54	4. IEEE 802.11e Hybrid Coordination Function

4.2.5Traffic Streams

To determine and classify MSDUs that are delivered within a QBSS with certain QoS requirements, 802.11e stations use so-called Traffic Streams (TSs) that are identified by Traffic Stream Identifiers (TSID). A TS is a set of MSDUs that are delivered using a certain EDCF parameter set. A TS is in this thesis referred to as stream. An 802.11e station has to be capable of supporting 8 streams from the HC to itself and 8 streams from itself to any other station, including the HC, at the same time. Before a stream is delivered, 802.11e stations have to negotiate with the HC the so-called Traffic Specification (TSPEC) for this stream. The TSPEC includes objectives and limits for traffic characteristics such as the MSDU frame body sizes, arrival rates, maximum MSDU Delivery delay, and variance. A socalled Traffic Classification (TCLASS) is used between the 802.11e MAC and the higher layer at the MAC SAP, in order to classify MSDUs into the TSPECs, by using the Traffic Identifier (TID). TID structure and TCLAS specification is implementation dependent and beyond the scope of the standard, as is the classification process itself.

4.2.6Default EDCF Parameter Set per Draft 4.0, Table 20.1

Table 4.2 is a short version of a table in IEEE 802.11 WG (2002c), and shows recommended default values for the EDCF parameter sets for the four ACs, as defined by 802.11e. An HC may use these values when setting up a QBSS, and may change them dynamically upon change of channel and traffic condition.

Table 4.2: 802.11e recommended default values (IEEE 802.11 WG, 2002c).

			TXOPlimit*	AIFSN	AIFS	earliest channel	PF
AC	CWmin *	CWmax *	(802.11a,
						access
			802.11g)
0	CWmin	CWmax	0	2	DIFS	DIFS+aSlotTime	32/16=2
1	CWmin	CWmax	1.5 ms	1	PIFS	PIFS+aSlotTime	32/16=2
2	(CWmin+1)	CWmin	3.0 ms	1	PIFS	PIFS+aSlotTime	32/16=2
	/2-1
3	(CWmin+1)	(CWmin+1)	1.5 ms	1	PIFS	PIFS+aSlotTime	32/16=2
	/4-1	/2-1

*) PHY dependent