Module 5 2010

CUTAJAR & CUTAJAR
©2010

 Students should be able to
◦ understand the basics of transmission methods in
communication
◦ distinguish between different categories of networks
◦ appreciate the purpose of a protocol in communication
◦ appreciate the wide range Internet related technical terms and
Internet applications

Communication & Networks 2

 Networks are an interconnection of computers. These
computers can be linked together using a wide variety of
different cabling types, and for a wide variety of different
purposes.
 The basic reasons why computers are networked are
◦ to share resources (files, printers,
modems, fax machines)
◦ to share application software
(distributed programs)
◦ increase productivity (make it easier
to share data amongst users)
◦ provide fast communication between
users.


There are FOUR basic elements involved in communications:
1. The SENDER which initiates the communication.
2. The MEDIUM which is the mechanism by which communication is
conveyed to the receiver
3. The RECEIVER which receives the communication
4. The MESSAGE, which is the information content that is transferred
between the sender and receiver via the medium.
message

Communication
Medium
Source
Sink

Transmitter
Receiver
Noise
SENDER MEDIUM RECEIVER


 Originally, communications depended on
codes transmitted by visual systems such
as mirrors, flags and smoke.
 Modern Communications make use of
electrical or optic signals to
communicate between one side and the
other.
◦ Electrical data communication systems
transmit codes by switching electrical
currents or voltages.
◦ Optic data communication systems
transmit codes by switching light pulses
through an optic fibre.


 Samuel F.B. Morse perfected the
telegraph,The first mass data
communication system based
on electrical power.

Telegraph used the Morse code to
transmit messages from one operator
to another.This code was very
difficult to automize


 Codes: Standard (agreed-on-in advance) interpretations
between signalling elements and characters. Some Codes
are used to represent characters within a computer.
 Signalling Elements: Representations of characters that are
transmitted over the transmission lines.
 Characters: Letters, signs and symbols on input/output
devices.

Signaling elements

Characters Encoding Decoding Characters


 Emile Baudot developed one of the
most successful codes, suited for
machine encoding and decoding.
However, it was limited because it
could only use five signalling
elements per character. He
introduced the LTRS and FIGS to
double his character set.


 ASCII: “as-key” is a code developed by ANSI. It uses 7 bits to represent
128 characters. It is the most popular code today. They are loaded in a
PC using the ANSI.SYS file.

“A” 100 0001 65
“a” 110 0001 97
“0” 011 0000 48

Internally the 8th bit, which is used in
transmission as the parity bit, is used to
extend the character set to 256 characters,
called the Extended ASCII Set.
Another Code called EBCDIC is an 8th bit
alternative to ASCII


 Unicode has the explicit aim of transcending the limitations
of traditional character encodings, such as those defined by
the ASCII code, which find wide usage in various countries
of the world, but remain largely incompatible with each
other. Many traditional character encodings share a
common problem in that they allow bilingual computer
processing (usually using Latin characters and the local
script), but not multilingual computer processing (computer
processing of arbitrary scripts mixed with each other).


Computer A Computer B

Application User-to-User communication Application
Process Process

Computer-to-Computer
communication
Communication Communication
Subsystem Subsystem

Computer-to-Network communications

Data Communication network


 Irrespective of the type of data communications facilities
being used, in most applications data is transmitted
between computers in a bit-serial mode whilst inside a
computer data is transferred in a word-parallel mode.
 It is thus necessary to perform a parallel-to-serial
conversion at the transmitter and vice-versa at the receiver

Bit Serial
Word Data
Parallel-to-Serial Serial-to-Parallel
Parallel conversion conversion Word
Data
Parallel
Data


 Once data is transmitted outside of a computer, there
is a much increased probability that bits are received in
error. It is therefore important to provide a means to
correct the data in case of error ( Error Control )
 The rate at which data is transferred between two
computers must also be controlled so as to assure that
all the information is received. ( Flow Control )
 If an intermediate network is involved, establishing a
communications path across a network is also
necessary ( Routing ).


 If only two computers are involved and both are in the
same room or office, then the transmission facility can
comprise just a simple point-to-point wire link.

Application Application
Process Process

Subsystem Subsystem


 If the two computers are located in different parts of a town or
country, public carrier facilities must be used. Normally this will involve
the Public Switched Telephone Network (PSTN) which requires a
device known as modem to transmit data.

Process Computer A
Process

Computer B
Subsystem Subsystem

Modem PSTN Modem


 In standards, data processing equipment (computers)
are known as Data Terminal Equipment ( DTE ).
 Modems are known as Data Circuit termination
Equipment ( DCE ). It is probably easier to remember it
as Data Communication Equipment, but this is not the
official name.

Network
DTE DCE

DCE DTE


When more than two computers Floor 1
are involved, a switched Site-wide
communication facility (network) (Backbone)
is normally provided to enable the MAN
computers to communicate with
Floor 2
each other. If all the computers
are installed within the same LAN A
building, it is possible to install
one’s own network. Such
networks are known as Local Area Floor 3 LAN B
Networks or LANs and
interconnect various LANS by
Terminals
means of a Metropolitan Area
Network or MAN Bridges
LAN C Transceivers


 When individual local area networks are located in different sites, the
public carrier facilities must again be used. The resulting network is
known as a Wide Area Network or WAN
Private Branch
Intelligent Exchange
Leased Lines SITE B
SITE A Multiplexer

Voice Voice
PBX PBX
IMUX PSTN IMUX
DSE DSE
Data Data

Company-wide Data Switching
backbone network Equipment


The Public Service Networks provide a public switched data services which
have been designed specifically for data transmission rather than voice.
Consequently, distributed networks use a Public Switched Data Network
(PSDN).

Computer
Computer

Communication
Subsystem Terminal
PSDN Controller

Interface
Standards

TC
Computer


 Alternatively, many public carriers are now converting their existing public
switched telephone networks to enable data to be transmitted without
the need of modems. The resulting networks, which operate in all digital
mode are known as Integrated Services Digital Networks (ISDN)
referring to both voice and data.
Voice
Voice
NTE
NTE Data
Data ISDN

Network Termination
Voice Equipment

NTE
Data


 Till now we have considered only intranetworking, in the sense that
communication is always within the same LAN or WAN.
 In some applications however, communication is also needed between
separate networks such as LAN-WAN-LAN connections. This type of
communication, is known as internetworking or internet.

