IEEE Wireless LAN

Transcription

1 IEEE The broadband wireless Internet Maximilian Riegel wlan-tutorial.ppt-1 ( )

2 WLAN Dream Finally Seems to Happen... Recently lots of serious WLAN activities have been announced Big players have invested in WLAN (Cisco, Intel, Nokia) Integrated WLAN solutions appearing (Apple, IBM, Dell,...) Wireless IP solutions have lots of momentum! People desire wireless IP terminals and access devices WLAN offers a good mobile solution for indoor IP access Added value for the user - Flexibility, user mobility Added value for ISP - solution for public high speed IP access WLAN standards are converging - IEEE b rules Interoperability has been the main obstacle wlan-tutorial.ppt-2 ( )

3 The market has taken off... In the past: Deployment of WLAN for vertical markets - moderate growth Now: Ubiquitous broadband wireless Internet access - the killer app IEEE802.11b 11 Mbps everywhere Airport Public W-LAN Campus Railway Station Office Semi-public W-LAN Hospital Congress hall, Hotel Corporate W-LAN Office Plant Remote Access Home W-LAN wlan-tutorial.ppt-3 ( )

4 IEEE Basic Architecture CPE + NIC local distribution network access router internet Netscape http tcp ip ppp Bluetooth IEEE ip apache http tcp ip ppp Bluetooth wlan-tutorial.ppt-4 ( )

5 What is unique about wireless? Difficult media interference and noise quality varies over space and time shared with unwanted devices shared with non-802 devices (unlicensed spectrum, microwave ovens) Full connectivity cannot be assumed hidden node problem Mobility variation in link reliability battery usage: requires power management want seamless connections Security no physical boundaries overlapping LANs Multiple international regulatory requirements wlan-tutorial.ppt-5 ( )

6 Industrial, Scientific and Medical (ISM) Bands 902 to 928MHz to GHz to 5.850GHz 26MHz 83.5MHz 125MHz FREQUENCY (GHz) Low bandwidth Polluted by cellular and cordless Relatively clean spectrum DS radios good at rejecting microwave interference Can fit several (11) WLAN Channels A band for the future No cost effective technology yet wlan-tutorial.ppt-6 ( )

7 Wireless IEEE Standard Approved June b approved September Standard supports 3 Physical Layers Frequency hopping Limited to 2Mbps data rate Requires more network overhead Has higher power density that can generate interference Direct sequence Only PHY to support the 11Mbps data rate Low power density to minimize interference Infrared Range limited wlan-tutorial.ppt-7 ( )

8 IEEE Ad Hoc Mode Peer-to-Peer Network Independent networking Use Distributed Coordination Function (DCF) Forms a Basic Service Set (BSS) Direct communication between stations Coverage area limited by the range of individual stations wlan-tutorial.ppt-8 ( )

9 IEEE Infrastructure Mode Wired Network Server BSS-A BSS-B Access Points (AP) and stations (STA) BSS (Basic Service Set): a set of stations controlled by a single coordination function Distribution system interconnects multiple cells via access points to form a single network Extends wireless coverage area and enables roaming wlan-tutorial.ppt-9 ( )

10 IEEE Network elements Distribution system Used to interconnect wireless cells multiple BSS connected together form an ESS, Extended Service Set Allows mobile stations to access fixed resources Not part of standard could be bridged IEEE LANs, wireless, other networks Distribution System Services are defined Access Points Stations select an AP and associate with it Support roaming Provide other functions time synchronization (beaconing) power management support point coordination function Traffic typically (but not always) flows through AP direct communication possible wlan-tutorial.ppt-10 ( )

11 MAC Functionality Independent and Infrastructure configuration support Each BSS has a unique 48 bit address Each ESS has a variable length address CSMA with collision avoidance MAC-level acknowledgment allows for RTS/CTS exchanges (hidden node protection) MSDU fragmentation Point Coordination option (AP polling) Association and Reassociation station scans for APs, association handshakes Roaming support within an ESS Power management support stations may power themselves down AP buffering, distributed approach for IBSS Authentication and privacy Optional support of Wired Equivalent Privacy (WEP) Authentication handshakes defined wlan-tutorial.ppt-11 ( )

12 CSMA/CA Explained Free access when medium is free longer than DIFS DIFS Busy Medium Defer Access DIFS PIFS SIFS Contention Window IFS: Inter Frame Space Backoff-Window Next Frame Slot time Select Slot and Decrement Backoff as long as medium is idle. Reduce collision probability where mostly needed. Stations are waiting for medium to become free. Select Random Backoff after a Defer, resolving contention to avoid collisions. Efficient Backoff algorithm stable at high loads. Exponential Backoff window increases for retransmissions. Backoff timer elapses only when medium is idle. Implement different fixed priority levels wlan-tutorial.ppt-12 ( )

13 Carrier Sense Multiple Access Collision Avoidance (CSMA/CA) Station 1 Station 2 Tx Data to STA 2 Rx data from STA 1 Short deferral Short interval ensures ACK is sent while other stations wait longer ACK to STA1 Distributed inter-frame deferral STA 3 s back-off is shorter than STA 4 s therefore it begins transmission first Station 3 Detects channel busy Detects channel busy Distributed interframe deferral Random back-off Tx Data Distributed inter-frame deferral Station 4 Detects channel busy Detects channel busy Distributed interframe deferral Random back-off Detects channel busy wlan-tutorial.ppt-13 ( )

