4.4 Improved Efficiency

57

NAV (timer)

transmission

	PIFS	CF-Poll	SIFS	aSlotTime
				frame exchanges within CAP

busy channel

NAV coverage from CF-Poll

time

Figure 4.12: NAV protection of a CAP.

CAPs are protected by the NAV, as indicated in Figure 4.12. Through the longer NAV setting, the HC can grab the channel again without colliding with other frames, and continue to allocate the channel.

4.4Improved Efficiency

In this last section about 802.11e, four important schemes to improve the efficiency of the protocol, that are under discussion at TGe, are briefly described and summarized. Contention free bursts help to improve the achievable throughput, they are described in the next section. The optional block acknowledgement allows an higher throughput and is described in the Section 4.4.2. The controlled contention is an additional random access scheme that was proposed at TGe but not accepted. It is described in Section 4.4.3. Controlled contention can be compared to the random access in HiperLAN/2, and enables stations to react more dynamically on changes of the radio environment, or on changes of the offered traffic. Finally, Section 4.4.4 summarizes a concept that will allow higher layers to exchange time information between different stations, for the purpose of synchronization.

4.4.1Throughput Improvement: Contention Free Bursts

The concept of transmitting more than one MSDU after winning the EDCF contention is referred to as Contention Free Bursts (CFBs). Figure 4.13 illustrates the CFB concept. With CFBs, a backoff entity may transmit many MSDUs within one EDCF-TXOP, however, for a duration that must not exceed the TXOPlimit [AC ]. The advantage of CFBs is the increased maximum achievable throughput at the cost of potentially increased MSDU Delivery delays in other streams, which do not necessarily utilize their complete EDCF-TXOP until the maximum allowed duration, because they deliver only one MSDU per TXOP. With CFBs, the number of collisions can be reduced (Tourrilhes, 1998).

58	4. IEEE 802.11e Hybrid Coordination Function

without CFBs:				new backoff after each
	Transmission Opportunity, TXOP			frame exchange
	RTS	DATA		RTS	DATA
		(MSDU)			(MSDU)
busy			A
		CTS	C	CTS
channel			K	time

with CFBs:

TXOP with CFBs, duration not exceeding TXOPlimit

	RTS	DATA	RTS	DATA
		(MSDU)		(MSDU)
busy	CTS	A	CTS	A
		C		C
channel		K		K

new frame exchange right after ACK, w/o backoff

Figure 4.13: One MSDU per EDCF-TXOP (top) and Contention Free Bursts (CFBs) (bottom).

4.4.2Throughput Improvement: Block Acknowledgement

With this concept, the MSDU Delivery throughput efficiency of the protocol is improved. The block acknowledgement frame exchange is an optional enhancement defined in 802.11e. Block acknowledgements allow a backoff entity to deliver a number of MSDUs being transmitted during one TXOP, separated by SIFS, and transmitted without individual ACK frames, consequently reducing the protocol overhead. The MSDUs that are delivered during the TXOP are referred to as block of MSDUs.

At the end of the block, all MSDUs are individually acknowledged through a bit pattern delivered in the block acknowledgement frame, thus reducing the overhead of control exchange sequences to a minimum of one acknowledgement frame per number of MSDUs delivered in a block. Each data frame in a block protects its succeeding frame by setting the NAV as illustrated in Figure 4.14. The subsequent frame may either be another data frame or a frame requesting the acknowledges for the preceding data frames.

The following block acknowledgement that is transmitted by the receiving station in response to the request includes the pattern bitmap of acknowledges for all received MSDUs. The block acknowledgement may be transmitted immediately after the request, or alternatively later, in the next available TXOP.

It is possible to transmit blocks of MSDUs over more than one TXOP. Before using the block acknowledgement, the transmitting backoff entity and the receiving station need to set up the block acknowledgement by exchanging some primitives as defined in the 802.11e.

4.4 Improved Efficiency

59

NAV (timer) transmission

PIFS QoS

CF-Poll

busy channel

time

		NAV coverages from frames
SIFS	QoS	SIFS	QoS	SIFS	QoS	SIFS	Block
	DATA		DATA		DATA		Ack
							Req
							SIFS	Block
NAV coverage from CF-Poll								Ack
					may be transmitted immediately,
						or delayed in next TXOP

Figure 4.14: A block acknowledgement frame exchange may consist of up to 16 data frames before the actual acknowledgement pattern is requested.

After the block acknowledgement mechanism was successfully established, the receiving station will not send individual ACK frames upon reception of data frames with a registered address. If the transmitting backoff entity has only a limited number of MSDUs to deliver to the particular receiving station, it may temporarily request individual acknowledgements for the sake of higher flexibility.