Gateway Satellite

Earth Station
PSDN PSDN

LAN LAN LAN LAN


 Until recently computer industry standards were
concerned primarily with either the internal operation of
a computer or the connection of a local peripheral device.
 This resulted in communication subsystems offered by
manufacturers only enabled their own computers to
exchange information.
 Such systems are known as closed systems.
 Initially, the services provided by most public carriers
were concerned primarily with data transmission, and
device interfacing with the network.
 This resulted in interface standards of multi-vendor
systems.


 In contrast to the closed system, the various international bodies
concerned with public carrier networks have formulated agreed standards
for connecting devices to these networks:
◦ V-Series Recommendations: DTE-Modem-PSTN connections
◦ X-Series Recommendations: DTE to PSDN connections
◦ I-Series Recommendations: DTE to ISDN connections
 Additionally they devised higher level standards concerned with the
format (syntax) and control of the of information (data) between systems.
 Consequently equipment from different manufacturers could be
exchanged as long as it adheres to these standards
 The resulting system is known as open system or open system
interconnection environment (OSIE)


To overcome the complexity of the communication subsystem,
the ISO (International Standards Organisation) has adopted a
layered approach for the reference model. The complete
subsystem was broken down in layers, each of which performs a
well defined function. Conceptually these layers can be
considered as performing one of two generic functions:-
◦ Network dependent functions
◦ Application oriented functions

Application Oriented

Network Dependent


There exist 3 operational environments:
a. The Network environment: This is concerned with protocols and standards
relating to different types of underlying communication networks.
b. The OSI environment: This embraces the network environment and adds
additional application oriented protocols and standards to allow the end
system to communicate with one another in an open way.
c. The Real system environment: This is concerned with the manufacturers own
proprietary software and services which have been developed to perform
a particular distributed information processing task.


Application Process Application Process

Presentation Presentation
Session Session
Transport Transport
Network Network
Data Link Data Link
Physical Physical

Data Communication Network

Network Environment
OSI Environment

Real System Environment

Open System Interconnection


End User Application Process

Distributed information service

Application Layer : FTP, Information Interchange, job transfer

Presentation Layer : Syntax negotiations, data representation transformations

Session Layer : Dialogue and Synchronization control for applications

Transport Layer : End to End message transfer
( connection management, error control, fragmentation and flow control ).

Network Layer : Network Routing, addressing, call setup and clearing

Data Link Layer : Datalink control ( framing, data transparency, error control )

Physical Layer :Mechanical and Electrical network interface definitions

Physical connection to the network terminating equipment

Data Communication Network : The real physical network carrying messages


Provides the following services in the form of normal function calls:
 Identification of the intended communication partner(s) by name or
by address
 Determination of the current availability of the partner
 Establishment of authority to communicate
 Agreement on privacy (encryption) mechanism
 Authentication of partner
 Selection of dialogue discipline, including initialisation and release
procedures
 Agreement on responsibility of error correction
 Identification of constraints


 This layer is responsible for the syntax of the data
transfer, transforming from abstract data syntax to
transfer or concrete syntax:
 Anecdote - Language translator.
 Issues handled by this layer are data encryption and
decryption, and key transfer for such a job.


 This layer is used for the organisation and
synchronisation of messages and setting up and
clearing a dialogue between two peer computers.
 Optional services offered by this layer are:
◦ Interaction management - Duplex/Half Duplex
◦ Synchronisation - If messages are too long
establishes synchronisation points
◦ Exception Reporting - Reports on non
recoverable exceptions


 This is one of the most important Layers and
interfaces the network-dependent protocols to the
application oriented layers and provides a message
transfer facility which is thus network independent.
Two classes of functions exist in this layer:

◦ Class 0 - basic connection and data transfer
◦ Class 4 - full error control and flow control


 The function of these layers varies from network to network and the
three layers which are included here are:
◦ Network Layer: This is responsible for establishing and clearing a network wide
connection, the routing of messages (addressing) and flow control of traffic in
the network.
◦ Link Layer: This layer is responsible for a reliable information transfer using
error detection and retransmission where needed.
 Two types of services exist:
 Connectionless - Self contained message entities or Datagrams
 Connection Oriented - Virtual Circuit
◦ Physical Layer: Responsible for the DCE - DTE connection - It provides the link
layer a means of transmitting a serial bit stream between two pieces of
equipment.


 Prior and concurrently with the
ISO standards activity, the End-user/Application process
United states Department of ISO
Defense has funded research Layers
which resulted in an
File transfer Protocol (FTP)
internetwork known as Remote terminal protocol (TELNET)
ARPANET which was extended Name Server Protocol (NSP)
(5-7)
to incorporate other internets Simple Network Management Protocol
(SNMP)
to form the now well know
Internet. (4) TCP UDP

 The internet Protocol Suite IP
known as Transmission Control (1-3) IEEE802.X / X.25
Protocol / Internet Protocol
(TCP/IP) or the User Datagram
Protocol (UDP/IP) has thus
been developed LAN / WAN


 In practice, transmission can occur in one of three modes, namely,
Simplex, Half-Duplex and Full-Duplex modes

• Simplex:Transmission in one direction
only
• Half-Duplex:Transmission in both
directions but not at the same time
• Full-Duplex:Transmission in both
directions simultaneously

Half-Duplex Communication


 In practice, transmitted electrical signals are attenuated ( reduced ) and
distorted ( misshapen ) by the transmission medium, so that at some stage
the reciever is unable to discriminate between the binary 1 and 0 signals.

