Data Communication & Networking -iisnelliady

Introduction to Communication

Data Communications & Networking

The components of the communication

• What is Communication? Communication is the exchange of information between two or more people in a way that creates understanding

Data Communication • The combination of communication technology and computer technology • It gives people access to online information, put services like banking and shopping into the home and linking professional together in complex computer networks • Basic components

•Sender •Message •Method •Receiver

– The Sending Device – A Communication Line – The Receiving Device

•Results

• The fundamental purpose is to exchange information between two agents

Data Communication (Cont…) • Communication System – A system built for communication

1

(Input Device)

6

3 4

5 Transducer

• Transmitter – Converts the electrical signal into a suitable form that can be send over the transmission channel

Transmitter 2

• Transducer – Converts the original signal into an electrical signal or vice versa (eg. Microphone = Voice Æ Electrical Signal Speakers = Electrical Signal Æ Voice Video Camera = Picture Æ Electrical Signal)

Source System Transducer

Data Communication (Cont…)

Transmission Medium

Receiver

(Output Device)

Destination System

• Data Transmission – Deals with the portion between 3 and 4 – The successful transmission of data depends principally on two factors; • The quality of the signal been transmitting • The characteristics of transmission media

(Block Diagram of a Communication System)

1

Data Communication (Cont…)

Signals

• Transmission Channel – The medium over which the signal is transmitted

•

• •

Based on the bit pattern of data, the sending side generates a signal on the selected transmission medium and then this signal propagates to the receiving end. z The receiving side extracts data from this signal.

(eg. Noise in telephone lines, Snow in television picture, Noise bursts due to lightening in radio, television, etc

– Note: All transmission channels add noise to the transmitted signal. Some channels (eg. Optical fibre) adds very little noise.

Analog Signals & Digital Signals

Signals (Cont…) A signal is generated by changing the strength of a particular form of energy with respect to time on a suitable medium. z Form of Energy used can be Electrical, Light, Electromagnetic. z The medium used can be Copper, Optical Fiber or even Free Space.

Main Characteristics of the signal z Amplitude

a t1

z

There are two basic types of signals z Analog

Signal (Broadband Signal)

It varies continuously with time

–

Digital Signal (Base band Signal) It takes only one of a predefined set of values

Main Characteristics of the signal (Cont… (Cont…) z Frequency

It indicates the strength of the signal at any given time

0

1 Æ ‘On’ On’ 0 Æ ‘Off’ Off’

z

• Receiver – Converts the received signal (transmitted signal) into an electrical signal • Noise – Unwanted signal added by the transmission channel

z

Signals are used to transmit data via a medium. Data in digit form:

•

– Copper wire: Carries electrical voltage/current. Used for telephones, computer networks – Open air: Carries electro magnetic waves. Used for radio, television, cellular phones, etc. – Optical fibre: Carries light. Used for telephone system (long distance), computer network.

Frequency F = 1/T cycles per second or Hertz (Hz) When a signal repeats it’s pattern, the frequency indicates how fast it repeats

time time T

T

2

Main Characteristics of the signal Cont… Cont…

Data Transmission Systems

•Phase

• Transmission Media

Phase is a relative measure If the two signals overlap with each other, then they are in phase. Otherwise they are out of phase

α

Characteristics of Transmission Media • Attenuation Input

Transmission Media

– A communication link is established by using some kind of transmission medium – In most cases it is a pair of electric conductors / fibre optical cables / free space

Characteristics of Transmission Media (Cont…)

Output

– When the signal travels over the transmission medium, it gets weaker as it gets away from the source – This is due to the loss of energy in transmit

Characteristics of Transmission Media (Cont…) – 0 dB Æ No signal loss – 10 dB Æ Output signal has 1/10 the power of the input signal – 20 dB Æ Output signal has 1/100 the power of the input signal – Low attenuation is better – Attenuation can be specified per unit distance (dB/m or dB/km)

– Repeaters may be used to amplify the signal level – Attenuation = 10 log Power of the I/P signal Power of the O/P signal – Unit Æ dB (decibels)

Characteristics of Transmission Media (Cont…)

