Performance Evaluation of Reservation MAC Protocols
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100 150 200 250 300 350 400 450
Utilization
Rare requests
Average packet size: 1500 bytes
Undisturbed
Lightly disturbed
Heavily disturbed
100 150 200 250 300 350 400 450
Number of stations
Number of stations
Frequent requests
Average packet size: 300 bytes
Undisturbed
Heavily disturbed
Lightly disturbed
Figure 6.43
Average network utilization networks with go-back-N ARQ
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100 150 200 250 300 350 400 450
Utilization
Number of stations
Number of stations
Rare requests
Average packet size: 1500 bytes
Undisturbed
Lightly disturbed
Heavily disturbed
100 150 200 250 300 350 400 450
Frequent requests
Average packet size: 300 bytes
Undisturbed
Heavily disturbed
Lightly disturbed
Figure 6.44
Average network utilization networks with ARQ-plus mechanisms
to the maximum possible (about 93%) in lightly disturbed networks and to 90% in heavily
disturbed networks.
The application of ARQ and ARQ-plus mechanisms improves the transmission delay
significantly, as shown in Fig. 6.45. As expected, the network using ARQ-plus mechanism,
which exploits possible retransmission gaps, achieves the shortest transmission delays.
The ARQ-plus mechanism without shifting
(ARQ
+ WS), achieves shorter transmission
delays than simple ARQ mechanism in low loaded networks. However, the transmission
delay remains longer than in the case of the ARQ-plus mechanism with shifting.
With the increasing network load, the transmission delay achieved in the network with
the ARQ-plus mechanism without shifting comes close to the delay achieved by a simple
ARQ. Beyond 200 stations in the network, the delays have practically the same value.
Thus, application of the ARQ-plus mechanism without shifting ensures good network
utilization (the same as ARQ-plus with shifting), but the transmission delay remains