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CS22 DATA COMMUNICATIONS AND NETWORKS
UNIT I:
Introduction: A brief History Applications Computer networks
Categories of networks
Standards and Standards Organizations Network architecture Open
Systems and OSI models-
TCP/IP Architecture.
Communication Media and Data Transmission:
Fourier Analysis-Analog and digital Data Transmission-Modulation
and Demodulation-
Transmission Media-Wireless Communication-Data Transmission
Basics-Transmission mode-
Interfacing-Multiplexing.
Data Link Control and Protocol Concepts: Flow Control-error
control-Asynchronous Protocols-High
Level Data Link Control (HDLC)
UNIT II:
Local Area Networks: Types of Networks and Topology-LAN
Transmission
Equipment-LAN Installation and Performance-Ethernet: IEEE
Standard802.3-Token Bus: IEEE
Standard 802.2-Token Ring: IEEE Standard 802.5-Fiber Distributed
Data Interface (FDDI)-
Distributed Queue Dual Bus (DQDB)-: IEEE Standard 802.6-LAN
Operating Systems and
Protocols-Ethernet Technologies.
Wide Area Network: WAN Transmission Methods-WAN Carrier
Types-WAN Transmission
Equipments-WAN Design and Multicast Considerations-WAN
Protocols.
UNIT III:
Integrated Services and Routing Protocols: Integrating
Services-ISDN Services-ISDN
Topology-ISDN Protocols-Broadband ISDN-Asynchronous Transfer
Mode (ATM)-Principal
characteristics of ATM-Frame Relay-Comparison of ISDN, ATM and
Frame Relay.
Wireless LANs:
WLAN Applications-Wireless LAN Requirements-Planning for
Wireless
LANs-Wireless LAN Architecture-IEEE 802.11 Protocol Layer-IEEE
802.11 Physical layer-
Designing the Wireless LAN Layout-WAP Services.
UNIT IV:
Internetworking: Principles of Internetworking-Routing
Principles-Internet work
Protocols (IP)-Shortcut of IPv4-IP Next Generation.
TCP Reliable Transport Services: Transport protocols-The
Services TCP provides to Applications-
End-to-End Services and Datagram-Transmission Control
Protocol-User Datagram Protocol
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UNIT V:
Network Applications: Client Server Model-Domain Name System
(DNS)-Telnet-
File Transfer and remove file access-Electronic Mail-World Wide
Web (WWW).
Network Management: Goal of Network management-Network
Management
Standards-Network Management Model-Infrastructure for Network
Management-Simple Network
Management Protocol (SNMP).
Text Book:
Data Communication and Computer Networks Second Edition
Brijendra Singh PHI,
2006.
Reference:
1. Computer Networks, Andrew S.Tanenbaum, 4th Edition.
2. Data Communication and computer Networks-Prakash C.Gupta,
Prentice Hall of
India.
3. Data and computer communications, William Stallings, PHI,
2007
4. Data Communication and Networking Behrouz A, Forouzan, TMH,
2005
5. Data Communications and Computer Networks, Brijendra Singh,
PHI, 2006
6. Data Communications and Networks-Achyut .S.Godbole, Tata
McGraw Hill,
2005
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INTRODUCTION
Data communication is the exchange of data between two devices
some
form of transmission medium (such as wire cable). Communicating
devices must be
part of a communication system made up of a combination of
hardware and software.
The effectiveness of data communication system depends on the
three characteristics:
delivery, accuracy, and timeliness.
There are five components in data communication systems as shown
in
figure 1.1.
1. Message : Message is the information (data message).
2. Sender : Device which sends the data message. 3. Receiver :
Device which receives the data message. 4. Medium : Physical path
by which a message travels from the sender to the
receiver.
5. Protocol : Set of rules that govern data communication.
A BRIEF HISTORY The fields of communications are certainly not
new: people have been
communication. Since the early days when humans grunted and
scratched pictures
on cave walls, which are form of communication based on the
auditory and visual
senses, where you either hear some one speaking or see letters
and symbols that
and symbols that define a message.
Communications changed drastically in 1837, after the invention
of the
telegraph by Samuel mores. Telegraph invention made possible to
send information
using electrical impulses over a copper wire.
In 1937, Howard A. Alien of Harvard University began work in the
design
of a fully automatic calculating machine using the concepts of
Babbage and those
used in punch cards in collaboration with the IBM. Seven years
later in January
1994. The design became a reality and was named MARKI.
Another event important to communications occurred in 1945 with
the
invention of the first electronic computer. ENIAC (electronic
numerical integrator
and calculator). It contains vacuum tubes, registers, capacitors
and switches and
was faster than MARK I.
The relation between computers and communications began to
emerge after
the invention of transistor of 1947 allowing smaller and cheaper
computers to be
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built. The new generation of computer that emerged during the
1960s made new
applications such as processing and routing telephone calls
economically feasible.
Another mile stone in computer-networking occurred with the
development
of ARPANET. It was developed by the US department of
defense.
The 1970s and 1980s saw the merger of the fields of computer
sciences and
data communications that profoundly changed the technology,
products and
companies of the now companied-communication industries.
The 1990s saw the emergence of the world wide web, an
application that
makes information from around the world easily accessible from
ones desk.
Computers and communication have progressed to the point where
most
businesses or schools can no longer function without them.
APPLICATIONS
Transferring data between computers is just one area of
communications. Data
communication networks have become an indispensable part of
business, industry
and entertainment. Some of the network application in different
fields are the
following
Electronic Messaging
Probably the most widely used network application is electronic
mail (e-mail).
With e-mail). With e-mail, it is possible to send a message to
remote locations from
the privacy of your own home.
Facsimile Machine (Fax)
A fax machine creates an electronic equivalent of an image on a
sheet of paper
and then sends the image over Telephone lines. A fax machine at
the other and
creates the original papers image.
Teleconferencing
Teleconferencing allows conference to occur without the
participants being in
the same place. Teleconferencing includes.
*Text conferencing, where participants communicate through their
keyboards and
computer monitors.
*Voice conferencing, where participants at a number of locations
communicate
simultaneously over the phone.
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*Video conferencing, where participants can see as well as talk
to one another.
Cellular Telephone
Hitherto, two parties wishing to use the services of a telephone
company had to
be linked by a fixed physical connection.
Information services
Information services include bulletin boards and banks. Bulletin
boards allow
the free exchange of some software, files or other
information.
Financial Services
Financial services include credit history searches, foreign
exchanges and
investment services, and electronic fund transfer.
Marketing and sales
Computer networks are used extensively in both marketing and
sales
organization.
COMPUTER NETWORKS
A network is a set of devices connected by media links. A node
can be a
computer printer, or any other capable of sending and receiving
data generated by
other nodes on the network. The links connecting the devices are
called
communication channels.
A computer network may be defined as an interconnected
collection of
autonomous computer.
Topology is the layout of the collection formed between
computer. To some
extent, the reliability and efficiency of a network is
determined by its structure.
Bus Topology
All computers attached to the cable can sense an electrical
signal any computer
can send data to any other computer.
Advantages:
Connecting a computer or peripheral to a linear bus is easy.
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This topology requires least amount of cabling to connect the
computers and
therefore, less expensive than other cabling arrangements.
It is easy to extend a bus since two cables can be joined into
one longer cable
with a connector.
Disadvantage:
Entire network shuts down if there is a failure in the
backbone.
Heavy traffic can slow down a bus because computers on such
networks do
not coordinate with each other to reserve time to transmit.
Star Topology
The star topology is the oldest communications design method,
with roots in
telephone switching systems. However, the advance in network
technology have made
the star technology a good option for modern networks.
A hub is a central device that joins single cable segments or
individual LANs
into one network.
A typical hub consists of an electronic device that accepts data
from sending
computer and delivers it to the appropriate destination.
Advantage:
Star topology is easy to install and wire.
The network is not disrupted even if a node fails or is removed
from the
network.
Fault detection and removal of faulty parts easier in star
topology.
Disadvantage:
It requires a longer length of cable.
If the hub fails, node attached to it or disabled.
The cost of the hubs makes the network expensive as compared to
bus and
ring topology.
