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Chapter 3 Wireless LANs

Chapter 3 Wireless LANs. Reading materials : 1. 第 8 章、第 9 章 [1] ， Part 4 in [2] 2.M. Ergen (UC Berkeley), 802.11 tutorial. Outline. 3.1 Wireless LAN Technology 3.2 Wireless MAC 3.3 IEEE 802.11 Wireless LAN Standard 3.4 Bluetooth. 3.1 Wireless LAN Technology. 3.1.1 Overview

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Chapter 3 Wireless LANs

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  1. Chapter 3 Wireless LANs Reading materials:1.第8章、第9章[1]，Part 4 in [2]2.M. Ergen (UC Berkeley), 802.11 tutorial

  2. Outline • 3.1 Wireless LAN Technology • 3.2 Wireless MAC • 3.3 IEEE 802.11 Wireless LAN Standard • 3.4 Bluetooth

  3. 3.1 Wireless LAN Technology 3.1.1 Overview 3.1.2 Infrared LANs 3.1.3 Spread Spectrum LANs 3.1.4 Narrowband Microwave LANs

  4. 3.1.1 Overview • WLAN Applications • WLAN Requirements • WLAN Technology

  5. 3.1.1.1 Wireless LAN Applications • LAN Extension • Cross-building interconnect • Nomadic Access • Ad hoc networking

  6. LAN Extension • Wireless LAN linked into a wired LAN on same premises • Wired LAN • Backbone • Support servers and stationary workstations • Wireless LAN • Stations in large open areas • Manufacturing plants, stock exchange trading floors, and warehouses

  7. Cross-Building Interconnect • Connect LANs in nearby buildings • Wired or wireless LANs • Point-to-point wireless link is used • Devices connected are typically bridges or routers

  8. Nomadic Access • Wireless link between LAN hub and mobile data terminal equipped with antenna • Laptop computer or notepad computer • Uses: • Transfer data from portable computer to office server • Extended environment such as campus

  9. Ad Hoc Networking • Temporary peer-to-peer network set up to meet immediate need • Example: • Group of employees with laptops convene for a meeting; employees link computers in a temporary network for duration of meeting

  10. 3.1.1.2 Wireless LAN Requirements • Throughput • Number of nodes • Connection to backbone LAN • Service area • Battery power consumption • Transmission robustness and security • Collocated network operation • License-free operation • Handoff/roaming • Dynamic configuration

  11. 3.1.1.3 Wireless LAN Technology • Infrared (IR) LANs • Spread spectrum LANs • Narrowband microwave

  12. 3.1.2 Infrared LANs • Strengths and Weakness • Transmission Techniques

  13. Strengths of Infrared Over Microwave Radio • Spectrum for infrared virtually unlimited • Possibility of high data rates • Infrared spectrum unregulated • Equipment inexpensive and simple • Reflected by light-colored objects • Ceiling reflection for entire room coverage • Doesn’t penetrate walls • More easily secured against eavesdropping • Less interference between different rooms

  14. Drawbacks of Infrared Medium • Indoor environments experience infrared background radiation • Sunlight and indoor lighting • Ambient radiation appears as noise in an infrared receiver • Transmitters of higher power required • Limited by concerns of eye safety and excessive power consumption • Limits range

  15. IR Data Transmission Techniques • Directed Beam Infrared • Ominidirectional • Diffused

  16. Directed Beam Infrared • Used to create point-to-point links (e.g.Fig.13.5) • Range depends on emitted power and degree of focusing • Focused IR data link can have range of kilometers • Such ranges are not needed for constructing indoor WLANs • Cross-building interconnect between bridges or routers

  17. Ominidirectional • Single base station within line of sight of all other stations on LAN • Base station typically mounted on ceiling (Fig.13.6a) • Base station acts as a multiport repeater • Ceiling transmitter broadcasts signal received by IR transceivers • Other IR transceivers transmit with directional beam aimed at ceiling base unit

  18. Diffused • All IR transmitters focused and aimed at a point on diffusely reflecting ceiling (Fig.13.6b) • IR radiation strikes ceiling • Reradiated omnidirectionally • Picked up by all receivers

  19. Typical Configuration for IR WLANs • Fig.13.7 shows a typical configuration for a wireless IR LAN installation • A number of ceiling-mounted base stations, one to a room • Using ceiling wiring, the base stations are all connected to a server • Each base station provides connectivity for a number of stationary and mobile workstations in its area

  20. 3.1.3 Spread Spectrum LANs • Configuration • Transmission Issues

  21. 3.1.3.1 Configuration • Multiple-cell arrangement • Within a cell, either peer-to-peer or hub • Peer-to-peer topology • No hub • Access controlled with MAC algorithm • CSMA • Appropriate for ad hoc LANs

  22. Spread Spectrum LAN Configuration • Hub topology • Mounted on the ceiling and connected to backbone • May control access • May act as multiport repeater • Automatic handoff of mobile stations • Stations in cell either: • Transmit to / receive from hub only • Broadcast using omnidirectional antenna

