18-03-2011, 06:31 PM
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 | M.Arsalan Qureshi | | Join Date: Oct 2008 Location: Garden Town, Multan Cantt
Posts: 616
Program / Discipline: BSTS Class Roll Number: 09-31 | |
 Wired LANs: Ethernet
Wired LANs: Ethernet
* Medium access methods can be categorized as random, controlled, or channelized. * In the carrier sense multiple-access (CSMA) method, a station must listen to the medium prior to sending data onto the line. * A persistence strategy defines the procedure to follow when a station senses an occupied medium. * Carrier sense multiple access with collision detection (CSMA/CD) is CSMA with a postcollision procedure. * Carrier sense multiple access with collision avoidance (CSMA/CA) is CSMA with procedures that avoid a collision. * Reservation, polling, and token passing are controlled-access methods. * In the reservation access method, a station reserves a slot for data by setting its flag in a reservation frame. * In the polling access method, a primary station controls transmissions to and from secondary stations. * In the token-passing access method, a station that has control of a frame called a token can send data. * Channelization is a multiple-access method in which the available bandwidth of a link is shared in time, frequency, or through code, between stations on a network. * FDMA, TDMA, and CDMA are channelization methods. * In FDMA, the bandwith is divided into bands; each band is reserved fro the use of a specific station. * In TDMA, the bandwidth is not divided into bands; instead the bandwidth is timeshared. * In CDMA, the bandwidth is not divided into bands, yet data from all inputs are transmitted simultaneously. * CDMA is based on coding theory and uses sequences of numbers called chips. The sequences are generated using Walsh tables.
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