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Ring Topology
The ring topology is a continuous path for data with no logical
beginning or
ending points and thus no terminators. Workstations and file
servers are attached to
the cable at points round the rings.
The ring topology is easier to manage than the bus because the
equipment used
to build the ring makes it easier to locate a defective node or
cable problem.
Advantages:
Ring topology is easy to install and reconfigure.
Every computer is given equal access to the ring, hence no
single computer
can monopolise the networks.
Disadvantages:
Failure in any cable or node breaks the loop and take down the
entire
network.
Maximum ring length and number of nodes are limited.
Tree Topology
A tree topology is a variation of star. As in star, nodes in a
tree are linked to a
central hub that controls the traffic to the network. However,
not every device plugs
directly into the central hub.
Advantages:
The distance to which a signal can travel increases as the
signal passes
through a chain of hubs.
Tree topology allows isolating and prioritizing communications
from
different nodes.
Disadvantages:
If the backbone line breaks, the entire segment goes down.
It is more difficult to configure wire than other
topologies.
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Mesh Topology
The mesh topology has a direct connection between every pair of
devices in the
network. This is extreme design. Communications becomes very
simple because there
is no common line.
Advantages:
The use of large number of links eliminates network
congestion.
If one link becomes unusable, it does not disable the entire
system.
Disadvantages:
The amount of required cabling is very large.
The amount of hardware required in this type of topology can
make it
expensive to implement.
Combined Topologies
Many computer networks use combinations of the various
topologies. It has a
common bus , sometimes called the backbones, which allows user
to access main
frames and high volume or frequency accessed storage.
Categories of Networks
There are two types of networks based on transmission
technology
1. Broadcast networks
2. Point to point networks
Broadcast network have a single communication channel that is
shared by all
the machines on the networks. Short messages- called packets.
When a packet with
this code is transmitted, it is received and processed by every
machine on the
network. This mode of operation is called broadcasting.
Point to point network consist of many connections between
individual pairs of
machines.
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Local Area Network (LAN)
LANs are used to interconnect distributed communities of
computer based data
terminal equipment located within a single building or localized
group of building.
LAN interconnect work station distributed around offices within
a single building such
as university campus, factory or hospital campus.
Metropolitan Area Network (MAN)
MAN is basically a bigger version of a LAN and normally uses
similar
technology. MAN is designed to extend over an entire city.
Wide Area Network (WAN)
A WAN is at the far end of the spectrum because it is for
reaching system of
networks that form a complex whole. One WAN is composed of two
or more LANs
that are connected across a distance of more than 30 miles.
Multimedia Network
The term multimedia is used to indicate that the
information/data being
transferred over the network may be composed of one or more of
the following media
types:
Text
Images
Audio
Video
There are 5 basic types of communication network that are used
to provide multimedia
communication services:
1. Telephone networks
2. Data networks
3. Broadcast television networks
4. Integrated services digital networks
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5. Broadband multi service networks
STANDARDS AND STANDARD ORGANISATIONS
The Need for Standards
The standards used in the computer industry by the various
international bodies
were concerned primarily with either the internal operation of a
computer or the
connection of a local peripheral devices.
Several national and international agencies play a strong roll
in establishing
network standards that ensure a common ground for communications
and network
equipments. Key among these agencies are:
American National Standard Institute (ANSI)
International Electro Technical Commission (IEC)
International Telecommunication Unions (ITU)
Institute Of Electrical And Electronics Engineers (IEEE)
International organization For Standardization (ISO)
Internet society and the associated Internet Engineering Task
Force (IETF)
Electronic Industries Alliance (EIA) and the associated
Telecommunications
Industry Association (TIA)
American National Standard Institute (ANSI)
ANSI is a private non-government agency where members are
manufactures
users and other interested companies.
International Electro-Technical Commission (IEC)
IEC is a non governmental agency devising standards for data
processing and
inter connections and safety in office equipment.
International Telecommunications Union (ITU)
ITU is an agency of the United Nations and has three
sectors:
1. ITU-R deals with radio communication.
2. ITU-D is a development sector.
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3. ITU-T deals with telecommunication.
Institute of Electrical and Electronics Engineering (IEEE)
The IEEE is the largest professional organization in the world
and consists of
computing and engineering professionals.
International Organizations for Standardizing (ISO)
The International Organization is a non-governmental
organization based in
Geneva, Switzerland, in which over 100 countries
participate.
Internet society and the associated Internet Engineering Task
Force (IETF)
IETF focuses on technical internet issues. Important
contributions include the
development of simple network Management Protocol (SNMP).
Electronic Industries Alliance (EIA) and the associated
Telecommunications
Industry Association (TIA)
TIA was created as a separate body within a EIA to develop
telecommunications and cabling standards.
NETWORK ARCHITECTURE
Network designers have developed general blue-print-usually
called a network
architecture that guides the design and implementation of
network.
Designing a network to meet these requirements is no small
tasks.
The essential elements of network architecture are:
Digital transmission lines for the transfer of streams of binary
information
between equipment.
Exchange of frames of information between adjacent equipment ;
those
frames contain delineation information as well as check bits for
error control.
Address to identify points of attachment to a network or a inter
network.
Exchange of packets of information between packet switches in a
network.
Congestion control mechanism may be used to prevent congestion
inside the
network.
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Inter networking provides connectivity across multiple, possibly
dissimilar,
networks by using gateways or routers.
A multiplicity of applications that built on the transfer of
messages between
computers.
OPEN SYSTEMS AND OSI MODEL
A set of protocols that would allow any two different systems to
communicate
regardless of their underlying architecture is called an open
system.
Layers of OSI Model
The OSI Model consists of the following layers:
1. Application layer
2. Presentation layer
3. Session layer
4. Transport layer
5. Network layer
6. Data link layer
7. Physical layer
Application layer protocols have been developed for file
transfer e-mail, network
management and other applications.
The presentation layer is responsible for presenting data in the
format the user
can understand.
The session layer allows application on two different computers
to establish a
session or logical connections.
The transport layer is responsible to ensure that data is sent
reliable from the
sending node to the destination node.
The network layer deals with routing strategies, which are
responsible for
delivery of a packet from source to destination.
The data link layer provides for the transfer of frames across a
transmission link
that directly connects two nodes.
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The physical layer deals with the transfer of bits over a
communication channel,
for example, copper wire pairs, coaxial cable, radio or optical
fiber.
TCP/IP ARCHITECTURE
When the OSI model was developed in 1978, many felt that it
would replace
proprietary, vendor-specific architecture such as IBMs system
network architecture,
which was in wide spread use in the late 1970s and 1980s.
In 1970s US Department of Defence wanted to interconnect
computer and
networks it had acquired from the different vendors.
The government Advanced Research Project Agency (ARPA) developed
a set
of protocols called the TCP/IP to enable the
interconnection.
TCP/IP is also the protocol of choice for most medium and large
sized
networks. It is one of the oldest protocols and is a proven
technology that is used by
millions of computer users around the globe.
The TCP/IP protocol suite is made of 5 layers:
Application layer
Transport layer
Internet layer
Network access layer
Physical layer
The first four layers provide physical standards, network
interface, internet working,
and transport functions that corresponds to the first 4 layers
of the OSI model.
A number of applications have been standardized to operate on
top of TCP/IP.
There of the most common here.
Simple Mail Transfer Protocol (SMTP)
Simple Mail Transfer Protocol (SMTP) provides a basic electronic
mail facility.
It provides a mechanism for transferring messages among separate
hosts. Features of
SMTP include mailing lists, return receipts and forwarding.
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File Transfer Protocol (FTP)
FTP is used to send files from one system to another under user
command. Both
text and binary files are accommodated, and the protocol
provides features for
controlling user access.
The file is transferred over the data connection, without the
overhead of any
headers of control information at the application level.
TELNET (Terminal Network)
The main task of the internet and its TCP/IP protocol suite is
to provide services
for users.
TELNET is an abbreviation of TErminaL NETwork. Client-server
application
program is called TELNET.
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Communication Media and Data Transmission
Fourier Analysis:-
A Famous Mathematician, JeanBaptiste Fourier developed a
theory
that any periodic function can be expressed as an Infinite
series of sums of sine and
cosine function of varying amplitude, frequency and phase shift
this series is called
Fourier series.
