Chapter Two

Introduction to Computer Networks

Data Communication and Computer Networks

(SE 3051)
Computer Network and its Applications

 At its most elementary level, a computer network consists of two computers

connected to each other by a cable that allows them to share data.

 All computer networking, no matter how sophisticated, stems from that simple

system.
 Computer networking arose as an answer to the need to share data in a timely

fashion.
 Personal computers are powerful tools that can process and manipulate large

amounts of data quickly, but they do not allow users to share that data efficiently.
 Before networks, users needed either to print out documents or copy document

files to a disk for others to edit or use them.

2
Contd.
 If others made changes to the document, there was no easy way to merge the

changes.
o This was, and still is, known as "working in a stand-alone environment."

 Copying files onto floppy disks and giving them to others to copy onto their

computers was sometimes referred to as the "sneakernet." This early form of

computer networking is one that many of us have used and perhaps still use today

3
Contd.

 connecting together of computers and other devices is called a network, and

the concept of connected computers sharing resources is called networking.

4
Contd.
 Two computers are said to be interconnected if they are able to exchange

information

 Components of a compute networks

Hardware:
 Computer Media:
 Network card Cable
 Routers Microwave …
 Modem …

Software: Network Design:

 Network OS Logical layout
 Utilities … Physical layout …
5
Uses of Computer Networks

6
Contd.
1. Resource sharing
 the goal of network is to make all programs, equipment, and especially data

available to anyone on the network without regard to the physical location

of the resource and the user.

 An obvious and widespread example is having a group of office workers

share a common printer.

 None of the individuals really needs a private printer, and a high-volume

networked printer is often cheaper, faster, and easier to maintain than a

large collection of individual printers.

 Information sharing is more important than physical resource sharing

7
Contd.

2. Means of communication
 E-mail
 Videoconferencing
 Chatting
 E-commerce
 Game
 ….
3. Centralizing administration and support
 Database
 Banks
 ….

8
Network Types: Networks of Many Sizes

9
 Network Types

 There are basically three categories of networks based on its size

and geographical coverage

1. Local Area Network (LAN)

2. Metropolitan Area Network (MAN)

3. Wide Area Network (WAN)

10
LAN
 A local area network (LAN) is the basic building block of any computer network.

 A LAN can range from simple (two computers connected by a cable) to complex

(hundreds of connected computers and peripherals throughout a major corporation).

 The distinguishing feature of a LAN is that it is confined to a limited geographic area.

 A local area network (LAN) is usually privately owned and links the devices in a single

office, building, or campus.

 Depending on the needs of an organization and the type of technology used, a LAN

can be as simple as two PCs and a printer in someone's home office; or it can extend
throughout a company and include audio and video peripherals.

 Currently, LAN size is limited to a few kilometers

11
Contd.

12
Contd.

 LANs are designed to allow resources to be shared between personal

computers or workstations. The resources to be shared can include hardware

(e.g., a printer), software (e.g., an application program), or data.

 In addition to size, LANs are distinguished from other types of networks by

their transmission media and topology.

 In general, a given LAN will use only one type of transmission medium.

 The most common LAN topologies are bus, ring, and star.

 Early LANs had data rates in the 4 to 16 megabits per second (Mbps) range.

Today, however, speeds are normally 100 or 1000 Mbps.

13  Wireless LANs are the newest evolution in LAN technology.

WAN
 A wide area network (WAN) provides long-distance transmission of data, image, audio, and
video information over large geographic areas that may comprise a country, a continent, or
even the whole world.
 A WAN can be as complex as the backbones that connect the Internet or as simple as a
dial-up line that connects a home computer to the Internet.

 We normally refer to the first as a switched WAN and to the second as a point-to-point WAN.

 The switched WAN connects the end systems, which usually comprise a router

(internetworking connecting device) that connects to another LAN or WAN.

 The point-to-point WAN is normally a line leased from a telephone or cable TV provider that

connects a home computer or a small LAN to an Internet service provider (lSP). This type of
WAN is often used to provide Internet access.
14
Contd.

15
MAN

 A metropolitan area network (MAN) is a network with a size between a LAN and a

WAN.

 It normally covers the area inside a town or a city.

 It is designed for customers who need a high-speed connectivity, normally to the

Internet, and have endpoints spread over a city or part of city.

