and device. Service success is very much dependent on the ability of operators to create and
deliver an experience that fulfills or exceeds users’ expectations. To maintain their position
as service provider, operators need to climb up the value chain and take a more active part
in service delivery. IMS is designed precisely for that purpose. IMS is access-independent: it
is the only open standardized way to deliver IP-based consumer and enterprise services,
enabled by one common core and control, to the fixed, mobile and cable communities.
It combines the quality and interoperability of telecoms with the quick and innovative
development of the Internet. IMS does this by making the unique values of the telecom
industry easily available to the application development community.When implemented
according to agreed standards, IMS enables operators to mix and match equipment and
applications from multiple vendors, and enables mobile users to access their personal set of
services wherever they roam, whichever operator network they are connected to. IMS
includes the tools and functions needed to handle numerous non-standardized services in a
standardized way – ensuring the interoperability, access awareness, policy support,
charging, security and quality of service functionality required to meet consumer demand
for attractive and convenient offerings.
‘Why IMS?’is one of the top strategic questions for any operator these days. There are many
good answers, but perhaps the key one is that IMS delivers innovative multimedia services
over fixed and mobile networks using open standards. IMS addresses key issues such as
convergence, service creation and delivery, service interconnection and open standards. IMS
can allow an operator to retain its existing business models, or evolve towards new ones.
WIRELESS LAN & Bluetooth
WIRELESS LAN INTRODUCTION
Over recent years, the market for wireless communications has enjoyed tremendous
growth. Wireless technology now reaches or is capable of reaching virtually every location
on the face of the earth. Hundreds of millions of people exchange information every day
using pagers, cellular telephones, and other wireless communication products. With
tremendous success of wireless telephony and messaging services, it is hardly surprising that
wireless communication is beginning to be applied to the realm of personal and business
computing. No longer bound by the harnesses of wired networks, people will be able to
access and share information on a global scale nearly anywhere they venture. This article
will try to answer some basic questions of why and where wireless local area networks can
be used, and present a brief description of some protocols that have been developed, with
emphasis on IEEE 802.11.
Mobile IP was suggested as a means to attain wireless networking. It focuses its attention at
the Network Layer, working with the current version of the Internet Protocol (IP version 4).
In this protocol, the IP address of the mobile machine does not change when it moves from
a home network to a foreign network. In order to maintain connections between the mobile
node and the rest of the network, a forwarding routine is implemented.
When a person in the physical world moves, they let their home post office know to which
remote post office their mail should be forwarded. When the person arrives at their new
residence, they register with the new post office. This same operation happens in Mobile IP.
When the mobile agent moves from its home network to a foreign (visited) network, the
mobile agent tells a home agent on the home network to which foreign agent their packets
should be forwarded. In addition, the mobile agent registers itself with that foreign agent on
the foreign network. Thus the home agent forwarded all packets, intended for the mobile
agent, to the foreign agent, foreign agent sends them to the mobile agent on the foreign
network. When the mobile agent returns to its original network, it informs both agents
(home and foreign) that the original configuration has been restored. No one on the outside
networks need to know that the mobile agent moved. This configuration works, but it has
some drawbacks. Depending on how far the mobile agent moves, there may need to be
some store and forwarding of packets while the mobile agent is on neither the home nor the
foreign network. In addition, Mobile IP works only for IPv4 and does not take advantage of
the features of the newer IPv6.
Bluetooth is an open standard and specification for small-form factor, low-cost, short range
radio links between mobile PCs, mobile phones and other portable devices. The technology
allows users to form wireless connections between various communication devices, in order
to transmit real-time voice and data communications. It extends a new era of
communication that eliminates and replaces cables which connects mobile phones, laptops,
palmtops, desktops and printers. It provides an absolute synchronization between
In February 1998, five companies as Bluetooth promoters - Ericsson, IBM, Intel, Nokia and
Toshiba have organized and founded a group known as the Bluetooth SIG (Special Interest
Group). The aim of this group is to study and develop a single standard for short range radio
connectivity, ensuring interoperability between devices of different manufacturers. Since
that time 3Com, Lucent, Microsoft, Motorola and more than 2000 member (adopters)
companies have joined the organization. In July 1999 first version of the Bluetooth
specification incorporating both radio protocols and control software was published.
The Bluetooth radio is built into a small microchip and operates in the 2.4Ghz band, a
globally available frequency band ensuring communication compatibility worldwide. It uses
frequency hopping spread spectrum, which changes its signal 1600 times per second, which
helps to avoid interception by unauthorized parties. In addition software controls and
identity coding built into each microchip ensure that only those units preset by their owners
The specification has two power levels defined; a lower power level that covers the shorter
personal area within a room, and a higher power level that can cover a medium range, such
as within a home. It supports both point-to-point and point-to-multipoint connections and
provides up to 720 Kbps data transfer within a range of 10 meters (up to 100 meters with a
power boost). The technology uses omni directional radio waves that can transmit through
walls and other non-metal barriers. If there is interference from other devices, the
transmission speed decreases but does not stop
With the current specification, up to seven slave devices can be set
to communicate with a master radio in one device. This connection of
devices (slaves and master) is called a piconet. Several piconets can be
linked together to form scatternets, which allow communication between
other device configurations.
Bluetooth range diagram is shown in figure 1.
Bluetooth communicates on a frequency of 2.45 GHz (starting from 2.402 GHz and stopping
at 2.480 GHz) , which has been set aside by international agreement for the use of industrial,
scientific and medical devices (ISM). A number of devices that you may already use take
advantage of this same radio-frequency band. Baby monitors, garage-door openers and the
newest generation of cordless phones all make use of frequencies in the ISM band. Making
sure that Bluetooth and these other devices don't interfere with one another has been a
crucial part of the design process. Bluetooth devices will coexist in the same frequency as
wireless LAN and microwave oven consequently the band has to be very robust.