  23. 3.1.3.2 Transmission Issues • Within ISM band, operating at up to 1 watt. • Unlicensed spread spectrum: 902-928 MHz (915 MHZ band), 2.4-2.4835 GHz (2.4 GHz band), and 5.725-5.825 GHz (5.8 GHz band). The higher the frequency, the higher the potential bandwidth

  24. 3.1.4 Narrowband Microwave LANs • Use of a microwave radio frequency band for signal transmission • Relatively narrow bandwidth • Licensed • Unlicensed

  25. Licensed Narrowband RF • Licensed within specific geographic areas to avoid potential interference • Motorola - 600 licenses (1200 frequencies) in 18-GHz range • Covers all metropolitan areas • Can assure that independent LANs in nearby locations don’t interfere • Encrypted transmissions prevent eavesdropping

  26. Unlicensed Narrowband RF • RadioLAN introduced narrowband wireless LAN in 1995 • Uses unlicensed ISM spectrum • Used at low power (0.5 watts or less) • Operates at 10 Mbps in the 5.8-GHz band • Range = 50 m to 100 m

  27. 3.2 Wireless MAC

  28. Wireless Data Networks • Experiencing a tremendous growth over the last decade or so • Increasing mobile work force, luxury of tetherless computing, information on demand anywhere/anyplace, etc, have contributed to the growth of wireless data

  29. Wireless Network Types … • Satellite networks • e.g. Iridium (66 satellites), Qualcomm’s Globalstar (48 satellites) • Wireless WANs/MANs • e.g. CDPD, GPRS, Ricochet • Wireless LANs • e.g. Georgia Tech’s LAWN • Wireless PANs • e.g. Bluetooth • Ad-hoc networks • e.g. Emergency relief, military • Sensor networks

  30. Wireless Local Area Networks • Probably the most widely used of the different classes of wireless data networks • Characterized by small coverage areas (~200m), but relatively high bandwidths (upto 50Mbps currently) • Examples include IEEE 802.11 networks, Bluetooth networks, and Infrared networks

  31. WLAN Topology Static host/Router Distribution Network Access Point Mobile Stations

  32. Wireless WANs • Large coverage areas of upto a few miles radius • Support significantly lower bandwidths than their LAN counterparts (upto a few hundred kilobits per second) • Examples: CDPD, Mobitex/RAM, Ricochet

  33. WAN Topology

  34. Wireless MAC • Channel partitioning techniques • FDMA, TDMA, CDMA • Random access

  35. Wireline MAC Revisited • ALOHA • slotted-ALOHA • CSMA • CSMA/CD • Collision free protocols • Hybrid contention-based/collision-free protocols

  36. Wireless MAC • CSMA as wireless MAC? • Hidden and exposed terminal problems make the use of CSMA an inefficient technique • Several protocols proposed in related literature – MACA, MACAW, FAMA • IEEE 802.11 standard for wireless MAC

  37. Hidden Terminal Problem • A talks to B • C senses the channel • C does not hear A’s transmission (out of range) • C talks to B • Signals from A and B collide Collision A B C

  38. Exposed Terminal Problem • B talks to A • C wants to talk to D • C senses channel and finds it to be busy • C stays quiet (when it could have ideally transmitted) Not possible A B C D

  39. Hidden and Exposed Terminal Problems • Hidden Terminal • More collisions • Wastage of resources • Exposed Terminal • Underutilization of channel • Lower effective throughput

  40. MACA • Medium Access with Collision Avoidance • Inspired by the CSMA/CA method used by Apple Localtalk network (for somewhat different reasons) • CSMA/CA (Localtalk) uses a “dialogue” between sender and receiver to allow receiver to prepare for receptions in terms of allocating buffer space or entering “spin loop” on a programmed I/O interface

  41. Basis for MACA • In the context of hidden terminal problem, “absence of carrier does not always mean an idle medium” • In the context of exposed terminal problem, “presence of carrier does not always mean a busy medium” • Data carrier detect (DCD) useless! • Get rid of CS (carrier sense) from CSMA/CA – MA/CA – MACA!!!!

  42. MACA • Dialogue between sender and receiver: • Sender sends RTS (request to send) • Receiver (if free) sends CTS (clear to send) • Sender sends DATA • Collision avoidance achieved through intelligent consideration of the RTS/CTS exchange

  43. MACA (contd.) • When station overhears an RTS addressed to another station, it inhibits its own transmitter long enough for the addressed station to respond with a CTS • When a station overheads a CTS addressed to another station, it inhibits its own transmitter long enough for the other station to send its data

  44. Hidden Terminal Revisited … • A sends RTS • B sends CTS • C overheads CTS • C inhibits its own transmitter • A successfully sends DATA to B RTS A B C CTS CTS DATA

  45. Hidden Terminal Revisited • How does C know how long to wait before it can attempt a transmission? • A includes length of DATA that it wants to send in the RTS packet • B includes this information in the CTS packet • C, when it overhears the CTS packet, retrieves the length information and uses it to set the inhibition time

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