ALOHA based wireless random access technique

Multiple Access ProtocolsMany algorithms exist for allocation of Multiple Access Channel. To begin with let us investigate representative algorithms:Pure ALOHASlotted ALOHAReservation ALOHA

Pure ALOHAIn 1970s, Norman Abramson and his team at University of Hawaii devised the algorithmThe basic idea used in the algorithm is applicable to any system in which uncoordinated users are competing for the use of single shared channelThe algorithm is referred to as Random Multiple Access Protocol or Pure ALOHA

Random Multiple Access ProtocolUsers transmit whenever they have data to be sentThere will be collisions and the colliding frames are destroyedHowever, due to the broadcasting nature of the channel, a sender can always find out whether or not its frame was destroyed by listening to the channel, the same way other users do

Random Multiple Access ProtocolWith a LAN, the feedback is immediateHowever, with a satellite, there is a delay of 270 ms, before the sender knows if the transmission was successfulIf the frame was destroyed, the sender just waits a random amount of time and sends it againThis kind of system where users share a common channel resource is referred to as Contention System

Pure ALOHA System

Pure ALOHA SystemFrames have the same length Whenever two frames try to occupy the channel at the same time, there will be collision and both fames are garbledQuestion: Can this system work? If yes, What is the efficiency or throughput of the system?

Formal Description of the AlgorithmTransmission Mode: Users transmit at any time they desire, encoding their transmission with an error detection codeListening Mode: After a message transmission, the user listens for the acknowledgement (ACK) from the receiver. Transmissions from different users will sometimes overlap in time, causing errors in the data in each of the colliding partners. The user then receives a negative acknowledgement (NAK)

Formal Description of the AlgorithmRetransmission Mode: When NAK is received, the messages are simply retransmitted. Colliding users retransmit after a random amount of delayTimeout Mode: If, after a transmission, the user does not receive either an ACK or NAK within a specified time, the user retransmits the message

Slotted ALOHABy introducing a small amount of coordination among users, the performance of the pure ALOHA can be improvedSuch a scheme is referred to as S-ALOHA or Slotted-ALOHA systemAs with pure ALOHA system, in S-ALOHA the packet size is constantPackets are required to be sent in the slot time between synchronization pulses and can be started only at the beginning of the time slot.

Slotted ALOHAThis simple change reduces the rate of collisions by half, since only packets transmitted in the same slot can interfere with one another.Normalized throughput of S-ALOHA system is thus given by:

Slotted ALOHA Operation

Slotted ALOHA ThroughputThe maximum value of throughput = 1/e=0.37This maximum occurs at G =1.0In S-ALOHA 37% of the Channel Resource can be utilizedThere exists tradeoff between Channel utilization and Coordination

Reservation ALOHA (R-ALOHA)Significant improvement in performance can be achieved over ALOHA system by using ReservationsThe R-ALOHA system has two basic modes: Unreserved Mode (Quiescent State)1. A time frame is established and divided into small reservation subslots2. Users use these subslots to reserve message slots3. After requesting a reservation, the user listens for an acknowledgement and a slot assignment

Reservation ALOHA (R-ALOHA)Reserved Mode:1. The time frame is divided into M+1 slots whenever a reservation is made2. The first M slots are used for message transmission3. The last is subdivided into subslots to be used for reservations4. Users send message packets only in their assigned portions of the M slots

5 Slots, 6 Subslots R-ALOHA System

R-ALOHA SystemIn the quiescent state, with no reservations, time is partitioned into short subslots for making reservationsOnce reservation is made, the system is configured so that 5 message slots followed by 6 reservation subslots becomes the timing formatIn the Figure the station seeks to reserve three message slotsThe reservation acknowledgement advises the station where to locate its data packets.

R-ALOHA SystemSince the control is distributed, all stations receive the downlink transmission and are aware of the reservation formatThe acknowledgement need not disclose any more than the location of the first slot to use.When there are no reservations taking place, the system reverts back to its quiescent mode

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