Carrier Sensed Multiple Access (CSMA) : CSMA is a network access method used on shared network topologies such as Ethernet to control access to the network. Devices attached to the network cable listen (carrier sense) before transmitting. If the channel is in use, devices wait before transmitting. MA (Multiple Access) indicates that many devices can connect to and share the same network. All devices have equal access to use the network when it is clear.
In other words, a station that wants to communicate 'listen' first on the media communication and awaits a 'silence' of a preset time (called the Distributed Inter Frame Space or DIFS). After this compulsory period, the station starts a countdown for a random period considered. The maximum duration of this countdown is called the collision window (Window Collision, CW). If no equipment speaks before the end of the countdown, the station simply deliver its package. However, if it is overtaken by another station, it stops immediately its countdown and waits for the next silence. She then continued his account countdown where it left off. This is summarized in Figure. The waiting time random has the advantage of allowing a statistically equitable distribution of speaking time between the various network equipment, while making little unlikely (but not impossible) that both devices speak exactly the same time. The countdown system prevents a station waiting too long before issuing its package. It's a bit what place in a meeting room when no master session (and all the World's polite) expected a silence, then a few moments before speaking, to allow time for someone else to speak. The time is and randomly assigned, that is to say, more or less equally.
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. Sep 28, 2005 - CSMA/CD (carrier sense multiple access/collision detection) is the most widely used protocol (i.e., set of rules) for determining how network devices respond in the. And simplify definitions with regard to computer networks.
Again, this is what we do naturally in a meeting room if many people speak exactly the same time, they are realizing account immediately (as they listen at the same time they speak), and they interrupt without completing their sentence. After a while, one of them speaks again. If a new collision occurs, the two are interrupted again and tend to wait a little longer before speaking again.
CSMA protocol was developed to overcome the problem found in ALOHA i.e. to minimize the chances of collision, so as to improve the performance. CSMA protocol is based on the principle of 'carrier sense'. The station senses the carrier or channel before transmitting a frame. It means the station checks the state of channel, whether it is idle or busy.
Even though devices attempt to sense whether the network is in use, there is a good chance that two stations will attempt to access it at the same time. On large networks, the transmission time between one end of the cable and another is enough that one station may access the cable even though another has already just accessed it.
The chances of collision still exist because of propagation delay. The frame transmitted by one station takes some time to reach other stations. In the meantime, other stations may sense the channel to be idle and transmit their frames. This results in the collision.
There Are Three Different Type of CSMA Protocols
(I) I-persistent CSMA
(ii) Non- Persistent CSMA
(iii) p-persistent CSMA
(i) I-persistent CSMA
• In this method, station that wants to transmit data continuously senses the channel to check whether the channel is idle or busy.
• If the channel is busy, the station waits until it becomes idle.
• When the station detects an idle-channel, it immediately transmits the frame with probability 1. Hence it is called I-persistent CSMA.
• This method has the highest chance of collision because two or more stations may find channel to be idle at the same time and transmit their frames.
• When the collision occurs, the stations wait a random amount of time and start allover again.
Drawback of I-persistent
• The propagation delay time greatly affects this protocol. Let us suppose, just after the station I begins its transmission, station 2 also became ready to send its data and senses the channel. If the station I signal has not yet reached station 2, station 2 will sense the channel to be idle and will begin its transmission. This will result in collision.
Even if propagation delay time is zero, collision will still occur. If two stations became .ready in the middle of third station's transmission, both stations will wait until the transmission of first station ends and then both will begin their transmission exactly simultaneously. This will also result in collision.
(ii) Non-persistent CSMA
• In this scheme, if a station wants to transmit a frame and it finds that the channel is busy (some other station is transmitting) then it will wait for fixed interval oftime. • After this time, it again checks the status of the channel and if the channel is.free it will transmit.
• A station that has a frame to send senses the channel.
• If the channel is idle, it sends immediately.
• If the channel is busy, it waits a random amount of time and then senses the channel again.
• In non-persistent CSMA the station does not continuously sense the channel for the purpose of capturing it when it detects the end of previous transmission.
Advantage of non-persistent
• It reduces the chance of collision because the stations wait a random amount of time. It is unlikely that two or more stations will wait for same amount of time and will retransmit at the same time.
Disadvantage of non-persistent
• It reduces the efficiency of network because the channel remains idle when there may be stations with frames to send. This is due to the fact that the stations wait a random amount of time after the collision.
(iii) p-persistent CSMA
• This method is used when channel has time slots such that the time slot duration is equal to or greater than the maximum propagation delay time.
• Whenever a station becomes ready to send, it senses the channel.
• If channel is busy, station waits until next slot.
• If channel is idle, it transmits with a probability p.
• With the probability q=l-p, the station then waits for the beginning of the next time slot.
• If the next slot is also idle, it either transmits or waits again with probabilities p and q.
