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Home > Protocol >  Aloha Protocol > Aloha Simulation Validation

  Aloha Protocol

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Aloha Simulation Validation
Theoretical results are based on Poisson distributed packet generation rates and uniform station traffic. Figure 31 shows the simulation results plotted against the theoretical function described earlier. Any noticable deviation is the result of simulation time limitations. The variable G represents initial transmission and retransmission attempts per slot while is described by the function:
The simulation results are indicated by *'s and were obtained using uniform station traffic similar to that used to validate TDMA.

Figure 1: Slotted ALOHA theoretical and simulation results.

Because access to vacant slots is controlled using Slotted ALOHA which was previously validated, no additional validation is necessary.

Simulation Execution Results

The Slotted ALOHA simulation results can be seen in Figures 2, 3, 4, 5 and 6.

Compared to GTDMA the utilization and delay results come up short, however the flexibility of Slotted ALOHA greatly exceeds that of GTDMA or TDMA. With respect to the station queue results the following should be noted. The collision of a packet is not determined by a station until it is read one propagation delay after its transmission on the rebroadcast. During this time the packet must be kept in some kind of hold queue buffer until the success of the transmission is known. This hold queue buffer must be of sufficient size to accommodate the number of packets which can be transmitted during one propagation delay. After the result of the transmission is determined, the packet is either purged from the hold queue or requeued in the transmission queue. Since the state of the hold queue is constant and the effects of collisions are seen in the size of the transmission queue, the packets in the hold queues are not represented in the graphs. With that said, the most remarkable statistic depicted in the queue buffer graphs is the almost non-existent average. This indicates that while these queue buffers do exhibit some peaks, mostly due to the burst factor, most of the time they are empty.


Figure 3: Slotted ALOHA Queue Activity for burst=1
Figure 4: Slotted ALOHA Queue Activity for burst=5
Figure 5: Slotted ALOHA Queue Activity for burst=10

Finally in the last graph, a very strong relationship between packet size and delay can be seen; the smaller the packet, the worse the delay. The intuitive explanation is that smaller packets represent smaller slots and a greater opportunity for collision.


Figure 6: Slotted ALOHA Packet Size vs. Delay

The Utilization vs. Delay graph shown in Figure 7 shows results similar to those obtained using GTDMA with some notable differences. For lower utilizations, the delay characteristic of GTDMA is better due to the lack of contention. But at higher utilizations, Reservation ALOHA has better delay results. This is due to the ownership of the slot for as long as needed. Obviously this protocol favors stream type messages as opposed to single packet messages.

Another important advantage is the lack of dependence on station population. In other words, stations could be added or removed without affecting the mechanics of the protocol.

The queueing characteristics seen in Figures 8, 9 and 10 are almost identical to those of GTDMA. This would seem to indicate that the contention at slot acquisition is offset by the contiguous assignment of that slot to the station.

Figure 7: Reservation ALOHA Utilization vs. Delay


Figure 8: Reservation ALOHA Queue Activity for burst=1
Figure 9: Reservation ALOHA Queue Activity for burst=5

Figure 10: Reservation ALOHA Queue Activity for burst=10

Predictably, the same relationship exists between packet size and delay with this protocol that existed with the Slotted ALOHA protocol. Once again it would seem that smaller packets and thus smaller slot sizes present more of an opportunity for collision which results in poorer delays. The difference here is that the performance stabilizes at a smaller packet size which provides an opportunity for reducing the queueing buffer requirements.


Figure 11: Reservation ALOHA Packet Size vs. Delay at 500 Mb/sec

Reservation Aloha

This channel allocation scheme divides the channel bandwidth into slot sizes equal to the transmission time of a single packet. This again assumes that the packet sizes are of constant length. The slots are organized into frames of equal size whose length spans the length of one propagation delay. A station makes an implicit reservation by successfully transmitting in an available slot. After a successful transmission the station is guaranteed that slot in succeding frames until it is no longer required. A slot becomes unused either by going empty in the previous frame or by collision in the previous frame. The remaining stations can then compete for unused slots using Slotted Aloha. If the frame size did not span the propagation delay, a slot could go unused successive times before stations sensed its availability.

Figure 12 shows several frames depicting the execution of this protocol. In frame 1 two stations (stations 2 and 5) collide in an attempt to acquire the same slot. They succeed in frame 2. The same scenario occurs in frames 5 and 6 except the collisions occur between stations 2 and 4, and 5 and 1 respectively.



Figure 12: Reservation Aloha Protocol


  • Efficiently handles bursty data traffic.


  • Inefficient for single packet messages.
  • Requires queueing buffers for retransmission of packets.
  • Requires tracking status of slots in previous frame.
  • If propagation delay is large, frame size could be excessive.

Because of Slotted ALOHA's limited utilization potential, various methods have been developed which improve upon its efficiency. One of these methods is known as Reservation Aloha. The main modification has to do with slot ownership after a successful packet transmission. With Slotted ALOHA any slot is available for use by any station regardless of its prior usage. With Reservation ALOHA the slot is considered owned temporarily by the station which used it successfully. When the station is through with the slot it simply stops sending. An idle slot is available to all stations on a contention basis.

Given the requirement that the frame size must span the propagation delay, the number of slots per frame varies with the channel speed. Table 6 shows the frame sizes in slots for a variety of channel speeds and packet sizes and a propagation delay of 0.113 seconds.
Table 6: Frame sizes for various channel speeds.
Channel Speed (Mb/sec) 400 500 600 700 800 900 1000 1100 1200 1300
.5 Mb Slot 91 113 136 159 181 204 226 249 272 294
1.0 Mb Slot 46 57 68 80 91 102 113 125 136 147
1.5 Mb Slot 31 38 46 53 61 68 76
91 98


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