Collision Domain Understanding – Cisco CCNA Lesson

By Youssef Edward
October 23, 2018


When two computers on the network send data to each other, the data will flow out to all other computers on the network and the MAC address is checked by every computer until the address is match with the correct recipient. The protocol that regulates this process is called CSMA/ CD (Short for carrier senses multiple access, Carrier Detect). Let us first see an overview of how this protocol works and see then how this is related to collision domains.

This above protocol is used to regulate traffic on the network. When one computer needs to send data to another computer, the hosts that wish to send must first check the line to see if any data is sent at this time. If there are data on the line, it suspends the transmission. If the line is free, it will begin to transmit and at the same time will check the line to see if another computer sends data at the same time. If it detects that data is sent at the same time, a collision is said to be occurred.

From the above discussion, it is seen that if the number of computer on one cable (called segment) is grown, the probability of collision will be larger. This means that the network will be slower because many collisions will be likely to occur. The reason a collision will make the network slower is because that the host will re-transmits the data and this will take a time.

The solution to the above problem is to break the network into group of hosts and limit the data broadcast to only one group. This group is called the collision domain. Thus, instead of the data to be flow to all the hosts on the network, it will flow only to all hosts in the collision domain. Thus the overall network is the major domain and the collision domain is one part of the network.

When one host in one collision domain needs to send data to another host in another one, it will send it to the gateway which forwards it to another collision domain but only in this case. The network device that performs this task is called the switch. The switch has many ports and each port corresponds to a collision domain on the network. Each port can carry several hosts through a hub. When one host on a port sends data to another device on a different port (which is on a different domains) the switch will forward the data only to the port the receiving hub belongs to. This of course will speed up the performance.

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