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

2Communication & Networks

Cutajar & Cutajar

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

Communication & Networks 4

◦ 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 isconveyed to the receiver

3. The RECEIVER which receives the communication

4. The MESSAGE, which is the information content that is transferredbetween the sender and receiver via the medium.


Communication

Medium

Receiver

Transmitter

Source

Sink

Noise

message

SENDER RECEIVERMEDIUM

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.


Characters Encoding Decoding Characters

Signaling elements

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).



Communication

Subsystem

Application

Process

Computer A

Communication

Subsystem

Application

Process

Computer B

Data Communication network

User-to-User communication

Computer-to-Computer

communication

Computer-to-Network communications

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


Serial-to-Parallel

conversion

Parallel-to-Serial

conversion

Word

Parallel

DataWord

Parallel

Data

Bit Serial

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.


Communication

Subsystem

Application

Process

Computer A

Communication

Subsystem

Application

Process

Computer B

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.


Communication

Subsystem

Application

Process Computer A

Communication

Subsystem

Application

Process

Computer B

Modem ModemPSTN

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.


DTE

DTE

DCE

DCE

Network

When more than two computers

are involved, a switched

communication facility (network)

is normally provided to enable the

computers to communicate with

each other. If all the computers

are installed within the same

building, it is possible to install

one’s own network. Such

networks are known as Local Area

Networks or LANs and

interconnect various LANS by

means of a Metropolitan Area

Network or MAN


Site-wide

(Backbone)

MAN

Floor 1

LAN A

Floor 2

LAN BFloor 3

LAN C

Terminals

Bridges

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


PSTN

PBX

DSE

IMUX

Voice

Data

PBX

DSE

IMUX

Voice

Data

SITE BSITE ALeased Lines

Company-wide

backbone network

Data Switching

Equipment

Private Branch

ExchangeIntelligent

Multiplexer

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).


ComputerComputer

TCComputer

PSDN

Communication

Subsystem

Interface

Standards

Terminal

Controller

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.


NTE

Voice

Data

NTE

Voice

Data

NTE

Voice

Data

ISDN

Network Termination

Equipment

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.


PSDN

LAN LAN

PSDN

LAN LAN

Gateway Satellite

Earth Station

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


Network Dependent

Application Oriented

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

Presentation

Session

Transport

Network

Data Link

Physical

Application Process

Application

Presentation

Session

Transport

Network

Data Link

Physical

Data Communication Network

Computer A Computer B

Network Environment

OSI Environment

Real System Environment

Open System Interconnection


End User Application Process

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

Data Communication Network : The real physical network carrying messages

Physical connection to the network terminating equipment

Distributed information service

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 United states Department of Defense has funded research which resulted in an internetwork known as ARPANET which was extended to incorporate other internets to form the now well know Internet.

The internet Protocol Suite known as Transmission Control Protocol / Internet Protocol (TCP/IP) or the User Datagram Protocol (UDP/IP) has thus been developed


End-user/Application process

File transfer Protocol (FTP)

Remote terminal protocol (TELNET)

Name Server Protocol (NSP)

Simple Network Management Protocol

(SNMP)

TCP UDP

IP

IEEE802.X / X.25

LAN / WAN

(5-7)

(4)

(1-3)

ISO

Layers

Cutajar & Cutajar

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.


0 1 0 0 1 1 0 1

0 1 0 0 1 0 0 1

Transmitted Data

Transmitted Signal

Typical Received

Signal

Sampling Instants

Received Data

time

time

Transmitting Electrical Signals

Distortion and attenuation

depend strongly on :

• The transmission medium,

• The bit rate of the data being

transmitted,

•The distance between two

communicating devices.

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 orVoltage 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


Because a wire acts as an antenna, several

techniques are used to reduce

electromagnetic interference (EMI). Most

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.


Has a much better noise immunity ( symmetrical pickup ) and reduced crosstalk

Types available: UTP ( Unshielded ) and STP ( Shielded ) Twisted Pairs:

Plastic

Jacket

Braided

Metal

Shield

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.


