Multiple Access

Definition - What does Multiple Access mean?

Multiple access is a technique that lets multiple mobile users share the allotted spectrum in the most effective manner.

Since the spectrum is limited, the sharing is necessary to improve the overall capacity over a geographical area. This is carried out by permitting the available bandwidth to be used simultaneously by different users. In computer networks and telecommunications, the multiple access method permits various terminals to connect to the same multi-point transmission medium to transmit over it and share its capacity.

A few examples of shared physical media include bus networks, wireless networks, star networks, ring networks, half-duplex point-to-point links, etc.

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Techopedia explains Multiple Access

A cellular system splits any allocated area into cells in which a mobile unit in every cell can communicate with a base station.

The key objective in the cellular system design is to offer increased channel capacity. This is to manage as many calls as possible in a particular bandwidth with an adequate standard of quality of service.

Multiple access techniques permit multiple access to a channel. A channel represents a system resource assigned to a given mobile user that enables the user to establish communication with other users in the network.

Depending on the channel type, specific multiple access technique can be used for communication. The channel types and the associated multiple access techniques are as follows:

Frequency Channels [Frequency Division Multiple Access (FDMA)] - Frequency band split into small frequency channels, and different channels are assigned to different users. One example is the case of FM radio where multiple users can transmit simultaneously; however, on different frequency channels.
Time-slot Within Frequency Bands [Time Division Multiple Access (TDMA)] - Every user is permitted to transmit only in specific time slots using a common frequency band. Various users can transmit at the same frequency band at different times.
Distinct Codes [Code Division Multiple Access (CDMA)] - Users can transmit simultaneously using the same frequency band, but with the help of different codes so that they can be decoded to recognize a specific user.