™ Bandwidth

fc

Frequency of the signal

– The attenuation in any transmission medium changes with the frequency of the signal – For a certain medium, it gets very large after a certain frequency – The bandwidth of a medium indicates the highest frequency of a signal, that will have “reasonable” attenuation – Bandwidth ~ Cutoff frequency (fc)

3

Characteristics of Transmission Media (Cont…) – Due to the bandwidth limit of the transmission links, all the frequency components will not be present at the receiver – The absence of some of the frequency components, makes it not possible to regenerate the exact original shape, hence the received signal will be distorted – High bandwidth is desirable

Characteristics of Transmission Media (Cont…) ™Transmission Delay

9 Time taken for the signal to travel from source to destination (eg: Satellite lines have long delay) 9 As the distance increases, fast components of one bit may catch up the slow components of the bit ahead, making the resultant digital signal distorted 9 We prefer transmission media with low delay

Media Used in Building Computer Networks Coaxial Cable

Magnetic Media

Twisted Pair

Characteristics of Transmission Media (Cont…) ™Signal-to-Noise Ratio (SNR) – Every transmission medium adds some noise to the signal – Output signal = Input signal + Noise – SNR = 10 Log Power of the output signal Power of the noise signal – Units Æ dB – Transmission media with high SNR are desirable

Characteristics of Transmission Media (Cont…) ™Cross talk 9When more than one signal sent over the transmission medium, one signal can ‘leak’ into another (interference) 9Cross talk is a measure of this interference 9 Cross talk = 10 Log

Power of actual signal . Power of interference signal

9 High Cross talk is desirable

Coaxial Cable – Two connectors on the same axis – The construction and shielding of the

coaxial cable give it a good combination of high bandwidth and noise immunity. – The energy loss due to skin effect is also minimized.

Optical Media

Electro Magnetic Media (Wireless Media)

Braided Copper Shielding Outer Jacket Insulation Copper Conductor

4

Twisted Pair Coaxial Cable (Cont…) Š Used to connect T-connectors Coaxial Cable

BNC Connector

• The oldest and still most common transmission medium is twisted pair • Consists of two insulated copper wires twisted together • Four twisted pairs within a plastic covering

Twisted Pair (Cont…) Š Can be used for either analog or digital transmission Š Twisting reduces the effects of the external interferences Š Common applications are the telephone system and Local Area Network of computers Š Types – UTP Æ Unshielded Twisted Pair – STP Æ Shielded Twisted Pair (Thin metal foil used to shield (protect) pairs from electro magnetic waves)

Optical Fibre (Cont…) Š Light source generates pulses of light ƒ 1 Æ Presence of light ƒ 0 Æ Absence of light

Š Two types of light sources are used; – Light Emitting Diode – Semi Conductor Laser

Š Transmission channel is a tiny fibre of glass

Optical Fibre

Š Optical transmission has three components; light source, transmission channel and detector Electrical Signal

Light Source

Optical Signal Transmission Channel

Electrical Signal

Light Detector

Š To send data over fibre, an electrical signal has to be converted to an optical signal (light)

Optical Fibre (Cont…) Š The light detector at the receiving end generates an electric pulse when light falls on it Š Photo diode is used to convert light to an electrical signal Š Light can travel long distance with very little attenuation

5

Optical Fibre (Cont…)

Optical Fibre (Cont…)

Š Optical Transmission is based on the theoretical principle called Total Internal Reflection in physics. Cladding

Core

αα

Light Source

Total Internal Reflection

Š Refraction indexes of the core and cladding are different Š If the angle (α) is above a certain value, light will not go out of the core

Optical Fibre (Cont…) Š There are two kinds of fibre in use – Multimode Fibre • Light travels in multiple “modes” • Mode – Bouncing at a different angle

• Wave length of the light is 850 nm, 1300 nm • Standard Diametres – 50/125 µm : core - 50 µm, cladding – 125 µm – 62.5/125 µm : core – 62.5 µm, cladding – 125 µm

Optical Fibre (Cont…) ŠAdvantages Low attenuation High Bandwidth Lighter in weight Not affected by electromagnetic interferences • Do not induce high voltages when lightening takes place • Secured from wire tapers • • • •

Core Æ a very narrow strand of high quality glass Cladding Æ Made from high quality glass Buffer/Outer Jacket Æ usually constructed from plastic