Cladding

Individual

Fiber Jacket

Reinforcing

MaterialSheath

Optical

Fiber

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



interface

cladding

core

jacket

Optical

transmitter

Optical

receiver

Impulse response

Normal

Refracted ray

Unrefracted ray

Less dense medium n2

More dense medium n1

Incident ray

1

2

Uses the principle of total

internal refraction: when light

passes from a more dense to a

lighter dense medium

The pulse is widened since not all the

rays starting at the same point take

the same path and thus arrive at

different time intervals

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. Becauseof this, rays traveling down this type of fiber have a tendencyto spread out. Consequently, a pulse of light propagating downa multi-mode step-index fiber is distorted more than with theother types of fibers.

◦ The bandwidth and rate of information transfer possible with this type of cable are less than the other types.



interface

cladding

core

jacket

Optical

transmitter

Optical

receiver

Impulse response

Decreasing

refractive

Index

The refractive index of the

core is decreased outwardly

so as to provide a gradual

change in direction of the

incident light

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.


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.


interface

cladding

core

jacket

Optical

transmitter

Optical

receiver

Impulse response

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.

Communication & Networks53

Base Station

User computers

Radio field

coverage of base

station

Fixed network

F2

F1

F1

F3

F2

F2

F1

F3

F3

F2

F1

F1

F3

Any signal carried on a

transmission medium

will be affected by

attenuation and noise.


0 1 0 0 1 1 0 1Transmitted Data

Transmitted Signaltime

timeCombined received

Signal

Sampling Instants

0 1 0 0 1 0 0 1Received Data

Caused by

Attenuation

Line (system)

noise

time

time

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 repeatersmust be inserted at intervals along the cable to restorethe received signal to its original level.

Attenuation increases with frequency and since a signalcomprises a range of frequencies amplifiers must bedesigned to amplify different frequency signals by varyingamounts. Alterenatively equalizers are used to equalizethe 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.


Bandwidth 3000Hz

Lower Cutoff frequency Upper Cutoff frequency

fL= 300Hz fH= 3300Hz

Gain

frequency

1

0

Signals whose frequency is out of this

region are attenuatedEXAMPLE :

Telephone Line

Bandwidth

◦ 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.


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.

The bit rate is the number of bits (1’s or 0’s) transmitted per second whilstBaud rate is the number (or frequency) of signalling elements per second.

Nyquist showed that the maximum data transfer rate C of a line ofbandwidth B, assuming M levels per signalling element is given by:


1

0

11100100

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

a maximum bit rate of 2.B.

With 4 levels per signaling element, 2 bits can be sent

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,


0.8 V

0.2 V

2.0 V

5.0 V1 representation

Intermediate

0 representation

Internal binary representation (TTL)

Cutajar & Cutajar

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 formcompatible with a speech signal at the sending end of theline and to reconvert this signal back into its binary format the receiver. The circuit that performs the firstconversion is called a modulator whilst the inverse functionis performed by a demodulator.


DTE

Telephone

Modem

PSTN

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)

◦ Frequency Shift Keying (FSK)


Carrier

Data

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.


Binary

signal

AM

1 1 1 10 0 0 0

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).


Binary

signal

FM

1 1 1 10 0 0 0

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.


F1F0

Signal

Level

Frequency

Bandwidth determined by the bit rate

and modulation method used

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


Sampling circuit

Quantization and

companding

Sampling

clock

Pulse amplitude

modulated signal

(PAM)

Digitized

voice signal

Analogue

voice signal

(A)

(B)

(C)

(D)

Time(A)

(B)

(D)

(C)

Voice communication tends to be short duration but continuous. Computer 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.

Circuit

Switching

Message

Switching

Packet

Switching


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.


Cutajar & Cutajar

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.

◦ Control Token for bus or ring networks

◦ Slotted versions of the above two.