F(t)=1/2C + an sin(2nft)+ an cos(2nft)
F=1/T is called fundamental frequency.
The number of times a signal oscillates per unit time is called
its frequency
It measurements are cycle per second or equivalently
,Hrtz(HZ).
T
sin (2kft) sin (2nft) dt{0 for k=n}
0 { T/2 for kn}
Only one term at summation survives an the bn summation vanishes
completely
C=2/T F(t)dt.
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Bandwidth limited signals:-
The transmission of the ASCII character b is encoded in an 8 bit
byte.
Transmitted is 01100010.
The Maximum Data rate of a channel:-
The maximum data rate =2hLog2 v bits/sec
Maximum number of bits/sec=hLog2h s/n)
Analog And digital Data Transmission:-
An example of analog data is the human voice.
Some one speaks; an analog wave is created in the air.
An example on digital data is data stored in the computer in the
form of 0
and 1.
Analog Signals:-
Analog signals can be classified simple or composite.
A composite analog signal is composed of multiple sine wave.
S (t) =A sin (2ft+)
S->Instaneous amplitude
A->peak amplitude
f->frequency
->phase
Frequency refers to the number at periods in one second
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Frequency is the inverse of periods f=1/T.
Unit Equivalent
Second(s) 1s
Millisecond(ms) 10power-3s
Microsecond(s) 10power-6s
Nanosecond(ns) 10power-9s
Picosecond(ps) 10power-12s
Unit Equivalent
Hertz(HZ) 1HZ
kilo Hertz (KHZ) 10power3HZ
mega Hertz(MHZ) 10power6HZ
giga Hertz(GHZ) 10power9HZ
Tera Hertz(THZ) 10power12HZ
The position of the waveform relative to time zero.
The phase is measured in degrees or radians.
The phase shift of 360 degree corresponds to a shift of
completed period.
180 degree corresponds to a shift one half of the period.
90 degree corresponds to a shift quarter of the period.
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Digital Signals:-
Data can be represented by a digital signal.
1can be encoded as a positive voltage
0can be encoded as a negative voltage
Analog transmission can use a Band pass channel.
Modulation and Demodulation:-
Computer are Digital devices,computer communication such as
terminal to computer or computer to disk transmission use
digital signals.
Modulation means Digital signal into analog signal.
Example:
PC and a telephone,PC and a digital signal via its modem port
Modem convert digital
signal to analog signal .
Telephone lines carry frequencies between 300 and 3300 HZ.its
used for voice
transmission.
Digital to Analog Conversion:-
1.frequency modulation
2.amplitude modulation
3. phase modulation
Frequency modulation:-
Its also called the frequency shift keying it assign the
frequency range 0 and 1
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Amplitude modulation:-
Its also called the amplitude shift keying it similar to
frequency
shift keying. it assign the range 0 and 1.
Phase modulation:-
Its also called the phase shift keying it similar to frequency
shift keying. it assign the
range 0 and 1.
Analog to Digital conversion:-
Reverse of modulation method. Incoming signals are frequency
amplitude
phase shifting.
Transmission Media:-
It can classified as guided or unguided .
Guided media provide physical path for example twisted pair
coaxial
cable, optical fibers.
Unguided media employee an antenna for transmitting air, vacuum
or
water
Twisted Pair:-
The Twisted pair is a Telephone System, telephones are connected
to the
telephone company office by a twisted pair
Twisted pair can run several kilometers and repeater are
needed.
The wires are connected in helical form.
Its used for either analog to digital transmission
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Electronic Industries association has developed the standards
for twisted pair
cables. It comes in several types.
1) category 3 twisted pair
1) category 5 twisted pair
Base band Coaxial Cable:-
It carry signals of higher frequency .it consistence of shift
copper wire.
It have outer conductor of metal foil, braid.
It contains the high bandwidth and excellent noise immunity.
Two kinds of cables 50 ohm(digital) 75 ohm (analog)
Base band Coaxial cable:-
It comes from telephone world. 4 KHZ used in transmission. It
used analog
transmission. It is also called the brand band.
Base band differ from broad band it need analog amplifiers to
strengthen.
Two types of broadband system.
Optical Fiber:-
Optical fiber uses light, not electricity, to transmit
information.
An optical fiber cable has a cylindrical shape and consists of
three sections.
1) The core
2) The Cladding
3) The jacket
Core is the innermost section and consists of fiber mode of
glass or plastic.
Core has a diameter in the range 8 to 100 m
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The jacket is composed at plastic and other material layered to
protect against
moisture
Single mode fiber
Multi mode fiber
Single mode fiber is manufacture in much smaller diameter than
that of multi mode
fiber and with substantially lower density.
Optical transmission system has three components
1)Light source
2)Transmission media
3) Detector
Wireless Communication:-
Wireless communication transport electromagnetic waves without
using
physical conductor.
They are several wireless media available for transmitting
network package.
1)Radio waves.
2) Infrared signals.
3) Micro waves
Radio Waves:-
Radio network transmission, signal is transmitted one or
multiple direction,
depending on the type of antenna
Electromagnetic waves having frequency between 3KH Z 1GHZ are
called
radio waves.
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Advantages:-
Inexpensive, option for portable communication.
Disadvantages:-
Not feasible, when higher speed communication are need.
Microwaves:-
Electromagnetic waves having frequency is between 1 and 300 GHZ
are called
microwaves.
It is used to link two are more ground based microwave
transmitted or receiver
know as earth station or ground station.
It handles two links uplink and downlink. uplink is used to
receives
transmission on one frequency band .
Down link is used to transmit it on another frequency band.
Single orbiting satellite operate on a number frequency called
transponders
channels.
Infrared :-
Can also be used as a media for network communication.
This communication used in remote control device using in
television and
stereo.
Does not penetrate walls.
Infrared transmission may not be feasible when high speed
communication is
needed.
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Data Transmission Basics:-
Data into a computer via a keyboard.
Each selected key elements an alphabetic or numeric
The two most widely used codes that have been adopted for this
function are
the Extended Binary Coded Decimal Interchange Code(EBCDIC).
The ASCII American Standards Committee for Information
Interchange.
EBCDIC is an 8 bit code .manufactured by IBM.
ASCII is an 7 bit code manufactured by ITUT.
Transmission Mode:-
They are three types of transmission mode.
1) Simplex
2) Half duplex
3) Full duplex
Simplex mode the communication is unidirectional as one way
street.
Only one transmit and receiver.
Half duplex mode communication both transmit and receivers.
Full duplex mode communication both transmit and receivers
simultaneously.
There are two categories of transmission:-
1) parallel transmission.
2) serial transmission
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parallel transmission:-
binary data consisting on 1 and 0 may be organized into group of
n bits each.
Parallel transmission means that a group of bits is transmitted
simultaneously by
using separate line.
Parallel transmission are commonly used when the distance
between the two
devices are short.
Example communication between computer and peripheral
devices.
Serial transmission:-
It means that a group of bits is transmitted one by one using
one line for all bits.
There are two types of serial communication:
1.asynchronous transmission.
2. synchronous transmission.
Asynchronous transmission means that bits are divided into small
groups.
Interfacing :-
Data terminal equipments do not connect to a network directly.
The DTE, DCE
interface has 4 important categories :
1.mechanical
2.electronical
3.functional
4.procedural
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The mechanical characteristics pertain to the actual physical
connection of the DTE
and DCE.
DTE and DCE are connected by pin conductors . Functions can be
classified into
the broad categories of data control timing and electrical
ground.
Procedural specify the sequence of events for transmitting data
based on the
functional .
The difference between x.21 and the RS standers is that x.21 was
defined as the
digital signaling interface.
x.21 is useful both as an interface to connect digital computer
to analog devices
Such as ISDN.
MULTIPLIXING:-
Multiplexing is the techniques by which simultaneously
transmission of multiple
signals through data link is possible.
Multiplexing techniques are:-
1) Frequency division multiplexing.
2) Time division multiplexing.
Frequency division multiplexing:-
It used with analog signals. Perhaps its most common use is in
television and
radio transmission.
It accept signals from multiple sources.
It has a specified bandwidth, the signals are combined into
another, more
complex signal with large bandwidth.