Distortion and attenuation Transmitted Data 0 1 0 0 1 1 0 1
depend strongly on :
Transmitted Signal
• The transmission medium, time
• The bit rate of the data being Typical Received
transmitted, Signal time

•The distance between two Sampling Instants
communicating devices.
Received Data 0 1 0 0 1 0 0 1

Transmitting Electrical Signals


 The type of transmission medium is important, since it
determines the maximum number of bits that can be
transmitted per second ( bps ) according to the
maximum bandwidth provided by the medium.
 The most commonly used media are:
◦ Two wire open lines
◦ Twisted Pair cables
◦ Coaxial Cables
◦ Optic Fibers


 Simplest form of transmission medium
 maximum distance: 50 m , maximum speed: 19.2 Kbps
 Working on Current or Voltage sensing
 Normally used for DTE-DCE connections
 Types available: multicore cable or flat ribbon cable
 Care must be taken to avoid cross coupling (capacitive
coupling between the two wires ) - crosstalk
 Open structure makes it susceptible to the pickup of
spurious noise signals caused by electromagnetic radiation -
picked up by just one wire


 Has a much better noise immunity ( symmetrical pickup ) and reduced
crosstalk
 Types available: UTP ( Unshielded ) and STP ( Shielded ) Twisted Pairs:

 Because a wire acts as an antenna, several
techniques are used to reduce Braided
Plastic Metal
electromagnetic interference (EMI). Most
Jacket Shield
wires are shielded, and some wires are also
twisted at 90º angles every so often. The
twists serve to additionally suppress EMI.
The attenuation of twisted wire pairs rises
rapidly with increasing frequency, and the
amount of crosstalk between adjacent pairs
also increases with frequency.
Twisted Pair


 used in token ring (4 or 16MBps), 10BaseT (Ethernet 10MBps), 100BaseT
(100Mbps)
 reasonably cheap
 reasonably easy to terminate [special crimp connector tools are necessary
for reliable operation]
 UTP often already installed in buildings
 UTP is prone to interference, and skin effect which limits speed and
distances
 low to medium capacity
 medium to high loss
 category 2 = up to 1Mbps (Telephone wiring)
 category 3 = up to 10Mbps (Ethernet and 10BaseT)
 category 5 = 100MBps (supports 10BaseT and 100BaseT)


 No skin effect and radiation effects at high frequencies
 maximum distance: 600 m , maximum speed 10 Mbps
 Applicable to both point to point and multipoint
topologies
 limited only by the maximum transmission frequency
through copper


 medium capacity
 Ethernet systems (10Mbps)
 slighter dearer than UTP
 more difficult to terminate
 not as subject to interference as UTP
 care when bending and installing is needed
 10Base2 uses RG-58AU
(also called Thin-Net or Cheaper Net)
 10Base5 uses a thicker solid core coaxial cable (also
called Thick-Net)


 Carries information in the form of a fluctuating beam of light in a
glass fibre. ( light waves have a much higher maximum
transmission frequency then electrical waves )
 Maximum distance : a few Kilometres, maximum speed: 100 Mbps
 Immune to electromagnetic radiation : thus can be employed in
electrically noisy environments
 Types available:
◦ Multimode Stepped index
◦ Multimode Graded index
◦ Monomode Stepped index. Reinforcing
Sheath Material Cladding

Optical
Fiber

Individual
Fiber Jacket

 relatively expensive
 used for backbones [linking LAN’s together] or FDDI rings
(100Mbps)
 high capacity [100Mbps]
 immune to electromagnetic interference and degrading
 low loss
 difficult to join (renders it more secure)
 connectors are expensive
 long distance


cladding jacket

Uses the principle of total interface
internal refraction: when light
passes from a more dense to a
lighter dense medium  core
Optical Optical
transmitter receiver

Unrefracted ray Impulse response
Normal

2 The pulse is widened since not all the
Less dense medium n2 Refracted ray rays starting at the same point take
More dense medium n1 the same path and thus arrive at
different time intervals
1

Incident ray


 Advantages
◦ Multimode step-index fibers are inexpensive and simple to
manufacture.
◦ It is easy to couple light into and out of multimode step-index
fibers; they have a relatively large source-to-fiber aperture.
 Disadvantages
◦ Light rays take many different paths down the fiber, which
results in large differences in their propagation times. Because
of this, rays traveling down this type of fiber have a tendency
to spread out. Consequently, a pulse of light propagating down
a multi-mode step-index fiber is distorted more than with the
other types of fibers.
◦ The bandwidth and rate of information transfer possible with
this type of cable are less than the other types.


cladding jacket

interface
The refractive index of the
core is decreased outwardly

so as to provide a gradual core
Optical Optical
change in direction of the transmitter receiver
incident light
Impulse response

Decreasing
refractive
Index


 Essentially, there are no outstanding advantages or
disadvantages of this type of fiber. Multimode graded-
index fibers are easier to couple light into and out of
than single-mode step-index fibers but more difficult
than multimode step-index fibers. Distortion due to
multiple propagation paths is greater than in single-
mode step-index fibers but less than in multimode
step-index fibers. Graded-index fibers are easier to
manufacture than single-mode step-index fibers but
more difficult than multimode step-index fibers. The
multi-mode graded-index fiber is considered an
intermediate fiber compared to the other types.


cladding jacket

interface

 core
Optical Optical
transmitter receiver

Impulse response

 Here light travels directly to destination or with some
total internal refraction.
 The power of the light source must be higher because
of the small acceptance angle. Thus lasers are normally
used as light sources instead of LED’s or ILD’s.


 There is minimum dispersion. Because all rays propagating
down the fiber take approximately the same path, they take
approximately the same amount of time to travel down the
cable. Consequently, a pulse of light entering the cable can
be reproduced at the receiving end very accurately.

 Because of the high accuracy in reproducing transmitted
pulses at the receive end, larger bandwidths and higher
information transmission rates are possible with single-
mode step-index fibers than with the other types of fibers.


 Because the central core is very small, it is difficult to
couple light into and out of this type of fiber. The source-
to-fiber aperture is the smallest of all the fiber types.
 Again, because of the small central core, a highly directive
light source such as a laser is required to couple light
into a single-mode step-index fiber.
 Single-mode step-index fibers are expensive and difficult
to manufacture.


 Terrestial Microwaves
◦ These are used in remote places where cables are difficult to reach
◦ Maximum distance: 50 Km.

 Radio
◦ Lower frequency radio transmission is also used in place of fixed wire links
over more modest distances using ground-based transmitters and receivers
such as wi-fi.

