Chapter 3 Wireless LANs.ppt

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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,Outline,3.1 Wireless LAN Technology3.2 Wireless MAC3.3 IEEE 802.11 Wireless LAN Standard3.4 Bluetooth,3.1 Wireless LAN Technology,3.1.1 Overview 3.1.2 Infrared LANs 3.1.3 Spread Spectrum LANs

2、3.1.4 Narrowband Microwave LANs,3.1.1 Overview,WLAN ApplicationsWLAN RequirementsWLAN Technology,3.1.1.1 Wireless LAN Applications,LAN ExtensionCross-building interconnectNomadic Access Ad hoc networking,LAN Extension,Wireless LAN linked into a wired LAN on same premises Wired LAN Backbone Support s

3、ervers and stationary workstationsWireless LAN Stations in large open areas Manufacturing plants, stock exchange trading floors, and warehouses,Cross-Building Interconnect,Connect LANs in nearby buildings Wired or wireless LANs Point-to-point wireless link is used Devices connected are typically bri

4、dges or routers,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,Ad Hoc Networking,Temporary peer-to-peer network set up to m

5、eet immediate need Example: Group of employees with laptops convene for a meeting; employees link computers in a temporary network for duration of meeting,3.1.1.2 Wireless LAN Requirements,Throughput Number of nodes Connection to backbone LAN Service area Battery power consumption Transmission robus

6、tness and security Collocated network operation License-free operation Handoff/roaming Dynamic configuration,3.1.1.3 Wireless LAN Technology,Infrared (IR) LANs Spread spectrum LANs Narrowband microwave,3.1.2 Infrared LANs,Strengths and WeaknessTransmission Techniques,Strengths of Infrared Over Micro

7、wave 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 Doesnt penetrate walls More easily secured against eavesdropping Less inte

8、rference between different rooms,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 pow

9、er consumption Limits range,IR Data Transmission Techniques,Directed Beam Infrared Ominidirectional Diffused,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 a

10、re not needed for constructing indoor WLANs Cross-building interconnect between bridges or routers,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 transmitte

11、r broadcasts signal received by IR transceivers Other IR transceivers transmit with directional beam aimed at ceiling base unit,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

12、 all receivers,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 num

13、ber of stationary and mobile workstations in its area,3.1.3 Spread Spectrum LANs,ConfigurationTransmission Issues,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 LA

14、Ns,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,3.1.3.2 Transm

15、ission 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,3.1.4 Narrowband Microwave LANs,Use of a microwave radio f

16、requency band for signal transmission Relatively narrow bandwidth Licensed Unlicensed,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

17、LANs in nearby locations dont interfere Encrypted transmissions prevent eavesdropping,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,3.2 Wire

18、less MAC,Wireless Data Networks,Experiencing a tremendous growth over the last decade or soIncreasing mobile work force, luxury of tetherless computing, information on demand anywhere/anyplace, etc, have contributed to the growth of wireless data,Wireless Network Types ,Satellite networks e.g. Iridi

19、um (66 satellites), Qualcomms Globalstar (48 satellites)Wireless WANs/MANs e.g. CDPD, GPRS, RicochetWireless LANs e.g. Georgia Techs LAWNWireless PANs e.g. BluetoothAd-hoc networks e.g. Emergency relief, militarySensor networks,Wireless Local Area Networks,Probably the most widely used of the differ

20、ent classes of wireless data networksCharacterized by small coverage areas (200m), but relatively high bandwidths (upto 50Mbps currently)Examples include IEEE 802.11 networks, Bluetooth networks, and Infrared networks,WLAN Topology,Distribution Network,Mobile Stations,Access Point,Static host/Router

21、,Wireless WANs,Large coverage areas of upto a few miles radiusSupport significantly lower bandwidths than their LAN counterparts (upto a few hundred kilobits per second)Examples: CDPD, Mobitex/RAM, Ricochet,WAN Topology,Wireless MAC,Channel partitioning techniques FDMA, TDMA, CDMARandom access,Wirel

