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In shared transmission networks all network participants meet on a common transmission medium such as wire or cable. However, this needs to be regulated to avoid collisions and chaos during transmission of data over a common channel or medium as this could lead to data loss or data destruction. CSMA/CD protocols provide provision for handling such collisions over early Ethernet/Local area networks which used shared bus network topology.
In today’s topic we will learn about the CSMA/CD media access control method, how it works, its features and limitations.
Understanding CSMA/CD
Carrier sense multiple access/collision detection or CSMA/CD is a CSMA protocol extension to detect collisions on shared bus topology networks. This creates a mechanism to regulate how communication will happen in a network which is using a shared transmission medium such as Ethernet cable / wire. The extension handles the situation wherein collision had already occurred due to two or more nodes having sent data packets simultaneously via shared transmission medium.
Components of CSMA/CD
Let’s look at individual components of CSMA/CD to understand its working in the next section.
- Carrier sense (CS) – detection ensures all network participants should check whether transmission medium is free or not before starting data transmission
- Multiple access (MA) – All network participants on common network share this medium
- Collision detection (CD) – is extension of original CSMA protocol to regulate how to proceed in the event collision is already occurred
How does CSMA/CD work?
The CSMA/CD process is based on group conversation. The medium is monitored by the station. As this continues until medium is occupied. Only when the transmission medium is free for a certain time station will send a data packet. The transmitter will continue to monitor the transmission medium to ensure if it detects any data collisions.
If no other network participant tries to send data via transmission medium by the end of transmission, and no collision occurs transmission is considered successful. In the scenario a collision is detected on the medium, the network participant immediately interrupts the transmission and instead sends an interference signal or JAM signal to all other participants to inform them of the collision.
The network participant waits for a random amount of time or back off before it tries to do transmission again. The back off is random so as to avoid collision. The transmission attempts are monitored and counted. If the maximum number of attempts (16 No) is reached the station reports an error and terminates transmission permanently.
For collision detection the station simultaneously checks if the transmitted signal is identical to the transmission medium. If that is not the scenario, another station performs transmission simultaneously and does the falsification of signals on the bus. Collisions like these happen regularly and usually part of normal operation in local area networks (LAN). Bigger the LAN size in terms of number of participants and length of transmission parts increase collision probability.
Pros and Cons of CSMA/CD
PROS
- CSMA/CS is widely used protocol in Ethernet networks and its simple to use and implement
- All devices have equal access for shared transmission medium to ensure fairness in transmission
- Efficient use of transmission medium to prevent unnecessary collisions and reduction in network congestion
CONS
- It does not provide any security features and vulnerable to attacks such as packet sniffing and packet spoofing
- Limitation in terms of scalability and not ideal for large networks having high number of network devices
- It can detect collisions but cannot prevent them from happening which could lead to data loss and data corruption so it is reactive but not a preventive mechanism
- It uses random back off algorithm which contributes to inefficient use of network bandwidth if device is experiencing collisions continuously
ABOUT THE AUTHOR
I am here to share my knowledge and experience in the field of networking with the goal being – “The more you share, the more you learn.”
I am a biotechnologist by qualification and a Network Enthusiast by interest. I developed interest in networking being in the company of a passionate Network Professional, my husband.
I am a strong believer of the fact that “learning is a constant process of discovering yourself.”
– Rashmi Bhardwaj (Author/Editor)
