CSMA .

• CSMA

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CSMA/CD

1 'Carrier Sense Multiple Access With Collision Detection' ('CSMA/CD') is a media access control method used

most notably in local area networking using early Ethernet technology


CSMA/CD

1 CSMA/CD is a modification of pure carrier sense multiple access (CSMA).

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 - Applications

1 IEEE Std 802.3, which defines all Ethernet variants, for historical

reasons still bears the title Carrier sense multiple access with collision detection (CSMA/CD) access method

and physical layer specifications.


GraphicsMagick

1 'GraphicsMagick' is a fork (software development)|fork of ImageMagick,

emphasizing stability of both programming API and command-line options. It was branched off

ImageMagick's version 5.5.2 in 2002 after irreconcilable differences emerged in the

developers' group.[http://marc.info/?l=imagemagick-

developerm=104777007831767w=2 imagemagick-developer mailing list: Introducing

GraphicsMagick Project, 15 March 2003]


GraphicsMagick

1 In addition to the programming language APIs available with

ImageMagick, GraphicsMagick also includes a Tcl API, called TclMagick.


GraphicsMagick

1 GraphicsMagick is used by several websites to process large numbers of uploaded

photographs.[http://www.kitchensoap.com/2009/04/03/slides-from-web20-expo-2009-and-somethin-else-interestin/ Slides from Web2.0 Expo 2009. (and somethin else

interestin’)][http://codeascraft.etsy.com/2010/07/09/batch-processing-millions-of-

images/ Etsy: Batch processing millions of images]


CSMA CA

1 'Carrier sense multiple access with collision avoidance' ('CSMA/CA') in computer networking, is a network

multiple access method in which Carrier wave|carrier sensing is used, but Node (networking)|nodes attempt to avoid

collisions by transmitting only when the channel is sensed to be idle. When they do transmit, nodes transmit their packet

data in its entirety.


CSMA CA

1 It is particularly important for wireless networks, where the

collision detection of the alternative Carrier sense multiple access with

collision detection|CSMA/CD is unreliable due to the hidden node

problem.


CSMA CA - Details

1 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.


CSMA CA - Details

1 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)


CSMA CA - Details

1 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


CSMA CA - Performance

1 Studies show that under ideal propagation conditions (simulations),

direct-sequence spread spectrum|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


CSMA CA - Usage

1 * Apple Computer|Apple's LocalTalk implemented 'CSMA/CA' on an

electrical bus using a three-byte jamming signal.


CSMA CA - Usage

1 * 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 line communication|power

lines, phone lines and ethernet over coax|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.


Carrier sense multiple access - CSMA with collision avoidance

1 In CSMA/CA 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.


Carrier sense multiple access - Virtual time CSMA

1 VTCSMA is designed to avoid collision generated by nodes transmitting signals

simultaneously, used mostly in hard Real-time computing|real-time systems. The

VTCSMA uses two clocks at every node, a virtual clock (vc) and a real clock (rc) which tells real time. When the channel is sensed to be busy, the vc freezes, when channel is free, it is reset. Hence, calculating vc runs

faster than rc when channel is free, and vc is not initiated when channel is busy.


Carrier sense multiple access - CSMA access modes

1 1-persistent CSMA is used in CSMA/CD systems including

Ethernet.



1 ; Non-persistent : Non persistent CSMA is a non aggressive transmission algorithm



1 p-persistent CSMA is used in CSMA/CA systems including Wi-Fi and other packet

radio systems.



1 O-persistent CSMA is used by CobraNet, LonWorks and the controller area network.


CSMA/CARP

1 Again, CSMA/CARP is similar to the functions of the

CSMA/CA


CSMA/CARP

1 In addition to having a different IFS per priority, a station in a

CSMA/CARP network will add a random backoff to its waiting period,

to reduce the collision probability between stations that have to

transmit packets in the same priority.


CSMA/CARP - Applications

1 * The ITU-T G.hn standard, which provides high-speed local area

networking over existing home wiring (power lines, phone lines and coaxial

cables) includes support for CSMA/CARP, although only during

certain periods of time called Shared Transmission Opportunities (STXOP).

During the rest of the time, G.hn uses a Time division multiple access Channel access method to ensure

Quality of Service.


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