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
Components of Conventional Computer Networks
1. Computers
2. Network Interface Cards (NIC)
–I/O device that plugs into the computer
Computer
C
Computer
D
Computer
E
Computer
A
Computer
B
Hub
Example of a Computer
Network
–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
• 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
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
Inexpensive
Limited connectivity
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
Bus
No server is required
One computer sends data to another by broadcasting the address of the
receiver and the data over the
bus
P2P
Computer
A
Computer
B
Star
Server
Computer
A
Computer
D
Computer
C
Computer
B
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
Ring
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
Compu
Compu
Compu
ter
Compu
ter
Computer
B
Bus
Computer
A
Computer
C
Bus: A high speed
cable
Computer
D
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)
Combination
Computer
B
Computer
A
Computer
C
Hub
Computer
E
Computer
D
Computer
F
Hub
27.6 Types of
Communication Channels
Wireless (Radio) LANs Are Becoming Popular27.7 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
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
Wire
– Copper
•
Twistedpair
• Coaxial
cable
– Optical
fiber
Wireless
–
Line-of-sight
• Microwa
ve
• Optical
–
Non-line-ofsight
• Satellite
• Radio
• Cellular
Key benefits:
–
Set-up time
–
Set-up cost
–
Maintenance cost
–
Cost
Key challenges:
– Security
& privacy
– Quality
of service
– Cost
Private
Network
Internet
Firewal
l
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 |