It’s my

turn

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 C

A

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 own address. If it matches, they continue copying all the data in the frame.

A C

A B C

D E

A C

Even so, two DTEs wishing to transmit a frame simultaneously sense no carrier and start transmitting simultaneously.

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.

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.

The stations involved back-off for a certain random time and then retry the transmission.


A B

A B

A B

A B

t = t

t = tp -t

t = tp

t = 2tp

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 backofflimit is reached.



frame ready for

transmission ?

Format frame

for transmission

Carrier

signal on ?

Start transmitting

after interframe gap

Collision

detected ?

Complete

transmission and

set status to OK Transmit jam sequence

Increment attempts

Attempts limit

reached?

Compute and wait

backoff time

Set Status to

NOT OK

YesNo

Yes

No

Yes

No

Another way of controlling

access to a shared transmission

medium is by a control token

(permission).

This token is passed from one

DTE to another according to a

defined set of rules. A DTE may

transmit a frame only when it is

in possession of the token and,

after it has transmitted the

frame, it passes the token on to

allow another DTE to access

the transmission medium.


Token-ring

A

C

BD

Token

D B

Token-ring

A

C

BD

Token

D B

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 active DTEs connected to the physical medium provide the 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 carries the responsibility for recovery and reinitialization.


May I have

another token

please ?

May I have another

ring please ?

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

After reading the data the receiving DTE modifies the pair of response bits.

If the DTE is inoperable, the response bits remain unchanged.

The Sender reads back the frame, checks the response bits and releases the token.


S

11

S

1100

S

1110

S

1101

Inoperable

Busy

NAK

ACK

1 1 1 1

DESTINATION

ADDRESS

8 8 N

Monitor Passed Bit

Start of Packet

SOURCE

ADDRESS

(Acknowledge) Response bits:

00 Busy

01 Accepted

10 Rejected

11 Ignored (not working)

DATA

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

Monitor passed bit = 0


Monitor


Empty Slot

The token is passed physically using

the bus around the logical ring.

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

maximum.

It then passes the token to its known

successor (downstream neighbor) on

the logical ring.


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.

A B C

EF D

P = F

S = B

P = A

S = C

P = B

S = D

P = C

S = E

P = D

S = F

P = E

S = A

logical

ring

The three MAC standards together with their associated physical

media specifications are contained in the following IEEE standards

documents:


IEEE 802..3 CSMA/CD bus

IEEE 802.4 Token bus

IEEE 802.5 Token ring

IEEE 802..11 Wireless

Physical Layer

Network Layer

802.2

802.3

Transmission Medium

802.4 802.11802.5

Data link

Layer

Logical link

control

Medium

access

control

Physical

ISO RM

IEEE 802

Cutajar & Cutajar

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.


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

Start bit Stop bits

Transmitted character

Parity bit

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).


B0

B1 B2

B3

B4 B5

B6

Even Parity

Odd Parity

(1)

(1)

(1)

(0)

(0)

(0)(0)

(1)(1)

(0)(0)

(0) (0)

(1)

(EXAMPLE 1001001)

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 ).



Row

Parity

(odd)

Column

Parity

(even)

P B6 B5 B4 B3 B2 B1 B0

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

0 0 1 0 0 0 0 0

1 0 1 0 1 1 0 1

0 1 0 0 0 0 0 0

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

Frame

Contents

P B6 B5 B4 B3 B2 B1 B0

0 0 0 0 0 0 1 0

1 0 1 0 1 0 0 0

0 1 0 0 0 1 1 0

0 0 0 0 1 0 0 0

1 0 1 0 1 1 0 1

0 1 0 0 0 0 0 0

1 1 0 0 1 0 1 1

1 0 0 0 0 0 1 1

1 1 0 0 0 0 0 1

Undetected

Error

Combination

Example

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.


Cutajar & Cutajar

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 !!

