Topologies

1. Introduction to Computer
Lecture # 14
Course Instructor : Sehrish Rafiq
Department Of Computer Science
University Of Peshawar

2. Lecture overview
 Topology
 Mesh
 Star
 Tree
 Ring
 Bus

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. 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.

5. Mesh topology

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. Advantages & Disadvantages
 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. 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 & Disadvantages
 Advantages
 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.
 Disadvantages
 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.

12. Tree Topology

13. 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.

14. 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.

15. Advantages & Disadvantages
 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.

16. 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.

17. Continued…
 As a signal travels along the back-bone, 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.

18. Advantages & Disadvantages
 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.

19. 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.

20. Advantages & Disadvantages
 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.

21. Hybrid Topologies
 Often a network combines several topologies as sub-networks
linked together in a large topology.
 For example, one department of a business 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 topology.