Aloha:

#)Aloha,in simple terms, refers to a simple communications scheme where each source (called as transmitter) in a network sends data whenever there is a frame to send.

#)If the frames uccessfully reaches the destination (called asreceiver), the next frame is sent.

#)Supose if the frame fails to be received at the destination, it will  be  sent again.

#)This protocol was originally developed at the University of Hawaii for use with satellite communication systems in the Pacific and this is called as Aloha Method

Controlled ALOHA:

#)The notion of an optimal retransmission delay was introduced by Roberts.

#)We extend the notion to include dynamic control of retransmission delay.

#)A controlled Aloha system has the property that its terminals adjust their retransmission behavior as a function of perceived channel utilization.

#)It will be shown that such adjustments are implementable in at least one way and that they improve system performance under heavy loads with the following example

#)An example for Controlled ALOHA is as follows:

#)A system and method for satellite based controlled ALOHA is provided.

#)The system and method configure VSATs so that they accept guidance from a centralized and/or distributed system controller to determine when, where, and/or on what portion of a satellite resource to attempt to access a channel.

#)The system and method take advantage of traffic patterns to maximize efficiency by utilizing a centralized and/or distributed control to determine which VSATs are and are not currently active, and to allocate to a portion of the inbound channel to the active VSATs and a portion of the inbound channel to the inactive VSATs.

#)In a distributed approach, each VSAT decides for itself whether it captures a certain part of the inbound resource or not.

#)This decision can be made based on previous transmissions that went through the part of the inbound intended to be captured.

#)Simple retransmission strategies are adopted to allow for the utilization of the ALOHA synchronous random access protocol.

#)Numerical simulations were performed using a model representing the set of mobiles and the base station of a cell.

#)The main propagation problems are progressively incorporated in the simulation, showing that the adaptive policy is always the best.

#)Controlled ALOHA and a traditional protocol such as AMRT are compared, defining for each case the limiting rate below which the proposed system is preferable.

Finite population ALOHA:

#)The finite-population ALOHA system can be bistable:  the system either is in low backlog or in high backlog.

#)Every now and then it hops from high backlog to low backlog, but it is unlikely to be in intermediate states, as it will rapidly drift away from these states.

#)The graph above represents the Bistability area for a network with N =100 users without capture (orange) and with imperfect capture (green) (capture ratio z = 10) in mobile channels.

#)The above figure gives the region for P0 and Pr where  the network with 100 terminals is bistable.

#)A conservative receiver threshold of 10 dB (z = 10) is considered, so these results may be somewhat pessimistic.

#)The Bistability area for ALOHA without capture was computed first by Onozato and Noguchi.

#)Areas mapping the stability of a slotted ALOHA network with capture were derived for the case that the population of terminals is divided into a group transmitting at high power and a group transmitting a low power.

#0Van der Plas and Linnartz estimated the area of bistability by trial and error with the technique used to obtain drift curves.

#)A uniform spatial distribution, shadowing (6 dB) and Rayleigh fading are considered.

#)When receiver capture occurs, the mobile network exhibits bistability at substantially higher packet traffic loads, even for a pessimistic receiver threshold of 10 dB (z = 10).

#)It appears that the network is always stable, irrespective of the retransmission probability Pr, if Pr < P0,max, with roughly P0,max = 2 . 10-3.

#)This agrees with the observation by Ghez, Verdu and Schwartz, that for packet arrival rates < the limit of the capture probability for collisions with infinitely many signals the channel is stable if the probability of capture is independent from slot to slot.

#)However, this may be a strong assumption if the retransmission waiting time is small.

#)If packets are retransmitted from the same location, they are received with the same power, as the initial transmission attempt.

#)If the same set of data packets collide again with the same powers for all signals involved, interference is likely to cause packet loss during all successive collisions.

Reservation-ALOHA:

#)Some of the caharacteristics of Reservation Aloha are as follows

#)Reservation-ALOHA is also called as R-ALOHA

#)Use ALOHA contention to reserve slots, TDMA for data transmission

#)Here time axis isdivided into frames, and frames are divided in slots

#)Some slots subdivided into reservation subslots

#)In unreservedmode, each user with data sends a reservation request in one of reservation subslots

#)Intended recipient sends ACK and slot assignment if reservation request does not suffer collision

#)In reservedmode, one slot assigned to hold reserv. subslots, all other slots pre-reserved data txslots

#)Node that has been granted slot sends its data in reserved slot

#)Reservation exchanges heard by all nodes

#)Nodes know which slots are reserved and not to send data during these slots

#)Flexible-TDMA

#)Contention-based reservation exchanges confined to short reservation subslots

#)Message slots shared among nodes with data to send in noninterferingmanner

#)Design tradeoff of how many reservation slots and how many data slots

#)Control distributed among all nodes in the network → each receiving node grants reservations when txnode requests a slot

• Written by Raaghavan Krishnamurthy.
• In category Engineering.
• 15 years ago.

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