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

Coexistence and interworking of two different types of wireless networks are discussed in the following. This chapter introduces a concept for interworking of wireless LANs in the 5 GHz unlicensed band, HiperLAN/2 and 802.11. As explained in Chapter 2, the 802.11a wireless LANs operate with an OFDM based transmission scheme defined in IEEE 802.11 WG (1999a) in the 5 GHz. We refer to this 802.11a in the following, when referring to 802.11. HiperLAN/2 operates with the same transmission scheme and channelization, with some minor modifications (ETSI, 2000a). Since 802.11 and HiperLAN/2 operate at the same frequency band, and because the transmission schemes are so similar, it is therefore natural to look at MAC enhancements that allow for interworking or coexistence.

This chapter provides in the next section a brief overview on HiperLAN/2, based on (Walke 2002; Walke et al. 2001). In Section 6.2, an interworking concept based on the integration of HiperLAN/2 into the 802.11 MAC protocol is developed, which was proposed as interworking solution between HiperLAN/2 and 802.11 in Mangold et al. (2001a), Mangold et al. (2001b), Mangold et al. (2001c), and Mangold et al. (2001d). One single communicating device coordinating a QBSS and being able to operate alternatively in 802.11 and HiperLAN/2 mode of operation is proposed to realize interworking there. Section 6.3 describes how this concept can serve as generic solution for the general coexistence problem. If this device can coexist with another co-located device of the same type, the coexistence problem of overlapping QBSSs, and between HiperLAN/2 and 802.11 stations is mitigated. As this is the focus of this thesis, Section 6.3 leads up to the following chapters, in which a game theoretic approach is developed and evaluated. This approach uses new adaptive methods for radio resource control for communicating devices in the presence of competing devices, with focus on radio spectrum sharing and QoS support for all coexisting wireless LANs.

6.1ETSI BRAN HiperLAN/2

ETSI BRAN HiperLAN/2 (referred to as HiperLAN/2) is a radio system for Wireless LAN and Home Networking applications. HiperLAN/2 is a standard for wireless transport systems including layer 1 and layer 2 protocols. A so-called convergence layer on top of the protocol allows the wireless extension of applications like Ethernet, IP, and IEEE 1394. For Logical Link Control (LLC), which is referred to as Error Control (EC) at ETSI BRAN, HiperLAN/2 defines its own EC scheme, which aims to provide a high level of QoS support. In the following, the reference model, the architecture, and the MAC protocol are summarized.

6.1 ETSI BRAN HiperLAN/2

119

For more details than given here, Walke (2002) provides an overview and performance evaluation of the HiperLAN/2 protocol.

6.1.1Reference Model (Service Model)

The HiperLAN/2 reference model is also referred to as service model, which is illustrated in Figure 6.1. The reference model comprises the Physical layer (PHY) and the Data Link Control layer (DLC) for stations and APs. A station is in HiperLAN/2 referred to as Mobile Terminal (MT), or if it is not mobile, as Wireless Terminal (WT). In this thesis, “station” is used instead of MT/WT. Various network types like Internet Protocol (IP), Ethernet, IEEE 1394 and the Asynchronous Transfer Mode (ATM) can be connected to the DLC layer by the Convergence Layer (CL) that performs the adaptation of the packet formats to the requirements of the DLC layer. For higher layers other than ATM, the CL contains a Segmentation and Reassembly (SAR) function.

The physical layer provides the basic transport functions for the DLC Protocol Data Units (PDU), that are equivalent to the MSDUs in 802.11. The DLC layer is divided into two parts, the control plane and the user plane. In the user plane, data are handled that arrive from higher layers via the User Service Access Point (U-SAP). This user plane contains the EC that implements the Automatic Repeat Request (ARQ) protocol. Therefore, HiperLAN/2 comprises a MAC and a LLC protocol; the combination of the two is referred to as DLC protocol. The HiperLAN/2 DLC protocol is connection oriented and provides multiple connection end points in the U-SAP. The control plane consists of the Radio Link Control (RLC) protocol that includes the connection control, the radio resource control, and association control functions. The HiperLAN/2 RLC is equivalent to a combination of MLME and SME in 802.11. Both planes access the physical medium via the MAC protocol.

6.1.2System Architecture

HiperLAN/2 is centrally controlled. An AP that is typically connected to a core network or a distribution system consists of an Access Point Controller (APC) and up to sixteen Access Point Transceivers (APTs). Each APT operates on a different frequency channel.

