Demand Assigned Multiple Access (DAMA) in Networking

Demand Assigned Multiple Access is generally used for the earth stations where the traffic condition of that particular station is been changing continuously. It is a process of allocation of satellite channels to a user on demand. There is a great increase in the number of simultaneous uses on demand of allocation of channels that can be served by the system. For example, let us consider telephone voice communication users i.e. they communicate at a random time, some talk for more than minutes, or hours, and some talk for less than minutes or hours. So each user is been allocated a fixed channel. That means, accordingly most of the time the channel will be idle resulting in better utilization of the system. The Demand Assigned Multiple Access [DAMA] has two different types of channels, they are:

• Common Signalling Channel (CSC).
• Communication Channel.

For entering the communication system the user first needs to call the controlling earth station using the Common Signalling Channel (CSC), and then a pair of channels is been allocated to that used for communication by the controller. Packet transmission techniques are been used widely in Demand Assigned Multiple Access (DAMA)because it determines the source and destination.

The Common Signalling Channel (CSC) as shown in the above image, is located at the ends of the transponder bandwidth. So, when a very small aperture terminal (VSAT) wants access to the satellite, it sends a control packet in CSC frequency to the satellite and waits for the reply. The control packets are then been received by the controlling earth stations, which then basically decode them to gain the address of the particular satellite destination for the call being made, DA, and the address of the requesting station, RA.

The packet also contains data that is used for identifying the controlling stations that it is a control packet. This control packet doesn’t carry any traffic data and a Cyclic Redundancy Check (CRC) i.e. used to check errors in the packets by the receiver. Next, the control station then allocates an Uplink frequency and a time slot of a specific duration to the very small aperture terminal (VSAT). The specified duration is in the outbound TDM frame, it is mostly a continuous TDM bit stream that is transmitted via a separate transponder as shown in the image above. The inbound and outbound channels are normally symmetric in nature, thus offering the same data rate in both directions. So, once both the inbound and outbound frequencies are been allocated to the very small aperture terminal (VSAT) station we can say that the connection is been completed, hence enabling voice communication or data transfer.

Need for DAMA

• For overcoming the downside of Frequency Access Multiple Access, the DAMA [Demand Assigned Multiple Access] was introduced.
• the DAMA [Demand Assigned Multiple Access]  is used by various multiple access systems to deal with the inefficiency of the FAMA [Frequency Access Multiple Access].
• DAMA helps in allowing the resources/channel allocation to the users basically on the basis of requirements.
• Hence, making the channel utilized by the users according to their needs, as there is no fixed allotment initiated.

Operations on DAMA

Demand Assigned Multiple Access [DAMA] generally has two fundamental methods, they are:

1. Polling method: In this method, in order to check for all the call request a master station sequentially polls all the earth stations that are connected to it. Once the receiving end i.e. the master station receives all the call requests then it basically allows the available slots from the frequency pool to the earth station which has sent the requests. Polling delay also increases with the increase in the number of users.
2. Centrally controlled random access method: Here in this method, the master station does not regularly poll the earth stations, instead the earth station requests the master station whenever it requires a slot from the channel. Once the slot is available upon receiving the request, then the master station allows it and the frequency returns to the pool once the session is over. Well, in this method there is a condition for the unavailability of the requested slot and in such cases, the master station keeps the blocked call request in the queue.

Advantages 

• There is a lesser chance of resource wastage in Demand Assigned Multiple Access [DAMA].
• It offers efficient use of resources by allotting the resources after the requests.
• It is cost-effective.
• DAMA offers a reduction in the satellite bandwidth requirements.

Disadvantages 

• Demand Assigned Multiple Access [DAMA] has higher complexity.
• In Demand Access, Multiple Access [DAMA] has a high load of network
• Due to the increased load of the network, the delay is quite large.