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$2\mathrm{.}2\mathrm{.}2$ Polling

Polling works with topologies in which one device is designated as a primary station and the other devices are secondary stations.

All data exchanges must be made through the primary device even when the ultimate destination is a secondary device.

The primary device controls the link; the secondary devices follow its instructions. It is up to the primary device to determine which device is allowed to use the channel at a given time.

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The primary device has

When the primary is ready to receive data

something to send

from the secondary

Figure $12\mathrm{.}19$ Select and poll functions in polling$-$access method

$12\mathrm{.}3$ CHANNELIZATION

Channelization $($or channel partition, as it is sometimes called$)$ is a multiple$-$access method in which the available bandwidth of a link is shared in time, frequency, or through code, among different stations.

In this section, we discuss three protocols:

FDMA

TDMA

CDMA.

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$12\mathrm{.}3\mathrm{.}1$ FDMA

In frequency$-$division multiple access $($FDMA$),$ the available bandwidth is divided into frequency bands.

Each station is allocated a band to send its data. In other words, each band is reserved for a specific station, and it belongs to the station all the time.

To prevent station interference, the allocated bands are separated by guard band.