122	6. Coexistence and Interworking between 802.11 and HiperLAN/2

6.2Interworking Control of ETSI BRAN HiperLAN/2 and IEEE 802.11

In this section, a concept to integrate HiperLAN/2 into 802.11 is described in detail. The concept is developed and introduced in Mangold et al. (2001a), Mangold et al. (2001b), Mangold et al. (2001c), and Mangold et al. (2001d). As explained in Chapter 2, 802.11 and HiperLAN/2 apply nearly the same OFDMbased transmission scheme and channelization, which facilitates interworking.

Interworking of 802.11 and HiperLAN/2 implies the communication between stations of similar and different types in a common integration protocol. The concept discussed in the following realizes this integration by a centrally coordinating device that is capable of operating in both, an 802.11 and HiperLAN/2 mode. As part of the interworking concept, regular HiperLAN/2 MAC frames with durations of 2 ms are integrated into the superframe of 802.11e. The concept is based on the QoS enhancements of 802.11 that are defined in the 802.11e MAC enhancements (IEEE 802.11 WG, 2002a). Interworking is realized by applying the HCF of the upcoming IEEE 802.11e QoS-enabled MAC. A combination of a HiperLAN/2 Central Controller (CC) and 802.11a/e Hybrid Coordinator

(HC), referred to as CCHC, is proposed for the interworking of 802.11a/e and HiperLAN/2 systems. The CCHC is placed in a device that must have 802.11a/e MAC/PHY and in addition, the HiperLAN/2 MAC/PHY implemented. The CCHC works as the HC to 802.11a/e stations and as the CC to HiperLAN/2 stations. The proposed CCHC relies on the HCF including QoS CF-poll as described in IEEE 802.11 WG (2002a).

Once this interworking concept is established, it can serve as a basis for also providing support for coexistence of HiperLAN/2 and 802.11, as well as coexistence of overlapping 802.11e QBSSs. It has been shown in Chapter 5 that 802.11e QBSSs suffer from unpredictable QoS reductions if they overlap and if more than one QBSS applies the HCF for controlled channel access.

6.2.1CCHC Medium Access Control

The CCHC as a single device is proposed that operates at one single frequency channel to coordinate the HiperLAN/2 and 802.11 networks, i.e., a HiperLAN/2 cell and an 802.11 QBSS. Within the limit of each radio resource allocated to stations under control of the CCHC, a station itself decides what data to transmit. This is exactly the concept used in 802.11e, when stations are polled by an HC. It appears natural to extend this concept for defining HiperLAN/2 MAC frames by the CCHC to cover the needs of HiperLAN/2 stations in an inter-

6.2 Interworking Control of ETSI BRAN HiperLAN/2 and IEEE 802.11

123

working scenario. It is assumed that the CCHC is able to execute both protocols completely to organize the interworking between any stations.

6.2.1.1CCHC Scenario

Figure 6.3 shows a CCHC based scenario, including the combined protocols used by the CCHC. One CCHC and one controlled station of each wireless network type are shown. The control over the stations is guaranteed by regularly allocating radio resources for some predefined duration to the 802.11 and HiperLAN/2 stations, by the CCHC that has full control over the radio channel.

Allocated time intervals are here referred to as resource allocations. A resource allocation is interpreted by an 802.11 station as a TXOP according to the 802.11e protocol, and by a HiperLAN/2 station as one or more consecutive HiperLAN/2 MAC frames, i.e., time intervals of 2 ms length that are started by a beacon (i.e., broadcast channel).

The interworking scenario addressed in Figure 6.3 allows the exchange of information between HiperLAN/2 and 802.11 stations via the CCHC device. If a HiperLAN/2 station has data to deliver to an 802.11 station, and if both stations are associated with the BSS that is coordinated by the CCHC, then the HiperLAN/2 station delivers this data during a MAC frame to the CCHC, which then forwards the data within a later resource allocation to the addressed 802.11 station, by using the respective communication protocol. The CCHC comprises the MAC layers of both communication protocol stacks, with an harmonized PHY and some common services on top of the two user planes of the MAC layer. An adaptation layer or convergence layer may be required, as it is already available in HiperLAN/2 (ETSI, 2000c). A central management entity within the CCHC MAC layer controls the alternating turns of operation of the two parallel user planes.