 A good example of a MAN is the part of the telephone company network that can

provide a high-speed DSL line to the customer (we will discuss DSL in later chapters) .

 Another example is the cable TV network that originally was designed for cable TV, but

today can also be used for high-speed data connection to the Internet.

16
 Interconnection of Networks: Internetwork
 Today, it is very rare to see a LAN, a MAN, or a LAN in isolation; they are connected to

one another.

 When two or more networks are connected, they become an internetwork, or internet.

17
Network, internet, and Internet

 A network is a group of connected communicating devices such as computers

and printers.

 An internet (note the lowercase letter i) is two or more networks that can

communicate with each other.

 The most notable internet is called the Internet (uppercase letter I), a

collaboration of more than hundreds of thousands of interconnected networks.

 Private individuals as well as various organizations such as government

agencies, schools, research facilities, corporations, and libraries in more than

100 countries use the Internet.
18
 Intranet and Extranet
 An organization may use an extranet to
provide secure and safe access to
individuals who work for a different
organizations, but require company data

 Intranet is a private connection of

LANs and WANs that belongs to
an organization, and is designed
to be accessible only by the
organization's members,
employees, or others with
authorization.
 Intranets are basically an internet
which is usually only accessible
from within the organization.
19
 Internet structure: network of networks
 roughly hierarchical

 at center: “tier-1” ISPs (e.g., UUNet, BBN/Genuity, Sprint, AT&T),

national/international coverage

 treat each other as equals

Tier-1 providers
also interconnect at
Tier-1 Tier 1 ISP public network
providers NAP access points
interconnect (NAPs)
(peer)
privately Tier 1 ISP Tier 1 ISP

20
Contd.

 “Tier-2” ISPs: smaller (often regional) ISPs

 Connect to one or more tier-1 ISPs, possibly other tier-2 ISPs

Tier-2 ISPs
also peer
o Tier-2 ISP pays Tier-2 ISP Tier-2 ISP privately with
tier-1 ISP for each other,
connectivity to Tier 1 ISP interconnect at
rest of Internet NAP NAP
o tier-2 ISP is
customer of tier-
1 provider Tier 1 ISP Tier 1 ISP Tier-2 ISP

Tier-2 ISP Tier-2 ISP

21
Contd.
 “Tier-3” ISPs and local ISPs

 last hop (“access”) network (closest to end systems)

local
Tier 3 local
ISP local local
ISP ISP
ISP ISP
Local and
Tier-2 ISP Tier-2 ISP
tier- 3 ISPs
are customers Tier 1 ISP
of higher tier NAP
ISPs
connecting
them to rest Tier 1 ISP Tier 1 ISP
Tier-2 ISP
of Internet local
Tier-2 ISP Tier-2 ISP
local local local ISP
22 ISP ISP ISP
Contd.
 a packet passes through many networks!

local
Tier 3 local
ISP local local
ISP ISP
ISP ISP
Tier-2 ISP Tier-2 ISP

Try a Tier 1 ISP

traceroute! NAP

Tier 1 ISP Tier 1 ISP

Tier-2 ISP
local
Tier-2 ISP Tier-2 ISP
local local local ISP
23 ISP ISP ISP
Contd.

24
 Ethiopian Network Structure

25
Network Type Based on Connection

 A network is two or more devices connected through links.

 A link is a communications pathway that transfers data from one device to

another.

 For visualization purposes, it is simplest to imagine any link as a line drawn

between two points.

 For communication to occur, two devices must be connected in some way to

the same link at the same time.

 There are two possible types of connections: point-to-point and

multipoint.
26
1. Point-to-Point

 A point-to-point connection provides a dedicated link between two devices.

 The entire capacity of the link is reserved for transmission between those two devices.

 Most point-to-point connections use an actual length of wire or cable to connect the

two ends, but other options, such as microwave or satellite links, are also possible.

 When you change television channels by infrared remote control, you are

establishing a point-to-point connection between the remote control and the

television's control system.

27
2. Multipoint
 A multipoint (also called multidrop) connection is one in which more than two specific

devices share a single link.

 In a multipoint environment, the capacity of the channel is shared, either spatially or

temporally.

 If several devices can use the link simultaneously, it is a spatially shared connection.

