Data Only Environment

Table 2.1 lists system parameters of a considered WLAN environment and values of PHY-related parameters, which are referred to specifications of IEEE 802.11 [3]. In the simulations, we compare the APP scheme with the BEB and the DDFC [15] schemes.

In the BEB scheme, two initial contention windows, W0=16 and W0=32, are assumed. In the DDFC scheme, the setting parameters are t0=100ms, ts=10ms and W0=16. Since

beginning of packet contention, not as the primary usage defined in [15]. In the following figures, results of APP are shown by numerical and/or simulation, while results of BEB and DDFC are given by simulation.

Table 2.1 Parameter Settings for a WLAN Environment

Slot Time, σ 20 µs

DIFS 60 µs

SIFS 10 µs

Propagation Delay 1 µs Bit Rate 11 Mbps PHY Overhead 192 µs

MAC Header 28 byte ACK Length 14 byte

Data Packet Payload, B 1028 byte Max Backoff Stage, BSmax 4 Initial Contention Window, W0 16

Transmission Retry Limit ∞

Figure 2.2 illustrates the collision probability pc of the APP, BEB, and DDFC MAC schemes. It reveals that APP with P0 =1/4 achieves an improvement of collision probability by 40% (38.8%) over DDFC (BEB with W0=16), when the number of stations is 8. The reason is that the proposed APP MAC scheme assigns every packet a permission probability P. When two stations count to zero simultaneously, the collision probability of APP is equal to P². Thus, APP has smallest collision probability; and the smaller the P0 is, the lower the collision probability would be. This phenomenon is equivalent to making the initial contention window larger. The figure also exhibits that the discrepancy between numerical and simulation results is less than 3.5%, thus this corroborates the collision probability analysis.

0

Figure 2.2 Collision probabilities of APP, BEB, and DDFC

Figure 2.3 depicts the system throughputs of the APP, BEB, and DDFC MAC schemes. It can be seen that the throughput increases first and then decreases. It is because increasing the number of stations not only raises the channel utilization but also enlarges the packet collision probability as shown in Fig. 2.2, so the throughput increases first and decreases due to high collision probability. Also, APP with P0 =1/4 achieves an improvement of throughput by 7% (6.5%) over DDFC (BEB with W0=16) when the number of stations is 8. The reason is that APP can reduce the collision probability and increase the transmission efficiency consequently. It can also be found that the smaller P0 will cause a lower system throughput when fewer stations are in the system. It is because the smaller P0 is equal to making a larger initial contention window.

This will increase the channel idle time and decrease the channel utilization. Noticeably,

justifies the validity of the throughout analysis.

Figure 2.3 System throughputs of APP, BEB, and DDFC

Figure 2.4 shows the mean delays of the APP, BEB, and DDFC MAC schemes. It indicates that the APP with P0 =1/4 achieves an improvement of mean delay by 6.6%

(6.1%) over DDFC (BEB with W0=16), when the number of stations is 8. It is because the APP enhances the channel utilization. It can also be found that the smaller P0 has a larger delay time when there are fewer stations in the system but a smaller delay time when there are more stations in the system. Also, the difference between numerical and simulation results is less than 3.23%, and this substantiates the delay analysis.

Figure 2.5 shows delay variances of the APP, BEB, and DDFC MAC schemes versus the number of stations by simulations. It can be found that the APP MAC scheme possesses the lowest delay variation, while the BEB MAC scheme (BEB with W0=16)

the highest. For example, the APP with P0 =1/4 achieves improvement of delay variation over DDFC (BEB with W0 =16) by 76.4% (79.4%), at the number of stations is 8. Also, the smaller the P0 is, the more the improvement of delay variation would be. The reason is the proposed APP scheme adaptively determines the permission probability of transmission according to a function of the number of retransmission (RT) and the number of re-backoff (RB). The APP scheme lets the ready packet with the longest delay time transmit first and delays the new packet, this makes the delay time of packet be close to the mean value.

0 0.005 0.01 0.015 0.02 0.025

2 3 4 5 6 7 8 9 10 11 12 13 14 15

The Number of Stations

Mean Delay (Sec)

BEB, W0 =16 (simulation) BEB, W⁰ =32 (simulation) DDFC (simulation) APP, P⁰ =1/2 (numerical) APP, P0 =1/2 (simulation) APP, P0 =1/4 (numerical) APP, P0 =1/4 (simulation) APP, P0 =1/16 (numerical) APP, P0 =1/16 (simulation)

Figure 2.4 Mean delays of APP, BEB, and DDFC

Besides, making P0 smaller is equivalent to making the W0 larger, thus lower collision probability. However, the large W0 in the BEB cannot greatly decrease delay

Figs. 2.2 – 2.4). It is because the APP scheme is not actually increase the size of W0, but provides another dimension (permission probability P) to avoid collision and makes the transmission efficiency, and thus the APP scheme has the smallest mean delay and highest system throughput.

0.0000 0.0005 0.0010 0.0015 0.0020 0.0025 0.0030

2 3 4 5 6 7 8 9 10 11 12 13 14 15

The Number of Stations

Delay Variance

BEB, W0 =16 (simulation) BEB, W⁰ =32 (simulation) DDFC (simulation) APP, P⁰ =1/2 (simulation) APP, P0 =1/4 (simulation) APP, P0 =1/16 (simulation)

Figure 2.5 Delay variances of APP, BEB, and DDFC

Figure 2.6 shows the system throughput and delay variance of APP with optimal

*

P and BEB with W0 opt given in [6] by simulations, where the BEB operates with Wopt to obtain the maximum system throughput and the APP uses the optimal P0 with fixed W0. It can be found that APP with optimal P loses the system throughput by 1.3% but ₀^* gains an improvement of delay variation by 15%, compared to BEB with Wopt. This shows that the APP MAC scheme can achieve maximum system throughput and support good delay fairness.

System Throuthput (Mbps) Delay Variance (ms2 )

*

0 0

P =P

*

0 0

P =P

Figure 2.6 performance of APP with optimal P and BEB with W₀^* opt