F1 F2 F3 F1 Radio field
coverage of base
station
F2
F3 F1 F2 F3 Fixed network
Base Station

F1 F2 F3 F1 User computers

53
Communication & Networks

 Any signal carried on a
transmission medium Transmitted Data 0 1 0 0 1 1 0 1

will be affected by Transmitted Signal
attenuation and noise. time

Caused by
Attenuation
time
Line (system)
noise time
Combined received
Signal time

Sampling Instants
Received Data 0 1 0 0 1 0 0 1

Bit error


 As a signal propagates along a transmission medium
(line) its amplitude decreases due to signal attenuation.
For long cables , amplifiers - also known as repeaters
must be inserted at intervals along the cable to restore
the received signal to its original level.
 Attenuation increases with frequency and since a signal
comprises a range of frequencies amplifiers must be
designed to amplify different frequency signals by varying
amounts. Alterenatively equalizers are used to equalize
the attenuation across a defined band of frequencies.


 The frequency of a channel is limited by the bandwidth
of the physical circuit.
 The bandwidth of a channel is the range of frequencies
that the circuit can pass without heavy attenuation.
Signals whose frequency is out of this
region are attenuated
Gain EXAMPLE :
Telephone Line
1 Bandwidth

Bandwidth 3000Hz

frequency
0
fL= 300Hz fH= 3300Hz

Lower Cutoff frequency Upper Cutoff frequency


 In the absence of a signal, a transmission line will ideally have zero electrical
signal present. In practice, however, there will be random perturbations on the
line. This is known as the line noise level. In the limit, as a transmitted signal
becomes attenuated, its level is reduced to that of the line (background) noise.
◦ Impulse Noise is caused by impulses of electrical
energy associated with external activity.
◦ Thermal Noise is caused by the thermal agitation
of electrons in the transmission line material. This
type is also known as White noise.
 An important parameter associated with a
transmission medium, therefore, is the ratio of
the power in a received signal, S, to the power
in the noise level, N. The ratio S/N is known as
the signal-to-noise ratio and is normally
expressed in bB.


 The bit rate is the number of bits (1’s or 0’s) transmitted per second whilst
Baud rate is the number (or frequency) of signalling elements per second.
 Nyquist showed that the maximum data transfer rate C of a line of
bandwidth B, assuming M levels per signalling element is given by:

C = 2.B.log2M bps.

The Bandwidth is a measure of frequency which takes
into account a whole wave cycle. So if with had just 2
possible levels per signaling element with would have 11
1 10
a maximum bit rate of 2.B. 01
0 With 4 levels per signaling element, 2 bits can be sent
00

per signaling element and thus the bit rate becomes
2.B.2


 A modem to be used with a PSTN uses an AM-PSK
modulation scheme with eight levels per signalling
element. If the bandwidth of the PSTN is 3100 Hz,
deduce maximum data transfer rate.
C = 2.B.log2M
= 2 x 3100 x log28
= 2 x 3100 x 3
Therefore C = 18600 bps
 In fact the data transfer rate will be less than this
because of other effects such as noise.


 The voltage inside a digital computer systems are mainly TTL
(Transistor Transistor Logic) with two nominal voltages – a 0V
represents the logic level 0 and 5V represents the logic level 1
 In practice there are two ranges to represent such levels – voltages
below 0.8V are considered a 0 and all voltages above 2V are considered
as 1,

5.0 V
1 representation

2.0 V
Intermediate
0.8 V
0 representation 0.2 V

Internal binary representation (TTL)


 Although the analogue PSTN was designed specifically for
voice communications, it is also possible to transmit data
using a modem. In the case of ISDN, calls can be set up and
data transmitted directly with a much higher bit rate.
 In the case of leased circuits, although in some
circumstances it is still necessary to use leased PSTN lines –
and hence modems – in most cases leased circuits are now
all-digital.


It is necessary to convert the binary data into a form
compatible with a speech signal at the sending end of the
line and to reconvert this signal back into its binary form
at the receiver. The circuit that performs the first
conversion is called a modulator whilst the inverse function
is performed by a demodulator.

DTE
PSTN
Modem
Telephone


 Various types of modulation are employed for
converting signals into a form suitable for
transmission on a PSTN.
◦ Amplitude Modulation (AM)
◦ Frequency Modulation (FM)
◦ Phase Modulation (PM)
 In converting binary signals keying is used and thus
the modulation techniques used are:
◦ Amplitude Shift Keying (ASK) Data
◦ Frequency Shift Keying (FSK)
Carrier


The level or amplitude of a single frequency audio tone
(carrier) switched or keyed between two levels at a rate
determined by the transmitted binary data signal.
Although the simplest type it is too much affected by
signal attenuation.
1 0 1 1 0 0 1 0
Binary
signal

AM


The frequency of a fixed amplitude carrier signal is
changed according to the binary stream to be transmitted.
Since only two frequencies ( audio tones ) are used for
binary data, this type of modulation is also known as digital
FM or frequency-shift keying (FSK).
1 0 1 1 0 0 1 0
Binary
signal

FM


 Let us consider we can share the bandwidth of a particular
medium by different channels, using modulation.
 The bandwidth occupied by a particular channel depends
on the type of modulation used and the maximum bit rate
of the channel.
Bandwidth determined by the bit rate
and modulation method used
Signal
Level

Frequency
F0 F1


 All the information relating to calls – voice and data – associated with most
public carrier networks is now transmitted between the switching
exchanges within the network in digital form. The resulting network is then
known as an integrated services digital network or ISDN since the user can
readily transmit data with voice without the use of modems.
 Voice transmissions are limited to a maximum bandwidth of less than 4KHz.
To convert such signals into digital form, the Shannon’s sampling theorem
states that their amplitude must be sampled at a minimum rate of twice the
highest frequency component.
 Hence to convert a 4Khz voice signal into digital form, it must be sampled
at 8000 times per second.

