What is Carrier Sense Multiple Access CSMA and How It Works

What is Carrier Sense Multiple Access?

A network technique called Carrier Sense Multiple Access (CSMA) enables devices to effectively share a communication channel without interfering with one another. It functions at the computer network’s Medium Access Control (MAC) layer.

“Listen before talk,” also known as “Carrier Sense,” is the basic tenet of CSMA. Before attempting to transfer data, a node listens (or senses the carrier) to the shared medium to ascertain whether another transmission is already underway.

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How It Works

These steps are typically followed in the process:

• Sense the carrier: A node listens to the channel to see if there are any other transmissions going on before sending.

• If idle: The node continues to transmit if no carrier is found.

• If busy: The node waits for the current transmission to finish before attempting to send if it detects a carrier.

• Collision potential: Due to propagation delays, even with sensing, collisions may happen if two or more nodes begin transmitting simultaneously after detecting an idle channel.

Vulnerability and Collision Potential

CSMA lessens the likelihood of collisions, but it does not completely prevent them.

• Simultaneous Sensing: When two or more nodes simultaneously detect that the channel is idle and start transmitting, collisions may result.

• Vulnerable Time: The time it takes for a signal to move from one device to another, or the propagation time (Tp), determines the susceptible time, which is the brief window of time during which this danger is present.

Variants of CSMA Protocol

CSMA protocols are differentiated based on how they handle potential conflicts. The two main types are differentiated by whether they detect collisions after they happen or try to prevent them beforehand:

CSMA/CA is essential for wireless networks because devices often cannot detect a collision while they are transmitting due to their own strong signal drowning out incoming signals, or due to issues like the hidden node problem.

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CSMA Access Modes (Persistence)

Several access modes that specify how persistently a device tries to broadcast onto the shared medium are part of the fundamental CSMA protocol:

• 1-Persistent CSMA: The device communicates right away if the channel is idle. If it is busy, it constantly scans the channel until it is idle, at which point it transmits unconditionally (probability=1). This transmission algorithm is regarded as aggressive.

• Non-Persistent CSMA: The node transmits right away if the channel is empty. Instead of continuously monitoring the channel status if it is busy, the station waits for an unpredictable (random) period of time before checking it again. This causes a longer initial delay but lowers the likelihood of a collision.

• P-Persistent CSMA: This strategy combines the two earlier methods. The node sends with probability $p$ if the channel is idle. It waits for a random amount of time and attempts again if it fails to transmit (with probability $1-p$). Wi-Fi and other CSMA/CA systems use P-persistent CSMA.

• O-Persistent CSMA: Every node is given a transmission command by a supervisory node. Nodes wait for their designated time slot before sending when the channel is idle.

CSMA variants

Two specialized protocols that control device access to a shared network medium at the Medium Access Control (MAC) layer are CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) and CSMA/CD (Carrier Sense Multiple Access with Collision Detection). These protocols are derived from the basic CSMA approach.

When it comes to handling data conflicts, the main distinction between them is that CSMA/CA aims to stop collisions before they happen, whereas CSMA/CD handles collisions after they happen and recovers.

Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA)

A network protocol called CSMA/CA is mostly used in wireless networks like Wi-Fi (IEEE 802.11 standard). Because it is difficult or impossible for a transmitting device to detect collisions in a wireless environment (the strong outgoing signal overwhelms any attempt to hear interference), its design is centered on avoiding collisions.

CSMA/CA Algorithm and Working Principle

In order to prevent data packet collisions, the CSMA/CA mechanism combines techniques to guarantee that only one device transmits at a time with a “listen before talk” strategy.

• Carrier Sense (Listen): A device first listens to the shared channel to see if it is busy (being used by another device) before sending a frame.

• Wait for Interframe Space (IFS): The station must wait for a brief, standardized amount of time known as the Interframe Space (IFS) if the channel is detected as idle. This guarantees that any previous gearbox has finished.

• Random Backoff: The basic collision avoidance method kicks in if the channel remains idle following the IFS. Within a Contention Window (also known as a backoff period), the device chooses a random number of time slots and delays transmission for that random duration. The likelihood that two devices that detected the channel idle at the same time will transmit at precisely the same time is reduced by this random waiting interval. Wi-Fi and other CSMA/CA systems use the p-persistent CSMA access mode.

• Transmission: The station sends its data frame when the backoff timer goes off.

• Acknowledgement (ACK): The station waits for an Acknowledgement (ACK) from the receiver after sending the frame and setting a timer. According to the 802.11 standard, every frame must have an ACK.

