Perspectiva Legal

tiated minimum PHY rates do not receive the QoS guarantees. However, we also mentioned in Section 4.3 that when a wireless LAN has some unutilized resource (i.e., the airtime), the AP may temporarily allocate more resources to the stations lowering their PHY rates — without violating other stations’ QoS — so as to support their QoS at lower PHY rates. This can be done via the HC in the HCCA by com- puting a new service schedule. In Section 4.3, we claim that these adjustments can be completed without any centralized control if using the enhanced EDCA, thanks to the autonomous distributed airtime control.

To simulate this scenario, we assume that the wireless LAN only admits 4 stations before t = 15 second, and stations 1, 2 and 4 carry a single stream and station 3 carries 2 streams. We again assume that each stream requires a 5-Mbps guaranteed rate and

Varying PHY rates of station 1: light load (throughput analysis in the EDCA) ˃ ˅˃˃˃˃˃˃ ˇ˃˃˃˃˃˃ ˉ˃˃˃˃˃˃ ˋ˃˃˃˃˃˃ ˄˃˃˃˃˃˃˃ ˄˅˃˃˃˃˃˃ ˃ ˄˃ ˅˃ ˆ˃ ˇ˃ ˈ˃ ˉ˃ ˊ˃ ˋ˃ ˌ˃ ˄˃˃ Time (second) T h ro u g h p u t (b p s) ̆̇˴ ̇˼̂ ́ ʳ˄ ʳʻ˄ ʳ̆̇̅˸ ˴ ̀ ʼ ̆̇˴ ̇˼̂ ́ ʳ˅ ʳʻ˄ ʳ̆̇̅˸ ˴ ̀ ʼ ̆̇˴ ̇˼̂ ́ ʳˆ ʳʻ˅ ʳ̆̇̅˸ ˴ ̀ ʼ ̆̇˴ ̇˼̂ ́ ʳˇ ʳʻ˄ ʳ̆̇̅˸ ˴ ̀ ʼ ̆̇˴ ̇˼̂ ́ ʳˈ ʳʻ˄ ʳ̆̇̅˸ ˴ ̀ ʼ

Station 1 lowers its PHY rate to 18 Mbps

Figure 4.16. Throughput of individual streams in the EDCA: station 1 lowers its PHY rate to 18 Mbps at t = 15 second. *The wireless LAN is not heavily loaded when station 1 lowers its PHY rate at t = 15 second. Therefore, station 1 can still receive the 5-Mbps guaranteed rate after t = 15. However, after t = 20 second, station 1 has to “relinquish” the extra airtime it is using so that station 5, which complies the minimum PHY rate of 54 Mbps receives the 5-Mbps guaranteed rate.

that all stations are required to transmit at 54 Mbps to maintain their QoS. We assume that station 1 lowers its PHY rate to 18 Mbps at t = 15 second. Unlike Scenario 3, the wireless LAN is still able to (but not necessarily has to) provide the QoS to station 1 without affecting other stations’ since there are only 5 streams asking a total amount of airtime (before t = 20 second)

4 ∗ 5 54 +

5

18 = 0.64 < 0.65 = EAedca. (4.17) We can observe in this figure that station 1 still obtains the required 5-Mbps guaran- teed rate even though it violates the agreement upon using a 54-Mbps transmission rate. Here, we do not need any additional adjustments as required in the HCCA.

Instead, station 1 automatically adjusts its airtime usage by contending the wireless medium more frequently via the enhanced EDCA, due to the build-up MAC buffer queue.

Varying PHY rates of station 1: light load (delay analysis in the EDCA) ˃ ˃ˁ˃ˈ ˃ˁ˄ ˃ˁ˄ˈ ˃ˁ˅ ˃ˁ˅ˈ ˃ˁˆ ˃ ˄˃ ˅˃ ˆ˃ ˇ˃ ˈ˃ ˉ˃ ˊ˃ ˋ˃ ˌ˃ ˄˃˃ Time (second) P ak ce t D el ay ( se co n d ) station 1 (1 stream) station 2 (1 stream) station 3 (2 stream) station 4 (1 stream) station 5 (1 stream)

