Carrier Sense Multiple Access and Collision Detection (CSMA/CD)

This method was developed to decrease the chances of collisions when two or more stations start sending their signals. Its happen in datalink layer . Carrier Sense multiple access requires that each station first check the state of the medium before sending.

Carrier Sense Multiple Access (CSMA)

The fundamental idea behind CSMA is to minimize data collisions. When a device connected to a network wants to transmit data, it first checks the channel to see if it’s currently in use by other devices. If the channel is idle (no ongoing transmission), the device can start sending its data. However, if the channel is busy, the device will wait for a random period and then check the channel again before attempting to transmit.

Characteristics of CSMA/CA

  • Carrier Sense: The device listens to the channel before transmitting, to ensure that it is not currently in use by another device.
  • Multiple Access: Multiple devices share the same channel and can transmit simultaneously.
  • Collision Avoidance: If two or more devices attempt to transmit at the same time, a collision occurs. CSMA/CA uses random backoff time intervals to avoid collisions.
  • Acknowledgment (ACK): After successful transmission, the receiving device sends an ACK to confirm receipt.
  • Fairness: The protocol ensures that all devices have equal access to the channel and no single device monopolizes it.
  • Binary Exponential Backoff: If a collision occurs, the device waits for a random period of time before attempting to retransmit. The backoff time increases exponentially with each retransmission attempt.
  • Interframe Spacing: The protocol requires a minimum amount of time between transmissions to allow the channel to be clear and reduce the likelihood of collisions.
  • RTS/CTS Handshake: In some implementations, a Request-To-Send (RTS) and Clear-To-Send (CTS) handshake is used to reserve the channel before transmission. This reduces the chance of collisions and increases efficiency.
  • Wireless Network Quality: The performance of CSMA/CA is greatly influenced by the quality of the wireless network, such as the strength of the signal, interference, and network congestion.
  • Adaptive Behavior: CSMA/CA can dynamically adjust its behavior in response to changes in network conditions, ensuring the efficient use of the channel and avoiding congestion.

Advantages of CSMA

  • Efficient Use of Bandwidth: CSMA allows multiple devices to share the same transmission medium, enabling efficient utilization of available bandwidth. Devices wait for the channel to be idle before transmitting data, reducing collisions and optimizing data throughput.
  • Simplicity and Cost-Effectiveness: Implementing CSMA doesn’t require complex infrastructure or additional hardware. It’s a relatively straightforward protocol that helps manage access to the network, making it cost-effective for deploying in various network environments.
  • Fairness in Access: CSMA provides a fair chance for all devices to access the medium. Since devices listen to the channel before transmitting, they wait for an idle period, minimizing the chances of one device dominating the channel continuously.
  • Dynamic Allocation: CSMA allows for dynamic allocation of the network channel. It doesn’t assign fixed time slots or priorities to devices, enabling flexibility in sharing the channel based on varying data transmission needs.
  • Collision Avoidance: While collisions may still occur in CSMA networks, collision avoidance mechanisms help reduce their frequency. CSMA/CD (Collision Detection) or CSMA/CA (Collision Avoidance) protocols manage collisions effectively, improving overall network efficiency.
  • Scalability: CSMA-based protocols are scalable and can accommodate a growing number of devices within a network. As the number of devices increases, CSMA manages access to the network medium, ensuring fair and efficient transmission for all connected devices.
  • Adaptability to Different Network Types: CSMA is versatile and adaptable, making it suitable for both wired and wireless networks. Variants like CSMA/CD are used in Ethernet networks, while CSMA/CA is employed in wireless technologies like Wi-Fi.

Overall, CSMA offers an effective means of managing network access, promoting fairness, and optimizing data transmission, making it a widely used protocol in various networking environments.

Collision Detection in CSMA/CD

CSMA/CD (Carrier Sense Multiple Access/ Collision Detection) is a media access control method that was widely used in Early Ethernet technology/LANs when there used to be shared Bus Topology and each node ( Computers) were connected By Coaxial Cables. Now a Days Ethernet is Full Duplex and Topology is either Star (connected via Switch or Router) or Point to Point ( Direct Connection). Hence CSMA/CD is not used but they are still supported though.

CSMA/CD is one such technique where different stations that follow this protocol agree on some terms and collision detection measures for effective transmission. This protocol decides which station will transmit when so that data reaches the destination without corruption.

How CSMA/CD works? 

  • Step 1: Check if the sender is ready for transmitting data packets.
  • Step 2: Check if the transmission link is idle.
    Sender has to keep on checking if the transmission link/medium is idle. For this, it continuously senses transmissions from other nodes. Sender sends dummy data on the link. If it does not receive any collision signal, this means the link is idle at the moment. If it senses that the carrier is free and there are no collisions, it sends the data. Otherwise, it refrains from sending data.
  • Step 3: Transmit the data & check for collisions.
    Sender transmits its data on the link. CSMA/CD does not use an ‘acknowledgment’ system. It checks for successful and unsuccessful transmissions through collision signals. During transmission, if a collision signal is received by the node, transmission is stopped. The station then transmits a jam signal onto the link and waits for random time intervals before it resends the frame. After some random time, it again attempts to transfer the data and repeats the above process.
  • Step 4: If no collision was detected in propagation, the sender completes its frame transmission and resets the counters.

Disadvantages of  CSMA/CD

  • Limited scalability: CSMA/CD has limitations in terms of scalability, and it may not be suitable for large networks with a high number of devices.
    Vulnerability to collisions: While CSMA/CD can detect collisions, it cannot prevent them from occurring. Collisions can lead to data corruption, retransmission delays, and reduced network performance.
    Inefficient use of bandwidth: CSMA/CD uses a random backoff algorithm that can result in inefficient use of network bandwidth if a device continually experiences collisions.
    Susceptibility to security attacks: CSMA/CD does not provide any security features, and the protocol is vulnerable to security attacks such as packet sniffing and spoofing.


Leave a Reply