1
CS101 Introduction to Computing
Lecture 27Computer Networks
2
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
3
don’t make me think!
4
think roadside billboard
rather than Dewan-e-Ghalib
5
Designs should be
consistent &
predictable (unified)
6
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!
7
Website Navigation
• The interface/controls that a Website provides to the user for accessing various parts of the Website
• It probably is the most important aspect of the design of a Website
8
Good designs assist the user in recovering from errors
9
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
10
All of you have used computer networks.
What is acomputer network?
11
Computer Network
Multiple computers that are connected together to share
information and other resources
12
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
13
ComputerC
ComputerD
ComputerEComputer
A
ComputerB
Hub
Example of a Computer Network
14
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
15
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
16
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
17
How Does a Conventional Network Work?
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
18
Hub (1)
• 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
19
Hub (2)
• 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
20
Packet (1)
• 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
21
Packet (2)
• All packets travel independently
• When all packets are received by the destination computer, it reassembles them to form the original message
22
Types of Computer Networksaccording to the network access policy
• Private
• Public
23
Private Networks (1)
• Organizations having many computers usually connect them in the form of private networks
• Access to these network is restricted to authorized computers only
24
Private Networks (2)
• 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
25
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
26
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
27
VPN: Virtual Private Network (2)
• 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
28
Types of Computer Networksaccording to the distance between nodes
• LAN: Local Area Network)
• WAN: Wide Area Network)
29
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
30
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
31
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
32
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
33
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
34
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
35
Modem (1)
• I/O device used for connecting two computers over telephone lines
• modem = modulator + demodulator
36
Modem (2)
• 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
37
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
38
P2P
ComputerA
ComputerB
39
P2P
• Inexpensive
• Limited connectivity
• Quite often used for connecting two LANs to form a WAN
40
Star
ComputerA
ComputerD
ComputerC
ComputerB
Server
41
Star (1)
• 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
42
Star (2)
• 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
43
BusComputer
AComputer
C
ComputerD
ComputerB
Bus: A high speed cable
44
Bus (1)
• 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
45
Bus (2)
• 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
46
Ring
ComputerA
ComputerD
ComputerC
ComputerB
47
Ring (1)
• 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
48
Ring (2)
• 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
49
Ring (3)
• New nodes are difficult to add
• Messages propagate in one direction only
• The network fails if a single node fails
50
Combination
ComputerB
ComputerA
ComputerC
Hub
ComputerE
ComputerD
ComputerF
Hub
51
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
52
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
53
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
54
Computer Networks
=Computers
+Communications
55
Types of Communication Channels
1. Wire
2. Wireless
A key characteristic of these channels is bandwidth
56
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)
57
Types of Communication Channels
Wire– Copper
• Twisted-pair• Coaxial cable
– Optical fiber
Wireless– Line-of-sight
• Microwave• Optical
– Non-line-of-sight• Satellite• Radio• Cellular
58
Wireless (Radio) LANs Are Becoming Popular
Key benefits:– Set-up time
– Set-up cost
– Maintenance cost
– Cost Key challenges:– Security & privacy– Quality of service– Cost
59
Network Security (1)
• 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
60
Network Security (2)
• 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
61
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
62
PrivateNetworkInternet
Firewall
63
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
64
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
65
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