Figure 4.17. Delay of individual streams in the EDCA: station 1 lowers its PHY rate to 24 Mbps at t = 15 second. *The wireless LAN is not heavily loaded when station 1 lowers its PHY rate at t = 15 second. Therefore, all streams’ delay bound are still satisfied after t = 15. However, after t = 20 second, station 1 has to “relinquish” the extra airtime it is using so that station 5, which complies the minimum PHY rate can receive the QoS. As a result, station 1’s stream experiences a delay greater than the required delay bound at t = 20 second.

station. When station 5 requests for admission at t = 20 second, the AP should admit it based on Eq. (4.5)

6 ∗ 5

54 = 0.55 < 0.65 = EAedca, (4.18) since all stations are required to transmit at Ri=54 Mbps. However, not all stations actually transmit at 54 Mbps. The total amount of airtime we really need to support QoS for all streams is

5 ∗ 5 54 +

5

18 = 0.73 > 0.65 = EAedca, (4.19) where stations 2-5 have 5 streams in total to transmit at 54 Mbps and stations has 1 stream to transmit at 18 Mbps. Obviously, station 1 should not receive the QoS (5-Mbps guaranteed rate). Figure 4.16 again shows this “expected” behavior and the most important fact is that such adjustment is again achieved automatically

(via the EDCA parameters) without any adjustment which is required in the HCCA.

t = 20 second, the delay bound of station 1 is satisfied even though station 1 violates

the minimum PHY rate requirement. However, such QoS is not guaranteed any more after t = 20 second, because station 5 joins the wireless LAN and complies with the minimum PHY rate requirement.

4.6

Conclusion

In this chapter, we provided a complete set of QoS solutions for the infrastructure- mode 802.11 wireless LAN using both the HCCA and the EDCA, and for the ad hoc-mode 802.11 wireless LAN. In order to provide parameterized QoS guarantees in the EDCA, we exploited the distributed airtime usage control developed in Chapter 3. We also extended the current QoS signaling of the HCCA to do admission control for the parameterized QoS in the EDCA. The simulation results showed that by using the EDCA, we are able to achieve the same level of parameterized QoS support as the HCCA, but results in less complexity than the centralized, polling-based HCCA scheme.

CHAPTER 5

Spectral-Agile Radios

The most important task of a network to support QoS is to provide users their required bandwidth. Therefore, as long as the network has sufficient system band- width, providing QoS support is a relatively easy task. Unfortunately, this is not the case in conventional wireless networks where the system bandwidth is a very precious and limited resource. Although such limitation is due to the scarcity of the wireless spectrum, it is the static spectrum allocation policy that prevents wireless networks from utilizing the spectrum more efficiently, and acquiring more usable bandwidth.

Under the current static spectrum allocation policy, wireless devices are only allowed to operate in designated spectral bands. For example, the IEEE 802.11b and 11g wireless stations are only allowed to operate in the unlicensed 2.4 GHz band, and so are the Bluetooth devices and cordless phones. These devices (in the crowded unlicensed bands) are prohibited from using other spectral bands even though those spectral bands may never or rarely be utilized by their designated users. As a result, these wireless devices get stuck in the heavily-used spectral bands, competing with each other for a very limited bandwidth, while many other spectral bands are left unused. One can expect that if the wireless devices (in crowded spectral bands) are allowed to explore and utilize the rarely-used spectral bands opportunistically, not only the performance of individual devices but also the overall spectrum efficiency can be improved.

In this chapter, we propose a new type of wireless communication based on op-

portunistic use of the wireless spectrum. This new type of communication, referred to

as the spectral-agile communication, relies on radio devices’ capability of seeking and utilizing (in real time) the spectral resources — in time, frequency and space domains.

From the perspective of QoS provisioning, using spectral agility helps a radio device acquire more spectral resources so as to provide users better QoS. Of course, the spectral-agile communication cannot be realized without developing new spectrum access mechanisms. Therefore, we propose a comprehensive framework along with resource monitoring and utilization functionalities to facilitate the adoption of spec- tral agility. Moreover, we establish a mathematical model to evaluate the potential performance gains of using the spectral agility.

This chapter is organized as follows. Section 5.1 describes the system model and assumptions for our development of spectral-agile communication. In Section 5.2, we present the mathematical model, and discuss and analyze the numerical results. Section 5.3 details the framework for spectral-agile communication, and the associ- ated functionalities. The ns-2 based simulation results are analyzed and discussed in Section 5.4. Finally, conclusions are drawn in Section 5.5.