MUX extracts and separates the individual components its carries
frequencies .
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Time division multiplexing:-
It has many input signals are combined and transmitted to
another its used with
digital signals. Multiple transmission can occupy a single
linked for specific time .
Each source are transmission is authorized .
Two basic forms are TDM are synchronous TDM and asynchronous
TDM.
synchronous TDM :-
The multiplexer allocates the exactly the same timeslot to each
transmission
devices at all times.
Time slots are grouped into frames. A frame consists of one
complete cycle of
time slots.
Asynchronous TDM :-
Asynchronous TDM are also called statistical time division
multiplexing it
avoid this type of waste .
The number of times slot is an Asynchronous TDM frame
To transmit any given point. Each slot is available to any of
the attached input
lines that has data to send.
DATALINK CONTROL AND PROTOCOL CONCEPTS
Protocol is a set of rules that governs the operation of
functional units to
achieve communication. The type of protocol used to establish a
link between two
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stations in accordance with the second layer of OSI model is
known as data link
protocol.
Data link protocol can be divided into two small subgroups:
asynchronous
and synchronous protocols.
Asynchronous protocols treat each character in a bit stream
independently.
Synchronous protocols take the whole bit stream and chop it into
characters of equal
size.
FLOW CONTROL:
Flow control is the technique which implies on the data link
layer that tells
the sender how much data it can transmit before it must wait for
an acknowledgement
from the receiver. Any receiving device has a limited amount of
memory in which to
store incoming data. So, the sending station must not send
frames at a rate faster then
the receiving station can absorb them.
Two techniques are developed to control the flow of data
across
communication links: stop-and-wait flow control and sliding
window flow control.
STOP-AND-WAIT FLOW CONTROL:
The sender waits for an acknowledgement from the receiver after
every
frame, which it transmitted by the source. It indicates the
willingness of the receiver to
accept another frame by sending back an acknowledgement to the
sender. The source
must wait until it receives the acknowledgement before sending
the next frame. The
destination can thus stop the flow of data simply by withholding
acknowledgement.
ADVANTAGES:
Simplicity.
DISADVANTAGE:
Inefficiency.
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SLIDING WINDOW FLOW CONTROL:
In the previous case, only one frame at a time can be in
transmitting and the
sender waits for an acknowledgement from the receiver after
every frame. In sliding
window, multiple frames can be transmitted at a time.
Suppose two stations A and B are connected via a full-duplex
link. Station B
allocates buffer space for F frames. Thus station B can accept F
frames, and station A
is allowed to send F frames without waiting for any
acknowledgement. To keep track
of which frames can be acknowledged, each is labeled with a
sequence number of the
next frame expected.
This acknowledgement also implicitly announces that station B is
prepared
to receive the next frame, beginning with the number specified.
The frames are
numbered modulo-n, which means they are numbered from 0 to
n-1.For example, if
n=8,the frames are numbered 0,1,2,3,4,5,6,7.
They have sender sliding window and receiver sliding window.
The
sliding window of the sender expands to the right when
acknowledgements are
received. .The sliding window of the receiver expands to the
left when
acknowledgements are received.
ERROR CONTROL:
Error control in the data link layers is based on automatic
repeat
request(ARQ)which means retransmission of data in three
cases:
1. Damaged frame
2. Lost frame
3. Lost acknowledgement
-
DAMAGED FRAME:
A recognizable frame does arrive, but some of the bits are in
error.
LOST FRAME:
A frame fails to arrive at the other side. for example, a noise
burst may
damage a frame to the extent that the receiver is not aware that
frame has been
transmitted.
LOST ACKNOWLEDGEMENT:
An acknowledgement fails to arrive at the source. The sender is
not
aware that acknowledgement has been transmitted from the
receiver.
The purpose of ARQ is to turn an unreliable data link into a
reliable one.
Three versions of ARQ have been standardized.
1. stop-and-wait ARQ
2. Go-back-N ARQ
3. selective-reject ARQ
STOP-AND-WAIT ARQ:
Stop-and-wait ARQis based on the stop-and-wait flow control
technique. The
sender transmits a single frame and then must await an
acknowledgement. No other
data frames can be sent until the receivers reply arrives at the
source station.
The sender sends a single frame to the receiver. There is a
chance that a
frame that arrives at the destination is damaged. The receiver
detects this by using the
error detection technique. To avoid such type of error, the
source station is equipped
with a timer. The sender waits for an acknowledgement for a
specified timing after
-
transmitting the frame. If no acknowledgement is received by the
time that the timer
expires, then the same frame is transmitted again.
GO-BACK-N ARQ:
The station A is sending frames to station B. After each
transmission, station A sets an acknowledgement timer for the
frame just transmitted.
Suppose that station B has previously successfully received
frame
(i-1) and A just frame i.e. We will illustrate go-back-n
technique based on damaged
frame, lost frame and lost acknowledgement.
SELECTIVE-REJECT ARQ:
In selective-reject ARQ, only the specific damaged or lost frame
is
retransmitted. If a frame is corrupted in transit, a NACK is
returned and the frame is
retransmitted out of sequence. The receiving device must be able
to send the frames it
has and insert the retransmitted frame into its proper place in
its sequence. We will
illustrate selective-reject ARQ based on damaged frame, lost
frame and lost
acknowledgement.
ASYNCHRONOUS PROTOCOLS:
Asynchronous protocols-used primarily in modems-feature start
and
stop bits and variable-length gaps between characters. A variety
of Asynchronous
protocols have been developed. That is
1. X-modem
2. Y-modem
3. Z-modem
4. Blocked Asynchronous transmission (BLAST)
-
X-MODEM:
File transfer communication for telephone line communication
between
PCs, designed by Ward Christiansen in 1979, is known as X-modem.
It is a half
duplex stop-and-wait ARQ protocol. In this protocol,
transmission begins with the
sending of a NACK frame from the receiver to the sender. each
time the sender sends
a frame, it must wait for an acknowledgement before the next
frame can be sent. A
frame can be resend either if response is not received by the
sender after a specified
period of time or if NACK is received by the sender.
Y-MODEM:
Y-modem is a protocol similar to X-modem, with the following
major
differences:
1. The data unit is 1024 bytes.
2. ITU-T CRC-16 is used for error checking
3. Multiple files can be sent simultaneously.
Z-MODEM:
Z-modem is a protocol which combines features of both X-modem
and Y-
modem.
BLAST:
BLAST is more powerful than X-modem. It is full duplex with
sliding window
flow control. It allows the transfer of data and binary
files.
SYNCHRONOUS PROTOCOLS:
Synchronous protocols can be divided into two classes. That
is
-
1. Character oriented protocols
2. Bit-oriented protocols.
CHARACTER ORIENTED PROTOCOLS:
Character oriented protocols also called byte-oriented protocols
interpret a
transmission frame or packets as a succession of characters,
each usually composed of
one byte. These are in use in both point-to-point and multipoint
applications. They are
characterized by the selected transmission control characters
used to perform the
various transmission control functions associated with link
management, flow control,
error control and data transparency.
In all data link protocols, control information is inserted into
the data stream
either as separate control frames or as additions to existing
data frames.
In character oriented protocols, this information is in the form
code words
taken from existing character sets such as ASCII or EBCDIC. The
binary Synchronous
communication protocol often referred to as BSC or
character-oriented data link
protocol was developed by IBM. It is used with Synchronous,
half-duplex
communications and uses a stop-and-wait flow control. Binary
Synchronous
communication protocol (BSC) does not support full-duplex
communication or sliding
window protocol.
BIT-ORIENTED PROTOCOLS:
In 1975, IBM pioneered the development of bit-oriented protocols
with
Synchronous data link control and lobbied the ISO to make SDLC
the standard. ANSI
modified SDLC and it became ADCCP and subsequently ISO modified
ADCCP to
HDLC. All of these protocols are based on the same stuffing for
data transparency.
Since 1981,ITU-T has developed a series of protocols called link
access protocols
such as:
1. Link access procedures, balanced (LAPB)
-
2. Link access procedures, D-channel (LAPD)
3. link access procedures, modem(LAPM)
All link access protocols are based on HDLC. All bit oriented
protocols are related to
the HDLC bit-oriented protocol published by ISO.
