Carrier-sense multiple access (CSMA) is a

medium access control In IEEE 802 LAN/MAN standards, the medium access control (MAC, also called media access control) sublayer is the layer that controls the hardware responsible for interaction with the wired, optical or wireless transmission medium. The MAC sublay ...

(MAC) protocol in which a node verifies the absence of other

traffic Traffic comprises pedestrians, vehicles, ridden or herded animals, trains, and other conveyances that use public ways (roads) for travel and transportation. Traffic laws govern and regulate traffic, while rules of the road include traffic ...

before

transmitting In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna. The transmitter itself generates a radio frequency alternating current, which is applied to th ...

on a shared

transmission medium A transmission medium is a system or substance that can mediate the propagation of signals for the purposes of telecommunication. Signals are typically imposed on a wave of some kind suitable for the chosen medium. For example, data can modulat ...

, such as an electrical bus or a band of the

electromagnetic spectrum The electromagnetic spectrum is the range of frequencies (the spectrum) of electromagnetic radiation and their respective wavelengths and photon energies. The electromagnetic spectrum covers electromagnetic waves with frequencies ranging from ...

. Under CSMA, a

transmitter In electronics and telecommunications, a radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna (radio), antenna. The transmitter itself generates a radio frequency alternating current, which i ...

uses a carrier-sense mechanism to determine whether another transmission is in progress before initiating a transmission. That is, it tries to detect the presence of a

carrier signal In telecommunications, a carrier wave, carrier signal, or just carrier, is a waveform (usually sinusoidal) that is modulated (modified) with an information-bearing signal for the purpose of conveying information. This carrier wave usually has ...

from another node before attempting to transmit. If a carrier is sensed, the node waits for the transmission in progress to end before initiating its own transmission. Using CSMA, multiple nodes may, in turn, send and receive on the same medium. Transmissions by one node are generally received by all other nodes connected to the medium. Variations on basic CSMA include addition of

collision In physics, a collision is any event in which two or more bodies exert forces on each other in a relatively short time. Although the most common use of the word ''collision'' refers to incidents in which two or more objects collide with great fo ...

-avoidance (

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 ...

), collision-detection (

CSMA/CD Carrier-sense multiple access with collision detection (CSMA/CD) is a medium access control (MAC) method used most notably in early Ethernet technology for local area networking. It uses carrier-sensing to defer transmissions until no other statio ...

) and collision-resolution techniques.

Access modes

Variations of CSMA use different algorithms to determine when to initiate transmission onto the shared medium. A key distinguishing feature of these algorithms is how aggressive or persistent they are in initiating transmission. A more aggressive algorithm may begin transmission more quickly and utilize a greater percentage of the available bandwidth of the medium. This is typically at the expense of an increased likelihood of collision with other transmitters. ; 1-persistent : 1-persistent CSMA is an aggressive transmission algorithm. When the transmitting node is ready to transmit, it senses the transmission medium for idle or busy. If idle, then it transmits immediately. If busy, then it senses the transmission medium continuously until it becomes idle, then transmits the message (a

frame A frame is often a structural system that supports other components of a physical construction and/or steel frame that limits the construction's extent. Frame and FRAME may also refer to: Physical objects In building construction *Framing (con ...

) unconditionally (i.e. with probability=1). In case of a

, the sender waits for a

random In common usage, randomness is the apparent or actual lack of pattern or predictability in events. A random sequence of events, symbols or steps often has no :wikt:order, order and does not follow an intelligible pattern or combination. Ind ...

period of time and attempts the same procedure again. 1-persistent CSMA is used in CSMA/CD systems including

Ethernet Ethernet () is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 198 ...

. ; Non-persistent : Non persistent CSMA is a non-aggressive transmission algorithm. When the transmitting node is ready to transmit data, it senses the transmission medium for idle or busy. If idle, then it transmits immediately. If busy, it jumps to the final random waiting step of 1-persistent CSMA directly before repeating the whole logic cycle again: it does not persist in checking the busy channel trying to get its transmission through, hence the name. This approach reduces the chance of collision and results in overall higher medium throughput but with a penalty of longer initial delay compared to 1–persistent. ; P-persistent : This approach lies between the 1-persistent and non-persistent CSMA access modes. When the transmitting node is ready to transmit data, it senses the transmission medium for idle or busy. If idle, then it transmits immediately. If busy, then it senses the transmission medium continuously until it becomes idle, then transmits with probability ''p''. If the node does not transmit (the probability of this event is ''1-p''), it waits until the next available

time slot Broadcast programming is the practice of organizing or ordering (scheduling) of broadcast media shows, typically radio and television, in a daily, weekly, monthly, quarterly or season-long schedule. Modern broadcasters use broadcast automation ...