When the overload condition ends and the computer becomes available to accept further characters, it returns a companion control character X-ON to inform the terminal control device that it may restart sending characters. This is known as handshaking.


X-OFF

X-ON

Computer Terminal

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.

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.


OK messagemessage

NOT OK

?

OK messagemessageExplicit

Implicit

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;



I(N)

I(N)

I(N+1)

I(N+1)

I(N+2) Primary P

Secondary S

start start startstop stop Timer

I(N)

ACK(N) ACK(N+1)

I(N+1) I(N+2)

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;


I(N)

I(N)

I(N)

I(N)

Primary P

Secondary S

start start

expired stop Timer

I(N)

ACK(N)

I(N)

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


Primary P

Secondary S

I(N)

I(N)

I(N)

I(N)

start start

expired stop Timer

I(N)

ACK(N) ACK(N)

I(N)

Duplicated Message

(discarded)

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;

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.



I(N)

I(N)

I(N)

I(N)

I(N+1) Primary P

Secondary S

start start start

stop stop Timer

I(N)

NAK(N) ACK(N)

I(N) I(N+1)

If S receives an I-frame containing transmission errors, the

frame is discarded an it returns a NAK (negative

acknowledgement) frame.

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.



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

N

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

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

N

N

N+1

N+2 N+3

N+1 N+2 N+3 N+4 N+5

N

N+1

N

N+2

N+1

N+4

N+3

N+2

N+1

N+5 V(S)

N+2

N+1

Secondary (S)

Primary (P)

timeN+4

N+3

N+2

N+1

N N N N

I(N+2)

N+4

N+3

N+2

N+1

Discarded frames


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

N

N

N+2 N+2

N+4

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

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

N

N

N+1

N+2 N+3

N+4

N+3

N+1 N+2 N+3 N+4 N+5

N

N+1

N

N+2

N+1

N+4

N+3

N+2

N+1

N+1

N+4

N+5 V(S)

N+1

N+5

N+3

N+2

N+3

N+1

N+4

N+2

N+3

Secondary (S)

Primary (P)

time

Cutajar & Cutajar

The distances which can be covered by a single LAN are limited and frequently there is a requirement to extend this range. This maybe due to:◦ Partitioning the whole network into groups of separate

entities for security reasons or to improve the 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 requirements and thus be from different vendors. Thus we speak of multivendor integration. The approach taken in the integration of these computers can take various viewpoints.


Multivendor

Integration

Network

driven

OS driven

Application driven

Internetworking InteroperabilityMultivendor

Integration=+

Application

Presentation

Session

Transport

Network

Data link

Physical

Interoperability

Internetworking

OSI MODEL

In reality, data is passed from one layer down to the next lower layer at the sending computer, till it's finally transmitted onto the network cable by the Physical Layer.


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 .

1

2

3

4

5

6

7

USER

1

2

3

4

5

6

7

USER

Physical Layer

Network Layer

Transport Layer

Application Layer

Presentation Layer

Session Layer

Data Link Layer

A B

Each layer acts as though it is communicating with

its corresponding layer on the other end.

Data

DataAH

DataAHPH

DataAHPHSH

DataAHPHSHTH

Headers

DataAHPHSHTHNH NT

DataAHPHSHTHNHDH NT DT

DataAHPHSHTHNHDHFH NT DT FT

Tails

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


Repeater Repeater

Repeaters also allow isolation of

segments in the event of failures or

fault conditions. Disconnecting one

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.


1

2

3

4

5

6

7

USER

1

2

3

4

5

6

7

USER

Station on

Segment A

Station on

Segment B

Repeater Station

Repeater

Thus the range of the network can be extended via a repeater.

Repeater



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

Communication & Networks

12

7

Bridges interconnect Ethernet segments. Most bridges today support filtering and

forwarding, as well as Spanning Tree Algorithm. The IEEE 802.1D specification is

the standard for bridges.

A bridge works at the MAC

Layer by looking at the

destination address and

forwards the frame to the

appropriate segment upon

which the destination computer

resides.