HIGH LEVEL DATALINK CONTROL:
PRIMARY STATION (CONTROL STATION):
Primary station is responsible for controlling the operation of
the link.
It means, the station manages dataflow by issuing commands to
other stations and
acting on their responses.
SECONDARY STATION (GUEST STATION):
This station operates under the control of primary station. It
means the
secondary station responds to commands issued by a primary
station.
COMBINED STATION:
This station combines the features of primary and secondary
station.
UNBALANCED CONFIGURATION:
Consists of one primary and one or more secondary stations ans
supports
both full duplex and half duplex transmissions.
BALANCED CONFIGURATION:
Consists of two combined stations and supports both full duplex
and half
duplex transmissions. HDLC defines three data transfer
modes:
1. Normal response mode
2. Asynchronous response mode
3. Asynchronous balanced mode
-
It is used with a balanced configuration. ABM is used in
configuration connecting
combined stations.
CHAPTER -6
LOCAL AREA NETWORKS (LAN)
Introduction:
Local area networks which we normally refer to simply as
LANs.
Local area networks or LANs are used to interconnect
distributed
communities of computer-based DTE is located within a single
building.
For example: University campus,
All the equipment is located within a single establishment; LANs
are
normally installed and maintained by the organization.
LAN standards have been developed by the IEEE 802 committee of
the
Institute Electronic Engineers (IEEE) and accredited in the area
of LAN by
the American National Standards Institute (ANSI).
The following are the examples of requirements that call for
higher-speed
LANs:
Centralized server farms:
Many applications are a need for user, systems to be able to
draw a
huge amount of data from multiple centralized servers called
server farms.
An example is a color publishing operation, in which servers
contain tens of
gigabytes of image data that must be downloaded to imaging work
stations.
The servers themselves has increased, the bottleneck has shifted
to the
network. Switched Ethernet alone would not solve this problem
because of
the limit of 10 Mbps on a single link to the client.
Power workgroups:
These groups typically consist of a small number of
cooperating
users who need to draw massive data files across the network.
Examples are
-
a software development group that runs tests on a new software
version or a
computer-aided design (CAD). In such cases, large amounts of
data are
distributed to several workstations, processed, and updated at
very high
speed for multiple iterations.
High-speed local backbone:
As the processing demand grows, LANs proliferate at a site,
and
high-speed interconnections are necessary.
6.1:TYPES OF NETWORKS AND TOPOLOGY :-
Each types of network require cabling, network equipment,
file
servers, workstations, software and training. Some types of
networks have
low start-up costs, but are expensive to maintain or upgrade.
The topology
is the physical layout of a network combined with its logical
characteristics.
The logical side of the network is the way the signal is
transferred from
point-to-point along the cable. The layout may be centralized,
with each
station physically connected to a central device that dispatches
packets
from workstation to workstation. Centralized layouts are like a
star with
workstations as its points.
There are three main topologies:
Bus topology, Ring topology, Star topology.
Client/server applications generate a medium to high level of
network
traffic, depending on the client/server software design.
Networks on which
there is frequent exchange of database information, scientific
programs and
publications software generate high levels of traffic. The
impact of hosts
and servers on a network is closely linked to the type of
software
applications that are used. The network topology for a small
business.
Heavily trafficked networks need high-speed data transmission
capabilities.
Security, i.e. the protection of data so that only authorized
persons have
access, is another issue that influences network design. It may
also use data
-
encryption, which encodes packets and allows only authorized
computers to
decode them. High-security networks use fiber-optic cable.
When a new LAN is installed, there are several factors that
affect its design,
including the following:
*Anticipated network traffic
*Redundancy requirements
*User movement
*Future growth
*Security consideration
*WAN connectivity
6.2: LAN Transmission Equipment:
LAN Transmission Equipment is used to connect devise on a
single
network, to create and connect multiple networks or
sub-networks, and to
set up a campus enterprise.
These are included the followings:
1. Network Interface Card 2. Repeaters 3. Hubs 4. Bridges 5.
Routers 6. Brouters 7. Switches 8. Gateway
6.2.1:Network Interface Card
It is used to enable a network device, such as a computer
equipment, to
connect to the network. The network connection requires four
components:
*An appropriate connector for the network medium.
-
*A transceiver
*A controller to support the Media Access Control data link
protocol
*Protocol control firmware
The connector and its associated circuits are designed for a
specific type of
medium, for example, coax, and twisted pair or optical fiber.
The cable
connector is attached to the transceiver, which may be external
to the NIC
or built into it.
The MAC controller unit and the firmware work together to
correctly
encapsulate source and destination address, the data to be
transported and
the CRC information into the service data unit.
The MAC controller and firmware are customized for particular
type of
network transport, which can be any one of the following:
Ethernet
Fast Ethernet
Gigabit Ethernet
Token ring
Fast token ring
Fast Distributed Data Interface
Asynchronous Transfer Mode Ethernet, Fast Ethernet, high speed
communication. It is able to handle both
half and full duplex transmission.
6.2.2:Repeaters:
It is an electronic device that operates on only the physical
layer of
the OSI model. It connects one or more cable segments and
retransmits any
incoming signal to all other segments.
For example the maximum distance that a single can travel on an
Ethernet
cable segment is 500 meters but one repeater cans double the
effective
length of an Ethernet to 1,000 meters.
Repeaters are not capable of connecting two dissimilar
network
technologies.
-
6.2.3:Hub:
It is a central network device that connects network nodes.
It
contains the star topology. Hub may be referred to as a
connector, and is a
device that can have multiple inputs and outputs, all active at
one time.
Provide a central unit from which to connect multiple nodes into
one
network.
Permit large numbers of computers to be connected on single or
multiple LANs.
Reduce network congestion by centralizing network design.
Provide multi-protocol services.
Consolidate the network backbone.
Enable high speed communication.
Provide connections for several different media types.
Enable centralizes network management. It also called
multistation access unit.
Operating as a central hub an MAU functions at the OSI physical
and data
link layers.
There are different kinds of hubs:
1. Passive hub(acts as path way) Data to follow from one device
to another.
2. Intelligent hub: It can detect errors and provide assistance
to a technician when
attempting to locate a failing component.
3. Active hub: Regenerate and process signals.
6.2.4:Bridge:
It is a network device that connects one LAN segment to another.
It
is high efficiency and security. It performs error detection,
frame
formatting, frame routing.
Bridges are used:
-
Extend a LAN when the maximum connection limit such as the
30-node limit on an Ethernet segment, has been reached.
Extend a LAN beyond the length limit, for example beyond
185metres with thin-net Ethernet.
Segment LANs to reduce data traffic bottlenecks. Prevent
unauthorized access to a LAN.
If the bridge knows that the destination of a frame is on the
segment as
the source of the frame, it drops the frame because there is no
need to
forward it.
If the bridge does not know the destination segment, the
bridge
transmits the frame to all segments except the source segments,
a process
that is called flooding.
Bridges can greatly enhance the performance of a network
because
they offer the ability to segment network traffic, limiting
traffic to those
networks where it belongs.
A firewall is software or hardware that sources data from
being
accessed outside a network and that can also prevent data from
leaving the
network through an inside source.
6.2.5:ROUTER:
A router performs some of the same function as a bridge.
Routers connect LANs at the network layer of the OSI model,
which
enables them to interpret more information from packet traffic
than bridges
can.
In general, routers are used to:
Efficiently direct packets from one network to another, reducing
excessive traffic.
Join neighboring or distant network. Connect dissimilar
networks. Prevent network bottlenecks by isolating portions of a
network. Secure portions of a network from intruders.
The logic that routers use to determine how to forward data is
called a
routing algorithm.
-
6.2.6:BROUTER:
A bridge router (brouter) performs both the functions of a
bridge (OSI
layer 2) and a router (OSI layer 3) in a single device.
A brouter is a network device that acts as a bridge in one
circumstance and
as a router in another.
Brouters are used to:
Handle packets efficiently on a multiprotocol network that
includes some protocols that can be routed and some that cannot
be.
Isolate and direct network traffic to reduce congestion. Join
networks. Secure a certain portion of a network by controlling who
can access it.