Digital


Analogue
voice signal
Time
(A)
(A)
Sampling
Sampling circuit clock
(B) (B)
Pulse amplitude
(C) modulated signal
(PAM) (C)

Quantization
and
companding

(D)
(D) Digitized
voice signal


 Voice communication tends
to be short duration but Circuit Message Packet
continuous. Computer Switching Switching Switching
communication tends to be
in burst with long periods
of no transmission. Because
of these differences, voice is
often transmitted over a
fixed, dedicated channel or
circuit while data is normally
transmitted in an occasional
packet, as needed, over a
temporary or shared
channel.


 Placing a phone call builds a physical
path or circuit from your phone to the
receiver's. When you hang up, the circuit
is broken and intermediate channels are
then available for other circuits to be
built for other phone calls. The circuit
from sender to receiver is dedicated
during the communication interval, so
no intermediate storage is required.
 However, the sender must wait for the
circuit to the receiver to be
constructed before transmission can
start.
 Delay is a function of the time required
to acquire exclusive use of the channel.


 The communication channel is shared, with a
message occupying the complete channel
during transmission. The entire message is sent
at once to an intermediate switch so there is
no wait for circuit construction all the way to
the receiver.
 However, the switch must be able to store
and forward the entire message, placing an
upper limit on the size of message that can be
transmitted to the lowest switch capacity along
the path.
 Because a message occupies the complete
channel during transmission, large messages
can cause considerable delay for other users
waiting to send messages.
 Also, since errors occasionally occur and large
messages are more likely to contain an error
than small ones, handling errors by resending
the message is potentially very costly.


 The channel is again shared.
 The message is broken up by the sender into smaller
packets of a maximum size that can be handled by
the intermediate switches.
 The switch stores each packet and forwards to
another switch along the way or to the receiver if
directly connected.
 Switches can receive and send packets
simultaneously, unlike message switching which must
receive the entire message before forwarding. This
reduces the overall time required to receive the
complete message since initial packets can be sent
on the communications channel without waiting for
the complete message.
 When errors occur only the bad packet must be
corrected (usually by resending) rather than the
complete message.
 Since the channel is shared, no one user has
exclusive control, other users packets can be
multiplexed onto the same channel, small packets
reduce the delay for other users sharing the channel.


 Here we are concerned with the mode of operation of the
different types of computer network that are used to
interconnect a distributed community and their various
interface standards and protocols.
 When the computers are distributed over a localized area –
such as a building – the network used is known as a Local Area
Network (LAN).
 Many LAN’s are linked together to form a Metropolitan Area
Network (MAN).
 When the computers are distributed over a wider
geographical area – such as a country – the network is known
as a Wide Area Network (WAN)


 LANs are used to interconnect distributed
communities of computer-based DTEs located within
say a single establishment.
 LANs are also referred to as private data networks as
they are normally installed and maintained by a single
organization.
 There are two quite different types of LAN:
◦ Wired LANs
◦ Wireless LANs
 We shall consider mostly the first type of LAN


 The most common network
topologies found are:
◦ Mesh - sometimes referred to
as distributed or network
◦ Star – All computers
connected to a central node.
◦ Bus – A common bus cable
links all computers
◦ Ring – All computers are linked
to form a ring of computers


 Most WANs, such as the PSTN, use a mesh (sometimes referred to as a network),
However, with LANs the limited physical separation of the DTEs permits simpler
topologies as the other four mentioned.

 There are two types of mesh topologies: full mesh and partial mesh:
 Full mesh topology occurs when every node has a circuit connecting it to every other
node in a network. Full mesh is very expensive to implement but yields the greatest
amount of redundancy, so in the event that one of those nodes fails, network traffic can be
directed to any of the other nodes. Full mesh is usually reserved for backbone networks.

 Partial mesh topology is less expensive to
implement and yields less redundancy than full mesh
topology. With partial mesh, some nodes are
organized in a full mesh scheme but others are only
connected to one or two in the network. Partial
mesh topology is commonly found in peripheral
networks connected to a full meshed backbone.


 The best example of a LAN based on a star topology is the digital
Private Automatic Branch Exchange (PABX).
 The need of modems are eliminated in modern PABXs by the use of
digital-witching techniques within the exchange and are therefore
referred to private digital exchanges (PDXs)


 Typically, with a bus topology the network cable is routed through all
those locations that have a DTE to be connected to the network and a
physical connection (tap) is made.
 Appropriate medium access control (MAC) circuitry and algorithms are
then used to share the available transmission bus among the various
DTEs attached.
 Bus extenders are used to link various bus sections

Bus

Bus
extender


 With a ring topology, the network cable passes from one DTE to another
until the DTEs are interconnected in the form of a loop or ring.
 The ring is unidirectional in operation and appropriate MAC algorithms
ensure the correct shared use of the ring.

DTE


 When a communication path is established between two DTEs through
a star network, the central controlling node ensures that the
transmission path between the two DTEs is reserved for the duration
of the call.
 However, with both ring and bus topologies this control is distributed
among the DTEs attached to the common transmission path.
 Two most common techniques adopted are:
◦ Carrier Sense Multiple-Access (CSMA) for bus topologies. It’s my
◦ Control Token for bus or ring networks turn
◦ Slotted versions of the above two.


 Carrier Sensing Multiple Access with Collision Detection
(CSMA/CD): In this method if a collision is detected
between two transmitting DTEs, transmission is
aborted and after a certain back-off time,
retransmission is attempted .


 In CSMA, two DTEs can attempt to transmit a frame over the cable at the
same time, causing data from both sources to get corrupted (collision).
 To reduce this possibility, before transmitting, the source DTE senses the
cable to check if a carrier is already present on the common line (frame in
transit).
 If a carrier is sensed (CS), the DTE defers the transmission until the
passing frame has been transmitted.

A A A

A C


 All DTEs are connected directly to the same cable, which is said to
operate in Multiple Access (MA) mode.
 To transmit data the sending DTE first encapsulates the data in a frame
headed with the destination address. The frame is then broadcast on the
bus.

 All stations listen to
the broadcast and
compare the
destination with their A B C
own address. If it A C A C
matches, they continue
copying all the data in
the frame.