 If users must take turns, it is a timeshared connection.

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 Network Topology

 The term topology refers to the way in which a network is laid out physically.

 Two or more devices connect to a link; two or more links form a topology.

 The topology of a network is the geometric representation of the relationship of all

the links and linking devices (usually called nodes) to one another.

 There are five basic topologies :

1. Star
2. Bus
3. Ring
4. Mesh
5. Hybrid
29
Star Topology

 In a star topology, each device has a dedicated point-to-point link only to a

central controller, usually called a hub/switch.

 The term dedicated means that the link carries traffic only between the two

devices it connects.

 The devices are not directly linked to one another.

 Unlike a mesh topology, a star topology does not allow direct traffic between

devices.

 The controller acts as an exchange: If one device wants to send data to

another, it sends the data to the controller, which then relays the data to the other
connected device
30
Contd.
 A star topology is less expensive than a mesh topology.

 In a star, each device needs only one link and one I/O port to connect it to any

number of others.
o This factor also makes it easy to install and reconfigure.

 Far less cabling needs to be housed, and additions, moves, and deletions

involve only one connection: between that device and the central device.

 Other advantages include robustness. If one link fails, only that link is

affected. All other links remain active. This factor also lends itself to easy fault
identification and fault isolation. As long as the central device is working,
it can be used to monitor link problems and bypass defective links.
31
Contd.

A SIMPLE STAR TOPOLOGY

A HIERARCHICAL STAR TOPOLOGY
32
Contd.

 One big disadvantage of a star topology is the dependency of the whole

topology on one single point, the central device(switch/hub).

 If the switch goes down, the whole system is dead.

 Although a star requires far less cable than a mesh, each node must be linked to

a central hub.

 For this reason, often more cabling is required in a star than in some other

topologies (such as ring or bus).

 The star topology is used in local-area networks (LANs).

 High-speed LANs often use a star topology with a central switch.

33
Bus Topology

 A bus topology, is multipoint connection: one long cable acts as a backbone to link all

the devices in a network

 Nodes are connected to the bus cable by drop lines and taps.

 A drop line is a connection running between the device and the main cable.

34
Contd.

 A tap is a connector that either splices into the main cable or punctures

the sheathing of a cable to create a contact with the metallic core.

 As a signal travels along the backbone, some of its energy is transformed

into heat.

 Therefore, it becomes weaker and weaker as it travels farther and farther.

 For this reason there is a limit on the number of taps a bus can support

and on the distance between those taps.

35
Advantages of Bus Topology.

 Advantages of a bus topology include ease of installation.

 Backbone cable can be laid along the most efficient path, then connected to the

nodes by drop lines of various lengths.

o In this way, a bus uses less cabling than mesh or star topologies.

 In a star, for example, four network devices in the same room require four

lengths of cable reaching all the way to the hub.

 In a bus, this redundancy is eliminated. Only the backbone cable stretches

through the entire facility.

 Each drop line has to reach only as far as the nearest point on the backbone.
36
Disadvantages of Bus Topology.
1. Difficult reconnection and fault isolation. A bus is usually designed to be optimally
efficient at installation. It can therefore be difficult to add new devices.
2. Signal reflection at the taps can cause degradation in quality. This degradation can be
controlled by limiting the number and spacing of devices connected to a given length
of cable.
o Adding new devices may therefore require modification or replacement of
the backbone.
 In addition, a fault or break in the bus cable stops all transmission, even between
devices on the same side of the problem.
 The damaged area reflects signals back in the direction of origin, creating
noise in both directions.
 Bus topology was one of the first topologies used in the design of early local area
networks.
37 o Ethernet LANs can use a bus topology, but they are less popular now.
Ring Topology

 In a ring topology, 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, until it reaches

its destination.

 Each device in the ring incorporates a repeater.

 When a device receives a signal intended for another device, its repeater regenerates the

bits and passes them along.

 A ring is relatively easy to install and reconfigure.

 Each device is linked to only its immediate neighbors (either physically or logically).

38 o To add or delete a device requires changing only two connections.

Contd.

 The only constraints are media and traffic considerations (maximum ring length and
number of devices).
 In addition, fault isolation is simplified.
 Generally in a ring, a signal is circulating at all times.
39
Contd.