Optical Fibre (Cont…) – Single Mode Fibre • Light travels in a single mode • Core diametre is smaller than multimode fibre • Typical diametres Æ 9/125 µm • Light can travel for longer distances than in multimode fibre • Wave length of the light is 1300 nm

Optical Fibre (Cont…) ŠDisadvantages – Expensive – Skilled people are needed for the

cabling – Making joints are expensive – Must be physically protected against damages

6

Comparison of Copper (UTP) and Fibre Media UTP Cost Bandwidth Attenuation Cross talk Security of Data

Fibre

Low Low High 64dB@100MHz Low

MM High High Low 0 High

SM High High Low 0 High

Wireless Media (Cont…) „ „

All electromagnetic waves travel at the speed of light ( 3 x 108 m/sec) The Radio, Microwave, Infrared and Visible Light portions of the electromagnetic spectrum can be used for transmitting information 104

106 Radio

108

1010

Microwave

1012

1014

Infrared

1016 UV

Wireless Media „

„

When electrons move, they create electromagnetic waves that can propagate through free space By attaching an antenna of the appropriate size to an electrical circuit, the electromagnetic waves can be broadcast efficiently and received by a receiver located some distance away

Wireless Media (Cont…) „

Radio Waves „ „ „ „ „ „ „

„

Travel Long Distance Easy to generate Penetrate buildings easily Pass through obstacles, but power falls sharply Absorbed by rain Omni directional Do not require dish antenna and need not be rigidly mounted to receive Subjected to electromagnetic interferences

Visible Light

Wireless Media (Cont…)

„

Satellite Microwaves „

„

„

Used to link two or more ground based microwave transmitter/receivers (Earth stations) Above 100MHz, the waves travel in straight lines and can therefore be narrowly focused Parabolic antennas are used

Wireless Media (Cont…) „ „

„

„

Do not pass through buildings Used to establish links, when laying physical cables are very expensive or impracticable The satellite receives transmissions on one frequency (uplink) and transmits on another frequency (downlink) The most important applications „ „ „

Television distribution Long distance telephone transmissions Private business network

7

Wireless LANs (WLAN) z z z z z

Wireless LANs (Con..)

Transmit data through the air using radio or infrared transmission A central wireless Access Point (AP) is a radio transmitter that plays the same a hub All devices in the WLAN use the same radio frequencies Computers must take turns using the one circuit since the signal travels in all directions from the AP The maximum range from the AP to the computers is determined by the amount of interference (e.g. concrete walls) but is typically 100 to 500 feet

Wireless LANs (Con..) z

z

The computers on the WLAN have a NIC inside the computer that is connected to an external transmitter that communicates with the AP The external transmitter that communicates transmits radio signals to a receiver that acts like a network hub and enables wireless computers to communicate with each other and with wired networks

Wireless LANs (Con..) z

z

Usually a set of APs are installed, so that there is complete wireless coverage in some area, enabling users to roam from AP to AP When configured with a wireless network, a set of laptops or PalmPalm-based devices becomes an effective way to enable workers to walk through a facility and have constant network access at any point (eg (eg.. Warehouse, hospital, airport)

Wireless LANs (Con..)

Bluetooth It is different from the wireless LAN technologies z Its goal is to provide seamless networking of devices in a very small area (30’ (30’ ~ 300’ 300’) z Bluetooth devices are small and cheap z A Bluetooth network is called a piconet and consists of no more than eight devices but can be linked to other piconets to form a large network z

8

Network Topologies • The geographic arrangement of the computer in the hardware 1. Bus topology 2. Ring topology 3. Tree topology 4. Star topology 5. Complete topology 6. Irregular topology

Ring Topology Computer network in which each computer is connected to other computers, forming a continuous loop or circle • Advantages •All computers have equal access • Disadvantages •Reconfiguration suspends entire network •One node failure can impact entire network

Star Topology

Hub

Network configuration consisting of a central host computers that connect to the computer to form a star pattern

• Advantages •Centralized management •Easy to add new users •Failure of one node doesn’t affect the whole network • Disadvantages •Manager must support multiple users •Large networks more time-consuming to support •Failure of the central hub downs entire network