Each channel is 1 MHz wide and so there are 79 different channels. Spread spectrum
technologies help to avoid interference between radio technologies. A Bluetooth device
changes its frequency in a pseudo random way 1600times per second. Interference often
occurs in a small portion of frequency band so hoping between different frequencies makes
the channel insensitive. Corrupt packets are resent on another frequency on which the same
interference may not exist. Packets are also small.
A Bluetooth channel always consists of a master and one or more slaves. Master initiates the
connection. The master decides on a hoping scheme that is related to its internal clock. The
slave calculate an offset which is the difference between master and slave clocks, and uses
this information to determine the frequency to which it will hop. This process enables the
master and its slaves to hop to the same frequencies at all times. The uplink and downlink
channels for one device are time multiplexed and use the Time Division Duplex and hence
use the same frequency hoping scheme.
Bluetooth uses radio medium to link devices instead of physical cable medium.Bluetooth
technology is hardware independent.For example, if a digital camera is to be connected to a
laptop/desktop, a cable system is necessary which is compatible with both devices Each
device follows a different set of interface/configuration to be connected.In the Bluetooth
approach, the devices are Bluetooth enabled thus eliminating the interface/configuration
conflict.This is true for all the hand-held devices which are communicating with each other
in Bluetooth mode technology.This is possible by embedding a tiny inexpensive chip in a
short-range transceiver into the mobile.
Bluetooth enabled devices are connected in two different networks ‘A’ and ‘B’ Up to eight
devices can be connected in a network.In order to communicate w with other devices, all
devices should work on the same frequency. The device, which initiates the connection, is
called ‘Master Device’ and the devices, which are connected to master device, are called
‘Slave Devices’ which are switched to the specified frequency for further communication.
If a subscriber is trying to connect his services (known device),
then the master device will send a ‘Page’ command which checks whether
the device is within the range.If so, the communication is established.
However, if an unknown device is trying to connect then the master
device sends an ‘inquiry’ command and gets all the information about the
unknown device and the database is updated.Thus, the devices within the
network or from other networks talk with each other.As seen earlier,
Master device sets hop sequence for the Slave devices on the network and
controls whether or not each device is active or inactive within the
network.Despite of Master/Slave designation, the network behaves more
of peer to peer relation.Instead of one device designated to control and
manage all resources. Each device has equal access to the entire network.
A slave device in a network can establish connection to the other networks
also.Thus, each slave device can participate with up to eight different
Application for End Users
Bluetooth technology supports all multimedia applications used by end users.A few
applications are as given below.
1. Provides an absolute synchronization among various digital devices (like
mobile phone, desktop, laptop and PDA).
2. Bluetooth enabled mobile phone can be used as a mobile router placed in a
3. Internet surfing is possible from laptop from anywhere
4. Passing on print command remotely for printing jobs.
Advantages of Bluetooth
1. Bluetooth provides flexible network that allows upto eight devices to share the
2. The network architecture allows one to add or remove nodes without
additional infrastructure involved.
3. The size of the implementation is small.
4. Power consumption is low.
5. It has the support of the security considerations like ‘encryption’
Bluetooth is going to emerge as one of the largest growing area in the field of
telecommunications in providing device to device wireless connectivity.2This standard is
now available for 2G networks and is in the process of evolution for 3G network standards
Broadband through WI FI and WIMAX
WI FI Introduction
Any two computers can be directly wired to each other using a crossover cable. When
number of computer exceeds, cables must be run from each computer to another computer
or to the central device. It can be time-consuming and difficult to run cables under the floor
or through walls, especially when computers sit in different rooms. The correct cabling
configuration for a wired LAN varies depending on the mix of devices, the type of Internet
connection, and whether internal or external modems are used.
Look around us at the moment, we have our keyboard connected to the computer, as well
as a printer, mouse, monitor and so on. What (literally) joins all of these together?, they are
connected by cables. Cables have become the bane of many offices. Most of us have
experienced the 'joys' of trying to figure out what cable goes where, and getting tangled up
in the details. Is there a technology to replace cable?
Wireless is the answer.
Wireless isn't just about the freedom to stay connected as we move around the office. It's
also about the freedom to connect our mobile laptop PC to the Internet from any room in
our home or whenever we take it on the road. Going wireless used to be complicated. It
meant dealing with different wireless standards and all the resultant hardware and
software. But the wireless industry settled on 802.11b (or Wi-Fi) as the predominant
standard in 1999, sending prices downward as demand surged.
Wi-Fi (or Wi-fi, WiFi, Wifi, wifi), short for "Wireless Fidelity", is a set of product compatibility
standards for wireless local area networks (WLAN).
Wi-Fi was intended to be used for mobile devices and LANs, but is now often used for
Internet and wireless VoIP phone access. It enables a person with a wireless-enabled
computer, a personal digital assistant (PDA), or a wireless VoIP phone to connect to the
Internet when in proximity of an access point The geographical region covered by one or
several access points is called a hotspot.
Wi-Fi is a trademark of the Wi-Fi Alliance (formerly the Wireless Ethernet Compatibility
Alliance), the trade organization that tests and certifies equipment compliance with the
Today WLAN technologies all follow one of the three main Wi-Fi communication standards.
The benefits of wireless networking depend on the standard employed:
802.11b was the first standard to be widely used in WLANs. The 802.11a standard is faster
but more expensive than 802.11b; 802.11a is more commonly found in business networks.
The newest standard, 802.11g, attempts to combine the best of both 802.11a and 802.11b,
though it too is more a more expensive home networking option.
Difference between WI FI and WIMAX
The main problem with WiFi access is that hot spots are very small, so coverage is sparse. Is
there a new technology that would provide high speed of broadband service, Wireless
rather than wired access, so it would be a lot less expensive than cable or DSL and much
easier to extend to suburban and rural areas and Broad coverage like the cell phone network
instead of the tiny little hotspots of WiFi. This technology is called WiMAX, short for
Worldwide Interoperability for Microwave Access.
