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CSMA vs MACA | Difference between CSMA and MACA

This page compares CSMA vs MACA techniques and mentions functional difference between CSMA and MACA. CSMA stands for Carrier Sense Multiple Access where as MACA stands for Multiple Access with Collision Avoidance.

Introduction: These protocols are used by MAC layer in wired and wireless systems. The basic function is to allow efficient access of transmission medium. The number of Medium Access Protocols have been developed till now such as ALOHA, CSMA, collision free protocols, WDMA etc. The popular among them are CSMA/CD and CSMA/CA. MACA stands for Multiple Access with Collision Avoidance designed to solves problem of hidden terminal found in CSMA.

CSMA | Carrier Sense Multiple Access

CSMA technique is used in wired and wireless systems. In this technique, nodes intending to transmit first senses the medium before transmission. If medium is idle, it transmits the frame. If medium is busy, it waits for random period of time and transmits the frame. There are two types of CSMA viz. CSMA/CD and CSMA/CA. CSMA/CD is used in wired networks (as per 802.3) and it is effective after a collision. CSMA/CA is used in wireless networks (as per 802.11 WLAN series) and it is effective before a collision.

What is CSMA-CD (Collision Detection) ?

CSMA/CD protocol can be in one of three states viz. contention, transmission, or idle. The fundamental behind listening the channel before transmission is to avoid collision and hence increase the throughput.

CSMA-CD
Figure:1 CSMA-CD

CSMA/CD is divided into following subtypes viz. 1-persistent, non-persistent and p-persistent. Refer difference between CSMA/CD versus CSMA/CA for more information.

Collision due to hidden terminal
Figure:2 Hidden terminal issue

The problem in CSMA-CD is when there is a hidden terminal. In wireless networking, hidden terminal is the node which is visible to AP (Access Point) or Router and not to those nodes which are in range of AP. Due to this, transmission from nodes collide as they are not aware of transmissions of each other. The collision detection does not work if there is hidden terminal as shown in the figure-3 and frame transmission is of long duration. CSMA/CA protocol solves this problem.

What is CSMA-CA (Collision Avoidance) ?

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-3.

CSMA-CA
Figure:3 CSMA-CA

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. For information on DIFS, SIFS, RTS and CTS frames, refer WLAN frame spacing and WLAN RTS and CTS frame.

Hidden Node in WLAN
Figure:4 Hidden Node in WLAN

Hidden node problem arises when two stations on opposite sides of an AP can hear activity from an AP, but not from each other, usually due to distance or an obstruction. In this case, 'A' and 'C' are called hidden terminals for each other. Both 'A' and 'C' can hear from 'B' but not from each other.

Exposed Node in WLAN
Figure:5 Exposed Node in WLAN

Exposed node problem is depicted in the figure above. In this case "B" is exposed to "C" terminal. Here though B wants to transmit to A, it does not do so as it receives "RTS signal" from C terminal and concludes that medium is busy.

What is MACA | Multiple Access with Collision Avoidance

It is used at MAC layer in wireless adhoc network. It can be used to solve collisions encountered due to hidden terminal and exposed terminal in wireless system. It is alternative to CSMA (Carrier Sense Multiple Access).

MACA protocol
Figure:6 MACA Protocol messages

It uses RTS/CTS mechanism similar to CSMA/CA. Nodes intending to transmit first send out "RTS signal". This is responded by receiving node using "CTS Signal". Once "CTS" is received it will start the "DATA" transmission to the node from where it has received "CTS signal". Once data transmission is completed it will be responded by "ACK" signal. The process is shown in the flow diagram above.

Difference between CSMA and MACA

Following points summarize major difference between CSMA and MACA.
➨RTS/CTS frames in CSMA/CA solves hidden terminal problem found in CSMA/CD. Exposed terminal problem can not be solved by RTS/CTS frames in CSMA/CA.
➨MACA is based on CSMA/CA. It adds "duration field" in RTS/CTS which intimates other terminals about their backoff time.
➨MACAW has been developed as improvement to MACA protocol. RTS/CTS/DATA/ACK messages are exchanged between terminals. Fast error recovery is implemented at link layer. It solves both hidden node and exposed node problems encountered in WLAN.


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