Multiple Access

* 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.