96	5. Evaluation of IEEE 802.11e with the IEEE 802.11a Physical Layer

5.2Contention Free Bursts

This section is based on Mangold et al. (2002d). Here, fairness problems between QBSSs are discussed that exist when coexisting, overlapping QBSSs share the radio channel. By applying a new 802.11e mechanism, called Contention Free Bursts (CFBs), it is shown that wireless LANs gain from intelligent radio resource control in a fair manner.

A simple radio resource control scheme based on the dynamic selection of PHY modes is introduced in the next section. The combination of this scheme with CFBs, and the gain of spectrum efficiency when using this combination are discussed in the following sections.

5.2.1Contention Free Bursts and Link Adaptation

The CFB concept is defined in 802.11e and described in detail in Section 4.2.5, p. 54.

Link Adaptation (LA) is the process of dynamically selecting a combination of PHY modes for the transmission of frames, under certain conditions such as the channel error probability, and required QoS. For example, the throughput optimization in the 802.11a wireless LAN via LA is presented in Qiao and Choi (2001).

For the analysis in this section, a simple open loop LA process is used, which counts the number of successful and failed transmissions and switches the PHY mode after a certain number of transmission successes or failures. A transmitting station that carries data for more than one station selects the PHY mode with respect to the addressed receiving station. Such a station is typically the AP. It has to alternate the PHY mode from frame exchange sequence to frame exchange sequence with high dynamics. Applying this simple LA process, a station ends up transmitting with the PHY mode that optimizes the throughput, by periodically attempting to increase it. This attempt occurs after 25 successful transmissions. This may then lead to higher probability of failed transmissions, which means that the station has to fall back to the original PHY mode, here after 4 unsuccessful transmission attempts. Finding an optimal algorithm for LA is beyond the scope of this discussion. The used algorithm is limited but allows to investigate the combination of CFBs with the radio resource control, i.e., with LA.

In principle, a frame can be transmitted with an individually optimized PHY mode, but in case of control frames under the following restriction. The 802.11a standard defines mandatory PHY modes, i.e., 6, 12, and 24 Mbit/s, which every 802.11 station must be able to operate with. As control frames (e.g.,

5.2 Contention Free Bursts

97

RTS/CTS/ACK) should be received not only by the addressed station but also by other active stations in the area close to the transmitting and receiving station, they must be transmitted using one of the mandatory PHY modes.

By applying dynamic LA, a station can select the optimal PHY mode in order to use the radio spectrum more efficiently. The duration of a frame exchange can be minimized when using dynamic LA. When CFBs are used in addition, the station may be able to transmit more MPDUs per TXOP. The advantages of the combination of LA and CFBs are discussed in the following.

5.2.2Simulation Scenario: two Overlapping QBSSs

Event-driven stochastic simulation is used for the analysis of CFBs. Figure 5.26 shows the scenario of two overlapping QBSSs with three stations in each QBSS. The two stations 2.1 and 1.1 are HCs, which deliver MSDUs to the other stations. Each HC generates the same mix of offered traffic of three data streams per station.

The three data streams are labeled with “high”, “medium”, and “low”, according to their priorities. The HC 1.1 transmits three data streams to station 1.2 and three data streams to station 1.3; the HC 2.1 transmits three data streams to station 2.2 and three data streams to station 2.3.

At the high priority AC, MSDUs of 80 byte are transmitted. The negativeexponentially distributed inter-arrival time has a mean of 2.5 ms for the offered traffic of 256 kbit/s. The high priority streams offer 256 kbit/s per stream throughout all simulation campaigns. The medium and low priority streams each transmit MSDUs of 1514 byte with negative-exponentially distributed inter-arrival times, each stream with variable rates.

35m

1m

stations of

QBSS2 transmit

station 2.3

station 2.2

station 2.1

at BPSK1/2

always

QBSS 2

QBSS 2

QBSS 2

1m

(AP)

stations of QBSS1

are capable of

station 1.3

station 1.2

station 1.1

dynamic link

adaptation

QBSS 1

QBSS 1

QBSS 1

(AP)

3 DL streams per station with different priorities

Figure 5.26: Simulation scenario. The two larger stations are the HCs that deliver MSDUs with three different priorities to their associated stations. All stations are in range to each other (no hidden stations).