Short Review on CSMA/CA

 

CSMA is a basic method that controls the communication of multiple participants on a shared and decentralized transmission medium. However, this is now available in three different variants, which depend on the transmission medium. While CSMA/CA is mainly used in wireless networks, CSMA/CD was developed for Ethernet, and CSMA/CR is used in controller area networks (CAN), which are mainly used in cars and machines.

To understand exactly what is behind Carrier Sense Multiple Access with Collision Avoidance, it makes sense to look at the components individually:

1. Carrier sense (CA): The initial idea is that participants may only send data over the network if the transmission medium is free. The carrier status detection checks the channel any time, and data is not sent until it’s available.


2. Multiple access (MA): Several stations share a transmission medium. It is crucial for functioning communication that all of them adhere to a binding protocol.


3. Collision avoidance (CA): A complex schedule tries to ensure that two or more participants do not start a transmission at the same time to avoid collisions. If overlapping does occur, this will be detected and the transmission will be tried again.

 

Distributed coordination function (DCF):

Within CSMA/CA, the distributed coordination function (DCF) controls the time a station waits before initiating transmission in a free medium. DCF also assigns certain time slots to network participants for further actions, creating a binding time structure. This procedure is the focus of collision avoidance: a complex time structure that makes it possible to avoid collisions. DCF takes various intervals into account when creating the time structure.

1. DCF interframe space (DIFS): In the first step, participants must monitor the network for the duration of the DIFS to determine whether it’s currently free. For CSMA/CA, this means that no other station within range is sending out a transmission at the same time. The DIFS results from the SIFS almost double the slot time, which is between 28 and 50 µs long.


2. Contention window: If participants determine that the channel is free, they wait a random amount of time before they start sending. This duration corresponds to the contention window. This time window doubles with each collision and corresponds to the binary exponential backoff (BEB) that is familiar from CSMA/CD.


3. Short interframe space (SIFS): After sending the data packet, the recipient node sends a notification – if the RTS/CTS procedure is also utilized. However, this station also waits for a fixed time before sending. SIFS is the time it takes to process a data package. The duration depends on the IEEE-802.11 standard and is between 10 µs and 16 µs.

 

The advantages and disadvantages of CSMA/CA

CSMA/CA solves some problems that occur in wireless networks and cannot be solved by CSMA/CD. However, the process is not without its disadvantages: on the one hand, certain problems cannot be solved completely and, on the other hand, CSMA/CA brings new difficulties along with it.

 

Advantages	Disadvantages
Helps prevent data collisions	Longer waiting times
Thanks to feedback, no data is unnoticeably lost	Causes additional traffic
Avoids unnecessary data traffic with the RTS/CTS extension	Solves the hidden station problem only by using RTS/CTS extension
	Creates the exposed station problem through using RTS/CTS