Prerequisite – Classification of MAC Protocols
First category which comes under classification of MAC protocols is MAC protocols using contention and Reservation mechanisms. First type of contention based protocols is Distributed Packet Reservation Multiple Access Protocol (D-PRMA).
Packet Reservation Multiple Access is already built protocol, difference is that in this it is distributed for use in Ad Hoc. So basically, it is updated version of PRMA. It is TDMA (Time Division Multiple Access) based technique.
In this, channel is divided in frame lengths. In every frame, there are some slots say, s slots. It means that there is channel, it’s small part is frame and further it’s small part is slot and then slots has it’s part as minislots.
There are two terminals named A and B. A has further named D and E. They have same channel, in which this process is going on. B is one to which data is to be transmitted. Now, as we know that there will be some kind of packet reservation.
First minislot of every slot is one which is compete for because if first minislot gets reserved, then next minislots (suppose (m-1)), which are left, they too gets reserved. So whole slot is assigned to that particular terminal. Like A, D and E are completing for slot-2’s minislot-1, and A won in that so every next minislot will be assigned to A only.
Now, Minislot has further assigned with RTS/BI and CTS/BI. RTS stands for Request to Send and CTS stands for Clear to Send. RTS/BI is used to deal with exposed terminal problems and CTS/BI is used to avoid hidden terminal problems.
A is sending CTS to everyone say, D and E. Till the time, there is value in CTS/BI, they know that they cannot transmit value as A is sending value to CTS/BI. Now suppose, for another packet say, that F wants to send to C terminal, where RTS is listening and responding but CTS is not. It means that F will not relate to CTS now and if it is getting RTS for terminal C, then F is allowed to transmit data. So, this avoids exposed terminal problems.
Basically, this protocol has 2 basic requirements –
- If the first mini-slot is reserved, then next mini slots will too get reserved for that terminal.
- If one slot is reserved and alternatively others too, then for that slots no other terminal will waste its resource.
In short, this protocol is used for voice traffic. Overall, if any node wants to transmit its package, then the first condition is to reserve the slot.
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