CS101 - Introduction to Computing - Lecture Handout 27

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Computer Networks

During the last lecture …

(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

Computer Network Usage

Components of Conventional Computer Networks

  1. Computers
  2. Network Interface Cards (NIC)
    • I/O device that plugs into the computer
    • 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
    1. 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


  • 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


  • 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

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

Public Networks

  • All networks that are not private, are … public
  • 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

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)


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


A network in which computers are separated by great distances, typically across cities or even continents
May consist of several interconnected LANs

  • 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


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


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


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


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

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

Network Topologies

Limited connectivity
Quite often used for connecting two LANs to form a WAN


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



No server is required
One computer sends data to another by broadcasting the address of the receiver and the data over the bus
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



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
The network fails if a single node fails



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


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)

Types of Communication Channels

Types of Communication Channels

Network Security

Keeping an eye on the security of private networks (e.g. LANs) is relatively easy
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

Network Security

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

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