Bus Topology

• System in which all stations or computer devices communicate by using a common distribution channel or bus • Advantages •Easy to setup •Economical •Reliable, widely used • Disadvantages •Difficult to trouble shoot •Slow when network traffic is heavy •Broken cable downs entire network

Tree Topology Computers connected this type network in which a hierarchy of nodes provides control and communication. Certain levels of control may be delegated down to intermediate branches

Complete Topology One computer connected to all other computers. This connection is very useful

9

Irregular Topology

Standards in Data Communications Communication Modes 1. Simplex

One computer connected to other computers. But not all computers

• • •

Communication Modes (Cont…) 2. Half Duplex

• • •

Receiver

Sender

Data is traveling only one direction – sender to receiver Only One-way communication is possible Examples : TV, Radio

Communication Modes (Cont…) 3. Full Duplex

Sender

Receiver

Sender

Receiver

Receiver

Sender

Receiver

Sender

Data can be sent only one direction at time Two-way communication is possible, but not simultaneously Example :– Walky-Talky

• Data can be sent both directions at the same time • Two-way simultaneous communication is possible • Example – Telephone System

Transmission Modes

Transmission Modes (Cont…)

• Serial Transmission 1 0 0 1 1 1 0 1 0 0 0 0 0 1 0 1 • Data are transmitted in a bit by bit fashion • Only one channel is needed for the link • Used in almost all the data communication applications

• Parallel Transmission

0 0 0 0 0 1 0 1

1 0 0 1 1 1 0 1

• All the bits in a byte or a word are transmitted all together (simultaneously)

• Disadvantages

• Multiple channels are needed for data transmission • Practically it is difficult to too long distance

10

Telephone Systems

Modulation Standards z

V-90 z Permits

upto 56,000 bps (56 kbps) downlink and 33,600 bps (33.6 kbps) uplink z To have 56 k downlink speed the remote modem must be connected via a digital telephone line z

V-34 z Permits

upto 33.6 kbps in both directions modems can adjust their modulation/speed at any time throughout the session

z The

(Cont..)

• Quantization:- The amplitude of each of the samples is represented as a binary value 0111 0110 0101 0100 0011 0010 0001 0000 1111 1110 1101 1100 1011 1010 1001 1000

0010

• Sampling:- Take samples of the signal at regular intervals

Analog & Digital Signals

0111

• An analog signal is converted to a digital signal in two steps

The modem is connected to the computer via the serial port z The cable used has 9 conductors (serial cable) z Computer/modem communication takes place at a speed of 300bps Æ 468,000bps [DTE speed] z Modem to modem communication takes place at speed 300bps to 56,000bps (depending on the modulation standard used and the quality of the telephone line) [DCE speed] z

0101

Analog & Digital Signals

Telephone Systems (Cont..)

0111

z

Telephone service is provided using two wires from the local exchange The modem (modulator/demodulator) converts the data (1’ (1’s and 0’ 0’s) into an analog signal, which can be sent over the telephone network

0011 0101

z

Types of Transmission Technology • Broadcast Network – All computers in the network share the same transmission channel – A computer can send information to all the other computers at the same time – Only one computer can send data – If two computers start transmitting at the same time, a ‘collision’ occurs – Local Area Networks (LAN) are generally broadcast networks

11

Types of Transmission Technology (Cont… (Cont…)

• Point – to - Point Network

– It consists of point-to-point links – A point-to-point link only connects two computers – Wide Area Networks (WAN) are generally point-topoint networks

Switching Systems • Circuit Switching – A physical circuit is established from the source to the destination – All the data sent must go in this physical circuit – Whether data is sent or not, once the circuit is established, the resources are allocated are not usable by someone else

• Packet Switching – No circuit is established – Data is broken down to packets – Each packet is sent independently of other packets

Switching Systems (Cont…) • Circuit Switching – Necessary to setup a “circuit” before data can be sent – Resources reserved during the initial setup – Unused resources are wasted – Not possible to lose a part of the message – Each part of the message travels over the same path

• Packet Switching – No circuit needs to be setup – Acquire and release resources as needed – Resources can be shared by other packets from other sources – Possible to lose packets due to congestion – Each packet may travel in a different path