6.2.7:SWITCHES:
A switch is a device that connects two or more network segments
and
allows different nodes to communicate smoothly with each other
as if they
are the only two connecting at the time.
Switches provide bridging capacity Along with the ability to
increase the
bandwidth on existing networks.
Switches used on LANs are similar to bridges.
A switch may act as a multiport bridge to connect devices or
segments
in a LAN.
A store-and-forward switch stores the frame in the input buffer
until
the whole packet has arrived. A cut-through switch on the other
hand.
Forwards the packet to the output buffer as soon as the
destination address
is received.
-
6.2.8:GATEWAYS:
Gateways usually operate at OSI layer 4 or higher, and
basically
translate the protocols to allow terminals on two dissimilar
networks to
communicate.
Gateways can be either/or combinations of hardware and
software.
An internet service provider (ISP), which connects users in a
home to the
Internet, is a gateway.
Gateways can suffer from slow performance.
A dedicated computer acting as a gateway, if it is of reasonable
speed,
usually eliminates any performance problems.
For examples, you might use a gateway to:
Convert commonly used protocols (e.g. TCP/IP) to a specialized
protocol (for example, an SNA: System Network Architecture).
Convert message formats from one format to another. Translate
different addressing schemes. Link a host computer to a LAN.
Provide terminal emulation for connections to a host computer.
Direct electronic mail to the right network destination. Connect
networks with different architectures.
6.3:LAN Installation and Performance:
Once the LAN has been selected based on the requirements of
the
organization, it must be installed by the people within the
organization.
Several suppliers of LAN hardware or software may be contacted
in the
course of evaluating and selecting the LAN and these companies
may offer
installation and maintenance services. Important tasks of
installation a LAN
are given below:
Install: New workstations NICs on existing workstations
-
Wiring or cabling Server hardware Bridges, routers, brouters, or
gateways LAN software Determine the access and capability required
by each user Document the LANs hardware and software configuration
Train the users Using the LAN and its new capabilities Troubleshoot
any startup problems
The performance of LAN is based on several factors, including
the
protocol that is used, the speed of the transmission, the amount
of traffic,
the error rate, the efficiency of the LAN software, and the
speed of server
computers and disks.
6.4:ETHERNET: IEEE STANDARD 802.3
IEEE 802.3 supports a LAN standard originally developed by
Xerox
and later extended by a joint venture between Digital
Equipment
Corporation, Intel Corporation and Xerox. This was called
Ethernet.
Ethernet has a bus topology. Stations contended for the segment
using
a form of the CSMA/CD contention protocol. It is commonly used
to
connect PCs, workstations, and printers and file servers and
even
mainframes.
Specifically, the data link layer is responsible for
accurate
communication between two nodes in a network. This involves
frame
formats, error checking a d flow control. Data link layer is
further divided
into two sub layers:
1. Logical Link Control(LIC)
2. Medium Access Control(MAC)
-
Medium access sub layer
Network can be divided into two categories: point-to-point
network
and broadcast network.
Broadcast channels are sometimes referred to as multi-access
channels or random access channels. The protocols used to
determine who
goes next on multi-access channel belong to a sub layer of the
data link
layer called the multiple access protocols. Many algorithms for
allocating
multiple access channels are known. Some of these are:
Pure Aloha
Slotted Aloha
Carrier Sense Multiple Access (CSMA)
CSMA with Collision Detection.
6.4.1:Pure Aloha
In pure Aloha, frames are transmitted at completely arbitrary
times.
We have made the frames all of the same length because the
throughput of
Aloha systems is maximized by having a uniform frame size rather
than
allowing variable length frames.
Whenever two frames try to occupy the channel at the same
time,
there will be a collision and both will be confused. If the
first bit of a new
frame overlaps with just the last bit of a frame almost
finished, both frames
will be totally destroyed and both will have to be transmitted
later.
-
6.4.2:Slotted Aloha
Slotted Aloha has double the capacity of an Aloha system. In
slotted
Aloha, time is divided into discrete intervals, each interval
corresponding to
one frame. This approach requires the users to agree on slot
boundaries.
In slotted Aloha, as against in pure Aloha, a computer is not
permitted
to send whenever a carriage return is typed. Instead, it is
required to wait
for the beginning of the next slot. Thus the continuous pure
Aloha is turned
into a discrete one.
6.4.3:Carrier Sense Multiple Access (CSMA) Protocols
Protocols in which stations listen for a carrier (transmission)
and act
accordingly are called carrier sense protocols.
The first carrier sense protocol is 1-persistent CSMA. When a
station
has data to send, it first listens to the channel to see if
anyone else is
transmitting at the moment. If the channel is busy, the station
waits until it
detects an ideal channel.
The second carrier sense protocol is non-persistent CSMA. In
this
protocol, a conscious attempt is made to be less greedy than in
the previous
one. Before sending, a station senses the channel. If no one
else is sending,
the station begins doing so itself.
6.4.4:CSMA with Collision Detection (CSMA/CD)
Persistent and non-persistent CSMA protocols are clearly an
improvement over Aloha because they ensure that no station
begins to
transmit when it senses the channel busy.
-
The interference between two signals is called a collision.
Technically, monitoring a cable during transmission is known as
collision
detection (CD), and the Ethernet mechanism is known as Carrier
Sense
Multiple Access with Collision Detection (CSMA/CD).
The access mechanism used in an Ethernet is called Carrier
Sense
Multiple Access with Collision Detection (CSMA/CD standardized
in IEEE
802.3).
6.5:Token Bus: IEEE Standard 802.4:
LAN have a direct application in factory automation and
process
control, where the nodes are computers controlling the
manufacturing
process. It is a real time processing with minimum delay is
needed.
Ethernet is not suitable for this purpose because the number of
collisions is
not predictable. It support for factory automation and process
control
application that required real time process. It combines
features of Ethernet
and collisions free. It is physical bus that operates as a
logical ring using
tokens. For example A-B-C-D. A send to D then it passes the
information
through B and C. The bus token specify the destination address
in the
source. A station receives a token from its predecessor and
sends a token to
its successor.
Token bus is limited to factory automation and process control
and
has no commercial application in data communication.
6.6:Token Ring: IEEE Standard 802.5:
It is defined by the IEEE standard 802.5. The token ring is a
MAC
protocol sitting between the Logical Link Control and the
physical layer in
the OSI model.
Station on a token ring LAN is connected in a ring using a
NIC.
-
All the stations are connected to the NIC and then connected to
another
one. The network access mechanism used by Ethernet is not
infallible and
may result in collision.
6.7:FIBRE DISTRIBUTED DATA INTERFACE (FDDI)
The fiber distributed data interface (FDDI) standard for a
100Mbps
fiber optic LAN was developed during the mid-1980s by a
subcommittee of
ANSI and was completed in 1990. LANs based on the IEEE 802
standards
reached capacity, optical fiber LANs based on the FDDI standard
became
an alternative growth path. FDDI LANs were used to provide
high-speed
backbone connections between distributed LANs
Two types:
Single mode fiber (SMF) and Multimode fiber (MMF)
Single mode fiber: it can deliver connectivity over longer
distances,
with higher performance than MMF.
Multimode fiber is usually used to connect devices within a
building or
a small geographically contained area.
FDDI has implemented over twisted pair copper wire. The
copper
distributed data interface (CDDI) called uses only shielded
twisted pair or
unshielded twisted pair category 5 cabling but supports
distances of 100
meters and data rates of 100 Mbps. FDDI network contains two
complete
rings one that is used to send data when everything is working
correctly,
and another that is used only when the first ring fails.
-
6.8:DISTRIBUTED QUEUE DUAL BUS (DQDB): IEEE
STANDARD 802.6
Local area networks are usually restricted to a single site.
Metropolitan Area Network (MAN) expands network coverage to
include
several buildings or sites within a limited area.
IEEE standard 802.6 defines the Distributed Queue Dual Bus
(DQDB)
which resembles a LAN standard. It is designed to be used in
MAN.
DQDB uses a dual bus configuration.
Each device in the system connects to two backbone links.
Access to these links is granted not by contention (as in 802.3)
or token
passing (as in 802.4 and 802.5) but by a mechanism called
distributed
queues.
This protocol specifies a dual-bus topology to carry data in
forward
and reverse directions.
