Lecture 03
COMPUTER NETWORK TOPOLOGIES
A Network Topology is the arrangement with which computer systems or network devices are
connected to each other. Topologies may define both physical and logical aspect of the network. 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 four basic topologies possible: mesh, star,
bus, and ring.
Mesh Topology
In this type of topology, a host is connected to one or multiple hosts. In a mesh topology, every device
has a dedicated point-to-point link to every other device. The term dedicated means that the link
carries traffic only between the two devices it connects.
Advantages:
1. 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.
2. A mesh topology is robust. If one link becomes unusable, it does not incapacitate the entire system.
3. 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.
4. 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 cause and solution.
Disadvantages:
1. Disadvantage of a mesh are related to the amount of cabling because every device must be
connected to every other device.
PARUL RASTOGI 1
2. Installation and reconnection are difficult.
3. The sheer bulk of the wiring can be greater than the available space (in walls, ceilings, or floors) can
accommodate.
4. The hardware required to connect each link (I/O ports and cable) can be prohibitively expensive.
Star Topology
In a star topology, each device has a dedicated point-to-point link only to a central controller, usually
called a hub. 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 .
Advantages:
1. 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.
2. Easy to install and reconfigure.
3. Far less cabling needs to be housed, and additions, moves, and deletions involve only one
connection: between that device and the hub.
4. Other advantage 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 hub is
working, it can be used to monitor link problems and bypass defective links.
Disadvantages:
One big disadvantage of a star topology is the dependency of the whole topology on one single point,
the hub. If the hub 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).
Bus Topology
In case of Bus topology, all devices share single communication line or cable. Bus topology may have
problem while multiple hosts sending data at the same time. Therefore, Bus topology either uses
CSMA/CD technology or recognizes one host as Bus Master to solve the issue. It is one of the simple
forms of networking where a failure of a device does not affect the other devices. But failure of the
shared communication line can make all other devices stop functioning.
Both ends of the shared channel have line terminator. The data is sent in only one direction and as
soon as it reaches the extreme end, the terminator removes the data from the line.
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.
PARUL RASTOGI 2
Advantages:
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. In this way, a bus uses
less cabling than mesh or star topologies.
Disadvantages:
Disadvantages include 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. 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. 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.
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.
Advantages:
A ring is relatively easy to install and reconfigure. Each device is linked to only its immediate neighbors
(either physically or logically). To add or delete a device requires changing only two connections. The
only constraints are media and traffic considerations (maximum ring length and number of devices).
Disadvantages:
Unidirectional traffic can be a disadvantage. 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.
Tree Topology
Also known as Hierarchical Topology, this is the most common form of network topology in use
presently. This topology imitates as extended Star topology and inherits properties of Bus topology.
This topology divides the network into multiple levels/layers of network. Mainly in LANs, a network is
bifurcated into three types of network devices. The lowermost is access-layer where computers are
attached. The middle layer is known as distribution layer, which works as mediator between upper
layer and lower layer. The highest layer is known as core layer, and is central point of the network,
i.e. root of the tree from which all nodes fork.
All neighboring hosts have point-to-point connection between them. Similar to the Bus topology, if
the root goes down, then the entire network suffers even though it is not the single point of failure.
Every connection serves as point of failure, failing of which divides the network into unreachable
segment.
PARUL RASTOGI 3
Hybrid Topology
A network structure whose design contains more than one topology is said to be hybrid topology.
Hybrid topology inherits merits and demerits of all the incorporating topologies.
Most WANs are connected by means of Dual-Ring topology and networks connected to them are
mostly Star topology networks. Internet is the best example of largest Hybrid topology.
PARUL RASTOGI 4