 In a simple ring, a break in the ring (such as a disabled station) can disable the

entire network.

 This weakness can be solved by using a dual ring or a switch capable of closing

off the break.

 Ring topology was prevalent when IBM introduced its local-area network Token

Ring.

 Today, the need for higher-speed LANs has made this topology less popular.

40
Mesh topology
 In a mesh topology, every device has a dedicated point-to-point link to every other

device.

 To find the number of physical links in a fully connected mesh network with n nodes,

we first consider that each node must be connected to every other node.

 Node 1 must be connected to n - I nodes, node 2 must be connected to n – 1 nodes, and

finally node n must be connected to n - 1 nodes.We need n(n - 1) physical links.

 However, if each physical link allows communication in both directions (duplex

mode), we can divide the number of links by 2. In other words, we can say that in a
mesh topology, we need n(n -1) /2 duplex-mode links.

 To accommodate that many links, every device on the network must have n – 1
41
input/output (I/O) ports to be connected to the other n - 1 stations.
Contd.
 A mesh offers several advantages over other network topologies.
 First, the use of dedicated links guarantees that each connection can carry its own data
load, thus eliminating the traffic problems that can occur when links must be shared
by multiple devices.
 Second, a mesh topology is robust. If one link becomes unusable, it does not
incapacitate the entire system.
 Third, there is the advantage of privacy or security. When every message travels along
a dedicated line, only the intended recipient sees it. Physical boundaries prevent other
users from gaining access to messages.
 Finally, point-to-point links make fault identification and fault isolation easy.
 Traffic can be routed to avoid links with suspected problems. This facility enables the
network manager to discover the precise location of the fault and aids in finding its
42 cause and solution.
Contd.

 The main disadvantages of a mesh are related to the amount of cabling and the

number of I/O ports required.

 First, because every device must be connected to every other device, installation and

reconnection are difficult.

 Second, the sheer bulk of the wiring can be greater than the available space (in walls,

ceilings, or floors) can accommodate.

 Finally, the hardware required to connect each link (I/O ports and cable) can be

prohibitively expensive.

 For these reasons a mesh topology is usually implemented in a limited fashion, for

example, as a backbone connecting the main computers of a hybrid network that can
43 include several other topologies.
Contd.
 One practical example of a mesh topology is the connection of telephone
regional offices in which each regional office needs to be connected to
every other regional office

Mesh topology

44
 Hybrid Topology
 A network can be hybrid. For example, we can have a main star topology

with each branch connecting several stations in a bus topology (star-bus

topology)

45
Network Models by Capability

1. Client/Server Model

 Microcomputer users, or clients, share services of a centralized computer

called a server.

46
Contd.
2. Peer-to-Peer Model

 Computers share equally with one another without having to rely on a

central server.

47
 Client-server model

Fig. Two clients using a single server at a time

48
 Client/Server
 A distinction exists between computers that make available network resources
(servers) and those computers that use the resources (clients or workstations).
Pros:
 Very secure

o Centralized servers easy to manage

o Physically centralized

o Secure OS Cons:
 Better performance  Require professional administration
 Centralized backups  More hardware intensive
 Reliability

o Simple job to do plus built in redundancy

49
 Peer-to-Peer (p2p)
 Computers on the network communicate with each others as equals and each computer is
responsible for making its own resources available to other computers on the network.
Pros:
 Uses less expensive computer networks

 Easy to administer

 No NOS (network operating system) required

 More built-in redundancy

 Shared resources – some machine will have what you need

Cons:
 Individual user performance easily affected

 Not very secure

 Tragedy of the commons – no guarantee others will administer their resources properly
(almost guaranteed with over 10 machines)
50
 Hard to back up.
 Client/Server vs. Peer-to-Peer: Advantages and Disadvantages

Client/Server Model Peer-to-Peer Model

Advantages:
Advantages:
o Uses less expensive networks.
o Very secure OS.
o Easy to configure
o Better performance.
o Contain both network operating system and
o Centralized servers, easy to
application software.
manage.
o Ideal for small business and home users (up
o Centralized backups.
to 10 computers).
o High reliability.
Disadvantages:
Disadvantages:
o Individual user performance easily affected.
o Expensive administration.
o Not very secure.
o More hardware intensive.
51 o Hard to back up.
 Q

52