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CSMA-CD vs CSMA-CA | Difference between CSMA-CD and CSMA-CA

This page compares CSMA-CD and CSMA-CA techniques and mentions functional difference between CSMA-CD and CSMA-CA. Here CSMA stands for Carrier Sense Multiple Access,CD stands for Collision Detection and CA stands for Collision Avoidance.

These are the protocol mechanism followed in mostly wireless systems such as WLAN (Wireless LAN). It helps in efficient use of wireless medium in effective time line. We have studied multiple access schemes such as TDMA and FDMA which allows use of common bandwidth by multiple users on time shared and frequency shared basis respectively.

CSMA-CD Carrier Sense Multiple Access Collision Detection

Figure:1 CSMA-CD

CSMA refers to Carrier Sense Multiple Access. ➨In this technique station listens before transmission.
➨If the channel is sensed as idle, it transmits the entire frame.
➨If the channel is sensed as busy, it defers the transmission.
This concept is similar to human analogy: "DO NOT Interrupt Others".
CSMA/CD can be in one of three states viz. contention, transmission, or idle.

CSMA-Carrier Sense Multiple Access
Figure:2 CSMA

The fundmental behind listening the channel before transmission is to avoid collision and hence increase the throughput. There are several types of CSMA.
• 1-persistent CSMA:
➨Sense the channel and do as follows:
•If busy, keep listening and transmit immediately when channel becomes idle.
•If idle, transmit a packet immediately.
➨If collision happens,
•Wait randome period of time and start all over again.
It is known as 1-persistent due to the fact that station transmits with probability of 1 wheneve it finds channel to be idle.
• Non-persistent CSMA:
➨Sense the channel and do as follows:
•If busy, wait for random period of time and sense or listen the channel again.
•If idle, transmit the packet immediately.
➨If collision happens,
•Wait for random amount of time and start all over again. • P-persistent CSMA: Here station transmits with probability of p.

The figure-2 implies that nonpersistent systems reduce chance of collisions and hence reduces efficiency, while persistent systems increase the chance for collisions. Let us understand difference between 1-persistent and non-persistent systems.

If B and C become ready in the middle of A's transmission,
• 1-Persistent: B and C collide • Non-Persistent: B and C probably do not collide

If only B becomes ready in the middle of A's transmission,
• 1-Persistent: B succeeds as soon as A ends • Non-Persistent: B may have to wait

Collision due to hidden terminal
Figure:3 Hidden terminal

The problem in CSMA-CD is when there is a hidden terminal. Collision if two or more nodes transmit at the same time. The collision detection does not work if there is hidden terminal as shown in the figure-3 and transmission is long. As packet is long the bandwidth is also wasted here.

The solution to this problem is CSMA/CA as described below.

CSMA-CA Carrier Sense Multiple Access Collision Avoidance

Figure:4 CSMA-CA

CSMA-CA stands for Carrier Sense Multiple Access with Collision Avoidance. The idea here is to avoid collision rather than allowing it to occur. This is done using RTS/CTS signaling mechanism and NAV (Network Allocation Vector). This is shown in the figure-4.

Following steps are followed in CSMA/CA protocol:
• Sender node transmits short RTS(Request to Send) packet. It indicates duration of transmission in RTS signal using NAV. NAV is broadcasted to all the stations.
• Receiver node replies with short CTS (Clear to Send) packet. It also indicates possible hidden nodes to the sender station/node.
• Hidden nodes will not transmit for specified duration (i.e. NAV).
In this way collision is avoided. Moreover as RTS and CTS is of short durations, collisions are less likely.

The NAV (Network Allocation Vector) is used for virtual carrier sensing. It is used at MAC layer and widely used in IEEE 802.11 (WLAN) and 802.16 (WiMAX) standards. It is a number which used as duration field. NAV specifies transmission time required for frame to be transmitted by the station. As this field NAV is broadcasted in CTS, it indicates other stations about how long they need to defer accessing the medium. This will limit unnecessary sensing of physical carrier and hence it saves power.
NAV is a counter which counts down to zero at uniform rate. When NAV is zero, it indicates that medium is idle and when NAV is nonzero, it indicates that medium is busy. For information on DIFS, SISF, RTS and CTS frames, refer WLAN frame spacing and WLAN RTS and CTS frame.

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