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CS601 - Data Communication - Lecture Handout 05

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


“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

Categories of TOPOLOGY

  • Question: What to consider when choosing a Topology????????
  • Answer: Relative status of the devices to be linked.

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



  • 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
  • 7 Nodes=21 Links

Advantages of Mesh Topology

  • 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

Disadvantages of Mesh Topology

  • 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

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

Advantages of Star Topology

  • Less Cabling
  • Less Expensive than Mesh as each device need sonly one link and one I./O Port
  • Easy to Install and Reconfigure
  • Robust, if a link fails , only that link fails
  • Easy Fault Detection

Disadvantages of Star Topology

  • 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

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

Advantages of Tree Topology

  • 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


  • 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



  • 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

Advantages of BUS TOPOLOGY

  • 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

Disadvantages of BUS Topology

  • 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


  • Topology
  • Categories of Topologies

Reading Sections

  • Section 2.2 “Data Communications and Networking” 4th Edition by Behrouz A. Forouzan