22、ine MAC Revisited,ALOHAslotted-ALOHACSMACSMA/CDCollision free protocolsHybrid contention-based/collision-free protocols,Wireless MAC,CSMA as wireless MAC?Hidden and exposed terminal problems make the use of CSMA an inefficient techniqueSeveral protocols proposed in related literature MACA, MACAW, FA

23、MAIEEE 802.11 standard for wireless MAC,Hidden Terminal Problem,A talks to BC senses the channelC does not hear As transmission (out of range)C talks to BSignals from A and B collide,A,B,C,Collision,Exposed Terminal Problem,B talks to AC wants to talk to DC senses channel and finds it to be busyC st

24、ays quiet (when it could have ideally transmitted),A,B,C,D,Not possible,Hidden and Exposed Terminal Problems,Hidden Terminal More collisions Wastage of resourcesExposed Terminal Underutilization of channel Lower effective throughput,MACA,Medium Access with Collision AvoidanceInspired by the CSMA/CA

25、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,Basis for MACA,In the context

26、 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!,MACA,Dialogue between send

27、er and receiver: Sender sends RTS (request to send) Receiver (if free) sends CTS (clear to send) Sender sends DATACollision avoidance achieved through intelligent consideration of the RTS/CTS exchange,MACA (contd.),When station overhears an RTS addressed to another station, it inhibits its own trans

28、mitter long enough for the addressed station to respond with a CTSWhen a station overheads a CTS addressed to another station, it inhibits its own transmitter long enough for the other station to send its data,Hidden Terminal Revisited ,A sends RTSB sends CTSC overheads CTSC inhibits its own transmi

29、tterA successfully sends DATA to B,A,B,C,RTS,CTS,DATA,CTS,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 packetB includes this information in the CTS packetC, when it overhears the CTS packet,

30、 retrieves the length information and uses it to set the inhibition time,Exposed Terminal Revisited,B sends RTS to A (overheard by C)A sends CTS to BC cannot hear As CTSC assumes A is either down or out of rangeC does not inhibit its transmissions to D,A,B,C,D,RTS,RTS,CTS,Cannot hear CTS,Tx not inhi

31、bited,Collisions,Still possible RTS packets can collide!Binary exponential backoff performed by stations that experience RTS collisionsRTS collisions not as bad as data collisions in CSMA (since RTS packets are typically much smaller than DATA packets),Drawbacks,Collisions still possible if CTS pack

32、ets cannot be heard but carry (transmit) enough to cause significant interferenceIf DATA packets are of the same size as RTS/CTS packets, significant overheads,MACA Recap,No carrier sensingRequest-to-send (RTS), Clear-to-send (CTS) exchange to solve hidden terminal problemRTS-CTS-DATA exchange for e

33、very transmission,MACAW,Based on MACADesign based on 4 key observations: Contention is at receiver, not the sender Congestion is location dependent To allocate media fairly, learning about congestion levels should be a collective enterprise Media access protocol should propagate synchronization info

34、rmation about contention periods, so that all devices can contend effectively,Back-off Algorithm,MACA uses binary exponential back-off (BEB)BEB: back-off counter doubles after every collision and reset to minimum value after successful transmissionUnfair channel allocation!Example simulation result:

35、 2 stations A & B communicating with base-station Both have enough packets to occupy entire channel capacity A gets 48.5 packets/second, B gets 0 packets/second,BEB Unfairness,Since successful transmitters reset back-off counter to minimum value Hence, it is more likely that successful transmitters

36、continue to be successfulTheoretically, if there is no maximum back-off, one station can get the entire channel bandwidthIdeally, the back-off counter should reflect the ambient congestion level which is the same for all stations involved!,BEB with Copy,MACAW uses BEB with CopyPacket header includes

37、 the BEB value used by transmitterWhen a station overhears a packet, it copies the BEB value in the packet to its BEB counterThus, after each successful transmission, all stations will have the same backoff counter Example simulation result (same setting as before: A gets 23.82 packets/second, B get