Types of Networks z Local

Area Networks (LAN)

z Covers

a small geographical area

z Single

Room, Building, Campus (Set of buildings)

z Distance

between two computers: 10m ~ 1km z Data Transfer Rate: 10 ~ 1000 mbps

Switching Systems (Cont…) • Circuit Switching (Cont..) – Not possible for parts of the message to reach destination in a different order – Congestion can occur only during setup – No store & forward – Charging per unit time

• Packet Switching (Cont..) – Possible for packets to reach destination in a different order – Congestion possible on every packets – Store & forward possible – Charging per packet

Types of Networks (Cont…) z Metropolitan z Covering

Area Networks (MAN)

a metropolitan area

z City

z Distance

z Wide

between two computers: ~ 10km

Area Networks (WAN)

z Covers

a large geographical area

z Country,

z Distance

z Data

Continent

between two computers: 100 ~ 1000 km

Transfer Rate: < 100 100 Mbps

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Network Standards z The

primary reason for standards is to ensure that hardware and software produced by different vendors can work together z The use of standards makes it much easier to develop software and hardware that link different networks

Standard making bodies z International

Organization for Standardization (ISO) z Makes

technical recommendations about data communication interfaces z ISO is a member of the International Telecommunication Union (ITU), whose task is to make technical recommendations about telephone, telegraph and data communication interfaces on a world wide basis

The standards making process z Specification z Consists

of developing a nomenclature (an organization) and identifying the problems to be addressed

z Identification z Identify

the various solutions and choose the optimum solution from among the alternatives

z Acceptance z Defining

the various solution and getting recognized industry leaders to agree on a single, uniform solution

Standard making bodies (Con..) z American

(ANSI) z ANSI

is a standardization organization, not a standards making body, in that it accepts standards developed by other organizations and publishes them as American Standards z Its role is to coordinate the development of voluntary national standards and to interact with ISO to develop national standards

z Protocols

is an agreed set of rules, governing the communication z Breaking the rules will make communication difficult or impossible

Reference Models

z It

z Protocol z To

Hierarchies

reduce design complexity, network hardware/software are organized as a series of layers or levels z Each layer has a specific task to perform z Each layer must interact with other layers to make communication possible

National Standards Institution

z “Standards” Standards”

defining the number of layers and the duties or the task each layer must perform z OSI

(Open System Interconnection) – 7 Layers (Transmission Control Protocol/Internet Protocol) – 4 Layers z Hybrid – 5 Layers z TCP/IP

13

Reference Models (Con..) z z z z z

OSI Reference Model

Each layer talks to the layer above it and below it. The lowest layer (layer 1) interacting with the transmission medium Each layer depends on the layer below it Each layer provide services to the layer above it Layer n of one computer communicates with layer n of the other computer. But in reality, no information is sent directly from layer n of one machine to layer n of another machine. Instance each layer passes the information to the layer immediately below it. On the receiving computer, information is received by layer 1 and is sent upwards to layer n

Layer

z Network

z Responsible

for transmitting data (i.e. 1’ 1’s and 0’ 0’s) on the communication channel

z Data

Link Layer

the raw transmission facility provided by the physical layer z Provide an errorerror-free transmission facility to the network layer z In a broadcast type network, it must decide, which computer gets to transmit next z

z Distance z The

to a source node node via which the distance is calculated B

Layer 4

Network Layer

Layer 3

Data Link Layer

Layer 2

Physical Layer

Layer 1

Layer

Dijkstra’s Algorithm (Cont…)

z Calculating

– A Router z Link – Link between two routers z We give labels to each node indicating

Layer 5

Transport Layer

for controlling the operation of the network z It must determine how data packets are routed from the source to the destination z It must know about the topology of the network and must be capable of selecting the optimal route to a given destination z Routing algorithms can be used to find optimal route (eg. eg. Dijkstra’ Dijkstra’s Algorithm) Router:Router:- A device that can operate at the network layer and connected to at least two networks

Dijkstra’s Algorithm z Node

Layer 6

Session Layer

z Responsible

z Take

the shortest path from a given source to the destination

Layer 7

Functions of Each Layer (Cont…)

Functions of Each Layer z Physical

Application Layer Presentation Layer

zA

label can be tentative or permanent z Tentative labels may be changed later as we find better paths z Permanent labels will not change z When all the labels are permanent, algorithm completes

A

Source 4 C (4,A)

4

D (8,C)

14

Dijkstra’s Algorithm (Cont…) Label all the nodes, except the source node with a distance of infinity. Label the source node with a distance of zero 2. Select the source node as the working node or the current node 3. Update the tentative labels of all the adjacent nodes of the working node, taking into account 1.

