COMPUTER NETWORKS: types of networks, networking topologies and protocols

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Introduction to Computing CS101

LESSON 27

COMPUTER NETWORKS

During the last Lesson ...

(Web Design for Usability)

We looked at the role of usability in Web design

We identified some of the factors affecting the usability of a Web page

Designs should be consistent & predictable (unified)

What's a Good Site?

The one that achieves the result that it was designed for

Generally, that result can only be achieved by giving the user what s/he wants, as quickly as

possible, without her/im expending much effort

One definition of usability: Let the user have what s/he wants, quickly, without much effort

"Quickly" is important!

Website Navigation

It probably is the most important aspect of the design of a Website

Good designs assist the user in recovering from errors

Today's Goals:

(Computer Networks)

We will become able to appreciate the role of networks in computing

We will look at several different types of networks

We will familiarize ourselves with networking topologies and protocols

Computer Network

Multiple computers that are connected together to share information and other resources

Examples of Computer Network Usage

I can send an eMail message to a remote computer using the SMTP protocol

I can browse documents residing on a remote computer using the HTTP protocol

·I can download or upload files to a remote computer using the FTP protocol

·I can run a program on a remote computer using the TELNET protocol

Example of a Computer

Network

Computer

Hub

Computer

Components of Conventional Computer Networks

1. Computers

2. Network Interface Cards (NIC)

I/O device that plugs into the computer

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Enables it to communicate over a network

3. Hub

The network traffic controller

Components of Conventional Computer Networks

4.Cables

Are either electrical or optical

Not required at all for wireless networks

5.Protocol

Rules governing communications over the network

How Does a Conventional Network Work?

1. Suppose computer A wants to send a message to D

2. Computer A sends the message to its NIC

3. The NIC translates the message into electrical pulses suitable for the computer network in use &

transmits it to the hub through the cable

4. The hub receives them and forwards them to all computers connected to the it

5. The NICs of all computers connected to the hub receive the forwarded electrical pulses

6. The NIC of computer D decides that the message is for it, & translates the pulses back to a form

suitable for the computer

Hub

A device that is used to connect several computers to form a network

A hub has several ports. The number generally is 8, 12, 16, 24, 32, or 48

Each computer in a network is connected to one of those ports through a cable

A computer wanting to send a message to one of the others in the network sends a message to the

hub, which, in turn, broadcasts the message to all others connected to it

Packet

The smallest unit of data transmitted over a computer network

A message to be transferred over the network is broken up into small packets by the sending

computer

Each packet contains the following info:

Sender's address

Destination address

Data

Error-recovery info

All packets travel independently

When all packets are received by the destination computer, it reassembles them to form the original

message

Types of Computer Networks

according to the network access policy

Private

Public

27.1 Private Networks

Organizations having many computers usually connect them in the form of private networks

Access to these network is restricted to authorized computers only

Organizations having many computers usually connect them in the form of private networks

Access to these network is restricted to authorized computers only

This allows computers from within the organization to exchange info, but keeps the info private

and protected from outsiders

All equipment on a private network is generally for the exclusive use of that organization

27.2 Public Networks

All networks that are not private, are ... public

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Example: Internet

Communication equipment used in these networks is generally being used by users belonging to

several (possibly thousands of) organizations as well as those belonging to no organization

27.3 VPN: Virtual Private Network (1)

From the user's point-of-view, a VPN looks like a secure, private network

VPNs use public telecom infrastructure, maintaining privacy through security procedures

VPNs provide secure network connections for distance computers without using dedicated, private

channels to supply the connection

Key benefit of VPNs over conventional PNs: Lower cost

Types of Computer Networks

according to the distance between nodes

LAN: Local Area Network)

WAN: Wide Area Network)

LAN

A network of computers located in the same building or a handful of nearby buildings

Examples:

Computer network at your PVC

Computer network of a University campus

WAN

A network in which computers are separated by great distances, typically across cities or even

continents

May consist of several interconnected LANs

Example:

The network connecting the ATM of a bank located in various cities

A network connecting the local and oversea offices of a SW house

Internet

Connecting LANs to other Networks:

Special-purpose devices are used to link LANs to other networks

They may belong to one of the following categories:

Routers

Bridges

Gateways

Modems

Router

A special-purpose computer that directs data traffic when several paths are available

A router examines the destination info in each arriving packet and then routes it through the most

efficient path available

The router either delivers the packet to the destination computer across a local network or forwards the

packet to another router that is closer to the final destination

Bridge

Used to form a connection between two separate, but similar networks

In a way, it creates an extended LAN by passing information between two or more LANs