The big difference between Wi-Fi and WiMAX is that we're going to use licensed spectrum to
deliver WiMAX. To date, all Wi-Fi technology has been delivered in unlicensed spectrum.
WiMAX will use one of the unlicensed frequencies, but we're also supporting two other
frequencies that are licensed. What that means is that you can turn up the output power
and broadcast longer distances. So where Wi-Fi is something that is measured in hundreds
of feet, usually WiMAX will have a very good value proposition and bandwidth up to several
Also WiMAX is designed to be a carrier-grade technology, which requires a higher level of
reliability and quality of service than are now available in typical Wi-Fi implementations.
Those fundamental differences make WiMAX more of a metropolitan area access technology
Thus WiMAX has the potential to do to broadband Internet access what cell phones have
done to phone access. In the same way that many people have given up their "land lines" in
favor of cell phones, WiMAX could replace cable and DSL services, providing universal
Internet access just about anywhere you go. WiMAX will also be as painless as WiFi -- turning
your computer on will automatically connect you to the closest available WiMAX antenna.
A WiMAX system consists of two parts: A WiMAX tower, similar in
concept to a cell-phone tower - A single WiMAX tower can provide
coverage to a very large area -- as big as 3,000 square miles (~8,000
square km). A WiMAX receiver - The receiver and antenna could be a
small box or PCMCIA card, or they could be built into a laptop the way
WiFi access is today.
A WiMAX tower station can connect directly to the Internet using a
high-bandwidth, wired connection (for example, a T3 line). It can also
connect to another WiMAX tower using a line-of-sight, microwave link.
This connection to a second tower (often referred to as a), along with the
ability of a single tower to cover up to 3,000 square miles, is what allows
WiMAX to provide coverage to remote rural areas.
What this points out is that WiMAX actually can provide two forms
of wireless service:
WiFi-style access will be limited to a 4-to-6 mile radius (perhaps 25
square miles or 65 square km of coverage, which is similar in range to a
cell-phone zone). Through the stronger line-of-sight antennas, the WiMAX
transmitting station would send data to WiMAX-enabled computers or
routers set up within the transmitter's 30-mile radius (3,600 square miles
or 9,300 square km of coverage). This is what allows WiMAX to achieve its
WiMAX outdistances WiFi by miles. WiFi's range is about 100 feet
(30 m). WiMAX will blanket a radius of (50 km) with wireless access. The
increased range is due to the frequencies used and the power of the
Wi-Fi kind of lives by what we call the "five minute rule". If you live
in a city, most likely you can walk five minutes and find a hotspot. Or if
you're in your car in the suburbs or a village, you can usually drive within
five minutes and find one of those. With WiMAX we're trying to offer that
same type of service without having to drive or walk five minutes. Even
though it's only five minutes, it's still five minutes.Eventually, you can just
open your notebook and get a connection, wherever you may be.
Wi-Fi is based on the IEEE 802.11 specifications. There are
currently four deployed 802.11 variations: 802.11a, 802.11b, 802.11g,
Bluetooth uses Frequency Hop Spread Spectrum (FHSS) to avoid
any interference. A Bluetooth channel is divided into time slots each 625
micro second in length. The devices hop through these timeslots making
1600 hops per second. This trades bandwidth efficiency for reliability,
integrity and security.
The range for Bluetooth communication is 0-30 feet (10 meters)
with a power consumption of 0dBm (1mW). This distance can be
increased to 100 meters by amplifying the power to 20dBm. The
Bluetooth radio system is optimized for mobility. Bluetooth communication
occurs between a master radio and a slave radio. Bluetooth radios are
symmetric in that the same device may operate as a master and also the
slave. Each radio has a 48-bit unique device address (BD_ADDR) that is
Two or more radio devices together form ad-hoc networks called
piconets. All units within a piconet share the same channel. Each piconet
has one master device and one or more slaves. There may be up to seven
active slaves at a time within a piconet. Thus, each active device within a
piconet is identifiable by a 3-bit active device address.
Beyond 3G Introduction
New data services, interactive TV and evolving Internet behavior will influence mobile data
usage. Long sessions in always-on mode will force a re-think of radio access technology to
achieve the required but not easy to attain capacity (Gbit/s/km) at low cost. The ideas
presented in this article can increase capacity by a factor of 500 with regard to expected
Coverage will be based on large umbrella cells (3G, WiMAX) and numerous Pico cells
interconnected to provide the user with seamless high data rate (several Mbs) sessions.
Scalable and progressive deployments are possible while protecting the operator’s long-
The 4G infrastructure operator will mix several technologies, each of which has its optimal
usage. The connection to one of them will result in a real-time trade-off which will offer the
user the best possible service. Some tools that genuinely improve the user’s multimedia
quality of experience (availability, response time, definition, etc) are also presented in this
4G will deliver low cost multi-megabit/s sessions any time, any place, using any terminal.
Voice was the driver for second generation mobile and has been a considerable success.
Today, video and TV services are driving forward third generation (3G) deployment and in
the future, low cost, high speed data will drive forward the fourth generation (4G) as short-
range communication emerges.
Service and application ubiquity, with a high degree of personalization and synchronization
between various user appliances, will be another driver. At the same time, it is probable that
the radio access network will evolve from a centralized architecture to a distributed one.
The evolution from 3G to 4G will be driven by services that offer better quality (e.g. video
and sound) thanks to greater bandwidth, more sophistication in the association of a large
quantity of information, and improved personalization. Convergence with other network
(enterprise,fixed) services will come about through the high session data rate. It will
require an always-on connection and a revenue model based on a fixed monthly fee. The
impact on network capcity is expected to be significant.