D E


A t = t B
 Even so, two DTEs wishing to transmit a frame
simultaneously sense no carrier and start
transmitting simultaneously.
A t = tp -t B
 A DTE monitors the data signal on the cable
when transmitting the contents of a frame on
the cable. If the transmitted and monitored
signals are different, a collision is assumed to
have occurred – Collision Detection. A t = tp B
 To ensure that the colliding parties are all aware
of the collision a random bit pattern (jam
sequence) is sent by the DTE detecting the
collision. A t = 2tp B
 The stations involved back-off for a certain
random time and then retry the transmission.
tp = worst case delay


 In the event of a collision, retransmission of the frame is attempted up
to a defined maximum number of tries known as the attempt limit.
 Since overloading the network leads to the network breakdown, the
MAC unit tries to adjust the load by progressively increasing the time
delay between repeated retransmission attempts. The scheduling of
retransmissions is controlled by a process called truncated binary
exponential backoff.
 When transmission of the jam sequence is over, and assuming the
attempt limit has not been reached, the MAC unit backs off a random
integral number R of slot times which is given by:
0  R  2K where K = min{N, backoff limit}
 Thus the backoff range doubles with every attempt until the backoff
limit is reached.


Set Status to
frame ready for
NOT OK
transmission ?

Format frame Compute and wait Yes
for transmission backoff time
No
Yes
Carrier Attempts limit
signal on ? reached?

Complete No
transmission and Start transmitting
set status to OK after interframe gap Transmit jam sequence
Increment attempts
No Yes
Collision
detected ?


 Another way of controlling A
access to a shared transmission
medium is by a control token
(permission). D Token-ring
B

 This token is passed from one D B
Token

DTE to another according to a
defined set of rules. A DTE may
transmit a frame only when it is C

in possession of the token and,
A

after it has transmitted the Token

frame, it passes the token on to
D Token-ring
B
allow another DTE to access D B

the transmission medium.

C


 The frame is repeated (that is, each bit is
received and then transmitted) by all DTEs in
the ring until it circulates back to the initiating
DTE, where it is removed.
 In addition to repeating the frame, the
intended recipient retains a copy of the frame
and indicate that it has done so by setting the
response bits at the end of the frame.
 A Sender DTE releases the token in one of
two ways:
◦ The token is released only after the frame
comes back and the response bits are received.
◦ The token is released after transmission of the
last bit of the frame ( early token release )


 Monitoring functions within the May I have
active DTEs connected to the another token
physical medium provide the please ?

basis for initialization and
recovery, both of the connection
and the logical ring and from
loss of token.
 Although the monitoring
functions are normally replicated
among all the DTEs on the
medium, only one DTE at a time May I have another
ring please ?
carries the responsibility for
recovery and reinitialization.


 The Physical medium need not be a ring topology; a token can also be
used to control access to a bus network.
 Thus we can have:
◦ A token ring and
◦ A token bus.

Physical Logical


S

 After reading the data the receiving DTE
modifies the pair of response bits. S 01 11
 If the DTE is inoperable, the response bits ACK
remain unchanged.
 The Sender reads back the frame, checks the S
response bits and releases the token. 11
Inoperable
11 8 8 N 11
DESTINATION SOURCE 10 11
ADDRESS ADDRESS
DATA NAK

Monitor Passed Bit S
(Acknowledge) Response bits:
00 Busy
Start of Packet 01 Accepted
10 Rejected
11 Ignored (not working)
00 11
Busy

 The monitor node, after initializing the ring with a fixed number of empty
slots, ensures that the number of bits in the ring remain constant.
 The monitor passed bit is used by the monitor to detect whether a DTE
fails to release the slot after transmitting the frame.
 The monitor node is the vulnerable node of the ring network.
 Frame segmentation and monitor vulnerability are the weak points of this
type of network.
Monitor passed bit = 1 Empty Slot

Monitor Monitor

Monitor passed bit = 0 Monitor passed bit = 1


 There is a single token and only the possessor of the token can transmit a
frame.
 All DTEs that can initiate the transmission of a frame are linked in the
form of a logical ring.
P=F P =A P=B
 The token is passed physically using S=B S=C S=D

the bus around the logical ring. A B C

 On receipt of the token from its
predecessor (upstream neighbor) on
the logical ring, a DTE may transmit
any waiting frames up to a defined
F E D
maximum. logical
 It then passes the token to its known ring
P=E P=D P=C
successor (downstream neighbor) on S =A S=F S=E

the logical ring.


 The three MAC standards together with their associated physical
media specifications are contained in the following IEEE standards
documents:

ISO RM
 IEEE 802..3 CSMA/CD bus Network Layer

 IEEE 802.4 Token bus
Logical link
 IEEE 802.5 Token ring control 802.2
Data link
Layer
 IEEE 802..11 Wireless Medium
access
control
802.3 802.4 802.5 802.11

IEEE 802 Physical Layer
Physical

Transmission Medium


 To ensure that the information received by the receiver is the same as
that transmitted by the transmitter there must be a way for the receiver
to deduce , to a high probability when the received information contains
errors. Furthermore, should errors be detected, a mechanism is needed
to obtain a (hopefully) correct copy of the information.
 There are two approaches for achieving this:
◦ Forward error control: in which each transmitted character or frame contain
additional (redundant) information so that the receiver can, not only detect
when errors are present but also determine where in the received bit stream
the errors are. The data can thus be corrected.
◦ Backward error control: in which each character or frame includes only sufficient
additional information to enable the receiver to detect when errors are
present but not their location. A retransmission control scheme is then used to
request another hopefully correct copy.


The most common method used for detecting bit errors with asynchronous
and character oriented transmission is the parity bit method. With this
method the transmitter adds an additional bit – the parity bit – to each
transmitted character prior to transmission.The parity bit used is a function
of the bits that make up the character being transmitted, such that it can be
recomputed by the receiver to verify the correctness of the character
received.

Transmitted character

Start bit Stop bits
Parity bit

1001001 1 (even parity) 1001001 0 (odd parity)


To compute the parity bit for a character, the number of 1 bits in the code
for the character are added together (modulo 2) and the parity bit is then
chosen so that the total number of bits (including the parity bit itself) is
either even (even parity) or odd (odd parity).

