Network collisions, detection and avoiding them
Introduction
A network cable must have just one data packet on it at any one time. When a packet is 'sent', it is actually broadcast to all the computers on the network but the computer with the correct IP address 'grabs' the packet, because it sees that it is addressed to them. If two different computers try to place a packet on the network at exactly the same time and try to broadcast their packets to all computers, then a 'collision' happens. The network protocol (typically TCP/IP) declares both packets void, and then asks both computers to resend their packets, but this time, ensuring that it is done at slightly different times. A user on the network will not notice this happening and any slowing down of the network on small networks, but if there are thousands of computers on a network, for example, across a university campus, there could be problems with the network slowing down if it has to deal with lots of network collisions.
Switches
A switch can reduce the number of collisions by being able to make multiple connections at the same time. for example, if Computer #1 wants to send a message to Computer #450, at the same time as Computer #300 wanting to send a message to Computer #2350, then the switch can set up links between these two pairs of computers to ensure that they can communicate at the same time, but not interfere with each other.
Ethernet
Ethernet is a widely-used design for LANs. Ethernet networks make use of collision avoidance and detection strategies as described above. As traffic increases on an Ethernet network, the number and frequency of collisions increase. This is because every station is broadcasting to every other one and the more workstations there are broadcasting, the more the communications on the network increase. Ethernet will try its best to prevent data collisions. When it does detect a collision, however, it will have the packets put on the network again but this time with a delay between the events.
Reducing data collisions on a 'swtched Ethernet LAN'
There are strategies for reducing the number of collisions on a network. We discussed one of them above using a switch, so that two pairs of computers can be given their own temporary communication link to ensure successful communication. A switch can also be used to split up a large network into separate areas that improve the efficiency of communication. It can be used to ensure each station is given a ‘time-slice’ in which to send data.