4.4.3Delay Improvement: Controlled Contention

With controlled contention, the MSDU Delivery delay efficiency of the protocol is improved. The controlled contention is not part of the 802.11e draft 4.0, but was an early proposal at TGe (draft 2.0). It is here briefly summarized because of its similarity to the HiperLAN/2 random access protocol. An additional random access protocol that allows fast collision resolution is referred to as controlled contention. In order for the HC to grant TXOPs for MSDU Delivery, it requires information about the data that is pending in the queues of the stations. In a dynamic environment, this information has to be updated from time to time. The controlled contention interval as illustrated in Figure 4.15 is a means for the HC to update which backoff entity requires radio resources, at what times and for what durations. The controlled contention interval is initiated by the HC via a frame referred to as controlled contention frame, as shown in the figure. Following this frame, backoff entities may request TXOPs by transmitting resource

Controlled Contention	NAV coverage				time
(transmitted by HC)
PIFS	PIFS	Controlled Contention Interval
	Res.	Res.	empty		Res.
busy	Req.	Req.			Req.
			SIFS	SIFS	SIFS
channel		SIFS

		NAV (timer)
		transmission	collision, two stations	Reservation Requests
			transmit at the same time
				(transmitted by stations)

Figure 4.15: Controlled contention.

60	4. IEEE 802.11e Hybrid Coordination Function

requests, without contending with other backoff entities. The controlled contention frame defines the number of so-called opportunity intervals available for the random access (i.e., intervals separated by SIFS) and some access priorities for the ACs. Each backoff instance that is allowed to transmit resource requests selects one opportunity interval and transmits a resource request frame containing the requested AC and TXOP duration, or the queue size of the requested AC. Resource requests are transmitted within randomly selected opportunity intervals, and may therefore collide when two or more backoff entities select the same opportunity interval. For fast collision resolution, the HC acknowledges the receptions by generating another control frame with a feedback field so that the requesting backoff entities identify collisions in the preceding controlled contention interval. Upon the successful reception of the initiating controlled contention frame, the EDCF backoff entities, but also legacy 802.11 stations set the NAV until the end of the controlled contention interval, as illustrated in the figure.

4.4.4Support of Time-Bounded Data: Improved Timer Synchronization

With this concept, an accurate synchronization of timers that are maintained by applications on top of MAC is supported. Time-bounded data of some audioand video applications such as IEEE 1394 require an accurate synchronization across stations. Different timers running in the layers on top of the 802.11 MAC may have to be mutually synchronized with each other. This requires that the information about other timers is provided by the MAC layer to the higher layer. The actual implementation of how these timers are synchronized is not focus of the standard and therefore implementation dependent. However, what needs to be included in the standard are additional primitives that have to be exchanged between the MAC layers and the higher layers via the MAC Layer Management Entity – Service Access Point (MLME-SAP). Further, management frames are defined by 802.11e that allow the set-up of the improved timer synchronization, if required. After setting-up the timer synchronization by determining which stream requires such a precise synchronization, all data frames transmitted within the respective stream carry an additional time field. This field includes the timer information from the transmitting backoff entity of the point in time when the last symbol of the transmitted data frame was transmitted. The receiving station will deliver this information to its higher layer, after adding some processing delay to that time. How this processing delay, which depends on the receiver only, is determined by the MAC layer, is again implementation dependent and not standardized.

Chapter 5

EVALUATION OF IEEE 802.11E WITH THE

IEEE 802.11A PHYSICAL LAYER

5.1	HCF Contention-based Channel Access	........................... 62
5.2	Contention Free Bursts.......................................................	96
5.3	Radio Resource Capture....................................................	102

5.4HCF Controlled Channel Access, Coexistence of

Overlapping QBSSs ...........................................................	108
5.5 Summary and Conclusion .................................................	115

THIS CHAPTER provides the analysis of the main 802.11e MAC enhancements, and identifies problems in QoS support in 802.11. It further highlights the advantage of Contention Free Bursts (CFBs), and shows that a coexistence problem exists when multiple Hybrid Coordinators (HCs) use the Hybrid Coordination Function (HCF) with controlled channel access at the same time. The contention-based channel access, i.e., the Enhanced Distributed Coordination Function (EDCF) is analyzed based on an analytical approach to model the achievable throughput per AC of the contention-based channel access, and with the help of stochastic simulation. As part of this analysis, a new analytical model is developed and evaluated. This new model allows calculating the achievable throughput per ACs and the mutual influences when different ACs with arbitrary EDCF parame-

ter sets are at the same time used by an arbitrary number of backoff entities.

The used simulation tool is described in Appendix A. The analytical approach makes use of a modified version of the analytical model described in Appendix D. This chapter is outlined as follows. In Section 5.1 a performance evaluation of the EDCF medium access is presented, where an analytical approach for ap-