The forward direction bus carries data while the reverse
direction
handles queuing and control information.
For example:
Two unidirectional buses are labeled Bus A and Bus B. Two
unidirectional buses (cables) to which all computers are
connected.
Each bus has a head-end. Each bus connects to the stations
directly through input and output
ports; no drops lines are used. To send data on one bus, a
station must
use the other bus to make a reservation.
-
6.9:LAN OPERATING SYSTEM AND PROTOCOLS
Several LAN operating systems are associated with specific
protocols
that are transported within Ethernet or Token ring.
LAN operating systems include the following:
Novell Netware Windows NT LAN Manage and LAN Server Apple
Talk
A local area network can transport several network protocols
individually or in combinations of two, three or more
protocols.
For example routers, are often set up to automatically configure
themselves
by recognizing the different protocols.
A single Ethernet LAN might host one protocol for a
mainframe
computer, a different protocol for Novell servers, and still
another protocol
for Windows NT servers.
Internet Packet Exchange (IPX) protocol is designed for use
with
Netware. The advantage of IPX over some other early protocols is
that it
can be routed, meaning that it can transport data over multiple
networks in
an enterprise.
Novell implemented a comparison protocol called Sequence
Packet
Exchange (SPX)
The native protocol for Windows NT is NetBEUI, which was
developed for LAN manager and LAN server before the creation
of
Windows NT. NetBEUI was developed when computer networking
primarily meant local area networking for a relatively small
number of
computers, from just a few to as many as 200.
The advantage of having multiple LAN protocols on a network is
that
such a network can perform many different functions on the same
LAN,
such as enabling Internet access and access to mainframe
computers and
servers.
-
The disadvantage is that some protocols operate in broadcast
mode,
meaning that they frequently send out packets to identify
devices on the
network causing a significant amount of redundant network
traffic.
The properties of a LAN protocols are similar to those of
other
communication protocols, but some LAN protocol were developed in
the
early days of networking.
In general, LAN protocols must provide the following:
Reliable network Links Relatively high speeds Source and
destination node address handling Adherence to network standards,
particularly the IEEE, 802 standards.
6.10:Ethernet Technologies:
Most common Ethernet Technologies are,
10 Base-2 Ethernet Base-5 Thick Ethernet Technology 10 Base-7
and 100 Base-T Gigabit Ethernet
6.10.1:10 Base-2 Ethernet:
Popular and more flexible coaxial cable only 0.25inches in
diameters. It is used in PC LANs. In 10 Base-2, 10 is stands for
10Mbps and 2 is denote for 200
meters.
It use bus topology and is the approximate maximum distance
between any two nodes.
A dozen PCs need 12 Ethernet cards. It is a cheap-net.
-
6.10.2:Base-5 thick Technology:
It is using standard coaxial cable, which is 0.4inch in
diameter. A speed of 10Mbps using base band transmission for a
maximum
distance of 500 meters. It is frequently called thick
Ethernet.
6.10.3:10 Base-T and 100 Base-T:
10Base-T transmits at 10Mbps and 100 Base-T Ethernet transmits
at 100Mbps.
100Base-T is also commonly called fast Ethernet. It uses a star
topology. It has a central office device called a hub. Each adapter
on each node has a direct, point-to-point connection to
the hub.
It is used in different building in the same campus.
6.10.4:Gigabit Ethernet:
It is highly successful 10Mbps and 100Mbps Ethernet standards.
It offering a raw data of 1000Mbps. It referred to as
IEEE802.3z
standard.
It uses CSMA/CD for shared broadcast channels. his allows a
full-duplex operation at 1000Mbps in both directions for
point-to point channels. It has a star topology with a hub or
switch at its centre. It is a backbone of the interconnecting
multiple 10Mbps and 100Mbps
Ethernet LANs.
-
Local Area Networks
Introduction:
Local Area Networks (LANs), discussed in Chapter 6, typically
cover small
geographical areas. They are designed around relatively simple
bus or ring topologies.
Some networks such as Wide Area Networks (WAN) however, cover
much larger
areas, sometimes panning several continents. In such cases the
LAN protocols are
inappropriate and new ones must be defined.
LAN uses include file transfer, electronic mail and file servers
just as for WAN.
WAN can be used for remote log-ins also (An application in which
a user in one
location logs into a computer at another). WAN protocols must
distinguish between
various applications.
There is a difference in routing between LAN and WAN. Routing
strategies are
more complex in WAN than in LAN. The fact that there are many
ways to go from
one point to another by itself makes the situation more
complex.
To add to the complexity, sometimes a link in a chosen route
experiences a
failure. What does the network protocol do with all the date
traveling in that route? In
some cases a route may prove to be so popular that too much data
travels over it. The
result is congestion and sometimes failures. Can network
protocols avoid such
situations? If they can not, what can they do to minimize their
effects? When data is
delayed due to failures and congestion, it must be stored
somewhere while WAN
protocols decide what to do with it. Network nodes must be
equipped with software
and buffers to do this.
LANs are controlled and managed by a single organization or
department. If a
problem occurs, users know whom to call. Some WANs such as the
Internet have
evolved mainly due to voluntary efforts of universities and
government agencies.
Consequently, there is no central authority responsible for
fixing problems or updating
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protocols so that problems do not recur. The success of such
network operations
depends on the cooperation of the organizations that use
them.
7.1 WAN TRANSMISSION METHODS
WAN transmission methods use different switching techniques.
Switching
techniques are use to create one or more data paths called
channels for transmitting
data. The channels may be created using one communication cable
or using several
cables that offer a range of paths along which data can be
transmitted. Switching can
enable multiple nodes to simultaneously transmit and receive
data or it can enable data
to be transmitted over different routes to achieve maximum
efficiency in terms of
speed and cost. The following are the common switching
techniques used in WANs:
Time Division Multiple Access (TDMA)
Frequency Division Multiple Access (FDMA)
Statistical Multiple Access
Circuit Switching
Message Switching
Packet Switching
7.1.1. Time-Division Multiple Access (TDMA)
TDMA divides the channels into distinct time slots. Each time
slot is
designated for a particular networks node, as if it were a
dedicated line. The WAN
switching deice rotates from time slot to time slot for each
channel. This is similar to
a 24-hour television programming, where the time has been
specified for a particular
program. TDMA does not guarantee the most efficient use of the
network medium
since transmission occurs only via one channel at a time. The
timing of node
transmission is also important, since a node may transmit at an
interval that is out of
synchronization with its time slot.
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7.1.2. Frequency Division Multiple Access (FDMA)
FDMA divides the channels into frequencies instead of time
slots. Each
channel has its own broadcast frequency and bandwidth. The
switching device
switches from frequency to frequency as it sends data. This is
similar to four listeners
with headsets sharing a radio modified to have four channels.
The first listener might
be listening to a classical station, the send to a talk show the
third to a base ball game
and thr fourth to the news. Each listener is at a different
frequency. The radio inputs
to each channel so quickly that none can tell it is quickly
switching from channel to
channel as it receives the signal on each frequency.
7.1.3. Statistical Multiple Access (SMA)
Statistical multiple access or statistical multiplexing, is sued
by many WAN
technologies, such as X.25, ISDN and frame relay. This method is
more efficient than
TDMA and FDMA, because the physical medium bandwidth is
dynamically allocated
according to the application need. The switching device
continuously monitors each
channel to determine the communication requirements. For
example, at one moment a
channel may need to transmit a large graphics file, and then be
quiet. Algorithms on
the switch determine the bandwidth needed to transmit the file.
After the file is
transmitted, the switch reallocates bandwidth to another
channel. This might be
compared to the way in which a workstation operating system
automatically decides
how much memory to give to three applications running at the
same time. It might
give 15 KB for an active word processing file, 7 MB for an image
from a scanner and
1.2MB for printing a graphic.
7.1.4. Circuit Switching
Circuit switching involves creating a dedicated physical circuit
between the
sending and receiving nodes. This acts as a straight channel on
which to send data
back and forth without interruption, similar to a telephone call
between two parties.
The transmission channel remains in the service until the two
nodes disconnect.
Communication via circuit switching implies that there is a
dedicated communication
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path between two stations. The path is a connected sequence of
links between
network nodes. On each physical link, a logical channel is
dedicated to the
connection. Communication via circuit switching involves there
phases.