(EXAMPLE 1001001)
(1) B0 (1)
(0)  (1)
B1  (0)
B2 (0)
 Odd Parity
(0) B3  (0) (0)
(1) B4
B5 
(0)  Even Parity
(0) B6
(1)
(1)


 Here when blocks of characters are being transmitted,
an extension to the error detecting capabilities
obtained by the use of a single parity bit per character
can be achieved , using an additional set of parity bits
computed from the complete block of characters in
the frame.
 In addition to the standard parity check (transverse or
row parity), an extra bit is computed for each bit
position (longitudinal or column parity ).


P B6 B5 B4 B3 B2 B1 B0
P B6 B5 B4 B3 B2 B1 B0 0 0 0 0 0 0 1 0

0 0 0 0 0 0 1 0 STX 1 0 1 0 1 0 0 0
0 1 0 0 0 1 1 0
1 0 1 0 1 0 0 0
0 0 0 0 1 0 0 0
0 1 0 0 0 1 1 0 1 0 1 0 1 1 0 1
0 0 1 0 0 0 0 0 Frame 0 1 0 0 0 0 0 0

1 0 1 0 1 1 0 1 Contents 1 1 0 0 1 0 1 1
1 0 0 0 0 0 1 1
0 1 0 0 0 0 0 0
1 1 0 0 0 0 0 1
1 1 1 0 0 0 1 1
1 0 0 0 0 0 1 1 ETX
1 1 0 0 0 0 0 1 BCC Column Undetected
Parity Error
Row (even) Combination
Parity Example
(odd)


 An alternative to retransmission of the blocks of data after an error has
been detected, is to build sufficient redundancy into the code to enable
the receiver to correct the error. The technique of detecting and
correcting the errors using an error correction code is known as Forward
error correction.
 The particular advantage of forward error correction is evident when
there is a long propagation delay, and thus since retransmission of the
message is remote, a lot of time is saved. This means that a continuous
stream of data can be transmitted with only a few interruptions for
retransmissions.
 An error correcting code can normally detect more errors than it can
correct. This scheme can detect single and double bit errors.


 In this case the most common alternative is based on
the use of polynomial codes.
 Simply said, The transmitter divides the message in
binary by another number (Generating Polynomial)
and appends the remainder to the tail of the message.
The receiver performs the same operation to check if
it obtains the same remainder. If the remainders agree,
the message is assumed to be correct.
 The computed check digits are referred to as the
frame check sequence (FCS) or the cyclic redundancy
check (CRC) digits.


 Error control is only one component of a data link
protocol. Another important and related component is
Flow control.
 As the name implies, it is concerned with controlling
the rate of transmission of frames on a link so that the
receiver always has sufficient buffer storage resources
to accept them prior to processing.
Enough !!


 A flow control facility is often invoked to ensure that a terminal does not
send any further characters until an overload condition has been cleared.
This mechanism is achieved by the computer sending a special control
character X-OFF to the controlling device within the terminal
instructioning it to cease transmission.
 When the overload condition
Computer Terminal
ends and the computer
becomes available to accept
further characters, it returns a
companion control character
X-ON to inform the terminal X-OFF
control device that it may
restart sending characters.
X-ON
This is known as handshaking.


In practice there are two basic types of ARQ:
 Idle RQ: used with character-oriented data
transmission schemes, implemented in either:
◦ Implicit Request or
◦ Explicit Request.
 Continuous RQ: used with bit-oriented transmission
schemes and employs either:
◦ Selective repeat or
◦ Go-back-N
retransmission strategies.


 The idle RQ error control scheme has been defined to
enable blocks of printable and formatting control chacters
to be reliably transferred – ie, to a high probability, without
error or replication and in the same sequence as they were
submitted. The information ( I-frames ) is transmitted here
between the sender (primary [P]) and the receiver
(secondary [S]) DTE’s across a serial data link.
 It operates in a half-duplex mode since the primary after
sending and I-frame, must wait until it receives an indication
from the scondary as to whether the frame was correctly
received or not. The primary then either sends the next
frame, if the previous frame was correctly received, or
retransmits a copy of the previous frame if it was not.


 There are two ways of implementing this sheme. In implicit
retransmission S only acknowledges correctly received frames and P
interprets the absence of an acknowledgement as an indication that the
previous frame was corrupted. Alternatively, in explicit request, when S
detects that a frame has been corrupted, it returns a negative
acknowledgement to request another copy of the frame.

?
message OK message Implicit

message OK message NOT OK
Explicit


The following can be noted from the following slides :
 P can have only one I-frame outstanding ( awaiting an acknowledgement
or ACK-frame) at a time;
 On receipt of an error-free I-frame, S returns an ACK-frame to P;
 On receipt of an error-free ACK frame, P can transmit another I-frame ;
 When P initiates the transmission of an I-frame it starts a timer;
 If S receives an I-frame or P receives an ACK-frame cantaining
transmission errors, the frame is discarded;
 If P does not receive an ACK-frame within a predefined time interval (the
timeout interval), then P retransmits the waiting I-frame;
 If an ACK-frame is corrupted, then S receives another copy of the frame
and hence this is discarded by S;


Note that:
 P can have only one I-frame outstanding ( awaiting an ACK-frame) at a time;
 On receipt of an error-free I-frame, S returns an ACK-frame to P;
 On receipt of an error-free ACK frame, P can transmit another I-frame ;
 When P initiates the transmission of an I-frame it starts a timer;

stop stop Timer
start start start

I(N) I(N+1) I(N+2) Primary P

I(N) I(N+1) I(N+2)
ACK(N) ACK(N+1)
I(N) I(N+1) Secondary S


 If S receives an I-frame or P receives an ACK-frame cantaining
transmission errors, the frame is discarded.

expired stop Timer
start start

I(N) I(N) Primary P

I(N) I(N)
ACK(N)
I(N) I(N) Secondary S


 If P does not receive an ACK-frame within a predefined time interval
(the timeout interval), then P assumes that the message has not been
received correctly and retransmits the waiting I-frame.
 If an ACK-frame is corrupted, then S receives another copy of the
frame and hence this is discarded by S;

expired stop Timer
start start

I(N) I(N) Primary P

I(N) I(N)
ACK(N) ACK(N)
Duplicated Message
(discarded)


 As with implicit acknowledgement sheme, on receipt of an
error free I-frame, S returns an ACK-frame to P;
 On receipt of an ACK-frame, P stops the timer and can
then initiate the transmission of another I-frame.
 If S receives an I-frame containing transmission errors, the
frame is discarded an it returns a NAK ( negative
acknowledgement) frame.
 If P does not receive an ACK-frame ( or NAK-frame)
within the timeout interval, P retransmits the waiting I-
frame.