38、s 23.32 packets/second,MILD adaptation,Original back-off scheme uses BEB upon collision, and resetting back-off to minimum value upon successLarge fluctuations in back-off valueWhy is this bad?MACAW uses a multiplicative increase and linear decrease (MILD) scheme for back-off adaptation (with factor

39、s of 1.5 and 1 respectively),Accommodating Multiple Streams,If A has only one queue for all streams (default case), bandwidth will be split as AB:1/4, AC:1/4, DA:1/2Is this fair?Maintain multiple queues at A, and contend as if there are two co-located nodes at A,A,B C D,Other modifications (ACK),ACK

40、 packet exchange included in addition to RTS-CTS-DATA Handle wireless (or collision) errors at the MAC layer instead of waiting for coarse grained transport (TCP) layer retransmission timeouts For a loss rate of 1%, 100% improvement in throughput demonstrated over MACA,Other modifications (DS),In th

41、e exposed terminal scenario (ABCD with B talking to A), C cannot talk to D (because of the ACK packet introduced)What if the RTS/CTS exchange was a failure? How does C know this information?A new packet DS (data send) included in the dialogue: RTS-CTS-DS-DATA-ACKDS informs other stations that RTS-CT

42、S exchange was successful,Other modifications (RRTS),Request to Request to SendConsider a scenario: A B C DD is talking to CA sends RTS to B. However, B does not respond as it is deferring to the D-C transmissionA backs-off (no reply to RTS) and tries laterIn the meantime if another D-C transmission

43、 begins, A will have to backoff again,RRTS (contd.),The only way A will get access to channel is if it comes back from a back-off and exactly at that time C-D is inactive (synchronization constraint!)Note that B can hear the RTS from A!When B detects the end of current D-C transmission (ACK packet f

44、rom C to D), it sends an RRTS to A, and A sends RTS,MACAW Recap,Backoff scheme BEB with Copy MILD Multiple streamsNew control packets ACK DS RRTS Other changes (see paper),IEEE 802.11,The 802.11 standard provides MAC and PHY functionality for wireless connectivity of fixed, portable and moving stati

45、ons moving at pedestrian and vehicular speeds within a local area. Specific features of the 802.11 standard include the following: Support of asynchronous and time-bounded delivery service Continuity of service within extended areas via a Distribution System, such as Ethernet. Accommodation of trans

46、mission rates of 1, 2,10, and 50 Mbps Support of most market applications Multicast (including broadcast) services Network management services Registration and authentication services,IEEE 802.11,The 802.11 standard takes into account the following significant differences between wireless and wired

47、LANs: Power Management Security Bandwidth Addressing,IEEE 802.11 Topology,Independent Basic Service Set (IBSS) Networks Stand-alone BSS that has no backbone infrastructure and consists of at-least two wireless stations Often referred to as an ad-hoc network Applications include single room, sale flo

48、or, hospital wing,IEEE 802.11 Topology (contd.),Extended Service Set (ESS) Networks Large coverage networks of arbitrary size and complexity Consists of multiple cells interconnected by access points and a distribution system, such as Ethernet,IEEE 802.11 Logical Architecture,The logical architectur

49、e of the 802.11 standard that applies to each station consists of a single MAC and one of multiple PHYs Frequency hopping PHY Direct sequence PHY Infrared light PHY802.11 MAC uses CSMA/CA (carrier sense multiple access with collision avoidance),IEEE 802.11 MAC Layer,Primary operations Accessing the

50、wireless medium (!) Joining the network Providing authentication and privacyWireless medium access Distributed Coordination Function (DCF) mode Point Coordination Function (PCF) mode,IEEE 802.11 MAC (contd.),DCF CSMA/CA A contention based protocolPCF Contention-free access protocol usable on infrastructure network configurations containing a controller called a point coordinator within the access pointsBoth the DCF and PCF can operate concurrently within the same BSS to provide alternative contention and contention-free periods,