The CSMA/CD Protocol

Each PC connected to an Ethernet network connects using a Network Interface Card (NIC), which controls data connectivity between the host PC and the network. Note: Each NIC has a unique identifier called the MAC address (which can be seen as the ‘Physical Address’ when using the ‘IPCONFIG /ALL command in MSDOS).

The Carrier Sense Multiple Access / Collision Detection protocol (CSMA/CD) means that all NICs connected to the network must listen and wait until there is no traffic on the network.

At this point each NIC has an equal chance to transmit data across the network. If any of the other NICs are transmitting data across the network, there will be a signal called a 'carrier' on the network that informs other NICs that the network is busy. This is defined as 'Carrier Sense'.

When there is no traffic on the network, each NICs has an equal chance of transmitting data, and this is defined as 'Multiple Access'.

As data can taken a defined period of time to travel across the network, and isn't instant (though to us it may appear to be so), there is a chance that two NICs may transmit data at the same time.

When this is detected by the nearest NIC at the point where the two transmissions meet, a signal is sent across the network that causes all transmissions to cease for a period of time before transmissions resume.

This is defined as 'Collision Detection', and when this happens each NIC resets an internal random delay that avoids the same collision occurring when transmissions can resume.

The CSMA/CD protocol by design allows an equal chance for each NIC to transmit across the network allowing the network to be shared by several NIC connected devices.