Name: Hanar Ahmed Star
Second Stage
C2
College of Science and Technology
Department of Information Technology
Physical Topology
Physical Topology: The physical topology is the way you physically lay out the
network, like a map, and the logical topology is the way the information flows on the
network. Usually, the physical and logical topology is the same, but sometimes they
can differ, such as in a physical star/logical ring topology.
What is Network Topology?
Network topology refers to the physical or logical layout of a network. It defines the
way different nodes are placed and interconnected with each other. Alternately,
network topology may describe how the data is transferred between these nodes.
There are two types of network topologies: physical and logical.
What is physical and logical?
High-level versus low-level. Logical implies a higher view than the physical. Users
relate to data logically by data element name; however, the actual fields of data are
physically located in sectors on a disk.
Types of Network Topologies :
Any computer network design can be said to have evolved from one of three basic
topologies. These three topologies can also be combined in a variety of ways to form
more complex hybrid topologies. These three topologies are:
Bus
Star
Ring
Bus
A bus topology is commonly referred to as a "linear bus" because all of the nodes are
physically connected in a straight line. A bus topology has a single backbone cable to
which computers and other devices are connected. This backbone is also known as a
segment or a trunk.
An Ethernet bus network, such as 10Base2, has a single communications channel
with each node connected to the main cable. The bus must be terminated at either
end by terminators. These absorb the signal and stop it being sent back along the
cable to corrupt any messages. If the bus is not terminated the signal will reflect back
and possibly collide with other data. When a node transmits its data it goes in both
directions. If a node recognizes that the message is addressed to it will take action,
otherwise it will ignore the message.
Star
In the star topology, cables from each computer are connected to a central device
known as a hub. Signals are transmitted from the sending computer through the hub
to all computers on the network. This topology has its origins in the early days of
computing when terminals were connected to a central mainframe computer.
The star network topology has the advantage of centralizing resources and
management, however, more cable is required than for other topologies. The star
topology also has a central point of failure, that is, if the hub at the center of the
topology fails then the whole network will be down.
If one of the computers (or the cable that connects it to the hub) fails on a star
network, only the failed computer will not be able to send or receive network data.
The rest of the network will continue to function normally.
This type of topology is the most commonly used. Most star topologies are Ethernet
networks, which mean that they use CSMA/CD as a means of media access.
Hubs have generally been replaced in star topologies by switches, which have the
advantage of creating multiple collision domains and effectively creating collision
free networks.
Ring
The picture shown is a classical ring topology and consists of a physical ring and a
logical ring. Each computer is linked to two others to form a closed loop. Each node
in this ring will act as a repeater by regenerating and cleaning up a signal before
passing it to the next node.
In a ring network communication takes place by the use of a token which is passed
from computer to computer around the ring and it is only the computer that has the
token that can send data. A ring network is unidirectional, ie: data can only go in one
direction.
Once a computer has the token it can send its message. Each successive computer on
the ring will examine the message to check if they are the message's destination.
Once the message has been received the token is put back on the network for the
next node that requires its use.
No matter what the size of the network or the distance covered there can only be
one token on a ring network. Ring networks are known as deterministic because
network access is determined by which workstation has the token.
The failure of one computer can have an impact on the entire network.
Other Topologies
Extended Star
An extended star topology has a main star with additional hubs at the end of each
link, which have, further nodes connected in a star topology.
Mesh Topology
A mesh topology is the most robust topology. Not only does it have the capabilities
of a star but each of the nodes has a direct connection to all other nodes. This
provides alternative links should there be failure on the network.