1

LLC

3

4

5

6

7

USER

Station on

Network A

Station on

Network B

Bridge Station

Bridge

PHY

MAC

1

LLC

3

4

5

6

7

USER

MACMAC

PHY


DA SA LENGTH


ERROR CS

Bridges are ideally used in environments where there a number of well defined workgroups, each operating more or less independent of each other, with occasional access to servers outside of their localized workgroup or network segment. Bridges do not offer performance improvements when used in diverse or scattered workgroups, where the majority of access occurs outside of the local segment.

Two types of bridging exists:◦ Transparent bridging: is used in Ethernet environments and relies on switching

nodes. ◦ Source-Route Bridging (SRB): used in Token Ring networks in which end systems

actively participate by finding paths to destinations, then including this path in data packets.


Workgroup BWorkgroup A

During initialization, the bridge learns about the network and the routes. Packets

are passed onto other network segments based on the MAC layer. Each time the

bridge is presented with a frame, the source address is stored. The bridge builds up

a table which identifies the segment to which the device is located on. This internal

table is then used to determine which segment incoming frames should be

forwarded to. The size of this table is important, especially if the network has a

large number of workstations/servers.


Hi ! A B

Ho ! B A

Who ? C B

BA

RightLeft

CA C

Me

Learning

Intrasegment traffic is

not forwarded to the

other segments

DTE A DTE C DTE B

The advantages of bridges are: ◦ increase the number of attached workstations and network

segments ◦ since bridges buffer frames, it is possible to interconnect

different segments which use different MAC protocols ◦ since bridges work at the MAC layer, they are transparent to

higher level protocols ◦ by subdividing the LAN into smaller segments, overall

reliability is increased and the network becomes easier to maintain

◦ used for non routable protocols like NETBEUI which must be bridged

◦ help localize network traffic by only forwarding data onto other segments as required (unlike repeaters)


The disadvantages of bridges are:

◦ the buffering of frames introduces network delays

◦ bridges may overload during periods of high traffic

◦ bridges which combine different MAC protocols require the

frames to be modified before transmission onto the new

segment. This causes delays

◦ in complex networks, data may be sent over redundant

paths, and the shortest path is not always taken

◦ bridges pass on broadcasts, giving rise to broadcast storms

on the network


operate at the MAC layer (layer 2 of the OSI model)

can reduce traffic on other segments

broadcasts are forwarded to every segment in learning phase

most allow remote access and configuration

often SNMP (Simple Network Management Protocol) enabled

loops can be used (redundant paths) if using spanning tree algorithm

small delays introduced

fault tolerant by isolating fault segments and reconfiguring paths in the event of failure

not efficient with complex networks

redundant paths to other networks are not used (would be useful if the major path being used was overloaded)

shortest path is not always chosen by spanning tree algorithm


Packets are only passed to the network

segment they are destined for. They work similar

to bridges and switches in that they filter out

unnecessary network traffic and remove it from

network segments. Routers generally work at

the network level.


Ethernet

Router

X.25

Router

Token Ring

Routers were devised in order

to separate networks logically.

For instance, an IP router can

segment the network based on

groups of IP addresses. Filtering

at this level (on IP addresses,

also known as level 3 switching)

will take longer than that of a

bridge or switch which only

looks at the MAC layer.


1

2

3

4

5

6

7

USER

1

2

3

4

5

6

7

USER

Station on

Network A

Station on

Network B

Router

Router Station

PHY PHY

IP

DLL DLL

DA SA LENGTH


DataAHPHSHTHNH NT

ERROR INFRouter

One of the most important design decisions is the assignment of IP addresses, 32-bit numbers that identify Internet hosts. These numbers are placed in the IP packet header and are used to route packets to their destination. Several things should be kept in mind about IP address assignment:◦ Prefix-based addressing. A basic concept of IP addressing is that initial prefixes of

the IP address can be used for generalized routing decisions. Prefix-based addressing has its origins in IP Address Classes, and has evolved into Subnetting.