Machine-to-machine transmission will involve two basic equipment types:
1. sensors (which measure parameters)
2. tags (which are generally read/write equipment)
It is expected that users will require high data rates, similar to those on fixed networks, for
data and streaming applications. Mobile terminal usage (laptops, Personal digital assistants,
hand-helds) is expected to grow rapidly as they become more user friendly. Fluid high
quality video and network reactivity are important user requirements.
Some of the key technologies required for 4G are briefly
The General Packet Radio System (GPRS) provides actual
packet radio access for Global System for Mobile
Communications (GSM) and time-division multiple access
The main benefits of GPRS are that it reserves radio
resources only when there is data to send and it reduces
reliance on traditional circuit-switched network
GPRS is a data service for GSM, the European standard digital cellular service. It is a packet-
switched mobile data service, a wireless packet based network. GPRS, further enhancing
GSM networks to carry data, is also an important component in the GSM evolution entitled
GSM+. High-speed mobile data usage is enabled with GPRS.
IF GPRS is compared to GSM data services, the following applies:
In GSM all the data that has to be sent, is sent via a circuit
switched connection. This means, that a link has to be established and is
used and maintained from setup until release. The data is sent via one
physical timeslot and has a maximum data rate of 9.6 kbps.
In GPRS all the data that has to be sent, is split into several smaller
data packets first. Those packets are then sent individually across the
GPRS network and each of those packets can travel on a different route.
The packets arrive at the right destination address and could be
reassembled in the right order, because every single packet contains the
destination address and information about the sequencing of the different
In GPRS, one user can occupy more than one timeslot or more than
one user can be on a single timeslot.
Depending on different aspects, a maximum data rate of 171.2
kbps could be achieved.
For GPRS the ETSI Standard introduces two new elements, the
Serving GPRS Support Node (SGSN) and the Gateway GPRS Support Node
(GGSN) (Shown in the diagram below as shadowed objects) is introduced
to create an end-to-end packet transfer mode.
The HLR is enhanced with GPRS subscriber data and routing
information. Two services are provided;
Point-To-Multipoint (PTM) (not yet specified by the Standards)
The European Telecommunications Standards Institute (ETSI) has
specified GPRS as an overlay to the existing GSM network to provide
packet data services. In order to operate a GPRS service over a GSM
network, new functionality has to be introduced into existing GSM network
elements and new network elements have to be integrated into the
existing operators GSM networks.The Base Station Subsystem (BSS) of
GSM has to be upgraded to support GPRS. The BSS works with the GPRS
Support Node (GSN) to provide GPRS service in a similar manner to its
interaction with the Switching subsystem for the circuit switched services.
DIGITAL SUBSCRIBER LOOP
The Digital Subscriber Line (DSL) technology is widely regarded as a
vehicle for offering wired broadband services to the mass market.
The services include high-speed access to the Internet, voice and
telephony services, interactive video services, e-commerce,
messaging, alerting and other multimedia services. Success of the
Internet and the World Wide Web has created the market demand
and high expectations for high-speed data services through DSL
Twisted pair - voice
Twisted pair - voice and data
It uses the existing copper pair for providing Broadband. The
previous diagram showed how the twisted pair is used for
transmitting voice and data. The DSL technology is now mature,
poised for rapid deployment and industry growth in the near future.
There are number of DSL types available to address the various
network environments and applications in the light of trade-offs
between rate and reach
1. HDSL -- High-rate DSL
2. ADSL-- Asymmetric DSL
3. SDSL – Symmetric DSL
4. RADSL – Rate Adaptive DSL
ADSL (Asymmetric Digital Subscriber Line)
Asymmetric Digital Subscriber Line (ADSL) technology was first
introduced in 1992 as a system capable of delivering Video-on-
Demand (VoD) service over telephone networks. ADSL utilizes the
same twisted two-wire facility (called the subscriber loop) as the
traditional telephone service. The traditional telephone service is
often referred to as “Plain Old Telephone Service” or POTS. With the
initial system architecture and design of ADSL, the same twisted pair
could simultaneously support both POTS and data. This is made
possible due to a frequency multiplexing technique that supports
POTS in the base band and data in a high frequency band above the
The ADSL data service is asymmetrical, in that, it has a higher
downstream data rate (up to 6 Mbps towards the subscriber) than
the upstream data rate (up to 640 kbps towards the service
provider). The main objective is to use ADSL as a technology choice
for offering VoD service. In 1994 it was proved that ADSL
technology could be used to offer high speed data services including
video. (Click to view flash movie on speed comparison). Mass-market
success of the Internet technologies – World Wide Web, the Internet
browser, and the Universal Resource Locator (URL) led to high
residential as well as business demand for broadband services.
A variation of ADSL technology more suitable for the T1/E1 market was envisioned
via a symmetric offering of ADSL, known as SDSL. The SDSL technology was
envisioned to be a 2-wire equivalent to the 4-wire HDSL service. The market
expectations were Ubiquitous and consistent high-speed data offering, support for
internet access protocols, automated provisioning, dynamic access to services
without requiring telephone company intervention, and inexpensive pricing to
enable a wider deployment of service. Note that all of these expectations have
shifted from service provider
In POTS, with the available voice band of 3.5kHz and allowed
S/N = 30dB, the theoretical upper limit on data rate will be roughly
35Kbps. Copper access lines can pass frequencies into the MHz
region. This feature is precisely what ADSL focuses on.
ADSL technology offers the asymmetric bandwidth
characteristics that are 1.544-8.448Mb/s in downstream and 32-
768kb/s in the upstream. This feature fits in with the requirements of
client-server applications, in which the client typically receives much
more data from the server then he is able to generate
ADSL System Architecture
The ADSL functions at the network end (central office end) are
performed by an ADSL Terminal Unit-Central office type (ATU-C)
together with a splitter function (S-C). The ATU-C interfaces with the
network switching, transport, and multiplexing functions and network
operations. The ATU-C functions are usually integrated within a
higher level network element, e.g. DSL access multiplexer (DSLAM).
