Computer Network
Resources
Data Communication & Networking, 2nd.
Ed., Forouzan, B.A., McGraw-Hill, 2001
Computer Networks, 4th, Ed., Tanenbaum,
A.S., Prentice Hall, 2002
Selected papers
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Syllabus
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Basic introduction
Network concepts
Network models
Signals
Encoding & modulating
Data transmission
Transmission media
Multiplexing
Error detection & correction
Data link control
Switching techniques
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Network concepts
Line configuration
Point-to-point, Multipoint
Topology
Tree, Bas, Mesh, Ring, Star
Transmission mode
Simplex, Half-duplex, full-duplex
Types of networks
LAN, WAN, MAN
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Network models
 OSI model
7 layers
 TCP/P
5 layers
 Application
 Presentation
 Session
 Transport
 Network
 Data link
 Physical
 Application
 Transport
 Network
 Data link
 Physical
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Signals
Analog signals
Digital signals
Periodic & aperiodic signals
Composite signals
Frequency signals
bandwidth
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Encoding & modulating
4 types
Digital-digital
Unipolar, polar, bipolar
Analog-digital
PAM, PCM, sampling rate
Digital-analog
ASK, FSK, PSK, QAM
Analog-analog
AM, FM, PM
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Data transmission
Digital data
Parallel, serial
DTE-DCE Equipment
Interface standards
EIA-449, EIA-530, X.21
Modems
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Transmission media
 Guided media
 Twisted-pair, coaxial, optical fiber
 Unguided media
 Terrestrial microwave, satellite, cellular telephony,
wireless
 Transmission impairment
 Attenuation, distortion, noise
 Performance
 Throughput, propagation speed, propagation time
 Wavelength
 Shannon capacity
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Multiplexing
FDM
WDM
TDM
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Error detection & correction
Types of error
Single-bit, burst error
Redundancy Detection
CRC
Error correction
Single-bit
Hamming code
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Data link control
Flow control
Stop-and-wait
Sliding window
Error control
ARQ (Automatic Repeat Request)
Stop-and-wait ARQ
Sliding window ARQ
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Switching techniques
Circuit switching
Packet switching
approaches
Datagram
Virtual circuit
Message switching
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Flash Back
 The past 3 centuries dominated by single technology
 18th century – mechanical system within Industrial Revolution
 19th century – steam engine
 20th century – information gathering, processing, and distribution;
telephone networks; radio & TV; communication satellites
 Computer technology made a spectacular progress in
short time
 First 2 decades, computer systems are highly centralized
(mainframes); now has become obselete
 Had been replaced by a large number of interconnected
computers doing the job
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The importance!
 Can a network design support company’s
expansion?
 PC evolution
Increase non-computer users awareness to usage of
computers; PC more user-friendly; does not burden on
users
Previously, PC does not allows data sharing
 Data being exchange through physical transmission
medium such as via sea, air, road etc; time-consuming,
 Created another inconveniences – allow users’ error, virus
spreading
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What is Data Communication?
Telecommunication – communicating via
distances
Data communication – the process
involving data exchanges (binary data)
between 2 devices via transmission
medium
Local data communication, remote data
communication
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What is Data Communication?
How effective your data communication
systems?
Depends on
Delivery – correct destination, intended recipient
Accuracy – data received must be the one that was
sent
Timeliness – must reach destination in time; very
important in real-time transmission (audio, video
data)
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What is Data Communication?
5 main components involve:
Message
Sender
Receiver
Transmission medium
Protocol
A set of rules which allows communicating devices to
‘talk’ to each others
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What is Networks?
A network is a set of devices connected by
a media links (channels); interconnected
collection of autonomous computers
Two computers are connected when they
able to communicate with each other,
exchanging information
What’s the different between a computer
network and a distributed system?