Gateway

A special-purpose computer that connects and translates between networks that use different

communications protocols

LAN's may use a gateway (or router) to connect to the Internet

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Modem

I/O device used for connecting two computers over telephone lines

modem = modulator + demodulator

Modulator converts computer messages to electrical pulses that are suitable for transmission over the

telephone lines

Demodulator converts electrical pulses received over telephone lines into messages that are

comprehensible for computers

27.4 Network Topologies

The pattern in which computers are connected to form a network

Popular patterns:

Point-to-point

Star

Bus

Ring

Networks are also formed by combining 2 or more of these 4 basic patterns

P2P

Inexpensive

LimitedComnecttivity

con pu er

Computer

Quite often used for connecting two LANs to form a WAN

Star

A computer sends the address of the intended receiver and the data to the server

The server then sends the message to the intended receiver

This topology allows multiple messages to be sent simultaneously

Costly, because it uses an additional computer to direct the data

Costly, because each node is individually wired to the hub

If the server goes down, so does the network

If any of the nodes goes down, the rest of the network is not affected

Computer

Star

Computer

Server

Computer

Bus

No server is required

One computer sends data to another by broadcasting the address of the receiver and the data over the

bus

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All the computers in the network look at the address simultaneously, and the intended recipient accepts

the data

A bus network, unlike ring or star networks, allows data to be sent directly from one computer to

another

However, only one computer at a time can transmit data. The others must wait to until the bus gets idle

If any of the nodes goes down, the rest of the network is not affected

Bus

Computer

Bus: A high speed

Ring

cable

No server is required

A computer sends the message to its neighbor. The neighbor examines the message to determine if it is

the intended recipient

If the data are not intended for that particular neighbor, it passes the message to the next computer in the

ring

This process is repeated until the data arrive at their intended recipient

This topology allows multiple messages to be carried, simultaneously

Data transmission is slow since each message is checked by each computer

New nodes are difficult to add

Messages propagate in one direction only

Compu

The network fails if a single node fails

Compu

ter

Compu

ter

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Combination

Computer

Hub

Computer

Hub

Computer

27.5 Networking Protocols

Networks use protocols, or rules, to exchange info through shared channels

Protocols prevent collisions of packets caused by simultaneous transmission between two or more

computers

Several protocols are available for various types of networks. Here we discuss two that are popular for

LANs: Ethernet; Token Ring

Ethernet Protocol

A computer using this protocol checks if a shared connection is in use before transmitting a message

If not, the computer transmits data

Two computers may sense an idle connection and may send packets simultaneously. To account for

such situations, transmitting computers continue to monitor the connection and re-transmit if a packet

collision occurs

Token Ring Protocol

This protocol passes a special message called a token through the network

A computer that receives the token is given permission to send a packet of information

If the computer has no packet to send, it passes the token to the next computer

Computer Networks = Computers + Communications

Types of Communication Channels

1 . Wire

2 . Wireless

A key characteristic of these channels is bandwidth

Bandwidth

Capacity of a communication channel for carrying data

Measured in bits/s (bps), kb/s, Mb/s, Gb/s, Tb/s

Optical fiber channels have the highest (1 Tb/s)

Telephone lines the lowest (56 kb/s)

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27.6 Types of Communication Channels

Wire

Wireless

Copper

Line-of-sight

· Twisted-

· Microwa

pair

· Coaxial

· Optical

cable

Non-line-of-

Optical fiber

sight

· Satellite

· Radio

· Cellular

Wireless (Radio) LANs Are Becoming Popular

Key benefits:

Key challenges:

Security & privacy

Set-up time

Quality of service

Set-up cost

Cost

Maintenance cost

27.7 Network Security

Keepin an eoston the security of private networks (e.g. LANs) is relatively easy

g C ye

However, their connections to other networks (e.g. the Internet) pose a security risk because the one has

no control over users on those networks

Network Security

Applications transferred from the Internet to the LAN may contain computer viruses

External, unauthorized users may gain access to sensitive data

A special type of gateway - a firewall can keep external users from accessing resources on the LAN

while letting LAN users access the external info

Firewall

A system that that guards a private network, enforcing an access/deny policy to all traffic going to and

coming from the Internet

It keeps an eye on all the packets that go in and out of the private network and blocks them or allows

them to continue to their destination according to the policy

Internet

Private

Firewall

Network

Firewall Policy: Example

One can configure a firewall to allow only eMail to enter the private network, thus shielding it from any

malicious attacks except for those via eMail

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Introduction to Computing CS101

In Today's Lecture:

We looked at the role of networks in computing

We looked at several different types of networks

We familiarized ourselves with networking topologies and protocols

Next Lecture:

Introduction to the Internet

To become able to appreciate the role of the Internet in today's computing

To become familiar with the history and evolution of the Internet

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