 If S receives an I-frame containing transmission errors, the
frame is discarded an it returns a NAK (negative
acknowledgement) frame.

stop stop Timer
start start start

I(N) I(N) I(N+1) Primary P

I(N) I(N) I(N+1)
NAK(N) ACK(N)


 Since with the idle RQ scheme the primary must wait for an
acknowledgement after sending a frame, it is also known as Stop-and-
Wait.
 With both schemes however, it is possible for S to receive two or
more copies a of a particular I-frame (duplicates). In ordeer for S to
discriminate between the next vaild I-frame and a duplicate, each
frame transmitted contains a unique identifier known as sequence
number (N, N+1 etc). To enable P to resynchronize, S returns an
ACK-frame for each correctly received frame with the related I-frame
identifier within it. The sequence number carried in each I-frame is
known as the send sequence number or N(S), and the sequence
number in each ACK and NAK frame as the receive sequence number
N(R)


In continuous RQ, the primary continues to send messages
without waiting for acknowledge ment. If something goes
wrong there are two possible retransmission schemes:
 Selective Repeat: where only the message in error is
retransmitted. This requires a certain amount of storage
space on the receiver side, to be able to re-order the
message sequence one the retransmitted messages arrives.
 Go-Back-N: where all the messages from the erroneous
message onwards are retransmitted. This requires no
storage space on the receiver side.


N N+1 N+2 N+3 N+4 N+5 N+5 V(S)

N+4 N+4 N+4

N+1
N+2
N+1
N+3
N+2
N+3
N+2
N+3
N+2
N+3
N+2
time
N N N N+1 N+1 N+1 N+1
Primary (P)

I(N) I(N+1) I(N+2) I(N+3) I(N+4) I(N+1) I(N+2)

Secondary (S)

I(N) I(N+2) I(N+3) I(N+4) I(N+1)

Discarded frames N N N N N N+1

N N+1 N+1 N+1 N+1 N+1 N+2 V(R)


N N+1 N+2 N+3 N+4 N+5 N+5 V(S)

N+4

N+1
N+2
N+1
N+3
N+2
N+3
N+2
N+1
N+4 N+1
time
N N N N+1 N+1 N+3 N+4
Primary (P)

I(N) I(N+1) I(N+2) I(N+3) I(N+4) I(N+1)

Secondary (S)

I(N) I(N+2) I(N+3) I(N+4) I(N+1)

N N+2 N+2 N+2 N+2
N+3 N+3 N+3
N+4 N+4
N+1

N N+1 N+1 N+1 N+1 N+1 N+5 V(R)


 The distances which can be covered by a single LAN
Network
are limited and frequently there is a requirement to driven
extend this range. This maybe due to:
◦ Partitioning the whole network into groups of separate
entities for security reasons or to improve the Multivendor
OS driven
Integration
performance of the network.
◦ Coupling together existing entities and form a new
cohesive structure. These may have been installed as
separate initiatives aimed at resolving unique Application driven
requirements and thus be from different vendors. Thus
OSI MODEL
we speak of multivendor integration. The approach Application
taken in the integration of these computers can take Interoperability
Presentation
various viewpoints.
Session
Multivendor Transport
Internetworking + Interoperability = Integration
Internetworking Network
Data link
Physical


 Each layer acts as though it is communicating with
its corresponding layer on the other end. A B
 In reality, data is
passed from one
layer down to the USER USER
7 7
next lower layer at Data
Application Layer
6 Presentation Layer 6
the sending Headers AH Data
5 Session Layer 5
computer, till it's PH AH Data Tails 4 Transport Layer 4
finally transmitted SH PH AH Data
3 Network Layer 3
TH SH PH AH Data
onto the network 2 Data Link Layer 2
NH TH SH PH AH Data NT
cable by the DH NH TH SH PH AH Data NT DT
1 Physical Layer 1

Physical Layer. FH DH NH TH SH PH AH Data NT DT FT

 As the data is passed down to a lower layer, it is encapsulated into a larger unit (in
effect, each layer adds its own layer information to that which it receives from a higher
layer). At the receiving end, the message is passed upwards to the desired layer, and as
it passes upwards through each layer, the encapsulation information is stripped off .


 Summary of Repeater features
◦ increase traffic on segments
◦ have distance limitations
◦ limitations on the number that can be used
◦ propagate errors in the network
◦ cannot be administered or controlled via remote access
◦ cannot loop back to itself (must be unique single paths)
◦ no traffic isolation or filtering

 Repeaters also allow isolation of
segments in the event of failures or
fault conditions. Disconnecting one Repeater Repeater

side of a repeater effectively
isolates the associated segments
from the network.


 At the simplest level of interconnection we can operate at the bottom
layer of the OSI model. If both peers are identical and the requirement is
simply to repeat and boost the digital signal transmission across similar
media, then a repeater is required.
 Thus the range of the network
can be extended via a repeater. Station on Station on
Segment A Segment B
FH DH NH TH SH PH AH Data NT DT FT

USER Repeater USER
7 7
6 6
Repeater 5 5
4 4
3 3
Repeater Station
FH DH NH TH SH PH AH Data NT DT FT 2 2
1 1


 In the case of bridges, a facility is provided which is closer to the concept
of providing multivendor integration since a repeater only couples similar
elements. A bridge normally connects LAN technologies and provides a
relay service at the MAC layer thus acting as a store-and-forward device
(where necessary). Data which is being forwarded needs to compete for
access on the output side.

Bridge

Token Ring

Ethernet


Module 5 2010

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