Carrier Sense Multiple Access
Encyclopedia
Carrier Sense Multiple Access (CSMA) is a probabilistic Media Access Control
(MAC) protocol in which a node verifies the absence of other traffic
before transmitting
on a shared transmission medium
, such as an electrical bus, or a band of the electromagnetic spectrum
.
"Carrier Sense" describes the fact that a transmitter
uses feedback
from a receiver that detects a carrier wave
before trying to send. That is, it tries to detect the presence of an encoded signal
from another station before attempting to transmit. If a carrier is sensed, the station waits for the transmission in progress to finish before initiating its own transmission.
"Multiple Access" describes the fact that multiple stations send and receive on the medium. Transmissions by one node are generally received by all other stations using the medium.
(CSMA/CD) is a modification of CSMA. CSMA/CD is used to improve CSMA performance by terminating transmission as soon as a collision is detected, and reducing the probability of a second collision on retry.
Carrier sense multiple access with collision avoidance
(CSMA/CA) is a modification of CSMA. Collision avoidance is used to improve the performance of CSMA by attempting to be less "greedy" on the channel. If the channel is sensed busy before transmission then the transmission is deferred for a "random" interval. This reduces the probability of collisions on the channel.
period of time and attempts to transmit again. 1-persistent CSMA is used in CSMA/CD systems including Ethernet
.
P-persistent : When the sender is ready to send data, it checks continually if the medium is busy. If the medium becomes idle, the sender transmits a frame with a probability
p. If the station chooses not to transmit (the probability of this event is 1-p), the sender waits until the next available time slot
and transmits again with the same probability p. This process repeats until the frame is sent or some other sender stops transmitting. In the latter case the sender monitors the channel, and when idle, transmits with a probability p, and so on. p-persistent CSMA is used in CSMA/CA systems including WiFi
and other packet radio
systems.
O-persistent : Each station is assigned a transmission order by a supervisor station. When medium goes idle, stations wait for their time slot in accordance with their assigned transmission order. The station assigned to transmit first transmits immediately. The station assigned to transmit second waits one time slot (but by that time the first station has already started transmitting). Stations monitor the medium for transmissions from other stations 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
, LonWorks
and the controller area network
.
Media Access Control
The media access control data communication protocol sub-layer, also known as the medium access control, is a sublayer of the data link layer specified in the seven-layer OSI model , and in the four-layer TCP/IP model...
(MAC) protocol in which a node verifies the absence of other traffic
Network traffic
Network traffic is data in a network. In computer networks, the data is encapsulated in packets.*Network traffic control*Network traffic measurement*Network traffic simulation...
before transmitting
Transmission (telecommunications)
Transmission, in telecommunications, is the process of sending, propagating and receiving an analogue or digital information signal over a physical point-to-point or point-to-multipoint transmission medium, either wired, optical fiber or wireless...
on a shared transmission medium
Transmission medium
A transmission medium is a material substance that can propagate energy waves...
, such as an electrical bus, or a band of the electromagnetic spectrum
Electromagnetic spectrum
The electromagnetic spectrum is the range of all possible frequencies of electromagnetic radiation. The "electromagnetic spectrum" of an object is the characteristic distribution of electromagnetic radiation emitted or absorbed by that particular object....
.
"Carrier Sense" describes the fact that a transmitter
Transmitter
In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...
uses feedback
Feedback
Feedback describes the situation when output from an event or phenomenon in the past will influence an occurrence or occurrences of the same Feedback describes the situation when output from (or information about the result of) an event or phenomenon in the past will influence an occurrence or...
from a receiver that detects a carrier wave
Carrier wave
In telecommunications, a carrier wave or carrier is a waveform that is modulated with an input signal for the purpose of conveying information. This carrier wave is usually a much higher frequency than the input signal...
before trying to send. That is, it tries to detect the presence of an encoded signal
Signal (electrical engineering)
In the fields of communications, signal processing, and in electrical engineering more generally, a signal is any time-varying or spatial-varying quantity....
from another station before attempting to transmit. If a carrier is sensed, the station waits for the transmission in progress to finish before initiating its own transmission.
"Multiple Access" describes the fact that multiple stations send and receive on the medium. Transmissions by one node are generally received by all other stations using the medium.
Protocol modifications
Carrier sense multiple access with collision detectionCarrier sense multiple access with collision detection
Carrier sense multiple access with collision detection is a Media Access Control method in which:*a carrier sensing scheme is used....
(CSMA/CD) is a modification of CSMA. CSMA/CD is used to improve CSMA performance by terminating transmission as soon as a collision is detected, and reducing the probability of a second collision on retry.
Carrier sense multiple access with collision avoidance
Carrier sense multiple access with collision avoidance
Carrier sense multiple access with collision avoidance , in computer networking, is a wireless network multiple access method in which:*a carrier sensing scheme is used....
(CSMA/CA) is a modification of CSMA. Collision avoidance is used to improve the performance of CSMA by attempting to be less "greedy" on the channel. If the channel is sensed busy before transmission then the transmission is deferred for a "random" interval. This reduces the probability of collisions on the channel.
CSMA access modes
1-persistent : When the sender (station) is ready to transmit data, it checks if the physical medium is busy. If so, it senses the medium continually until it becomes idle, and then it transmits a piece of data (a frame). In case of a collision, the sender waits for a randomRandomness
Randomness has somewhat differing meanings as used in various fields. It also has common meanings which are connected to the notion of predictability of events....
period of time and attempts to transmit again. 1-persistent CSMA is used in CSMA/CD systems including Ethernet
Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....
.
P-persistent : When the sender is ready to send data, it checks continually if the medium is busy. If the medium becomes idle, the sender transmits a frame with a probability
Probability
Probability is ordinarily used to describe an attitude of mind towards some proposition of whose truth we arenot certain. The proposition of interest is usually of the form "Will a specific event occur?" The attitude of mind is of the form "How certain are we that the event will occur?" The...
p. If the station chooses not to transmit (the probability of this event is 1-p), the sender waits until the next available time slot
Time division multiple access
Time division multiple access is a channel access method for shared medium networks. It allows several users to share the same frequency channel by dividing the signal into different time slots. The users transmit in rapid succession, one after the other, each using its own time slot. This...
and transmits again with the same probability p. This process repeats until the frame is sent or some other sender stops transmitting. In the latter case the sender monitors the channel, and when idle, transmits with a probability p, and so on. p-persistent CSMA is used in CSMA/CA systems including WiFi
WIFI
WIFI is a radio station broadcasting a brokered format. Licensed to Florence, New Jersey, USA, the station is currently operated by Florence Broadcasting Partners, LLC.This station was previously owned by Real Life Broadcasting...
and other packet radio
Packet radio
Packet radio is a form of packet switching technology used to transmit digital data via radio or wireless communications links. It uses the same concepts of data transmission via Datagram that are fundamental to communications via the Internet, as opposed to the older techniques used by dedicated...
systems.
O-persistent : Each station is assigned a transmission order by a supervisor station. When medium goes idle, stations wait for their time slot in accordance with their assigned transmission order. The station assigned to transmit first transmits immediately. The station assigned to transmit second waits one time slot (but by that time the first station has already started transmitting). Stations monitor the medium for transmissions from other stations 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
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...
, LonWorks
LonWorks
LonWorks is a networking platform specifically created to address the needs of control applications. The platform is built on a protocol created by Echelon Corporation for networking devices over media such as twisted pair, powerlines, fiber optics, and RF...
and the controller area network
Controller Area Network
Controller–area network is a vehicle bus standard designed to allow microcontrollers and devices to communicate with each other within a vehicle without a host computer....
.