Download Basic Concepts of Computer Networks

Basic Concepts of Computer Networks
Network Topology

Lesson Objectives
At the end of this lesson, the learner should be able
to achieve the following three objectives:
 Clearly define the term network topology.
 Identify the types of network topologies and their
individual advantages and disadvantages.
 How to properly choose the appropriate topology
for a given situation.
DEFINITION: What Is a Topology?
A network topology describes the arrangement
of systems on a computer network. It
defines how the computers, or nodes,
within the network are arranged and
connected to each other.
(TechTerms.com, 2007)
Commonly Used Network Topologies
The most commonly used network topologies are:
 Bus
 Ring
 Star
 Mesh
The Bus Topology
 The bus topology is the simplest and
most common.
 It is often used when a network
installation is small, simple, or
temporary.
 It is a passive topology. This means
that computers on the bus only
“listen” to the data being sent. They
are not responsible for “moving” the
data from one computer to the next.
The Bus Topology
 In an active topology network, the computers
regenerate signals and are responsible for moving
the data through the network.
 On a bus network, all the computers are connected
to a single cable.
 When one computer sends a signal using the cable,
all the computers on the network receive the
message, but only the addressee accepts it. The
other computers disregard the message.
Advantages of the Bus Topology
 The bus is simple and very reliable in very
small networks.
 The bus network requires the least amount of cable
to connect the computers together and is,
therefore, less expensive than other cabling
configurations.
 Failure of one network node does not effect the
rest of the network.
Disadvantages of the Bus Topology
 Heavy network traffic can slow a bus considerably
 A break in the cable or lack of proper termination
can shutdown the entire network.
 It is extremely difficult to troubleshoot a bus if the
entire network shuts down.
Appropriate Use of a Bus Topology
 The network is small.
 The network will not be frequently reconfigured.
 The least expensive solution is necessary.
 Network growth is not predicted.
The Ring Topology

In a ring topology, each computer is
connected directly to the next computer
in line, forming a circle of cable.

It uses a token to pass the information
from one computer to the next.

Each computer is connected to the next
computer in the ring and retransmits
what it receives from the previous
computer.

The message flow around the ring is
unidirectional.

The ring is classified as an active
topology because there is no
termination point in the ring.
Token Passing in the Ring Topology
 Token passing is a method of sending data in a ring
topology.
 A small packet, called the token is passed around the
ring to each computer in turn.
 If a computer has information to send, it modifies the
token, adds address information and data and sends it
down the ring.
 The information travels around the ring until it either
reaches it’s destination or returns to the sender.
 A token can circle a ring 200 meters in diameter at a
speed of around 10,000 times per second.
Advantages of Ring Topology
 All the computers have equal access to the
network.
 Even with multiple users, network performance is
balanced. There is no “bottle neck”.
 Allows for error checking and acknowledgement.
Disadvantages of the Ring Topology
 Failure of one computer can affect the entire
network. Data cannot be transmitted successfully.
 Data packets must pass through every computer.
This makes the network slower in nature.
 It is difficult to troubleshoot the ring network.
 Adding or removing computers disturbs the
network.
Appropriate Uses of the Ring Topology
 The network must operate reasonably under a
heavy load.
 A higher speed network is required.
 The network will not be frequently reconfigured.
The Star Topology

In a star topology, each device has a
dedicated point-to-point link to a central
controller, sometimes referred to as a hub,
server, or host.

Each computer on a star network
communicates with the central hub that
then resends the message to the
appropriate computer(s).

The hub can be active or passive in nature.

An active hub regenerates the electrical
signal and sends it to all the computers
connected to it.
Hubs in the Star Topology
 The type of hub used in a star topology is
commonly referred to as a multi-port repeater.
 Active hubs require electrical power to operate.
 A passive hub, such as a wiring panel, acts as a
connection point and does not amplify or
regenerate the signal.
 Passive hubs do not require electrical power to
operate.
Advantages of the Star Topology

A star network is easy to modify and add new computers.

During the addition or removal of nodes, the network
continues to function normally.

The central hub can be easily upgraded when the network
capacity is exceeded.

The central hub provides for centralized monitoring and
management of the network, thereby increasing security.

A single computer failure does not necessarily bring down the
entire star network.
Disadvantages of the Star Topology
 If the central hub fails, then the entire network
ceases to operate. The attached nodes are disabled.
 There is a larger upfront cost to create a star
network in terms of cabling and hardware.
 A star network requires a dedicated server.
Appropriate Uses of the Star Topology
 The network requires easy addition or removal of
client computers.
 Star network topologies are recommended for easy
troubleshooting ability.
 The network is large in nature.
 The network is expected to grow in the future.
The Mesh Topology
 In a mesh topology, every device
has a dedicated point-to-point link
to every other device.
 A fully connected mesh network
has n(n – 1)/2 physical channels to
link n devices.
 To accommodate the many links,
every device on the network must
have n -1 input/output ports.
Advantages of the Mesh Topology
 Because there is a dedicated link, there is no traffic
between computers.
 Failure of one node computer does not affect the
rest of the network.
 Because of the dedicated link, privacy and security
are easier to maintain.
 Point-to-point links make fault identification and
fault isolation easier to determine.
Disadvantages of the Mesh Topology
 Due to the number of cables and input and output
ports, it can be more expensive to establish and
maintain.
 A large amount of space is necessary to run each of
the cables.
Review of Common Network Topologies
Conclusions
 Topologies are the essence of computer network
designs.
 Efficient networks can only be built based on the
complete knowledge and understanding of the
previously explored topologies.
 Knowledge of communication devices is of equal
importance when determining the best options for
network requirements.
 Resources and time can be wasted when choosing the
wrong topology to fit the requirements of the network
to be built.
References
Web References
 http://compnetworking.about.com/od/networkdesi
gn/ig/Computer-Network-Topologies/
 http://www.techterms.com/definition/networktop
ology
 http://en.wikipedia.org/wiki/Network_topology
 http://www.youtube.com/watch?v=kfEDPQAYH4k