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EE360: Lecture 11 Outline Annoucements One homework, due May 30 Remarks on Paper Summaries Wireless LAN Design Wireless LAN Predictions and Reality Wireless LAN Protocols “Comparison of 802.11a and HyperLan” Tom Bruns. Rate Adaptive Techniques “A rate adaptive MAC protocol” Jie Weng Paper Summary Remarks Should convey the main ideas, techniques, results, contributions, and possible extensions of the papers Write for an audience that has not read the paper Do not write a “review” of the paper Analysis should be briefly described Should compare and contrast results in all papers, not just separately summarize each one Give an overview of the general topic these papers address Give the “big picture” and how these papers fit in Collaborative summaries must be collaborative, not just a division of labor All collaborators must read and discuss all papers The writeup should be collaborative and read/critiqued by all Wireless LAN Overview Wireless LANs provide high-speed wireless access over short distances (10s-100s of feet) Typically operate in the “free” bands Interference rejection/avoidance needed Geared for low-mobility users Slow channel variations Infrequent handoff Wireless LAN Design Physical (link) layer design MAC design Network architecture centralized or ad-hoc Interaction with higher layer protocols TCP QOS mechanisms Physical Layer Want high speeds and good performance Must compensate for ISI and slow fading Must compensate for interference Most designs use DSSS or OFDM Wideband techniques for high speeds Both techniques compensate for ISI DSSS rejects interference while OFDM allocates subchannels to avoid interference MAC Design CSMA or centralized control (reservations) CSMA efficient for short data bursts Centralized control (with QOS) needed for media streaming Centralized control does not work well for multihop networks What will be the “killer” application for wireless LANs??? Network Architecture Centralized (Star) Ad-Hoc Implications Single hop versus multihop Centralized versus distributed control Tradeoffs: Infrastructure, performance, flexibility, and range Higher Layer Protocols TCP Congestion control protocol designed for the Internet (transport layer) Lost/delayed/corrupted packets trigger congestion control Doesn’t work well for wireless links QOS Mechanisms ATM guarantees QOS (delay, rate, BER, etc.) Needed for media streaming applications Hard to provide QOS on wireless channels Flavors: Guaranteed, predictive, best effort Performance Gap with Wired Systems 100 M ATM 100,000 Ethernet 10,000 FDDI wired- wireless Ethernet bit-rate "gap" 1000 User Bit-Rate (kbps) 2nd gen DATA RATES 100 WLAN 1st gen Polling WLAN 10 Packet 1 Radio .1 .01 1970 1980 1990 2000 YEAR 802.11a promises about 5 times higher data rates than 802.11b - Still 2-10 times slower than wired LANs, for a single user! Wireless LAN Predictions “802.11 is on fire” Benchmark Capital, 12/00. “Although its been the pot of gold just around the corner for the last five years, we still don’t have a hughly profitable business case for wireless data” Dr. David Roddy, Chief Telecom Economist, Deloitte & Touche, 4/98 “Up to now, the wireless data party has been more flop than fete, with precious few suscribers showing even a hint of interest in using unwired services for anything more than voice calls. This year may finally see wireless data make its long-expected splash” Meg McGinity, Wireless Editor, 4/98 “Wireless LANs finally appear poised for the growth and popularity long expected of them. The wireless LAN market in the U.S. will enjoy a sixfold expansion by the year 2000, reaching more than 1 billion in revenue” Morris Edwards, Communications News, May, 1997. Wireless LAN Reality Internet users 100 Cellular users 90 80 USA market 70 pager users millions 60 50 40 laptop users 30 20 dedicated wireless 10 data subs 0 1995 2000 Closing Thoughts 802.11a promises 50 Mbps speeds (shared rate) at relatively low cost QOS support not really available Is this the breakthrough that will finally realize the wireless LAN promise? What killer applications will need to be supported on wireless LANs? What technical breakthroughs are still needed?
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