Although copper pairs are widely available,
several line conditions may prevent the delivery of ADSL: first, if the
telephone line to the customer premises is longer than 5.5km,
second, existing of the load coils or an excessive number of bridged
taps and third, that some portions of the telephone line is carried to
the premises on fiber optic cable .
DSLAM contains the access interface (network termination –
NT) to the appropriate next device in the network, e.g., Tier2, Tier1
Switch etc. ADSL functions at the customer end (remote end) are
performed by an ADSL Terminal Unit-Remote end type (ATU-R)
together with a splitter function (S-R). At the customer premises,
ATU-R may present the interfaces to the local distribution for
broadband services via service modules (SM). The SM contains
necessary decoders and terminal interfaces for the given service and
customer control interfaces.
Splitters are three node devices that allow the telephony
signals and the ADSL signal to reside on the same copper loop
without interfering one with the other. The splitter provides a low
pass filter to the basic voice and control telephony signal (below 4
kHz) and a high pass filter for the ADSL signals, starting
approximately at 25 kHz or above. Most POTS splitter designs are
passive, that is without powering requirements. The advantages of
passive filters are in their reliability, because they enable continuous
telephone service even if the modem fails (for example, due to a
The connectivity architecture of DSLAMs is as shown in figure.
The DSLAMs are connected to the Tier 2 Switches and again, the
Tier2 switches are connected to Tier1 Switch. In turn the Tier1
Switch is connected to "Broadband Remote Access Server"(BBRAS)
which is routed to core router through which it gets access to the
The below diagram shows the connectivity portion of DSLAM
with the Tier2 switch. If the distance between the DSLAM and the
Tier2 switch is less than 10 Kms, then Dark Fibre could be used for
connectivity. (Dark fibre means a spare fibre). If the distance
exceeds 10 kms, then we can use the STM medium with electrical to
optical converters at both the ends, as both DSLAM as well as Tier2
switch has optical interface whereas the output from STM is
There are a large number of different kinds of servers that can
be accessed by an ADSL system Video on Demand service is one of
the most interesting aspect of ADSL. By using MPEG coded video it is
possible to deliver video quality movies over existing copper loops to
customers. A video quality can be achieved by only 1.5 Mbps data
rate. Together with pure VoD services there might exist combined
movie information and advertiser services in which commercial and
non commercial information that providers and advertisers can
deliver their information.
The downstream uses DMT tones 7 - 255 (echo cancellation) or 32 - 255 (FDM). The
upstream channel uses tones 7 - 31. Bit loading is adaptive and varies from 2 to 15
bits per tone (sub channel) depending on the relative noise of each carrier. When
high noise levels are detected in a given sub channel, the DMT modem can shut
down a particular sub channel altogether.
The ADSL transmission signal is modulated onto discrete multi-
tones (DMT). As already seen, there are 256 independent parallel sub
channels available in the 1.1MHz ADSL bandwidth. Each sub channel
is separated by approximately 4 kHz and has a distinct carrier
frequency in the center of this 4 kHz band. While DMT is the physical
transmission level, framing and encoding (error correction) occurs at
a higher level. The modulation technique used in each of the discrete
multi-tone (DMT) channels is Quadrature Amplitude Modulation
(QAM) where both the phase angle and the amplitude of the carrier
band are modulated to represent the information being transmitted.
In a typical ADSL modem, the main sections are
1. The Digital Interface (e.g. ATM)
2. The Framer/FEC plus Encoder/Decoder
3. The DMT Modulator
4. The AFE (Analog Front End)
The Framer multiplexes serial data into frames, generates FEC
(Forward Error Correction), and interleaves data. FEC and data
interleaving corrects for burst errors. This allows DMT-based ADSL
technology to be suitable for support of MPEG-2 and other digital
video compression techniques
For the transmit signal
The Encoder encodes frames to produce the constellation data for the DMT
Modulator. It assigns the maximum number of bits per tone (based on measured
SNR of each carrier) and generates a QAM constellation where each point represents
a digital value. Each constellation point is one of N complex numbers, x + iy, where x
and y are the phase and amplitude components. The summation of bits in all
carriers, multiplied by the frame rate (4kHz), represents the data rate
For the receive signal
The Decoder converts QAM symbols back into the data bit stream.
In the DMT Modulator, a frequency domain processor implements FFT/IFFT and
associated processing. In the transmit path, the Inverse Fast Fourier Transform (IFFT)
module accepts input as a vector of N QAM constellation points and duplicates each
carrier with its conjugate counterpart so the 2N output samples are real. The 2N
time domain samples have the last 2N/16 samples appended as a cyclic prefix, and
are then delivered to the DAC (Digital to Analog converter). The set of time domain
samples represents a summation of all the modulated sub channels, for the duration
of one data frame
NGN BASIC CONCEPTS
Definition & Features of NGN
1. Tender for 200 KC IP TAX equipment, which is Pilot Project for introduction of
NGN in transit network, has been awarded
2. Plan to introduce 6.4 million circuits capacity in 2007-2008 through IP TAX
3. Strengthen SSTP Networks
4. Trials for migration of PSTN access to NGN and for introduction of NGN in
access network underway
5. Migration to IMS expected to roll out from 2009
6. Full migration to NGN with replacement of PSTN by 2012
7. IP Multimedia Subsystems
WiMax-Access to NGN
WHAT IS NGN?
10. NGN is a network infrastructure that will enable the
provisioning of the existing telecommunications services
and innovative applications of the next generation. It is a
converged network capable of carrying voice, data and
video over the same physical network, with all traffic
carried as IP (Internet Protocol).NGN is a multi-service
network , which enables operators to implement
converged and new services in addition to POTS. From the
users’ perspective, the convergence of services will enable
the “desired” services from any type of access network.
