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Information about Carrier Sense Multiple Access

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 physical medium, such as an electrical bus, or a band of electromagnetic spectrum.

"Carrier Sense" describes the fact that a transmitter listens for 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 node waits for the transmission in progress to finish before initiating its own transmission.

"Multiple Access" describes the fact that multiple nodes send and receive on the medium. Transmissions by one node are generally received by all other nodes using the medium.

Collisions

Concurrent transmission by multiple nodes results in frame collisions. The multiple transmissions interfere with each other so that all are garbled and receivers are unable to distinguish the overlapping received signals from each other. It is impossible to entirely prevent collisions in CSMA networks, but there are three ways to address them:
  • In pure CSMA, only the carrier sense is used to avoid collisions. If two nodes try to send a frame at nearly the same time, neither detects a carrier so both begin transmitting. The transmitters do not detect collisions, so transmit the entire frame (thus wasting the bandwidth used). Receivers cannot distinguish between collisions and other sources of frame errors, so collision recovery relies on the ability of the communicating nodes to detect frame errors and invoke an error recovery procedure. For example, the receiver may not send a required ACK, causing transmitters to time out and retry.
  • In Carrier Sense Multiple Access With Collision Avoidance (CSMA/CA), each node listens the channel to see if it's free for transmission. If the channel is actually free, the node begins the transmission with the first data frame. However, collisions are still possible, and are not detected so have the same consequences as in pure CSMA.
  • In Carrier Sense Multiple Access With Collision Detection (CSMA/CD), sending nodes are able to detect when a collision occurs and stop transmitting immediately, backing off for a random amount of time before trying again. This results in much more efficient use of the media since the bandwidth of transmitting the entire frame is not wasted. However, it is not possible with all media (e.g., radio), and requires extra electronics (not really an issue with today's technology, but one reason Apple used CSMA/CA-based LocalTalk instead of the then much more expensive Ethernet).
  • In Carrier Sense Multiple Access with Bitwise Arbitration (CSMA/BA, also known as CSMA/CR - collision resolution), all of the nodes on the bus are assigned an identification number or priority code. When a collision occurs, one of the nodes that are attempting to send at the same time will be given priority to transmit according to its identification number or priority code (as opposed to waiting a random amount of time and then retransmitting, as in CSMA/CD). Used in CAN communications, often found on vehicles.
  • There is another newer technology called Carrier Sense Multiple Access with Collision Prevention (CSMA/CP).
There are three types of collisions

Protocols Using CSMA

See also

Probability is the likelihood that something is the case or will happen. Probability theory is used extensively in areas such as statistics, mathematics, science and philosophy to draw conclusions about the likelihood of potential events and the underlying mechanics of
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Media Access Control (MAC) data communication protocol sub-layer, also known as the Medium Access Control, is a part of the data link layer specified in the seven-layer OSI model (layer 2).
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In telecommunications, a carrier wave, or carrier is a waveform (usually sinusoidal) that is modulated (modified) with an input signal for the purpose of conveying information, for example voice or data, to be transmitted.
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In computing, an ACK (also known as an acknowledgment code) is a signal passed between communicating processes or computers to signify acknowledgement, or receipt of response, as part of a communications protocol.
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Carrier Sense Multiple Access With Collision Avoidance. In CSMA, a station wishing to transmit has to first listen to the channel for a predetermined amount of time so as to check for any activity on the channel.
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Carrier Sense Multiple Access With Collision Detection (CSMA/CD) is a network control protocol in which
  • a carrier sensing scheme is used.
  • a transmitting data station that detects another signal while transmitting a frame, stops transmitting that frame, transmits

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LocalTalk is a particular implementation of the physical layer of the AppleTalk networking system from Apple Computer. LocalTalk specifies a system of shielded twisted pair cabling, plugged into self-terminating transceivers, running at a rate of 230.4 kbit/s.
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Ethernet is a family of frame-based computer networking technologies for local area networks (LANs). The name comes from the physical concept of the ether. It defines a number of wiring and signaling standards for the physical layer, through means of network access at the Media
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A local collision, in CSMA/CD computer networks, is a collision that occurs at the NIC as opposed to on the wire.

On UTP cable a local collision is detected on the local segment only when a station detects a signal on the RX pair at the same time it is sending on the TX pair.
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A remote collision, in CSMA/CD computer networks, is a collision that occurs when a frame having length less than minimum length and with an incorrect FCS (Frame Check Sequence), is transmitted. This frame causes a collision at the remote end which will not be detected by the transmitter.
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Late Collision is a type of collision found in the CSMA/CD protocol standard. If a collision error occurs after the first 512 bit times of data are transmitted by the transmitting station[1], a late collision is said to have occurred.
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ALOHAnet, also known as ALOHA, was a pioneering computer networking system developed at the University of Hawaii. It was first deployed in 1970, and while the network itself is no longer used, one of the core concepts in the network is the basis for the widely used Ethernet.
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Distributed Coordination Function (DCF) is the fundamental MAC technique of the IEEE 802.11 wireless LAN standard. DCF employs a CSMA/CA distributed algorithm and an optional virtual carrier sense using RTS and CTS control frames.
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Ethernet is a family of frame-based computer networking technologies for local area networks (LANs). The name comes from the physical concept of the ether. It defines a number of wiring and signaling standards for the physical layer, through means of network access at the Media
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A duplex communication system is a system composed of two connected parties or devices which can communicate with one another in both directions. (The term duplex is not used when describing communication between more than two parties or devices.
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A duplex communication system is a system composed of two connected parties or devices which can communicate with one another in both directions. (The term duplex is not used when describing communication between more than two parties or devices.
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LocalTalk is a particular implementation of the physical layer of the AppleTalk networking system from Apple Computer. LocalTalk specifies a system of shielded twisted pair cabling, plugged into self-terminating transceivers, running at a rate of 230.4 kbit/s.
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IEEE 802.11 is a set of standards for wireless local area network (WLAN) computer communication, developed by the IEEE LAN/MAN Standards Committee (IEEE 802) in the 5 GHz and 2.4 GHz public spectrum bands.

Although the terms 802.
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The introduction to this article provides insufficient context for those unfamiliar with the subject matter.
Please help [ improve the introduction] to meet Wikipedia's layout standards.

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IEEE 802.15 is the 15th working group of the IEEE 802 which specializes in Wireless PAN (Personal Area Network) standards. It includes five task groups (numbered from 1 to 5) [1] :

Task group 1 (WPAN/Bluetooth)


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Carrier Sense Multiple Access With Collision Detection (CSMA/CD) is a network control protocol in which
  • a carrier sensing scheme is used.
  • a transmitting data station that detects another signal while transmitting a frame, stops transmitting that frame, transmits

..... Click the link for more information.
A local collision, in CSMA/CD computer networks, is a collision that occurs at the NIC as opposed to on the wire.

On UTP cable a local collision is detected on the local segment only when a station detects a signal on the RX pair at the same time it is sending on the TX pair.
..... Click the link for more information.
A remote collision, in CSMA/CD computer networks, is a collision that occurs when a frame having length less than minimum length and with an incorrect FCS (Frame Check Sequence), is transmitted. This frame causes a collision at the remote end which will not be detected by the transmitter.
..... Click the link for more information.


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