6.2.1.2CCHC and Legacy 802.11

Besides using a HiperLAN/2 MAC frame and the high priority access through 802.11e HCF, 802.11 stations may wish to operate in the prominent EDCF mode, by contending for medium access whenever they want to transmit. This mode of operation would be no problem for the CCHC concept proposal, as long as stations follow the EDCF instead of the legacy DCF. The EDCF does not allow stations to allocate the radio channel for longer durations than the TXOPlimit, and thus can be easily coordinated by the CCHC. Stations that operate according to the legacy DCF are here referred to as legacy 802.11 stations and should not be allowed to associate with a QBSS coordinated by a CCHC, because

124	6. Coexistence and Interworking between 802.11 and HiperLAN/2

they would violate the TXOPlimit, see Chapter 4, p. 41. Besides to not allowing legacy 802.11 stations to associate with a QBSS coordinated by the CCHC, a function existing in 802.11 is proposed to keep legacy stations silent. The function helps to prevent legacy 802.11 stations from interfering with CCHC resource allocations. The Extended Interframe Space (EIFS) specified to be able to operate under hidden station interference is proposed to be exploited by the CCHC, in order to silence down legacy duration stations. There is a mechanism specified in the IEEE 802.11 MAC protocol that allows QBSSs coordinated by a CCHC that are co-located with 802.11 stations to force 802.11 stations to defer from medium access for a long time, i.e. EIFS duration. This mechanism has been originally defined to reduce interference of hidden stations. The 802.11 MAC protocol specifies a concept called virtual carrier sensing, as explained in Chapter 3.

An 802.11 station that detects a valid preamble, but that is not able to successfully receive the complete frame, assumes a hidden station scenario and is forced to defer from medium access for a long duration, called EIFS. A Frame Check Sequence (FCS) that is part of any frame in 802.11 is incorrect in case of unsuccessful frame reception. The CCHC should take advantage of this. By using the same preambles and headers, but different FCSs or different PHY modes for the rest of the frames, legacy stations that detect frames from the CCHC will operate with EIFS instead of DIFS.

detection ranges

802.11 station

HiperLAN/2

station CCHC

vectors indicate "has control over"

CCHC protocol with harmonized

PHY and MAC

Applications

(for example, IP, 1394.1)

Adaptation-/ Convergence-

Layer

	Hiper-	802.11	harmo-
	LAN/2	(e,h)	nized
	MAC	MAC	Manage-
			ment
	harmonized PHY		Plane

Figure 6.3: CCHC coordinating 802.11 and HiperLAN/2 stations. The detection ranges indicate that all stations are in the range of CCHC, which is required for QoS support. Note that this requirement exists for all standard QBSSs.

6.2 Interworking Control of ETSI BRAN HiperLAN/2 and IEEE 802.11

125

It is possible for a QBSS coordinated by a CCHC to protect itself from interference of legacy 802.11 stations by regularly transmitting preambles and PLCP headers9 during the EIFS duration, which then would set the NAV in the 802.11 stations again for another EIFS duration.

6.2.1.3CCHC Working Principle

Figure 6.4 shows the proposed CCHC frame structure. It can be seen that within the CCHC superframe with optional Contention Free Period (CFP), the CCHC allocates TXOPs in order to allow the periodic resource allocation for HiperLAN/2 MAC frames.

To enable the alternated operation of 802.11 and HiperLAN/2 in subsequent resource allocations, the HiperLAN/2 stations receive a periodic AP-Absence announcement by the CCHC, a concept in HiperLAN/2 to allow the Hiper- LAN/2-AP or CC to stop transmitting the periodic beacon for some defined time interval. Originally, AP-Absence is defined to let the AP/CC perform channel measurements (ETSI, 2000c).

The HCF is the basis for the new CCHC interworking concept. The QoS CFPoll can be used by the CCHC to allocate TXOPs within the Contention Period (CP) with high priority, i.e., after PIFS idle time. The CCHC may initiate a frame exchange right after PIFS during the CP by immediately transmitting a data frame, preceded by or without RTS/CTS followed by an HiperLAN/2 MAC frame. One HiperLAN/2 MAC frame is shown to be transmitted after a CF-Poll in the CFP in Figure 6.4.

According to Figure 6.4, a superframe between two TBTTs is starting with an 802.11 beacon as the first frame. Information fields in the beacon announce the superframe duration and inform all stations whether a CFP will start right after the beacon. Further, the TXOPlimit and the EDCF parameters are broadcasted by the CCHC via information fields in the beacon and can be used to control the impact of the EDCF background traffic on the resource allocations scheduled by the CCHC.

In the example shown in Figure 6.4, there is a CFP with two HiperLAN/2 MAC frames, ending with the CF-end frame. This example will be discussed in the following.

9See Chapter 3, p. 19, for the explanation of the frame structure in 802.11, including the PLCP and MAC headers.