• This process is repeated till either frame has been transmitted or another station has begun transmitting.
• In case of the transmission by another station, the station acts as though a collision has occurred and it waits a random amount of time and starts again.
Advantage of p-persistent
• It reduces the chance of collision and improves the efficiency of the network.
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'.[1][2] 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.
CSMA/CA is unreliable due to the hidden node problem.[3][4]
CSMA/CA is a protocol that operates in the Data Link Layer (Layer 2) of the OSI model.
Simplified algorithm of CSMA/CA
Details[edit]
Collision avoidance is used to improve the performance of the CSMA method by attempting to divide the channel somewhat equally among all transmitting nodes within the collision domain.
Carrier Sense: prior to transmitting, a node first listens to the shared medium (such as listening for wireless signals in a wireless network) to determine whether another node is transmitting or not. Note that the hidden node problem means another node may be transmitting which goes undetected at this stage.
Collision Avoidance: if another node was heard, we wait for a period of time (usually random) for the node to stop transmitting before listening again for a free communications channel.
Request to Send/Clear to Send (RTS/CTS) may optionally be used at this point to mediate access to the shared medium. This goes some way to alleviating the problem of hidden nodes because, for instance, in a wireless network, the Access Point only issues a Clear to Send to one node at a time. However, wireless 802.11 implementations do not typically implement RTS/CTS for all transmissions; they may turn it off completely, or at least not use it for small packets (the overhead of RTS, CTS and transmission is too great for small data transfers).
Transmission: if the medium was identified as being clear or the node received a CTS to explicitly indicate it can send, it sends the frame in its entirety. Unlike CSMA/CD, it is very challenging for a wireless node to listen at the same time as it transmits (its transmission will dwarf any attempt to listen). Continuing the wireless example, the node awaits receipt of an acknowledgement packet from the Access Point to indicate the packet was received and checksummed correctly. If such acknowledgement does not arrive in a timely manner, it assumes the packet collided with some other transmission, causing the node to enter a period of binary exponential backoff prior to attempting to re-transmit.
Although CSMA/CA has been used in a variety of wired communication systems, it is particularly beneficial in a wireless LAN due to a common problem of multiple stations being able to see the Access Point, but not each other. This is due to differences in transmit power, and receive sensitivity, as well as distance, and location with respect to the AP.[5] This will cause a station to not be able to 'hear' another station's broadcast. This is the so-called 'hidden node', or 'hidden station' problem. Devices utilizing 802.11 based standards can enjoy the benefits of collision avoidance (RTS / CTS handshake, also Point coordination function), although they do not do so by default. By default they use a Carrier sensing mechanism called 'exponential backoff', or (Distributed coordination function) that relies upon a station attempting to 'listen' for another station's broadcast before sending. CA, or PCF relies upon the AP (or the 'receiver' for Ad hoc networks) granting a station the exclusive right to transmit for a given period of time after requesting it (Request to Send / Clear to Send).[6]
IEEE 802.11 RTS/CTS Exchange[edit]
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 partially solve the hidden node problem that is often found in wireless networking.[7][8]
Performance[edit]
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[edit]
GNET – an early proprietary LAN protocol
Apple'sLocalTalk 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
Bus networks
The ITU-TG.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[edit]
References[edit]
^'Federal Standard 1037C'. Its.bldrdoc.gov. Retrieved 2012-09-09.
^'American National Standard T1.523-2001, Telecom Glossary 2000'. Atis.org. Archived from the original on 2008-03-02. Retrieved 2012-09-09.
^'Study of different CSMA/CA IEEE 802.11-based implementations, Universitat Politècnica de Catalunya'(PDF). Archived from the original(PDF) on 2012-03-06. Retrieved 2012-09-09.
^ abViral V. Kapadia; Sudarshan N. Patel; Rutvij H. Jhaveri (2010). 'Comparative study of hidden node problem and solution using different techniques and protocols, Journal of Computing'(PDF). Retrieved 2016-01-05.
^Kaixin Xu; Mario Gerla; Sang Bae. 'How Effective is the IEEE 802.11 RTS/CTS Handshake in Ad Hoc Networks?'(PDF). UCLA. Retrieved 28 September 2012.
^Park, Kihong. 'Wireless Lecture Notes'(PDF). Purdue. Retrieved 28 September 2012.
^Comer, Douglas. (2009). Computer Networks and Internets. Upper Saddle River, NJ: Pearson Education Inc. ISBN0-13-504583-5.
^'MIT Lecture - Communication Systems Engineering. Dr. Eytan Modiano'(PDF). Retrieved 2012-09-09.
Computer Networks: a Systems Approach. Peterson & Davie. Morgan Kaufmann, Burlington, MA, USA. ISBN978-0-12-385138-3. pp128–139
External links[edit]
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