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 Distributed systems
Existence of multiple autonomous computers is
transparent to the user
Users type in command, run the program
OS will determine which processor will run the program
 Computer networks
User explicitly log onto a machine; explicitly submit jobs
remotely, explicitly moves files around and generally
handle all the network management personally
 Thus, a distributed system s a software system
built on top of a network
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Uses of computer networks
To companies
Resource sharing – regardless of physical
location of the resource and the user
To provide high reliability – alternative sources
of supply
Saving money – client-server architecture
More scalable – allowing extension of network
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Uses of computer networks
 To people
Access to remote information
 Online payment, online banking, online shopping
 Online personalized newspaper
 Online digital library
 WWW
Person-to-person communication
 Email
 Virtual meeting
Interactive entertainment
 Video on demand (killer application) – possible to select
any movie or television program in any country and have it
displayed on the screen instantly
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Social issues
Widespread of networking technology
introduce new social, ethical, political
problems
Obscene and abusive images and messages
Offensive forum
Cyber-terrorism, cyber-warfare
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Network criteria
 What is an effective and efficient networks?
Performance – measured by transit time or response
time
 Number of concurrent users – too many will slow down
response time
 Types of transmission medium
 Hardware
 Software
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Transform raw data to transmittable signals
To route signals to destination
To ensure signals are error free
Transformation to original data
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Network criteria
Reliability
Failure frequency
Recovery time after a failure
Catastrophic disaster
Security
Ensure no unauthorized access
• User authentication – password, encryption
• Host authentication
Virus protected
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Performance evaluation
Transit time
Time required for a message to travel from one
device to another
Response time
Elapsed time between an enquiry and a
response
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Network hardware
2 types of transmission technology
Broadcast network
Single communication channel shared by all
machines on the network
Packets sent will be received by all the others
Address field attached to packet will determined who
can have the packet
Multicasting – transmission to a subset of the
machines
Generally used by smaller, geographically localized
networks
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Point-to-point network
Consists of many connections between individual
pairs of machines
A packet will be routed to any intermediary machines
which exist between the source and the intended
recipient based on certain algorithm
Generally used by large dispersed networks
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Local Area Network (LAN)
 Privately-owned network
 Single building or campus; few kilometers in size
 Connecting PCs and workstations
 Allowing sharing of resources and information
exchange
 LANs are distinguished according to
Their size
Their transmission technology
 Single cable attaching all machines (bus, ring)
 Bus – master-slave / CS; ring – token ring
Their topology
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Wide Area Network (WAN)
 Spans a large geographical area
 Consists of a collection of machines intended for running
user programs (hosts)
 Hosts are connected by subnet which carries messages
from host to host
 Subnet
 Transmission line
 Switches (router) – specializes computers/device to connect two
or more transmission lines
 Packet forwarding based either on point-to-point, store-andforward or packet-switching principles
 Nearly all WAN applied the store-and-forward principle
 Point-to-point need to based on certain topology either ring, star,
tree, mesh, irregular, intersection ring
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Wireless networks
What is wireless networking?
What is mobile computing?
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Protocols
 A system may consists of several entities – anything
capable of sending and receiving information such as
application programs, browser, ftp packages, dbms,
email sw
 To reduce design complexity, networks are organized as
a series of layers
 Each layer will offer services to the higher layers
 For 2 entities to send stream of bits, they must agree on
a protocol – defines what is communicates, how it is
communicated and when it is communicated
 Between each pair of adjacent layers, there is an
interface which defines the operations and services the
lower layer offers to the upper layer
 A set of layers and protocols is called NETWORK
ARCHITECTURE
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Interfaces and services
 Services are available at SAP (Service Access
Points)
The layer n SAPs are the places where layer n + 1 can
access the services offered
2 layers to exchange information
 Has to had an agreed upon set of rules about the interface
 Layer n+1entity pass IDU (interface Data Unit) to layer n
entity through SAP
 IDU consists of control information and SDU (Service
Data Unit) which information will be passed across the
network to the peer entity
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Types of services
 2 types of services
Connection-oriented services
 Need to establish the connection, use the connection and
terminate connection when finish
 Reliable – data will not lose; acknowledgement required
 Eg: File trasfer
Connectionless services
 No prior establishment required
 Data not guarantee to arrive at the intended machine
 Also known as datagram service
 Eg : Electronic mail
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Standards
 Products may be used on each other regardless
of its manufacturers
 Provides guidelines to manufacturers, vendors,
service providers to ensure connectivity
 2 types : de jure & de facto
 De facto:
proprietary – invented by commercial orgs and fully
owned by them – also called closed standards
Nonproprietary – developed by groups or committees
and passed it to public domain – also called open
standards
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Standards organizations
International Standards Organization (ISO)
Inter. Telecommunications UnionTelecommunications Standards Sector
(ITU-T)
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