Short  Review on CSMA/CA

CSMA/CA:

Carrier-sense multiple access with collision avoidance (CSMA/CA) in computer networking, is a network multiple access method in which carrier sensing is used, but nodes attempt to avoid collisions by beginning transmission only after the channel is sensed to be "idle". When they do transmit, nodes transmit their packet data in its entirety.

It is particularly important for wireless networks, where the collision detection of the alternative CSMA/CD is not possible due to wireless transmitters desensing their receivers during packet transmission.

The most important rule, which you’ll know from communication situations when there are several participants (e.g. a conversation with the family at the dinner table), is: only one person may send their information at once. If everyone speaks at the same time, it’s confusing and no-one can understand anything. The posts can also overlap in networks, in this case in the form of data packets. This is called a collision, when the data packets meet and alienate their contents.

CSMA/CA tries to reduce the frequency of these collisions and provide a plan at the same time on how to proceed if a collision does occur. The protocol is also important because the transmissions in the wireless networks cannot run in the same order (due to the technology used) as they would have done with a cable. In a decentralized network, it is necessary for all participants to follow a set of rules and organize the communication among themselves.

 

The hidden station problem:

The technical differences between wired and wireless networks also lead to the so-called hidden station problem. Stations in a wireless network have a limited range, so that it could be that participants in a network don’t even recognize each other.

It is conceivable, but not improbable, for two stations not to recognize one another, but to want to simultaneously reach a station that is located between them. Transmissions can overlap at the receiving node, meaning data gets lost. Both transmitters do not detect the collision and don’t start a new delivery attempt. CSMA/CA alone cannot solve this problem, which is why an optional extension was created: RTS/CTS (“Request to Send” and “Clear to Send”).

 

 

 

CSMA/CA vs. CSMA/CD:

CSMA/CA adapts the collision handling method (CSMA/CD) used in half-duplex Ethernet networks to face the challenges that wireless networks pose. CSMA/CD tries less to avoid collisions. Instead, the protocol understands collisions as a matter of fact and establishes a mechanism for network participants instructing them how to proceed in the event of a collision so that another one doesn’t happen straightaway on the next attempt. Backoff – a random amount of time that stations must wait after a failed transmission so that both participants don’t start sending information at the same time – accomplishes this.

 

Wireless networks cannot be monitored as securely as wired networks. Collisions may be caused by a second transmitter out of the range of the first, and neither can recognize the other’s attempts to send information. It’s necessary, then, to reduce the probability of collisions. CSMA/CA brings the backoff forward in the process and uses it before the first delivery process, making it less likely for participants to simultaneously start a transmission and cause a collision.

 

How CSMA/CA works:

The basic idea behind CSMA/CA is the “Listen Before Talk” (LBT) principle. This means that the line has to be checked to see if it’s free (“idle”) before the station can start a transmission. But this is just the first step. Further functions within the procedure ensure that collisions can be avoided to a large extent.

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.

 

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.

 

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.

 

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.

Request to send and clear to send (RTS/CTS):

The frames “Request to Send” (RTS) and “Clear to Send” (CTS) are part of the optional extension CSMA/CA RTS/CTS. This procedure is upstream of the actual data transmission. If a participant determines that the transmission medium is free, the device first sends an RTS frame to the participant that is to receive the data. With this, the output computer makes it clear that it wants to start a transmission and will occupy the transmission medium for a certain time.

 

The receiver, in turn, sends a CTS frame to the original sender. As with the RTS frame, all other participants in the range are informed that the transmission is currently occupied and the transmitter is enabled for transmission. Only then does the original device start transmitting the data. Now it is not possible for the participants in a wireless network to detect collisions or other interference during transmission. For this reason, the receiving station needs to send an acknowledgement (ACK) when the data packet has arrived correctly.

 

If the ACK frame doesn’t appear, the sender of the data assumes that a complication has occurred and resends the data packet. The station has a preferential right to use the medium and doesn’t have to wait again for the channel to be free. The three frame types each consist of several fields.

By extending the CSMA/CA protocol with RTS/CTS, it’s possible to reduce collisions on the initial RTS frame. It is still likely that two participants will send a request to send to the same station at the same time. In cases like this, however, the receiver does not send a CTS frame because the RTS frames have not arrived correctly. So, RTS/CTS can solve the hidden station problem: even if the two transmitters don’t recognize each other due to the limited range, only the RTS frames are at risk and not the actual data. CSMA/CA then takes effect and the transmission can take place in an orderly fashion.

 

The RTS/CTS exchange solves the hidden station principle, but the extension causes another difficulty: the exposed station problem. The situation is initially the same as with the hidden station problem: one station is in the middle of two other stations so that they cannot reach each other. One of the two devices now wants to send data to the station in the middle. All accessible nodes receive the CTS frame that stops them from broadcasting. This avoids the hidden station problem, but creates a new one.

A third station is now prevented from transmitting, even if a completely different, fourth station would have been the destination of the transmission. This transmission would not cause a collision, but it still has to be prevented, which leads to the entire network slowing down.