Two operation modes are defined for the HiperLAN/2 DLC: centralized mode and direct mode. In the centralized mode, stations communicate via the AP with other stations. In the direct mode, stations communicate user data under the control of the AP directly over direct links with each other, similar to what is

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

defined for 802.11e DiL. In both modes, the AP assigns radio resources, schedules the transmission times, and controls the communication in the radio cell. A radio cell in HiperLAN/2 is equivalent to a (Q)BSS in 802.11(e).

Data are transmitted via an RLC connection either between station and AP or between directly communicating stations. A connection must be established prior to the data transmission, using signaling functions of the HiperLAN/2 control plane. Therefore, HiperLAN/2 is centrally controlled, like the PCF in 802.11. Point-to-point connections are bidirectional, point-to-multipoint and broadcast connections are unidirectional from the AP to stations in the radio cell. Connections are realized by means of logical channels.

Because of the centralized approach and the connection orientation of HiperLAN/2, support of QoS is achieved. Each connection can be associated to QoS parameter requirements that define throughput, delay, delay variation, bit error probability, and others.

6.1.3Medium Access Control

In HiperLAN/2 the controlling station transmits a beacon every 2 ms, providing the timing of the Medium Access Control frame (MAC frame) that follows the beacon. Whereas in 802.11, a MAC frame refers to a data frame, in HiperLAN/2 a MAC frame refers to the periodic time interval of 2 ms.

The station that coordinates the MAC frame is referred to as Central Controller (CC) and has the full control over the frequency channel it is operating at. The CC is equivalent to an HC in 802.11e, and typically resides in an AP, but might reside in any other station.

transport layer

4

network layer

3

data link control 2 layer

physical layer

1

OSI reference model

Figure 6.1: HiperLAN/2 reference model.

									e
				e				n
								la
			n					p
			la				l
			p				ro
		r					t
	e					n
s					o
u					c
		Convergence Layer (CL)
		User-SAP				Control-SAP
Error Control (EC)					Radio Link Control
					(RLC)

Medium Access Control (MAC)

Physical Layer (PHY)

ETSI BRAN HiperLAN/2

6.1 ETSI BRAN HiperLAN/2

121

For the exclusive medium access, Dynamic Frequency Selection (DFS) is required in the HiperLAN/2 standard. With DFS, HiperLAN/2 systems are able to avoid that co-located CCs operate at the same frequency channel.

The HiperLAN/2 MAC frame comprises four generic phases, all controlled by the CC. Order and duration of the phases are determined by the CC and may change from MAC frame to MAC frame depending on the instantaneous QoS requirements. Typically, after the beacon sent to the associated stations, the unicast data from the CC to the stations follows in downlink, before transmissions from stations are scheduled for data in uplink direction. In HiperLAN/2, data transmission is performed by transmitting so-called Protocol Data Unit (PDU) trains, i.e., groups of consecutive DLC PDUs that carry the user data. The lengths of the PDU trains vary depending on what duration is scheduled by the CC for data transmission. The direct link phase, during which data is transmitted directly from station to station, is optionally allowed at any time during the MAC frame. During the random access phase of the MAC frame, stations may send short control frames, called Resource Requests (RRs) to ask for transmit capacity for the subsequent MAC frame. A random access phase ends a MAC frame and the next MAC frame starts immediately with the beacon data transmitted. The assignment of resources to the individual stations and to their connections is not static but may change dynamically from one MAC frame to the next MAC frame, depending on what the CC will schedule.

Figure 6.2 illustrates the operation of HiperLAN/2. The CC transmits the beacon (broadcast channel) at the start of a periodic MAC frame even if there is no user data to be transmitted. The beacon contains control information like network identity, transmission power levels, scheduled transmission starting points, and durations of random access phases. It is transmitted using the most robust modulation scheme available, i.e., BPSK1/2.

beacon channels	transmitted
	by CC (AP)

transmitted by stations

	one H2 MAC frame	uplink and direct link
		time

unicast downlink

random access

2ms

2ms

2ms

2ms

2ms

2ms

Figure 6.2: HiperLAN/2 timing. The MAC frame comprising beacon (broadcast) phase, down-, directand up-link phase, and random access phase, is repeated every 2 ms. The access to the radio channel is centrally controlled by one station, the CC. HiperLAN/2 requires an exclusive frequency channel.