◦ Per-interface assignment. IP addresses are assigned on a per-interface basis, so a host might possess several IP addresses if it has several interfaces. An IP address doesn't really refer to a host, it refers to an interface

◦ Dynamic addressing. Many hosts, particularly user workstations, do not need to be assigned any particular IP address, and can be dynamically addressed.


The Domain Name Service (DNS) is a hierarchical naming system built on a distributed database for computers, services, or any resource connected to the Internet or a private network. It associates various information with domain names assigned to each of the participating entities. Most importantly, it translates domain names meaningful to humans into the numerical identifiers associated with networking equipment for the purpose of locating and addressing these devices worldwide.

An often-used analogy to explain the Domain Name System is that it serves as the phone book for the Internet by translating human-friendly computer hostnames into IP addresses. For example, the domain name www.example.com translates to the addresses 192.0.32.10 (IPv4).

The Domain Name System makes it possible to assign domain names to groups of Internet resources and users in a meaningful way, independent of each entity's physical location. Because of this, World Wide Web (WWW) hyperlinks and Internet contact information can remain consistent and constant even if the current Internet routing arrangements change or the participant uses a mobile device. Internet domain names are easier to remember than IP addresses such as 208.77.188.166 (IPv4). Users take advantage of this when they recite meaningful Uniform Resource Locators (URLs) and e-mail addresses without having to know how the computer actually locates them.


The address classes differ in size and number. Class A addresses are the largest, but there are few of them. Class Cs are the smallest, but they are numerous. Classes D and E are also defined, but not used in normal operation

Internet routing works like this: A router receiving an IP packet extracts its Destination Address, which is classified (literally) by examining its first one to four bits. Once the address's class has been determined, it is broken down into network and host bits. The routers ignored the host bits, and only need to match the network bits to find a route to the network. Once a packet reaches its target network, its host field is examined for final delivery.


0

Class A

netid hostid8 16 24 32

1 0

Class B

netid hostid16 32

1 1 0

Class C

netid hostid

24 32

1 1 1 0

Class D

multicast address32

1 1 1 1

Class E

netid hostid

24 32

internetwide netid part hostid

8/ 16/ 24 32

internet routing part Local part

1 0

Class B

netid hostid16 32

Class A/B/C

subnet

use dynamic routing

operate at the Internet Protocol level (or Network Layer)

remote administration and configuration via SNMP

support complex networks

the more filtering done, the lower the performance

provides security

segment networks logically

broadcast storms can be isolated

often provide bridge functions too

more complex routing protocols can be used


A brouter (pronounced BRAU-tuhr or sometimes BEE-rau-tuhr) is a network

bridge and a router combined in a single product.

A bridge is a device that connects one LAN to another LAN. If a data unit on one

LAN is intended for a destination on an interconnected LAN, the bridge forwards

the data unit to that LAN; otherwise, it passes it along on the same LAN. A bridge

usually offers only one path to a given interconnected LAN.

A router connects a network to one or more other networks that are usually part

of a WAN and may offer a number of paths out to destinations on those networks.

A router therefore needs to have more information than a bridge about the

interconnected networks. It consults a routing table for this information.

Since a given outgoing data unit or packet from a computer may be intended for an

address on the local network, on an interconnected LAN, or the wide area

network, it makes sense to have a single unit that examines all data units and

forwards them appropriately.


Operating at the network level or above, a gateway is used to interconnect two dissimilar networks.

In this case significantly more translation between the two networks takes place making gateways a slower and more expensive device.


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2

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USER

1

2

3

4

5

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7

USER

Station on

Network A

Station on

Network BGateway

Gateway Station

PHY PHY

DLL DLL

NET NET

TRA TRA

SES SES

PRE PRE

APP APP

gateway

DECNET SNAGateway


Data

Gateway

There are many types of hubs. Passive hubs are simple splitters or combiners that group workstations into a single segment, whereas active hubs include a repeater function and are thus capable of supporting many more connections.

