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# CS601 - Data Communication - Lecture Handout 05

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# TOPOLOGY

“The Topology is the geometric representation of the relationship of the links and the linking devices (Nodes) in a Network”

Or

“Topology defines the physical or the Logical Agreement of Links in a Network”

Topology of a Network is suggestive of how a network is laid out. It refers to the specific configuration and structure of the connections between the Links and the Nodes. Two or more devices connect to a Link and two or more Links form a Topology

## Categories of TOPOLOGY

• Question: What to consider when choosing a Topology????????

Two relationships are possible in a network

• PEER-TO-PEER: Devices share the link equally
• PRIMARY-SECONDARY: One device controls traffic and the others must transmit through it

## MESH TOPOLOGY

• Every device has dedicated a point-to-point link to every other device
• Dedicated: Means that the link carries traffic only between these two devices Links to connect ‘n’ devices
• Each device must have I/O Ports

### Example Mesh Topology

In figure above, we have 5 Nodes, therefore:

• No. of Links= 5(5-1)/2 = 10
• No. of I/O Ports= 5-1 = 4

This increase exponentially with increase in No. of Nod

• e.g. for 6 nodes = 15 Links

• Use of Dedicated links guarantees that each connection can carry its own load. This eliminates Traffic Problems as in case of Shared Link
• Mesh Topology is robust. If one link fails, it does not effect other links
• Security & Privacy due to dedicated links
• Point – to –Point links make Fault Identification easy

• Amount of Cabling
• Makes Installation & Reconfiguration difficult
• Sheer bulk of wiring can be greater than the available space
• Number of I/O Ports Required
• Hardware required to connect each link can be prohibitively expensive

Therefore, Mesh topology has limited use

## Star Topology

• Each device has a dedicated point-to-point link to a central controller ( Hub)
• Devices are not directly connected to each other
• Controller (Hub) acts as an exchange
• If one device wants to send data to the other, it sends the data to the controller , which then relays it to the other connected device

• Less Cabling
• Less Expensive than Mesh as each device need sonly one link and one I./O Port
• Easy to Install and Reconfigure
• Easy Fault Detection

• Although Cabling required is far less than Mesh
• Still each node must be connected to a Hub , so Cabling is still much more than some other Topologies

## Tree Topology

• A variation of Star Topology
• Nodes in a Tree are linked to a central hub that controls the traffic to and from network
• Difference b/w star and tree is not all the devices plug directly into the central HUB
• Majority connects to secondary hub that is connected to central hub

### CENTRAL HUB in Tree Topology

• Central Hub in a Tree is an ACTIVE HUB
• ACTIVE HUB contains a repeater
• Repeater is a hardware device that regenerates the received bit pattern before sending them out.
• Repeater strengthens TX. And increases the distance a signal can travel

### Secondary HUB in Tree Topology

• Secondary Hub in a Tree may be Active or Passive HUB
• Passive Hub simply provides physical connection between attached devices

• Because of Secondary Hub, More devices can be attached to a Central Hub and therefore increase the distance a signal can travel
• Enables Differentiated Services: Allows to prioritize communication, e.g. computers attached to one secondary hub can be given priority over others
• Therefore, TIME SENSITIVE data will not have to wait for access to the network
• Rest of the advantages are almost the same as STAR

Example Tree Topology: Cable TV

CABLE TV

• Main cable from main office is divided into many branches and each branch is divided into smaller branches and so on
• Hubs are used when cable is divided

## BUS TOPOLOGY

• Drop Lines and Taps
• Drop Line is the connection between device and the main cable (Backbone)
• Tap is a connector that;
Splices into the main cable or
Punctures the sheathing of a cable to create connection with the metallic core

• Signal degrades as it travels, therefore there is a limit on:
• The number of Taps a Bus can support and
• The distance between those Taps

• Easy to install
• Backbone can be laid on the most efficient path and then rest of the nodes can be connected using Drop Lines
• Less cabling than Mesh , Star or Tree
• Difference b/w Star Cabling and Bus Cabling

• Difficult Reconfiguration
• Difficult to add new devices
• adding new devices may require modification of backbone
• No Fault Isolation
• A fault or break in backbone can disable communication even on the same side of the problem
• Damaged area reflects signals back in the direction of origin creating Noise in both directions

## Summary

• Topology
• Categories of Topologies