11. NGN i.e. the Next Generation Network refers to the
convergence of different telecom services i.e. voice, data
and video over a unified packet network utilising Internet
Protocol (IP). NGN can be thought of as a packet-based
network where the packet switching and transport
elements (e.g. routers, switches and gateways ) are
logically and physically separated from the service/ call
control intelligence. This control intelligence is used to
support all types of services over the packet-based
transport network including everything from the basic
voice telephony services to data, video, multimedia,
ARCHITECTURE OF NGN
The architecture of Next Generation Network is shown below.
It is a horizontally layered network architecture instead of the
present vertically separated networks for each service.It uses packet-
based transport for all services (including voice).The access,
switching, transport, control and service functions which are
integrated in today’s switches are separated into individual network
layers, which inter-work via interfaces based on open standards.The most
significant aspect is the separation of call control from switching and transport
NGN APPLICATIONS – THE KEY TO
1. Voice Telephony
NGNs will likely need to support various existing voice
telephony services (e.g., Call Waiting, Call Forwarding, 3-Way
Calling, various AIN features, various Centrex features, and
various CLASS features).Note, however, that NGNs are not
trying to duplicate each and every traditional voice telephony
service currently offered. Rather, they will likely attempt to
support only a small percentage of these traditional services,
with an initial focus on the most marketable voice telephony
features and the features required from a regulatory
2. Data (Connectivity) Services
Allows for the real-time establishment of connectivity
between endpoints, along with various value-added features
(e.g., bandwidth-on-demand, connection reliability/resilient
Switched Virtual Connections [SVCs], and bandwidth
management/call admission control).
1. Multimedia Services
Allows multiple parties to interact using voice, video,
and/or data. This allows customers to converse with each other
while displaying visual information.
2. Virtual Private Networks (VPNs)
Voice VPNs improve the inter location networking
capabilities of businesses by allowing large, geographically
dispersed organizations to combine their existing private
networks with portions of the PSTN, thus providing subscribers
with uniform dialing capabilities.Data VPNs provide added
security and networking features that allow customers to use a
shared IP network as a VPN.
1. Unified Messaging
Supports the delivery of voice mail, email, fax, and
pages through common interfaces. Through such interfaces,
users will access, as well as be notified of, various message
types (voice mail, email, fax , etc.), independent of the means
of access (i.e., wire line or mobile phone, computer, or wireless
1. Call Center Services
A subscriber could place a call to a call center agent by
clicking on a Web page.The call could be routed to an
appropriate agent, who could be located anywhere, even at
home (i.e., virtual call centers).Voice calls and e-mail
messages could be queued uniformly for the agents.Agents
would have electronic access to customer, catalog, stock, and
ordering information, which could be transmitted back and
forth between the customer and the agent.
1. Interactive gaming
Offers consumers a way to meet online and establish
interactive gaming sessions (e.g., video games).
2. Home Manager
With the advent of in-home networking and intelligent
appliances, these services could monitor and control home
security systems, energy systems, home entertainment
systems, and other home appliances
IP Multimedia subsystem (IMS)
IMS (IP Multimedia Subsystem) enables and drives efficient converged service
offerings. It is the key to delivering multimedia services with telecom-grade quality
of service across fixed and mobile accesses. It creates new opportunities for
operators who want to deliver attractive, easy-to-use, reliable and profitable
multimedia services – including voice, pictures, text and video, or any combination of
these –with existing services. Users benefit by being able to enjoy attractive
converged multiple services regardless of access network and device.pService
success is very much dependent on the ability of operators to create and deliver an
experience that fulfills or exceeds users’ expectations.. IMS is designed precisely for
that purpose. IMS is access-independent: it is the only open standardized way to
deliver IP-based consumer and enterprise services, enabled by one common core
and control, to the fixed, mobile and cable communities.
It combines the quality and interoperability of telecoms with the quick and
innovative development of the Internet. IMS does this by making the unique values
of the telecom industry easily available to the application development
community.When implemented according to agreed standards, IMS enables
operators to mix and match equipment and applications from multiple vendors, and
enables mobile users to access their personal set of services wherever they roam,
whichever operator network they are connected to. IMS includes the tools and
functions needed to handle numerous non-standardized services in a standardized
way – ensuring the interoperability, access awareness, policy support, charging,
security and quality of service functionality required to meet consumer demand for
attractive and convenient offerings.
IMS offers a standardized way to deliver convenient IP-based consumer and
enterprise services to fixed, mobile and cable community – enabled by one common
core and control. It is the cornerstone of the evolution of current networks to a
single, all-IP based network where all types of services (messaging, telephony, etc.)
and media (voice, video, pictures, text etc.) can be integrated into a single user
experience. For consumer, IMS opens communication options that seamlessly
combine ongoing voice sessions with multimedia elements (sharing video while
talking, for example) or enrich shared applications with voice communication (for
instance, talking while playing a multiplayer game).
Wireless broadband technology sends data over a 'wireless'
communications network, typically using radio frequency.
These technologies are a strong and popular platform for
delivery of high-speed Internet services and wireless broadband, is
emerging as a legitimate local access platform for the delivery of
high-quality digital data, video and voice services.
In many regional areas where telecommunications
infrastructure such as fibre optic or coaxial cable is limited, wireless
technologies offer a competitive broadband access solution.Rather
than stringing thousands of miles of fiber, coax or twisted-pair
wiring, a wireless operator installs a headend and transmission tower
and is open for business.
Wireless technologies can provide area coverage from
anywhere between 5km to 40km depending on the local terrain and
strength of the transmitter signal.
The advantages of wireless broadband:
1. Access - a wireless network provides high-speed access to the Internet
without the need for expensive wire or cable infrastructure.
2. Flexibility - the capacity (number of customers) of a wireless network can be
expanded when required.
3. Versatility - wireless services are suitable for both lightly populated areas, but
can also be deployed to provide customised services in highly populated
4. Costs - without the need for expensive equipment and/or infrastructure, the
cost of wireless broadband products can be lower than wired products.
