3. Topology
The term topology refers to the way a network is laid out, either
physically or logically.
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
each other.
The topology describes that how the devices in a network are
interconnected rather than their physical arrangement.
4. Topology continued…
A consideration when choosing a topology is the relative status of the devices to
be linked.
Two relationships are possible.
Peer-to-Peer: where the devices share the link equally.
Primary-Secondary: where one device controls traffic and the others can
transmit through it.
Five basic topologies
Bus
Star
Mesh
Tree
Ring
Note: Ring & Mesh topologies are more convenient for Peer-to-Peer transmission while star &
Tree topologies are better for primary–secondary transmission. A bus topology is convenient for
either.
6. Mesh topology
In 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.
A fully connected mesh network has n(n-1)/2 physical channels to
link n devices and every device must have n-1 I/O ports.
7. Mesh topology continued…
Advantages:
Each connection can carry its own data load.
This topology is robust. If one link becomes unusable, it does not affect the entire
system.
Privacy or security
Easy fault identification.
Disadvantages:
Large amount of cabling & I/O ports are required.
Installation & reconfiguration are difficult because every device must be connected to
every other device.
The bulk of wiring can be greater than the available space( in walls, ceilings or floors)
can accommodate.
The hardware(I/O ports & cabling) required to make a fully connected mesh network
can be prohibitively expensive.
8. 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 connected to each other.
Like mesh a star topology does not allow direct traffic between devices.
9. Star Topology Continued…
If one device wants to send data to another, It sends data to the controller
which then relays the data to the other connected device.
Usually two types of Hubs are available.
Passive Hub
Active Hub
10. Types of Hubs
Passive Hub:
A passive Hub is used to connect computers in a broadcast network.
The signal sent to a passive hub is sent to all workstations without any
regeneration or amplification.
Passive hubs do not require any external power source.
Active Hub:
An active hub uses an external power source and regenerates the signal,
therefore greater cable distances are allowed.
A switched hub directs the signal directly to the recipient.
Active hubs also contain diagnostic features to aid in network trouble shooting.
Active Hub is more expensive than a passive Hub.
11. Advantages of using star topology
A star topology is less expensive than a mesh topology.
Each device needs only one cable and one I/O port to connect with any
number of devices.
Easy to install and easy to reconfigure.
Far less cabling is required therefore space problems are rare.
Addition, deletions and moves involve only one connection between that
device and hub.
If a link fails only that link is affected.
Easy fault identification & fault isolation because of central Hub.
12. Disadvantages of using star topology
More cabling than any other topology(such as bus, ring or tree ) except Mesh
because every device must be connected to the central hub.
Active hub is expensive.
A central hub failure can damage the whole network.
14. Tree topology
A variation of star topology.
As in star, nodes in a tree are linked to a central Hub that controls the traffic to
the Network.
However, not every device is directly plugged in to the central Hub.
The majority of the devices connect to a secondary Hub that in turn is
connected to the central Hub.
The central Hub in the tree is an active Hub.
15. Tree topology continued…
An active Hub contains a repeater which is a hardware device that regenerates
the received bit patterns before sending them out.
Repeating strengthens transmissions and increases the distance a signal can
travel.
The secondary Hubs may be active or passive hubs.
A passive Hub provides a simple physical connection between the attached
devices.
16. Advantages & Disadvantages of using Tree
The advantages and disadvantages of tree are almost the same to that of star.
However, because of the addition of secondary Hubs more devices can be
connected with the central Hub and can increase the distance a signal can
travel.
It allows the network to isolate and prioritize communications from different
computers.
By means of prioritizing, the network designers and operator can guarantee that
time-sensitive data will not have to wait for access to the network.
17. Bus Topology
A Bus topology is a multipoint.
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.
A tap is a connector that either splices in to the main cable or punctures the
sheathing of a cable to create a contact with the metallic core.
18. Bus topology Continued…
As a signal travels along the backbone,some of its energy is transformed in to
heat.
Therefore it becomes weaker and weaker.
For this reason there is a limit on the number of taps a bus can support and on
the distance between those taps.
19. Advantages & Disadvantages of using bus
topology
Advantages:
Ease of installation.
Less cabling than mesh, star or tree.
Only one backbone cable stretches through the entire facility.
For n devices to connect only one backbone cable and n drop lines are needed.
Disadvantages:
Difficult reconfiguration and fault isolation.
It can be difficult to add new devices.
Signal reflection problem.
Adding new devices may require replacement of the backbone.
A fault in backbone can damage the entire network causing noise problems in
both directions.
20. Ring Topology
In ring topology every device has a dedicated point to point line configuration
only with 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.
Token passing is frequently used in ring topology.
21. Advantages & disadvantages of using ring
topology
Advantages:
Easy to install and easy to reconfigure.
Adding and deleting a device only involves two connections.
Easy fault isolation.
If one device does not receive a signal within a specified period,it can issue an
alarm.
Disadvantage:
Uni-directional traffic.
A break in a ring can disable the entire network.
This weakness can be solved by a dual ring.
22. Hybrid Topologies
Often a network combines several topologies as subnetworks
linked together in a large topology.
For example, one department of a buisness may have decided to
use a bus topology while another department has a ring.
All can be connected with a central controller in a star toplogy.
