Carrier sense multiple access with collision avoidance (CSMA/CA), in computer networking, is a wireless network multiple access method in which:

• a carrier sensing scheme is used.
• a node wishing to transmit data has to first listen to the channel for a predetermined amount of time to determine whether or not another node is transmitting on the channel within the wireless range. If the channel is sensed "idle," then the node is permitted to begin the transmission process. If the channel is sensed as "busy," the node defers its transmission for a random period of time. Once the transmission process begins, it is still possible for the actual transmission of application data to not occur.^[1][2]

CSMA/CA is a modification of carrier sense multiple access.

Collision avoidance is used to improve CSMA performance by not allowing wireless transmission of a node if another node is transmitting, thus reducing the probability of collision due to the use of a random truncated binary exponential backoff time.

Optionally, but almost always implemented, an IEEE 802.11 RTS/CTS exchange can be required to better handle situations such as the hidden node problem in wireless networking.^[3]

CSMA/CA is a layer 2 access method, not a protocol of the OSI model.^[4]

  Simplified Algorithm of CSMA/CA

  Details

Collision avoidance is used to improve the performance of the CSMA method by attempting to divide the wireless channel somewhat equally among all transmitting nodes within the collision domain. CSMA/CA differs from CSMA/CD due to the nature of the medium, the radio frequency spectrum. Collisions cannot be detected while occurring at the sending node, thus it is vital for CSMA/CA or another access method to be implemented. CSMA/CA is used in 802.11 based wireless LANs and other wired and wireless communication systems. One of the problems of wireless data communications is that it is not possible to listen while sending, therefore collision detection is not possible. Another reason is the hidden terminal problem, whereby a node A, in range of the receiver R, is not in range of the sender S, and therefore cannot know that S is transmitting to R.^[5]

  IEEE 802.11 RTS/CTS Exchange

CSMA/CA can optionally be supplemented by the exchange of a Request to Send (RTS) packet sent by the sender S, and a Clear to Send (CTS) packet sent by the intended receiver R. Thus alerting all nodes within range of the sender, receiver or both, to not transmit for the duration of the main transmission. This is known as the IEEE 802.11 RTS/CTS exchange. Implementation of RTS/CTS helps to solve the hidden node problem that is often found in wireless networking.^[3]

  Performance

CSMA/CA performance is based largely upon the modulation technique used to transmit the data between nodes. Studies show that under ideal propagation conditions (simulations), Direct Sequence Spread Spectrum (DSSS) provides the highest throughput for all nodes on a network when used in conjunction with CSMA/CA and the IEEE 802.11 RTS/CTS exchange under light network load conditions. Frequency Hopping Spread Spectrum (FHSS) follows distantly behind DSSS with regard to throughput with a greater throughput once network load becomes substantially heavy. However, the throughput is generally the same under real world conditions due to radio propagation factors.^[4]

  Usage

• GNET - an early proprietary LAN protocol
• Apple's LocalTalk implemented CSMA/CA on an electrical bus using a three-byte jamming signal.
• 802.11 RTS/CTS implements virtual carrier sensing using short request to send and clear to send messages for WLANs (802.11 mainly relies on physical carrier sensing though).
• IEEE 802.15.4 (Wireless PAN) uses CSMA/CA
• NCR WaveLAN - an early proprietary wireless network protocol
• HomePNA
• Bus networks
• The ITU-T G.hn standard, which provides a way to create a high-speed (up to 1 Gigabit/s) local area network using existing home wiring (power lines, phone lines and coaxial cables), uses CSMA/CA as a channel access method for flows that don't require guaranteed quality of service, specifically the CSMA/CARP variant.

  See also

• Carrier sense multiple access
• Carrier sense multiple access with collision detection
• CSMA/CARP
• IEEE 802.11 RTS/CTS
• Truncated binary exponential backoff
• Network Allocation Vector

  References

  External links

• CSMA/CA RTS/CTS Simulation