Short for wireless fidelity, Wi-Fi technologies include the approved IEEE 802.11a, b
and g specifications, as well as the yet-to-be-ratified 802.11n specification. Wi-Fi is
the first high-speed wireless technology to enjoy broad deployment, most notably in
hotspots around the world including homes and offices, and increasingly cafes,
hotels, and airports.
Wi-Fi hotspots became popular almost immediately and have been applauded by
road warriors for their ability to improve productivity. Wi-Fi is limited, however, by
its range: high-speed connectivity is possible only as long as a user remains within
range of the wireless access point, which is optimum within 300 feet.
Wi-Fi was one of the earliest high-speed wireless data technologies and now benefits
from a broad availability of supporting products and technologies. Some of the
newest platforms even support multiple Wi-Fi standards (e.g. 802.11a, b and/or g)
for compatibility among several wireless networks.
WiMAX is an emerging technology that will deliver last mile broadband connectivity
in a larger geographic area than Wi-Fi, enabling T1 type service to business
customers and cable/DSL-equivalent access to residential users. Providing canopies
of coverage anywhere from one to six miles wide (depending on multiple variables),
WiMAX will enable greater mobility for high-speed data applications. With such
range and high throughput, WiMAX is capable of delivering backhaul for carrier
infrastructure, enterprise campuses and Wi-Fi hotspots.
WiMAX will be deployed in three phases. Phase one will see WiMAX technology
using the IEEE 802.16d specification deployed via outdoor antennas that target
known subscribers in a fixed location. Phase two will roll out indoor antennas,
broadening the appeal of WiMAX technology to carriers seeking simplified
installation at user sites. Phase three will launch the IEEE 802.16e specification, in
which WiMAX-Certified* hardware will be available in portable solutions for users
who want to roam within a service area, enabling more persistent connectivity akin
to Wi-Fi capabilities today.
CABLE MODEM BASICS
Current Internet access via a 28.8–, 33.6–, or 56–kbps modem
is referred to as voiceband modem technology. Like voiceband
modems, cable modems modulate and demodulate data signals.
However, cable modems incorporate more functionality suitable for
today's high-speed Internet services.
Cable modem is capable of delivering up to 30 to 40 Mbps of
data this is approximately 500 times faster than a 56–kbps modem
In the conventional case the TV set receives the signal directly
from the cable operator through Co-axial cable as shown in the
figure1. When internet data combined with the TV signal is received a
splitter is required to separate the signals(figure 2).
FIGURE 1 FIGURE
CABLE MODEM BASICS
The separated data is taken through co-axial cable to the
cable-modem which in turn is connected to the PC through
A subscriber can continue to receive cable television service
while simultaneously receiving data on cable modems to be delivered
to a personal computer (PC) with the help of a simple one-to-two
A device called a cable modem termination system (CMTS),
located at the local cable operator's network hub, controls access to
cable modems on the network.It is for support of data services that
integrates upstream and downstream communication over a cable
data network. The number of upstream and downstream channels in
a given CMTS can be engineered based on serving area, number of
users, data rates offered to each user, and available spectrum
A cable headend (combiner)combines the downstream data
channels with the video, pay-per-view, audio, and local advertiser
programs that are received by television subscribers. The combined
signal is then transmitted throughout the cable distribution network.
Traffic is routed from the CMTS to the backbone of a cable
Internet service provider (ISP), such as Road Runner, which, in turn,
connects to the Internet. With newer cable modem systems, all
traffic from the CMTS to the cable modem is encrypted to ensure
privacy and security for users
To try to promote cable modem rollouts, as well as relieve technological confusion,
CableLabs, an industry trade organization, drafted a standard for cable modem
products in 1996 called DOCSIS (Data Over Cable Service Interface Specification). The
standard was developed to ensure that cable modem equipment built by a variety of
manufacturers is compatible, as dial-up modems are.
Today, CableLabs continues to manage a rigorous testing process for DOCSIS cable
modems, stamping the products that
POINT TO POINT
Point-to-Point Protocol over Ethernet is a proposal specifying
how a host personal computer (PC) interacts with a broadband
modem (i.e. xDSL, cable, wireless, etc) to achieve access to the
growing number of High-speed data networks. Relying on two widely
accepted standards, Ethernet and the point-to-point protocol (PPP),
the PPPoE implementation requires virtually no more knowledge on
the part of the end user other than that required for standard Dial up
In addition, PPPoE requires no major changes in the
operational model for Internet Service Providers (ISPs) and carriers.
The significance of PPP over Ethernet has to do with its far greater
ease of use versus competing approaches. By making high-speed
access easier to use for end consumers, and more seamless to
integrate into the existing infrastructure for carriers and ISPs, PPPoE
could speed the widespread adoption of High-speed access services
Also, PPP over Ethernet provides a major advantage for service providers by
maximizing integration with - and minimizing disruption of - service providers'
existing dial network infrastructures. Through tight integration with existing back
office automation tools that ISPs have developed for dial customers, PPPoE enables
rapid service deployment and cost savings. From authentication, accounting and
secure access to configuration management, PPPoE supports a broad range of
existing applications and services.
Why would PPPoE be used?
PPPoE is used to allow Internet Service Providers (ISPs) the use of their existing
Radius authentication systems from their Dial-Up service on a Broadband / Ethernet
based service. Dial-Up is PPP; most broadband connections are Ethernet, hence
Point to Point Protocol over Ethernet. It also allows for ISPs to resell the same line
multiple times. IE: Rated services, Broadband specific content (movies, etc.),
metered services, etc
1. ADSL modem, splitter
Customer Premises – Connectivity
1. Unzip the zipped RASPPPoE folder to a temp directory of your choice
2. Right click on "Network Neighborhood" (in ME "My Network Places"). Choose
"Properties" Click "Add..."