. If the transmission medium is not busy, it transmits again with the same probability ''p''. This probabilistic hold-off repeats until the frame is finally transmitted or when the medium is found to become busy again (i.e. some other node has already started transmitting). In the latter case the node repeats the whole logic cycle (which started with sensing the transmission medium for idle or busy) again. p-persistent CSMA is used in CSMA/CA systems including

Wi-Fi Wi-Fi () is a family of wireless network protocols, based on the IEEE 802.11 family of standards, which are commonly used for local area networking of devices and Internet access, allowing nearby digital devices to exchange data by radio wave ...

and other packet radio systems. Note that ''p = 0''-persistent CSMA is different from non-persistent CSMA. Both can only transmit at the start of the procedure (if the channel is idle), but their behaviour on a busy channel differs: non-persistent CSMA doesn't attempt to sense the channel and restarts its logical cycle, whilst ''p = 0'' necessarily gets stuck in an infinite loop of waiting (since it has zero probability of transmission even if the channel goes back to being idle). ; O-persistent : Each node is assigned a transmission order by a supervisory node. When the transmission medium goes idle, nodes wait for their time slot in accordance with their assigned transmission order. The node assigned to transmit first transmits immediately. The node assigned to transmit second waits one time slot (but by that time the first node has already started transmitting). Nodes monitor the medium for transmissions from other nodes and update their assigned order with each detected transmission (i.e. they move one position closer to the front of the queue). O-persistent CSMA is used by

CobraNet CobraNet is a combination of software, hardware, and network protocols designed to deliver uncompressed, multi-channel, low-latency digital audio over a standard Ethernet network. Developed in the 1990s, CobraNet is widely regarded as the firs ...

LonWorks LonWorks or Local Operating Network is an open standard (ISO/IEC 14908) for networking platforms specifically created to address the needs of control applications. The platform is built on a protocol created by Echelon Corporation for networking ...

and the

controller area network A Controller Area Network (CAN bus) is a robust vehicle bus standard designed to allow microcontrollers and devices to communicate with each other's applications without a host computer. It is a message-based protocol, designed originally for mu ...

Protocol modifications

When broadcasting over vehicular ad hoc networks, the original 1-persistence and p-persistence strategies often cause the

broadcast storm A broadcast storm or broadcast radiation is the accumulation of broadcast and multicast traffic on a computer network. Extreme amounts of broadcast traffic constitute a "broadcast storm". It can consume sufficient network resources so as to render ...

problem. To improve performance, engineers developed three modified techniques: weighted p-persistence, slotted 1-persistence, and slotted p-persistence. ;

Carrier-sense multiple access with collision detection Carrier-sense multiple access with collision detection (CSMA/CD) is a medium access control (MAC) method used most notably in early Ethernet technology for local area networking. It uses carrier-sensing to defer transmissions until no other statio ...

:CSMA/CD is used to improve CSMA performance by terminating transmission as soon as a collision is detected, thus shortening the time required before a retry can be attempted. CSMA/CD is used by

. ;

Carrier-sense multiple access with collision avoidance 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 channe ...

:In CSMA/CA collision avoidance is used to improve the performance of CSMA. If the transmission medium is sensed busy before transmission, then the transmission is deferred for a random interval. This random interval reduces the likelihood that two or more nodes waiting to transmit will simultaneously begin transmission upon termination of the detected transmission, thus reducing the incidence of collision. CSMA/CA is used by

. ;CSMA with Collision Resolution :CSMA/CR uses priorities in the frame header to avoid collisions. It is used in the

Controller Area Network A Controller Area Network (CAN bus) is a robust vehicle bus standard designed to allow microcontrollers and devices to communicate with each other's applications without a host computer. It is a message-based protocol, designed originally for mu ...

. ;Virtual time CSMA :VTCSMA is designed to avoid collision generated by nodes transmitting signals simultaneously, used mostly in hard

real-time systems Real-time computing (RTC) is the computer science term for Computer hardware, hardware and computer software, software systems subject to a "real-time constraint", for example from Event (synchronization primitive), event to Event (computing), ...

. It uses two clocks to prioritize messages based on their deadline.

References

;General *

Andrew S. Tanenbaum Andrew Stuart Tanenbaum (born March 16, 1944), sometimes referred to by the handle ast, is an American-Dutch computer scientist and professor emeritus of computer science at the Vrije Universiteit Amsterdam in the Netherlands. He is the author ...

, ''Computer Networks''. Prentice Hall, Upper Saddle River, NJ (2003). 892 pp. {{Channel access methods Media access control

Access modes

Protocol modifications

See also

References