Unlike wired networks, packets transmitted on wireless channels are often subject to burst errors which cause back to back packet losses. Most wireless LAN link layer protocols recover from retransmitting lost segments. channel is in a burst error state, most retransmission attempts fail, thereb causing poor utilization of the wireless channel Furthermore, in the event of multiple sessions sharing a wireless link, FIFO packet scheduling can cause the HOL blocking effect, resulting in unfair sharing of the bandwidth. This observation leads to a new class of acket dispatchne ! characteristics into consideration in making packet dispatching decisions. We compare a variety of channel state dependent packet (CSDP) scheduling methods with a view towards enhancing the performance of transport layer sessions. Our results indicate that by employing a CSDP scheduler at the wireless LAN device driver level, significant improvement in channel utilization can be achieved in typical wireless LAN configurations.acket losses b b h e n the wireess P in methods which explicitly ta f e wireless chan-

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The emerging widespread use of wireless LAN systems together with the users' desire for such systems to interoperate has created a requirement for standards. Many standards bodies are currently defining standards for wireless systems that relate to different layers of the networklng protocol stack. Of these, two influential physical and data link layer standard3, IEEE 802.11 and the European HIPERLAN, are described. The article then considers the network layer by discussing extensions that are bcing made to the widely used Internet Protocol (IP) to deal with mobility (wired or wireless). The final standards that are discussed relate to wireless link management. The article concludes by speculating on future directions for wireless LAN systcms. GHz ISM band.The discussion that follows treats several types or emerging standards which impact wireless LAN systems. We begin with a description of two influential physical-and data-link-layer standards, IEEE 802.11 and HIPERLAN. Following this, we briefly examine some developments concerning the U.S. personal communication services (PCS) bands, future spectrum allocations, and wireless asynchronous transfer mode (ATM) systems. After describing these physical-and link-layer developments, we focus o n the network layer. We discuss the extensions being made to the widely used Internet Protocol (IP) to deal with mobility (wired or wireless). Finally, we describe some emerging standards for wireless link management in which interfaccs are specified to provide wireless link information to protocol stacks and applications on the mobile client. In the conclusion, we speculate on futurc directions of wireless LAN systems. IEEE 802.11 WIRELESS LAN STANDARDT he IEEE 802.11 committee has been working on the establishment of a standard for wireless LANs. Having begun its work in 1990, the 802.11 committee is nearing completion of the standard, which is expected to be finalized in mid-1996. Much of the standard appears to have reached final form at the current time (early 1996), so we can descrlbe the main features of the architecture, the multiple physical layers, arid the common medium access control (MAC) sublayer [l]. ARCHITECTUREWe introduce the general architecture and terminology deIined by the 802.11 cornmillee [l]. As shown in Fig. 1, two primary topologies are supported by the 802.11 standard: one in which the stations access the backbone network1 (distribution system in 802.11 nomenclaturc) via access points (Le., base stations), and one in which a group of stations commnnicate directly with each other in an ad hoc network, independent of any infrastructure or base stations. The first topology is useful for providing wireless covcrage of building or campus areas by deploying multiple access points whose radio coverage areas overlap to provide complete coveragc. The stations associated with a given access point are referred to as its basic service set (BSS) in the 802.11 standard, but more commonly as the members of the acccss point's cell. The second topology, the one for a...

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