Nowadays, intelligent hub concentrators are being very popular. These are very sophisticated and offer significant features which make them radically different from the older hubs.

These hubs provide each client with exclusive access to the full bandwidth, unlike bus networks where the bandwidth is shared. Each workstation plugs into a separate port, which runs at the full port bandwidth and is for the exclusive use of that workstation, thus there is no contention to worry about like in Ethernet.

These hubs also include buffering of packets and filtering, so that unwanted packets (or packets which contain errors) are discarded. SNMP management is also a common feature.


Hubs dedicate the entire bandwidth to each port (workstation). The workstations attach to the hub using UTP. The hub provides a number of ports, which are logically combined using a single backplane, which often runs at a much higher data rate than that of the ports.

Ports can also be buffered, to allow packets to be held in case the hub or port is busy, and since each workstation has it's own port, it does not contend with other workstations for access,

The ports on a hub all appear as one Ethernet segment. In addition, hubs can be stacked or cascaded (using master/slave configurations) together, to add more ports per segment.


port 1

Backplane

port 2 port 3 port 4 port 5 port 6 port 7 port 8

HUB

As hubs do not count

as repeaters, this is a

better option for

adding more

workstations than the

use of a repeater

Hub options also include an SNMP (Simple Network Management Protocol) agent. This allows the use of network management software to remotely administer and configure the hub. Detailed statistics related to port usage and bandwidth are often available, allowing informed decisions to be made concerning the state of the network.

In summary, the advantages for newer hubs are, ◦ each port has exclusive access to its bandwidth (no CSMA/CD)

◦ hubs may be cascaded to add additional ports

◦ SNMP managed hubs offer good management tools and statistics

◦ utilize existing cabling and other network components

◦ becoming a low cost solution


Header Hub (master)

Intermediate Hub (slave)

Hub cascading

Ethernet switches increase network performance by decreasing the amount of extraneous traffic on individual network segments attached to the switch. They also filter packets a bit like a router does.


Segment A Segment B

Ethernet Switch

In addition, Ethernet switches work and function like bridges at the MAC layer, but instead of reading the entire incoming Ethernet frame before forwarding it to the destination segment, usually only read the destination address in the frame before retransmitting it to the correct segment.

In this way, switches forward frames faster than bridges, offering less delays through the network, hence better performance.

As packets arrive at the switch, it looks

at the MAC address in the header, and

decides which segment to forward the

packet to. Higher protocols like IPX and

TCP/IP are buried deep inside the

packet, so are invisible to the switch.

Once the destination segment has been

determined, the packet is forwarded

without delay.


1

LLC

3

4

5

6

7

USER

Station on

Network A

Station on

Network B

Ethernet Switch

Ethernet Switch

PHY

MAC

1

LLC

3

4

5

6

7

USER

MACMAC

PHY

DA SA LENGTH



Switches divide the network into smaller collision domains [a collision domain is a

group of workstations that contend for the same bandwidth]. Each segment into the

switch has its own collision domain (where the bandwidth is competed for by

workstations in that segment).

Each segment attached to the switch is considered to be a separate collision domain.

However, the segments are still part of the same broadcast domain [a broadcast

domain is a group of workstations which share the same network subnet, in TCP/IP

this is defined by the subnet mask]. Broadcast packets which originate on any

segment will be forwarded to all other segments (unlike a router). On some

switches, it is possible to disable this broadcast traffic.


Collision domain Collision domain

Ethernet Switch

existing cabling structure and network adapters is preserved

switches can be used to segment overloaded networks switches can be used to create server farms or

implement backbones technology is proven, Ethernet is a widely used

standard improved efficiency and faster performance due to low

latency switching times each port does not contend with other ports, each

having their own full bandwidth (there is no contention like there is on Ethernet)


Documents

Module 5 2010