3. Switch

A network cable can only have one data packet in it at any instant.

So if two or more computers want to place a data packet on to the network at exactly the same time, then a 'data collision' will take place. 

The network protocol is set up to deal with this. Basically it declares the collided data as unusable and forces the two computers to re-send their data packets at a slightly different time.

This is fine for a lightly loaded network with only a few computers on-line. You will not notice the small delay caused by data collisions.

 But imagine what happens when a hundred PCs are sharing the same network and they are all wanting to send their data packets. This will most likely result in thousands of data collisions per second - each one costing a small amount of time. You will certainly notice the network 'slowing down'.

The clip below shows how a network SWITCH can help with this problem.

A switch has a number of ports and it stores the addresses of all devices that are directly or indirectly connected to it on each port. 

As a data packet comes into the switch,  its destination address is examined and a direct connection is made between the two machines.

This added sophistication compared to a hub means that virtual networks can be set up where only a sub-set of connected computers can see each other. As far as these computers are concerned there are no other machines on the network.

Virtual networks are very useful. Consider a school network that is using a switch. The network manager may have set it to create the following networks

• Student virtual network, all students use this to do their work
• Administration virtual network, only authorised staff have access

Performance

Some switches are able to control the bandwidth of each port, for example one port may be set for a 100 Mbps link which is fine for every-day use whilst another may be set for 1 Gbps perhaps to support video streaming over the network.

A switch may also be able to prioritise some ports over others so if a data packet is passing through a high priority port this is allowed to pass before a packet from a lower priority port. This 'Quality of Service' feature means that delay-sensitive feeds can be improved. For instance you instantly notice stuttering in a video conference but you would not be sensitive to a file being opened a few milliseconds later.