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Chapter 12
Computer Networks
Chapter Outline
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Introduction
Basic concepts in computer
networking
• Communication links
• Local area network (LAN)
• Wide area network (WAN)
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Network services
The Internet
The World Wide Web
Basic Concepts
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A computer network is a set of
independent computer systems
connected by telecommunication
links that together are called
interconnection network.
Individual computers in the network
are referred to as nodes or hosts.
Communication Links
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Vary widely in the physical
characteristics, error rate, and
transmission speed.
Switched dial-up telephone lines.
• The term “switched” means that when
we dial a phone number, a circuit (path)
is temporarily established between
called and the callee. The circuit lasts
for the duration of the call.
Analog vs. Digital
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Voice-oriented dial-up telephone network
is (in part) an analog medium.
However, a computer produces digital
information.
Use modem (modulator/demodulator) to
convert binary information into analog and
vice versa.
Telephone line has a rather low bandwidth.
Physical limits on transmission speed over
the telephone line are being reached.
(56000 bits per second)
Asymmetric Digital Subscriber Line (ADSL)
Amplitude Modulation
Dedicated Communication Links
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Twisted-pair copper wire
Coaxial cable
• ISDN: 128 Kbps
• T1: 1.544Mbps
• T3: 44.736 Mbps
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Fiber-optic cable
• OC-3: 155.5 Mbps
• OC-12: 622 Mbps
• OC-48: 2.488 Gbps (gigabit networking)
Transmission Time
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16mb color image
Line type
speed
time
Dial-up phone line
33.6 Kbps
7.9 min
Dedicated ISDN
128 Kbps
2.1 min
T1
1.544 Mbps
10.4 s
T3
44.737 Mbps
0.36 s
Fiber-optic OC-3
155.5 Mbps
0.1 s
Fiber-optic OC-12
622 Mbps
0.026 s
Fiber-optic OC-48
2.488 Gbps
0.006 s
Wireless Data Communication
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Use radio, microwave, infrared
signals instead of wires/cables.
Mobile computing
In the future,
• Fiber-optics to support high-speed
error-free wired connections
• Radio and microwave-based wireless
links to for places that fiber does not or
cannot reach.
Local Area Networks
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A local area network connects hardware
devices such as computer, printer, and
mass storage devices that are all in close
physical proximity.
Owner of the computers is also owners of
the means of communications.
A private network.
Most widely used LAN: Ethernet.
Ethernet
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Developed in the mid-1970s by computer
scientists at Xerox PARC.
Originally designed to operate at 10 Mbps
using coaxial cables.
Fast Ethernet: 100 Mbps.
Two ways to construct an Ethernet LAN:
• Cable, single wire + transceiver or multiple
cable + bridge
• Hub
Constructing Ethernet
Communication Protocol
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The rules describing how messages
are transmitted between network
nodes are called communication
protocols.
Protocols are “algorithm of
communication”. They describe the
step-by-step procedures required to
guarantee an orderly exchange of
information across a network.
Ethernet Protocol
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Use contention-based transmission technique.
Users compete for access to a single shared
communication line.
• When a node wants to send a message, it first listens to
the line to see if it is currently in use.
• Broadcast message if line is not busy.
• Address field attached to the front of the message that
identifies the destination.
• Every node reads the address filed, if not its own,
discards remainder of the message. If it is, accepts and
store it.
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What to do in a collision?
Control of Ethernet network is distributed. More
robust.
Wide Area Network
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A wide area network (WAN) connects devices that
may be across town, across the country, or
across the ocean.
Cross public property, must purchase services
from external providers.
Point-to-point links.
WAN Protocol
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Most WANs use a store-and-forward,
packet switching communication protocol.
Unit of transmission in WAN is a packet:
an information block with fixed maximum
size.
A message is “chopped” into N separate
packets and sent independently through
the network.
At the destination node, the packets are
reassembled to restore the original
message.
Packet Switching
Routing
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Selecting the specific path to use for
relaying data.
Routing Algorithm
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Shortest path problem: O(N2)
Highly complex because of:
• Massive amount of data
• Topological changes: routing table must
be re-computed often.
• Network failures
Router
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A router transmits message between
two distinct networks that uses
different protocols.
LAN/WAN connection.
Network Services
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Resource sharing:
• Physical resources: printer or disks. 
printer server
• Logical resources: data file or software.
 file server
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Client-server computing
Information sharing: electronic data
interchange
Information utility
Network Services (cont’d)
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Groupware: support group efforts in
producing shared documents.
E-mail
Bulletin board
News groups
Electronic commerce
Network Security
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Authentication problem
Encryption/decryption problem
Fault-tolerance problem