ch01(Modified).ppt

Computer Network
Textbooks
•Data Communications and Networking: B. A.
Forouzan, 4th Edition, TMH
•Computer Networks: A. S. Tanenbaum, 4th Edition,
Pentice Hall

Introduction
•The term telecommunication means communication at a distance
(tele is Greek for “far”).
•The word data refers to information presented in whatever form is
agreed upon by the parties creating and using the data.
•Data communications are the exchange of data between two
devices via some form of transmission medium such as a wire cable.
•A computer network is a number of devices (often referred to as
nodes) connected by communication links.
•Two computer connected to the network can communicate with
each other through other nodes if they are not directly connected.
•Some of the nodes in the network may not be computer at all but
they are network devices (like switches, routers etc.) to facilitate
communication.

1.3
Criteria of Effectiveness of Data Communication
Delivery:
Data must arrive at the correct destination.
Accuracy:
Data must be deliver accurately without any error.
Timeliness:
Data must be delivered in timely manner.
Jitter:
Variation in the data arrival time at the destination.
Different applications have different requirements on these criteria

1.4
Components (Q)
Figure 1.1 Five components of data communication
• Message : The information (data) to be communicated.
• Sender : The device that sends the data message.
• Receiver : The device that receives the data message.
• Transmission medium : The physical path by which a
message travels from sender to receiver.
• Protocol : A set of rules that govern data communications.

1.5
Data Flow
Figure 1.2 Data flow (simplex, half-duplex, and full-duplex)

1.6
Data Flow Examples
 Simplex mode
 Keyboard, mouse, traditional monitor
 GPS device
 Half duplex
 Walkie-talkie
 Citizen band radio
 Advantage: entire bandwidth (capacity of a channel)
can be used for transmission
 Disadvantage: not suitable for high interactive
application
 Full duplex
 Telephone network

1.7
Network Criteria
• Performance: Evaluated by two metrics
1. Throughput: average rate of successful
message delivery
2. Delay: Due to traffic congestion
However these two criteria are often contradictory.
• Reliability: Frequency of failure, recover time, and
robustness
• Security: Unauthorized access, protecting data
from damage etc
• becoming more important now

1.8
Physical structure
Figure 1.3 Types of connections: point-to-point and multipoint
Whether the link capacity is shared or not
Point-to-Point Line Configuration

Point-to-Point Line Configuration

1.11
Figure 1.4 Categories of network based on topology(connectivity)

1.12
Figure 1.5 A fully connected mesh topology (five devices)
1
2
3
4
5
n(n-1) half-duplex links
n(n-1)/2 duplex links
Example: Telephone regional offices
Advantages: Eliminating traffic problem, robustness and privacy
Disadvantages: Many cabling/installation and need many I/O ports on nodes

Figure 1.6 A star topology connecting four stations
Example: Ethernet switch/hub, Wifi access point and LAN
Advantages: robust against a node’s failure or a link failure
Disadvantages: If hub/switch goes down, the whole system is dead
Q. Compare star and mess topology with advantages and
disadvantages

1.14
Figure 1.7 A bus topology connecting three stations
The preceding two are point to point connections, but it is a multipoint connection.
Example: Ethernet LAN can use bus topology
Advantages: Less cabling, Easy of installation,
Disadvantages: Any tap device failure or break in bus will cause the whole system
fail.
Limitation on the number of taps a bus can support because signal becomes weaker
and weaker as it travels farther and farther, difficult to add new device.
The damaged area reflects signals back in the direction of origin, creating noise in
both direction.

1.15
Figure 1.8 A ring topology connecting six stations
Each device has a dedicated point-to-point connection with only the two devices on
either side of it. A signal is passed along the ring in one direction from device to device.
When a device receives a signal intended for another device, its repeater regenerates the
bits and passes them along.
Example: IBM token ring LAN (less popular now).
Advantages: Easy to install a new device just by changing two connections
Disadvantages: Unidirectional traffic, A break in ring can disable the whole network

Figure 1.9 A hybrid topology: For e.g. a star backbone with three bus networks
Q. For n devices in a network, what is the number of cable
links required for a mesh, ring, bus and star topology?
Ans. Mesh n(n-1) half-duplex links
n(n-1)/2 duplex links
Star n
Ring n
Bus n+1

1.17
Categories of Networks based on network size

1.18
Figure 1.10 LAN (range few km)

1.20
Figure 1.11 MAN (Range town or a city)

1.21
Figure 1.12 WAN (Large geographic areas)

1.22
THE INTERNET
Internet: Interconnection of massive network of
networks. It connects millions of computers together
globally, forming a network in which any computer
can communicate with any other computer as long as
they are both connected to the Internet.
Before Internet:
Many isolated Local Area Networks (LANs) existed
Those LANs had very different hardware and
network protocols
 Protocol example: TCP/IP, IPX (from Novell)

Internet, internet and Intranet (Contd.)
• Internet with a capital "I" refers to the entire Internet
(meaning the biggest collection of networks on the
planet) . Internet with a lowercase "i" refers to any
group of networks that are connected together.
• Intranet refers to a single inter-connected network
within one organization that uses HTTP/Web
technologies for the sharing of information internally.
In other words we could string together intranets to
make our own private internet. We could then
connect our internet to the Internet.

1.24
Figure 1.13 Internetwork or Internet

1.25
PROTOCOLS AND STANDARDS
Protocols: A set of rules that governs data communications
(similar to human language). It defines what is
communicated, how it is communicated, and when it is
communicated.
The key elements of a protocol are syntax, semantics, and
timing
Syntax: It refers to the structure or format of the data.
Semantics: It refers to the meaning of each section of data.
Timing : It refers when data should be sent and how fast
they can be sent.

Standards – public recognized protocols for open market
De facto: A format, language, or protocol that has become a standard not because it has been
approved by a standards organization but because it is widely used and recognized by the
industry as being standard.
E.g. Microsoft’s Windows operating system, along with commonly used business applications
such as Microsoft Word and Excel, has long been the de facto standard for business and home
users. De facto standards can also become de jure standards over time as, for example, HTML
and PDF have done.
De jure: De jure, from Medieval Latin, means from law. that have been endorsed by an official
standards organization.
E.g. TCP/IP, ASCII
Standards Organizations: ANSI, IEEE etc

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