FDMA is a continuous transmission method. Channel can use one phone circuit at any instant of time.
Basically, there are two types of FDMA multiple access methods.
Types of FDMA :
- FAMA
- DAMA
FAMA - FAMA denoted as Fixed assignment multiple access. In FAMA the channel is assigned in a predetermined manner and distributed so that random changes in the capacity are not allowed.
DAMA - DAMA denoted as Demand assignment multiple access. If there are multiple stations there is a change of capacity and after that, the channel can be allocated to the demand.
FAMA - FDMA :
FDMA deal with link accessing using different frequency band and by multiple stations. It is concerned with the logical link between the stations that are preassigned.
FAMA-FDMA is preassigned. It is not flexible in accordance with the change in traffic.
DAMA - FDMA :
Demand access signifies the allocation of satellite channel to a user on demand, rather than continuously. Therefore demand assignment multiple access is used for earth stations where the traffic conditions are continuously changing.
One demand allocation of channel greatly increases the number of simulations uses who can be served by the system.
For example, telephone voice users communicate at random times, some for less than a minute to voice users communicate at random times, some for less than a minute to several minutes.
If each user were allocated a fixed channel, most of the time the channel will be idle, resulting efficiency utilization of the entire system is low. Therefore demand allocation remarkably increases the number of telephone user served by the system.
Demand access system requires two different types of channel :
- Common signaling channel
- Communication channel