The Effect of Parameter θ on Absolute Throughput Support

Parameter θ is a deescalating factor that is used to reduce the fair share of RT-MSs in comparison to the fair share based on the normal weight calculation. The range of θ is between 0 and 1 (0 < θ ≤ 1). The smaller the value of θ, the smaller is the fair share of RT-MSs. According to Eq. (3.17), the higher the value of overload ratio (ω/θ), the larger is the amount of traffic from RT-MSs that can be supported while absolute throughput is maintained. Therefore, to achieve a higher amount of supported traffic from RT-MSs, we can simply increase the value of the overload ratio. Two ways that the overload ratio can be increased: a) decreasing θ (discussed below) or b) increasing ω (discussed in Section 4.3).

The first method to prolong the absolute throughput support is to calculate an appropri- ate value of θ. The appropriate value of θ can be calculated by Eq. (4.1) during the phase of network design where the maximum throughput requirement of RT-MS can be projected or estimated. Assume that we would like to support the traffic from RT-MSs up to 9.5 Mbps and traffic from AT-MSs up to 0.5 Mbps, i.e., total of 10 Mbps in 2 Mbps WLAN. According to Eq. (4.1), the value of θ must be equal to or less than 0.5 to achieve this objective.

θ ≤ ω · λe− P ∀i∈AT,AD λ[i] P ∀j∈RT λ[j] (4.1)

To demonstrate the effectiveness of θ, we consider a new scenario 1AT+19RT+05TT. Sce- nario 1AT+19RT+05TT is similar to scenario 1AT+19RT, except the value of θ is changed from 1.0 to 0.5. In scenario 1AT+19RT+05TT, Figure 4.4 (a) shows that the AT-MS re- ceives as much throughput as it requires for the entire simulation. During 200 < t < 250, the total throughput requirement in the WLAN is 10 Mbps. This result matches the calculation discussed previously. We believe that this level of aggregate throughput requirement of 10 Mbps in a 2 Mbps WLAN is extremely high and sufficient in most situations. However, without admission control, absolute throughput support cannot be guaranteed. Additional traffic can still be injected into the network, since there is no mechanism to prevent it from doing so. If the amount of throughput requirement from RT-MSs is higher than 10 Mbps (although unlikely), the ability to provide absolute throughput will deteriorate. The results in terms of aggregate throughput, MAC delay, jitter, collision rate, and backoff interval are

also provided in Figure 4.4 (b) - Figure 4.4 (f). Due to the higher offered load, the MAC delay, jitter, collision rate, and backoff interval of 1AT+19RT scenario are higher than those of 10RT scenario. However, the aggregate throughput remains the same. Note that, the ag- gregate throughput of scenario 10RT drops sooner than that of scenario 1AT-19RT because there are fewer MSs in scenario 10RT than scenario 1AT+19RT. Since each MS remains active for the same 240s, the last MS of scenario 10RT becomes inactive sooner than the last MS of scenario 1AT+19RT. Next, we will evaluate the second method, namely DSG-RT, to maintain the ability to provide absolute throughput support by limiting offered load in a distributed manner.

4.1.4 Distributed SafeGuard for Absolute Throughput Support in Relative Throughput Flows

The objective of DSG-RT is to limit the excessive traffic from RT-MSs to prevent deterio- ration of absolute QoS support. This objective is achieved by each RT-MS independently comparing the experienced throughput with the overload threshold. A new flow or RT-MS will not be allowed to continue if the overload threshold is violated. The overload threshold is the throughput level that can be calculated independently by each MS from the specified quantum rate, ω, and θ at each RT-MS, Eq. (3.22).

To demonstrate the performance of DSG-RT, we consider the scenario 1AT+19RT-DSG where θ is kept at 1.0. Without DSG-RT, the throughput of AT-MS begins to deteriorate at t = 110s or when the 11th_{RT-MS becomes active (Figure 4.3). With DSG-RT, absolute}

throughput support can be maintained since additional traffic from RT-MSs after t > 110s will not be allowed to continue (Figure4.5). The experienced throughput of AT-MS remains equal to the specified throughput requirement while the experienced throughput of accepted RT-MSs is proportional according to its weight. The overload threshold is set at 200 Kbps. If the experienced throughput of RT-MSs is smaller than the overload threshold, a new MS will not be allowed. The main advantage of DSG-RT is that the mechanism is fully distributed and does not require exchanging any information from other MSs. Each RT-MS monitors the experienced throughput and accepts or rejects a new flow independently.

0 50 100 150 200 250 300 350 400 450 0 1 2 3 4 5 6x 10 5 Throughput (bps) Time (sec) DRAFT−AT − 500Kbps DRAFT−RT − 500Kbps 4.4 (a): Throughput 0 50 100 150 200 250 300 350 400 450 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2x 10 6 Throughput (bps) Time (sec) Scneario − 1AT+19RT Scneario − 10RT 4.4 (b): Aggregate Throughput 0 50 100 150 200 250 300 350 400 450 0 1 2 3 4 5 6 Delay (sec) Time (sec) DRAFT−AT − 500Kbps DRAFT−RT − 500Kbps 4.4 (c): MAC Delay 0 50 100 150 200 250 300 350 400 450 −0.05 −0.04 −0.03 −0.02 −0.01 0 0.01 0.02 0.03 0.04 0.05 Jitter (sec) Time (sec) DRAFT−AT − 500Kbps DRAFT−RT − 500Kbps 4.4 (d): Jitter 0 50 100 150 200 250 300 350 400 450 0 0.1 0.2 0.3 0.4 0.5 0.6 Collision Rate Time (sec) DRAFT−AT − 500Kbps DRAFT−RT − 500Kbps

4.4 (e): Collision Rate

0 50 100 150 200 250 300 350 400 450 0 20 40 60 80 100 120 140 160 BI (ts) Time (sec) DRAFT−AT − 500Kbps DRAFT−RT − 500Kbps 4.4 (f): Backoff Interval

0 50 100 150 200 250 300 350 400 450 0 1 2 3 4 5 6x 10 5 Throughput (bps) Time (sec) Overloading Threshold Accepted MSs Rejected MSs DRAFT−AT − 500Kbps DRAFT−RT − 500Kbps

Figure 4.5: Absolute Throughput Support with DSG-RT, 1 AT-MS & 19 RT-MSs