1. 2.

The distance of the working node The metric of the link from the working node to the adjacent node

7 2 2

1 6 F

4

H

D Destination 2

2 E

Functions of Each Layer (Cont…) z Session

Layer

z Allow

Layer

z Accepts

3 G

A

Source

Example… Example…

z Transport C 3

2

Examine all tentative labels and make the labels with the smallest distance permanent. Also select the corresponding node as the working node 5. If there are more tentative labels, goto step 3 6. Algorithm completed 4.

Functions of Each Layer (Cont…)

Example B

Dijkstra’s Algorithm (Cont…)

users to establish different sessions between machines (Remote Logins, File Transfers) z Token Management

data from the session layer, breaks the data into smaller units (if necessary) and hands them to the network layer z Responsible for establishing and releasing connection

Functions of Each Layer (Cont…) z Application

Layer

z Interacts

with the user z Provide services to the user z Electronic

Mail Access z File Transfer z Remote Login z Domain Name Service z Web

z Presentation z Responsible z Encoding

Layer for presentation of data

of data (ASCII)

15

Functions of Each Layer (Cont…)

TCP/IP Reference Model Application Layer

Layer 4

z Network

Transport Layer

Layer 3

z Similar

Network Layer

Layer 2

Host-to-Network Layer

Layer 1

z Transport

Layer

z Similar

to the transport layer in the OSI model z There are two protocols used

Functions of Each Layer z Host

Layer

to the network layer of the OSI model z Protocol use is the Internet Protocol (IP)

z TCP

– to - Network Layer

z UDP

– Transmission Control Protocol – User Datagram Protocol

z It

has to connect to the network, so that it can send network layer packets

Transport Layer (Cont…) z Transmission

Control Protocol

is set up by Connection Oriented Routing between the sender and receiver z The Transport Layer software sends a special packet, called a SYN or Synchronization character to the receiver, requesting that a connection be established z The receiver either accepts or rejects the connection and together they settle on the packet sizes the connection will use z Once the connection is established, the packet flow between the sender and receiver following the same route through the network

Transmission Control Protocol (Con..) z

z It

User Datagram Protocol z z

z z z

z

No connection is established The sender simply sends the packets as separate unrelated entities, and it is possible that different routes through the network, depending on the type of routing used and the amount of traffic They may arrive out of sequence at their destination The sender’ sender’s network layer therefore puts a sequence number on each packet The receiver’ receiver’s network layer must reassemble them in the correct order before passing the message to the application layer Eg: Eg: Sending real time data (Data Conferencing, Internet Telephony)

z

z z z

All packets in the same message arrive at the destination in the same order in which they were sent After receiving a packet, the receiver sends either an acknowledgement (ACK), if the packet was received without error, or a negative acknowledgement (NAK), if the message contained an error. If it is NAK, the sender resends the previous message TCP uses the continuous ARQ technique to make sure that all packets arrive and to provide flow control When the transmission is completed, the sender sends a special packet, called a FIN to close the connection Eg: Eg: HTTP, SMTP, FTP

Functions of Each Layer (Cont…) z Application

Layer

z Corresponds

to the Application Layer of the OSI model z Has a number of protocols defined for various applications z Remote

Logins (Telnet Protocol) Transfer (FTP Protocol) z E mail (SMTP Protocol) z Web Services (HTTP Protocol) z File

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Network Security Threats

Hybrid Reference Model Application Layer

Layer 5

Transport Layer

Layer 4

Network Layer

Layer 3

Data Link Layer

Layer 2

Physical Layer

Layer 1

Š Disruption is the loss of or reduction in network service Š It may be minor or temporary Š For example, a network switch might fail or a circuit may be cut, causing part of the network to cease functioning until the failed component can be replaced Š Some users may be affected, but others are not Š Some disruptions may also be caused by or result in the Destruction of data Š For example, a virus may destroy files or the “crash” of a hard disk may cause files to be destroyed

