lesson 3

Network Design

Topology

Topology means the shape made by the wires and computers in a network. 3 Topologies are common

Bus

Here the computers are connected along the length of a wire. The wire is the bus.

Star

Ring

There are also hybrid topologies such as the star bus.

Our college network is a star-bus.

Network Adapter Cards

A network adapter card is an expansion card that plugs into a PC and provides a socket into which the network cable can be plugged.

Every network adapter card ever made has a unique identification number called its ’MAC address’. This is fixed inside its chips when it is made. This means that every computer on a network has a unique MAC address.

Bus Topology In More Detail

Bus topology is also known as linear bus topology – linear means ‘in a line’.

The bus is also known as the trunk or backbone.

We shall study how data is sent from one computer to another in more detail later, but here are the basics:

1. All data is cut into pieces called packets. A typical packet may be between ½KB and 4KB in size. The data is transmitted one packet at a time, then put back together again at the receiving end.

2. Only one packet may be sent at a time. (The cable can only handle one packet.) If one computer is sending a packet, all of the other computers must wait before they can send a packet.

3. Every packet has two MAC addresses at the beginning: the address of the sending computer and the address of the computer the packet must go to.

4. Every computer on the network receives the packet. Each one looks at the target address. It is not theirs they ignore it.

The most common cable system for PC bus networks is called ‘thinnet coaxial’. A separate piece is used between each computer. Together they form the bus.

The terminators stop packets bouncing back into the cable when they get to the end.

These networks are getting less common because they have a major problem: if any of the connections is broken, the whole network stops working.

The bus topology is a passive topology. This means that each computer sees each packet, but need take no further action. Computers are not responsible for actively passing the packet on to the next machine.

Bus networks can be made longer by using a repeater. This is device makes the signal stronger then passes it on.

Star Network In More Detail

A machine called a hub makes this topology possible.

Whatever comes into the hub on any wire is copied out on all other wires. (A hub can also be called a multi-port repeater.)

If machine ‘A’ wishes to send a packet of data to machine ‘D’, it addresses the packet correctly then sends it to the hub. The hub then sends it to machines ‘B’, ‘C’ and ‘D’. ‘B’ and ‘C’ ignore it and ‘D’ reads it.

Hub based networks normally use a cable called UTP – unshielded twisted pair. The big advantage is that if one connection is broken, only one computer is affected, the rest keep on working.

Some hubs are passive, simply letting the signal pass through them. Most hubs are active – they make the signal stronger as they pass it.

The Ring Topology In More Detail

In bus and star technologies if a computer wants to send a packet, it waits until the cable is free. Ring topology networks use a different system.

A special packet called a token is passed from computer to computer around the ring.

When a computer wants to send it:

1. Waits for the token to come round. When it does, the sender eats the token and sends out an addressed data packet.

2. The data packet is passed around the ring until reaches its destination. The receiving computer takes in the packet and sends out a special ‘confirmation of receipt’ packet.

3. This confirmation message is passed around the ring until it gets back to the sender. The sender eats it and sends out a new token. Another computer can now send data once it receives the packet.

Some networks really do use a ring of cables, but there is an obvious problem – if there is a break anywhere in the ring the whole network fails.

The most common token ring system uses special hubs that make a star topology work like a ring.

Here the special hub sends tokens and packets to each computer in turn. If any connection is broken, the hub disconnects that arm and the rest of the network works normally.

This is called a star-ring or star wired ring. Physically it is a star but logically it is a ring.

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