3. Select "Protocol". Click "Add..." . Select "Have Disk". .Select "Browse" Then
choose the folder you unpacked the RASPPPoE programs to. Then select any
one of the INF files, it does not matter which one Click "OK“. Then "OK" again
4. In the "Select Network Protocol" window make sure "PPP over Ethernet
Protocol" is highlighted and click "OK". .Click "OK" on "Network" window. Click
"Yes" to Reboot.
5. Go to your "Start" menu, and then choose "Run". Type in RASPPPoE and
"RASPPPoE" window opens. Click Exit
6. Double click the Dial-Up short cut on your desk top
7. Enter your userid as provided by your ISP
Note: you may need to include your ISP's domain as part of your userid
The term meaning the provisioning of different
telecommunication services by Telecom Service Providers on wired
and wireless networks , such as Broadband Internet access, TV,VOD,
VPN, Telephony/VOIP and mobile phone service.
Broadband Multi-Play network focuses on the augmentation of
Broadband Access Network supporting multi-play services like Video
on Demand, IP TV, VoIP, VPN service etc with guaranteed control of
critical parameters like latency, throughput, jitter to ensure high
grade delivery of real time, near real time, non real time and best
The Multiplay network is a three tier Architecture consisting of
the Access Network, Aggregation Network and Core & Content
BROADBAND NETWORK GATEWAY
The Redback SmartEdge 800 Multi-Service Edge Router is
deployed as Broadband Network Gateway (BNG) in BSNL Multi Play
project. BNG act as Gateway of the broadband traffic towards the
SmartEdge MSERs provide a comprehensive IP routing
foundation required for the evolving Multi-Play broadband services.
SmartEdge 800 MSER offers a diverse range of interface options:
Ethernet, Packet over SONET (PoS) and channelized connections.
All SmartEdge MSER interface modules are hot-swappable and
highly resilient with full session and state redundancy in the event of
a failure or replacement. Support for high-performance multicast is
provided, including protocol independent multicasting (PIM), Internet
group management protocol (IGMP) and multicast routing.
1. Session level reliability: Supports Non Stop Forwarding and keeps
SubscribeSessions running uninterrupted during a Route Processor fail-over.
2. Resilient software architecture: Modular design provides stability and protects
against crashes and protocol errors.
3. Carrier-Grade Design: Engineered to carrier standards and deployed in
carrier networks worldwide.
Network Connectivity Diagram
The network connectivity of aggregation networks of various cities are illustrated in
the following diagrams.
Broadband Multiplay – A1 & A2 Cities
The network connectivity of aggregation networks of A3 and A4 cities is shown
Broadband Multiplay – A3 & A4 Cities
The Various services offered by Multiplay are as follows:
1. Internet Services
2. Layer3 VPN Services
3. Video Services
4. IP Telephony Services
5. Data/Voice/Video Services
The network path and the traffic flow of various services are
explained in the following section.
FIBER TO THE HOME –
Fiber to the home (FTTH) is the ideal fiber-optics architecture.
In this architecture, fiber deployment is carried all the way to the
customer’s home (premises).This chapter will address the solution,
which is a fiber-optics architecture called FTTH
FTTH has been developed in response to several residential
access market drivers, including the following:
1. The Internet explosion, second line growth, the desire for higher speeds,
alternative strategies such as voice over DSL (VoDSL), voice over IP (VoIP),
voice over ATM (VoATM), and cable modems
2. The increased competition in the market due to the growing number of
competitive local-exchange carriers (CLECs), an increase in services offered
by application service providers (ASPs), and deregulation and pending
Federal Communications Commission (FCC) rulings
3. The declining costs of optical equipment Technology
Fiber To The Home – FTTH
How FTTH works
In an FTTH system, equipment at the head end or CO is interfaced into the public
switched telephone network (PSTN) and is connected to ATM or Ethernet interfaces.
Video services enter the system from the cable television (CATV) head end or from a
A switched-mode power supply, switch-mode power supply, or SMPS, is an
electronic power supply unit (PSU) that incorporates a switching regulator — an
internal control circuit that switches power transistors (such as MOSFETs) rapidly
on and off in order to stabilize the output voltage or current. Switching regulators
are used as replacements for the linear regulators when higher efficiency, smaller
size or lighter weight are required. They are, however, more complicated and their
switching currents can cause noise problems if not carefully suppressed. As with
any offline electronic systems employing peak-hold AC-DC conversion, simple SMPS
designs may have a poor power factor. The power output to cost crossover point
between SMPS and linear regulating alternatives has been falling since the early
1980s as SMPS technology was developed and integrated into dedicated silicon
chips. In early 2006 even very low power linear regulators became more expensive
than SMPS when the cost of copper and iron used in the transformers increased
abruptly on world markets.
Power plant is an equipment, which gives uninterrupted DC power supply to the
telecommunication systems. Because telecommunication systems require electrical
1. Conversion of speech signals to electrical signals
2. For operating switching, transmission equipments
Need of D.C. Power supply
1. Harmonics of A.C may affect the speech signals.
2. Relays used in telecom systems are more sensitive to D.C than A.C
3. Transistors and I.C.s etc. being unidirectional devices, the use of D.C has
4. Arranging standby source to A.C is difficult compare to D.C for which
secondary cells can be used as S/B source.
5. Not hazardous to human life
Sources of power:
Telecommunication systems need uninterrupted power supply round the clock and
throughout the year. For any uninterrupted power supply system, two sources are
1. Normal or Main source
Main Source is D.C derived from commercial source.
2. Secondary or standby source
By name we can define Normal source is one which supplies power to the load round
the clock and secondary source is one which supplies power to the load only during
the absence of power from normal source.
Hence it is a must to convert AC from commercial mains to D.C.
A.C to D.C conversions
Previously M.G (Motor-Generator) sets were used for A.C to D.C conversion. In this
A.C motor rotates on commercial A.C. supply. To the shaft of this AC motor, D.C.