Network Security Threats (Con…) Š Natural or human made Disasters may occur that destroy host computers or large sections of the network Š For example, fires, floods, earth quakes, mudslides, tornadoes and terrorist attacks can destroy large parts of the buildings and networks in their path Š Unauthorized access is often viewed as hackers gaining access to organizational data files and resources from across internet

Network Controls •

Controls are mechanisms, that reduce or eliminate the threats to network security • There are three types of controls 1. Prevent • •

• •

Preventing Viruses • •

• •

Viruses cause unwanted events The best way to prevent the spread of viruses is to not copy or download files of unknown origin, or atleast to check every file you do copy or download Many antivirus software packages are available to check disks and files to ensure that they are virus free Always check all diskettes and files for viruses before using them

It mitigates or stops a person from acting or an event from occurring For example, A password can prevent illegal entry into the system A guard or a security lock on a door may deter an attempt to gain illegal entry

Firewalls •

•

It is a router, gateway or special purpose computer that filters packets flowing into and out of a network No access to the organization’s networks is permitted except through the firewall

Internet

Firewall

Organization’s backbone network

17

Network Controls (Con…)

Firewalls •

Two commonly used types of firewalls are, –

A packet level firewall • •

–

2. Detect • •

It examines the source and destination address of every network packet that passes through it It allows into or out of the organization’s networks only those packets that have acceptable source and destination addresses

An application level firewall • •

•

It acts as an intermediate host computer between the internet and the rest of the organization’s networks Anyone wishing to access the organization’s networks from the internet must log into this firewall and can access only the information he or she is authorized for, based on the firewall account profile he or she access

Network Controls (Con…) 3. Correct • •

It provides remedy to an unwanted event or a trespass Either computer programmes or humans verify and check data to correct errors or fix a security breech, so it will not occur in the future

Error Correction via Re-Transmission (Con..) • There are two types of ARQ 1. Stop-and-Wait ARQ • •

• •

The sender stops and waits for a response from the receiver after each data packet transmitted After receiving a packet, the receiver sends either an acknowledgement (ACK), if the packet was received without error, or a negative acknowledgement (NAK), if the message contained an error If it is an NAK, the sender resends the previous message If it is an ACK, the sender continues with the next message

It reveals or discover unwanted events For example, software that looks for illegal network entry or a virus can detect those problems It also document an event when a situation or a trespass, providing evidence for subsequent action against the individual’s or organizations involved or to enable corrective action to be taken

Error Correction via Re-Transmission •

•

•

The simplest, most effective, least expensive and most commonly used method for error correction method A receiver, that detects an error simply asks the sender to retransmit the message until it is received without error This is often called Automatic Repeat Request (ARQ)

Stop-and-Wait ARQ (Con..) Sender

Receiver Packet A

No errors detected

ACK Packet B

Errors detected

NAK Packet B A

No errors detected

ACK

• It is by definition, a half duplex transmission technique

18

Error Correction via Re-Transmission (Con..) 2. Continuous ARQ • • • •

The sender does not wait for an acknowledgement after sending a message; it immediately sends the next one Although the messages are being transmitted, the sender examines the stream of returning acknowledgements If it receives an NAK, the sender retransmits the needed messages The packets that are retransmitted may be only those containing an error

Network interface card (NIC) „

„

Sender

Receiver Packet A

Devices used for connecting network segments „

Continuous ARQ (Con..) Packet B ACK A Packet C ACK B Packet D NAK C Packet C ACK D ACK C

• It is by definition, a full duplex transmission technique, because both the sender and the receiver are transmitting simultaneously

Devices used for connecting network segments (Cont…) „

Repeaters „

It must be installed in each computer on the network It manages the flow of network information to and from the computer in which they reside

„

„

It connects different segments of the same network It copies signals received on one part to the other if necessary, it amplifies the signal Repeater Ports

Devices used for connecting network segments (Cont…)

Devices used for connecting network segments (Cont…) „

Hubs „

„

„

It copies the signal received on one port to all the other ports It can be considered as a repeater with more than two ports Hubs

Bridges „

It is used to interconnect two networks Hub

Bridge Network 1

Hub Network 2

Stations

19

Devices used for connecting network segments (Cont…) „

Switches „

It can be considered to be a bridge with more than two ports Switch Hub

Hub

THANK YOU

Hub

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