Phase I: Circuit establishment.
Before any signals can be transmitted an end-to-end (station to
station) circuit
must be established. For example, station A sends a request to
node4, requesting a
connection to Station E. Typically, the link from A to 4 is a
dedicated line, so that part
of the connection already exists. Node 4 must find the next leg
in a route leading to
node 6 based on routing information and measures of availability
and perhaps cost.
Node 4 selects the link to node 5, allocates a free channel
(using FDM or TDM) on
that link and sends a message requesting connection to E. So
far, a dedicated path has
been established from A through 4 to 5. Because a number of
stations may attach to 4,
it must be able to establish internal paths from multiple
stations to multiple nodes.
The remainder of the process proceeds similarly. Node 5
dedicates a channel to node
6 and internally ties that channel to the channel from node 4.
Node 6 completes the
connection to E. In completing the connection, a test is made to
determine if E is busy
or is prepared to accept the connection.
Phase II: Data Transfer.
Information can now be transmitted from A through the network to
E. The data
may be analog or digital, depending on the nature of the
network. As the carriers
evolve the fully integrated digital networks, the use of digital
(binary) transmission for
both voice and data is becoming the dominant method. The path
is: A-4 link, internal
switching through4; 4-5 channel, internal switching through 5;
5-6 channel, internal
switching through 6; 6-E link. Generally the connection is fully
duplex.
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Phase III: Circuit disconnect.
After some period of data transfer, the connection is
terminated, usually by the
action of one of the two stations. Signals must be propagated to
nodes 4,5 and 6 to de-
allocate the dedicated resources.
Note that the connection path is established before data
transmission begins.
Thus, channel capacity must be reserved between each pair of
nodes in the path and
each node must have available internal switching capacity to
handle the request
connection. The switches must have the intelligence to make
these allocations and to
devise a route through the network.
Circuit switching can be rather inefficient. Channel capacity is
dedicated for the
duration of a connection, even if no data are being transferred.
For a voice
connection, utilization may be rather high, but it still does
not approach 100 per cent.
For a terminal-to-computer connection, the capacity may be ideal
during most of the
time of the connection. In terms of performance, there is delay
prior to signal transfer
for call establishment. However, once the circuit is
established, the network is
effectively transparent to the users. Information is transmitted
at a fixed data rate with
no delay other than the propagation delay through the
transmission links. The delay at
each node is negligible.
Circuit switching was developed to handle voice traffic but is
now also used for
data traffic. The best known example of a circuit-switching
network is the public
telephone network. This is actually a collection of national
networks intern-connected
to form the international service. Although originally designed
and implemented to
service analog telephone subscribers, it handles substantial
data traffic via modem and
is gradually being converted to a digital network. Another well
known application of
circuit switching is the private branch exchange (PBX) used to
interconnect telephones
within a building or office
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7.1.5 Message Switching
Message switching uses a store-and-forward communication method
to transmit
data from the sending to the receiving node. The data is send
from one node to
another, which stores is temporarily until a route towards the
datas final destination
becomes available. Several nodes along the route store and
forward the data until it
reaches the destination node. Message switching is used for
example, when you send
an e-mail message on an enterprise network with file servers
acting as post offices.
The message goes from one post office to the next until it
reaches the intended
recipient.
7.1.6 Packet Switching
Circuit switching was designed for voice communication. In a
telephone
conversation, for example once a circuit is established it
remains connected for the
duration of the session. Circuit switching creates temporary
(dialed) or permanent
(leased) dedicated links that are well suited to this type of
communication.
A key characteristic of circuit-switching networks is that
resources within the
network are dedicated to a particular call. For voice
connections, the resulting circuit
will enjoy a high percentage of utilization because most of the
time, one party or the
other is talking. However, as the circuit-switching network
began to be used
increasingly for data connections, two shortcomings became
apparent.
1) In a typical user/host data connection (for example, a
personal computer user
logged on to a database server) much of the time the line is
idle. Thus, with data
connections, a circuit-switching approach is inefficient.
2) In a circuit-switching network, the connection provides for
transmission at a
constant data rate. Thus, each of the two devices that are
connected must transmit and
receive at the same data rate as the other. This limits the
utility of the network in
interconnecting a variety of the host computers and
workstations.
To understand how packet switching addresses these problems, let
us briefly
summarize the packet-switching operation. Data are transmitted
in short packets. A
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typical upper bound on packet length is 1000 octets (bytes). If
the source has no
longer message to send, the message is broken up into a series
of packets as shown in
Figure 7.2 Each packet contains a portion (or all for a short
message) of the users
data, plus some control information that the network requires to
be able to route the
packet through the network and deliver it to the intended
destination. At each node en
route, the packet is received, stored briefly and passed on to
the next node.
Now assume that Figure 7.2 depicts a simple packet-switching
network.
Consider a packet to be send from Station A to Station E. The
packet includes control
information that indicates that the intended destination is E.
the packet is send from A
to node 4 stores the packet, determines the next leg of the
route (say 5) and queues
the packet to to-out on that link (the 4-5 link). When the link
is available, the packet is
transmitted to node 5, which forwards the packet to node 6 and
finally to E. This
approach has a number of advantages over circuit switching:
Line efficiency is greater, because a single node-to-node link
can be
dynamically shared by many packets over time. The packets are
queued up and
transmitted as rapidly as possible over the link. By contrast,
with circuit switching,
time on a node-to-node link is pre-allocated using synchronous
time-division
multiplexing. Much of the time, such a link may be ideal because
a portion of its time
is dedicated to a connection that is ideal.
A packet-switching network can perform date-rate conversion. Two
stations of
different data rates can exchange packets because each connects
to its node as its
proper data rate.
When traffic becomes heavy on a circuit-switching network, some
calls are
blocked; that is, the network refuses to accept additional
connection requests until the
load on the network decreases. On a packet-switching network,
packets are still
accepted, but delivery delay increases.
Priorities can be used. Thus, if a node has a number of packets
queued for
transmission; it can transmit the higher-priority packets first.
These packets will
therefore experience less delay than lower-priority packets.
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If the station has a message to send through a packet-switching
network that is of
length greater than the maximum packet size, it breaks the
message up into packets
and sends these packets, one at a time, to the network. A
question arises as to how the
network will handle this stream of packets as it attempts to
route them through the
network and deliver them to the intended destination. There are
two approaches that
are used in contemporary networks: datagram and virtual
circuit
In the datagram approach to packet switching, each packet is
treated
independently from all others. Even when one packet represents
just a piece of a multi
packet transmission, the network (and network layer functions)
treats it as though it
existed alone. Packets in this technology are referred to as
datagrams. Figure 7.1
shows how the datagram approach can be used to deliver three
packets from Station A
to Station E. In this example, all the three packets (or
datagram)belong to the same
message, but may go by different paths to reach their
destination. This approach can
cause the datagrams of transmission to arrive at their
destination out of order. It is the
responsibility of the transport layer in most protocols to
reorder the datagrams before
passing them on to the destination port. The link joining each
pair of nodes can
contain multiple channels. Each of these channels is capable, in
turn, of carrying
datagrams either from several different sources or from one
source. Multiplexing can
be done using TDM or FDM.
In the virtual circuit approach to packet switching. The
relationship between all
packets belonging to a message or session is preserved. A single
route is chosen
between the send and receiver at the beginning of the session.
When the data are sent,
all packets of the transmission travel one after another along
that route. The difference
from the datagram approach is that, with virtual circuits, the
node need not make a
routing decision for each packet. It is made only once for all
packets using the virtual
circuit.
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7.2 WAN CARRIER TYPES
There are several physical signaling or carrier methods for
transporting data on
WANS. Some of the most common include the following:
Point to point
T-carrier
SONET
ISDN
Wireless
7.2.1 Point-to-Point
Point-to-point carrier communications through public dial-up
lines and leased
telephone lines represent the most basic WAN carrier
communications. For example,
a simple WAN is established every time you employ a modem to
make a modem-to-
modem to make a modem-to-modem connection over a dial-up line.
The modem at
the other end may be connected to a network or to a computer
that is a few miles away
or a few thousand mi