Carrier Sense Multiple Access with Collision Detection is abbreviated to CSMA/CD.
This is an Ether Layer Protocol (i.e. Layer 2 ), and it defines how two or more devices attempt to use a data channel simultaneously & overcome a data collision. It also defines how long a device should wait if an other device has already sent some data in the cable. As the medium is used by multiple devices. CSMA/CD is persistent & used over 802.3 Ethernet Standards. It decides when an other device is allowed to send the data in the medium if no device is sending or one device has already sent data and get ACK(nowledged).
Carrier Sense : It states “Listen before Talk” , this prevents from collision of data and prevents from causing noise.
Multiple Access : It states “When a device should allowed to speak on a multiple endpoint network”
In fig 1.1,
There are 6 PC’s connected in Bus topology such that 1 is sending some data to 4 in the time lag till 4 receive the data no other can use the link 1 to 4 , which means CSMA/CD protocol in NIC’s of each PC’s will recognize the busy link and it will make reserved for single transmission of data frames.
CSMA/CD uses the FCS (Frame Check Sequence) is a 4 bytes flag used in trailer of a Frame in layer 2. CSMA/CD makes you and me safe from collision of data , Suppose if you are sending the data to one of your friend and at same time your other friend is also sending him some data but it got collapsed or collide , which may turn the irregularity of data transmission that is not good for a healthy Network. So we use CSMA/CD to prevent the data from collision and maintains the delay in data transmission.
In fig 1.2
Chart shows the process of CSMA/CD and Frame Check Sequence while the data collision detection.
CSMA/CD is an important data collision detection mechanism in Layer 2, it makes life “easier”.