• Retransmission: The entire process begins, frequently with a new, possibly longer, random backoff time (Binary Exponential Backoff), assuming a collision happened (data was lost) if the ACK is not received before the timer expires.

RTS/CTS Handshake

The Request to Send (RTS)/Clear to Send (CTS) handshake is an optional addition to CSMA/CA that helps with the hidden node issue.

• RTS: A brief Request to Send packet is sent from the sender device to the recipient, which is frequently an access point.

• CTS: A Clear to Send packet is sent back by the recipient.

• Reservation: This exchange notifies every node that is within the sender’s or recipient’s range to set their Network Allocation Vector (NAV) timer and stop sending during the impending data transport.

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Advantages of CSMA/CA

• Collision Reduction: By determining whether a network channel is available before transmitting data, CSMA/CA lowers the likelihood of collisions. In wireless networks, where communication might be disrupted by collisions, this is helpful.

• Better for Wireless Networks: Because it maintains a smoother connection and prevents collisions, it is appropriate for wireless systems.

• Efficient Channel Use: Effective channel utilization reduces interruptions and allows for easier communication only when protocol verifies that the channel is clear.

• Energy Efficient: In order to prevent needless energy waste, the protocol waits till the channel is clear.

Disadvantages of CSMA/CA

• Lower Throughput: The network may become slower as a result of extra overhead, such as waiting for acknowledgment, which lowers overall speed.

• Delay and Latency: CSMA/CA continuously monitors if the channel is clear, which causes delays, particularly in networks with heavy traffic. Because of this, it is less useful for real-time applications that require fast reaction times.

• Complex Implementation: Because of its acknowledgement and collision avoidance techniques, it necessitates a more complex setup than other simpler protocols.

• Inefficiency in High Traffic: When numerous devices utilize the same network, CSMA/CA wait times might rise, making the network slower and less effective.

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

In conventional, half-duplex wired networks, like Ethernet (IEEE 802.3 standard), the contention technique is CSMA/CD. After a collision, it works. Wired devices are able to identify a collision in real time because they can readily detect changes in voltage or signal power on the cable during transmission. By reducing the amount of time lost on a jumbled gearbox, CSMA/CD enhances CSMA performance.

CSMA/CD Algorithm and Working Principle

The 1-persistent access mode is frequently used by the aggressive transmission method known as the CSMA/CD mechanism.

• Carrier Sense (Listen): A node detects the connected medium by carrier sense (listen). It transmits right away if the channel is idle.

• Monitor While Transmitting: As the station transmits its data, it keeps listening to the channel.

• Collision Detected: A collision has occurred if the station detects interference (a jumbled signal or an unanticipated rise in signal energy) brought on by another device transmitting concurrently.

• Jam Signal: To ensure that all other stations promptly detect the collision, the transmitting station instantly pauses providing the data frame and broadcasts a brief, reinforcing signal known as a jam signal.

• Backoff and Retry: After waiting for an arbitrary amount of time, the station tries the process once more (retransmitting the data). Immediate, recurrent collisions are less likely with this delay.

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Advantages of CSMA/CD

• Efficient for Wired Networks: It is optimal and efficient for wired networks, such as Ethernet, because it is simpler to identify collisions and respond promptly.

• Reduces Collisions: As soon as a collision is detected, data transmission is promptly stopped, saving time and maintaining network efficiency.

• Balances Network Traffic: When a collision is detected, CSMA/CD causes each device to wait for a random period of time. This balances network traffic and aids in avoiding recurring collisions.

• Simple to Implement: It is simpler to deploy in wired environments, particularly in networks with controllable traffic and less collision probability.

Disadvantages of CSMA/CD

• Not Suitable for Wireless Networks: Since CSMA/CD necessitates the ability to reliably detect collisions, its implementation in wireless networks will be challenging because these networks interfere with one another, rendering collision detection useless.

• Adds Latency: In certain applications where a quick response time is required, CSMA/CD’s waiting period for collision detection and subsequent retransmission can pose issues.

• Lower Efficiency in Busy Networks: After detecting a collision, CSMA/CD pauses for specific periods of time before retransmitting data. Therefore, frequent collisions in a network with significant traffic might cause delays and decreased efficiency.

• Less Common in Today’s Era: With the advent of new technology that helps prevent collisions, including network switches and full duplex Ethernet, it is becoming less prevalent every day in the modern world.

Key Differences Between CSMA/CA and CSMA/CD

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