Network Topology and Types of Networking

Overview

Recall questions on physical star and logical bus topologies and their operation, and on peer-to-peer and client-server networking.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   Describe a physical star topology.

   Model answer

   In a physical star topology every device is connected by its own dedicated cable to a single central node, typically a switch or hub. No device is cabled directly to any other; all traffic passes through the central node, which is responsible for getting it to its destination. Drawn out, the cables radiate from the centre like the points of a star.

2. 2

   Explain the operation of a physical star network.

   Model answer

   A device that wants to transmit sends the data along its own cable to the central switch. The switch reads the destination address in the frame, looks it up in the table of addresses it has learned for each of its ports, and forwards the frame out of only that one port to the intended recipient. Because every device has a private link to the centre and the switch can carry several such conversations at once, two pairs of devices can communicate simultaneously without interfering.

3. 3

   Describe a logical bus topology.

   Model answer

   In a logical bus topology all devices share one common communication channel. Any data transmitted is broadcast along that shared channel and reaches every device on the network. Each device inspects the destination address of what it receives, keeps the data if the address is its own and ignores it otherwise. Only one device can transmit successfully at a time, so the devices must take turns.

4. 4

   Explain the operation of a logical bus network.

   Model answer

   A device with data to send broadcasts it onto the shared channel. The signal travels to all attached devices, each of which reads the destination address and either accepts or discards the frame. Because the channel is shared, if two devices transmit at once their signals interfere and both transmissions are spoiled, so a bus protocol is needed to decide who may transmit and when, and to recover when two devices do transmit together.

5. 5

   Differentiate between a physical star topology and a logical bus topology.

   Model answer

   They describe two different things. Physical star is about the cabling: each device has its own dedicated link to a central node. Logical bus is about the behaviour of the data: every device shares one channel and every transmission is broadcast to all of them. In a physical star, traffic is directed by the central node and separate conversations can run in parallel; in a logical bus, traffic is seen by everyone and only one device can transmit at a time. Because one describes wiring and the other describes data flow, a network can be both at once.

6. 6

   Explain how a network physically wired as a star can behave as a bus network.

   Model answer

   The physical layout and the logical behaviour are independent. If the devices are all cabled to a central node but that node simply repeats whatever arrives on any port out of every other port — and the devices run a bus protocol under which they share one channel, listen before transmitting and read the destination address of everything they receive — then, as far as the data is concerned, all devices are on one shared broadcast channel. The star is the physical topology; the bus is the logical topology. It is achieved by using a bus protocol together with appropriate physical switching at the central node.

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