14 CSMA/CA + ACK protocol DIFS Src Dest Other Data SIFS Ack DIFS Defer Access Contention Window Next MPDU Backoff after Defer Defer access based on Carrier Sense. CCA from PHY and Virtual Carrier Sense state. Direct access when medium is sensed free longer then DIFS, otherwise defer and backoff. Receiver of directed frames to return an ACK immediately when CRC correct. When no ACK received then retransmit frame after a random backoff (up to maximum limit). wlan-tutorial.ppt-14 ( )

15 IEEE Point Coordination Function (PCF) CFP repetition interval Contention Free Period Contention Period Access Point Beacon D1+Poll D2+Poll CF end Stations U1+ACK U2+ACK Optional PCF mode provides alternating contention free and contention operation under the control of the access point The access point polls stations for data during contention free period Network Allocation Vector (NAV) defers the contention traffic until reset by the last PCF transfer PCF and DCF networks will defer to each other PCF improves the quality of service for time bounded data wlan-tutorial.ppt-15 ( )

16 Hidden Node Provisions Problem Stations contending for the medium do not Hear each other Solution Optional use of the Duration field in RTS and CTS frames with AP STA B cannot receive data from STA A CTS-Range RTS-Range DIFS STA B Access Point STA A STA A AP STA B RTS CTS Data Ack STA B cannot detect carrier from STA A Time period to defer access is based on duration in CTS Next MPDU Back off after defer wlan-tutorial.ppt-16 ( )

17 Frame Formats MAC Header Bytes: Frame Control Duration ID Addr 1 Addr 2 Addr 3 Sequence Addr 4 Control Frame Body CRC Bits: Protocol Version Type SubType To DS From DS More Frag Retry Pwr Mgt More Data WEP Rsvd MAC Header format differs per Type: Control Frames (several fields are omitted) Management Frames Data Frames Includes Sequence Control Field for filtering of duplicate caused by ACK mechanism. wlan-tutorial.ppt-17 ( )

18 Physical Layer Convergence Protocol (PLCP) PLCP Protocol Data Unit SYNC SFD SIGNAL SERVICE LENGTH CRC (gain setting, energy detection, antenna selection, frequency offset compensation) (Start Frame Delimiter; bit synchronization) (rate indication; 1, 2, 5.5, 11 Mbit/s) (reserved for future use) (number of octets in PSDU) (CCITT CRC-16, protects signal, service, length field) wlan-tutorial.ppt-18 ( )

19 Three PHYs Frequency Hop Spread Spectrum 2.4GHz band, 1 and optional 2Mbps 2GFSK, 4GFSK (Gaussian Frequency Shift Keying) 2.5 hops/sec over 79 1MHz BW channels (North America) Direct Sequence Spread Spectrum 2.4GHz band, 1 and 2Mbps DBPSK, DQPSK (Differential Binary/Quadrature Phase Shift Keying) 11 chip Barker sequence 2.4GHZ band, 5.5 and 11Mbps CCK Baseband IR Complex spread functions Diffused infrared, 1 and 2Mbps, 16-PPM and 4-PPM (Pulse Position Modulation) wlan-tutorial.ppt-19 ( )

20 Direct Sequence Spread Spectrum RF Energy is Spread by XOR of Data with PRN Sequence 1 0 Data 1 bit period 11 Bit Barker Code (PRN*) chips PRN Out chips 1 bit period * PRN: Pseudorandom Number Signal Spectrum Transmitter baseband signal before spreading Receiver baseband signal before matched filter (Correlator) Transmitter baseband signal after spreading Receiver baseband signal after matched filter (De-spread) wlan-tutorial.ppt-20 ( )

21 DSSS Transmit Spectrum and Channels Transmit Spectrum Mask 0 dbr Unfiltered Sinx/x -30 dbr -50 dbr fc -22 MHz fc -11 MHz fc fc +11 MHz fc +22 Mhz Cannel USA ETSI Japan MHz 2412 MHz N/A MHz 2417 MHz N/A MHz 2422 MHz N/A MHz 2427 MHz N/A MHz 2432 MHz N/A MHz 2437 MHz N/A MHz 2442 MHz N/A MHz 2447 MHz N/A MHz 2452 MHz N/A MHz 2457 MHz N/A MHz 2462 MHz N/A 12 N/A 2467 MHz N/A 13 N/A 2472 MHz N/A 14 N/A N/A 2484 MHz wlan-tutorial.ppt-21 ( )

22 Power Management Mobile devices are battery powered. Power Management is important for mobility. Current LAN protocols assume stations are always ready to receive. Idle receive state dominates LAN adapter power consumption over time. How can we power off during idle periods, yet maintain an active session? Power Management Protocol: allows transceiver to be off as much as possible is transparent to existing protocols is flexible to support different applications possible to trade off throughput for battery life wlan-tutorial.ppt-22 ( )

23 Power Management Approach Allow idle stations to go to sleep station s power save mode stored in AP APs buffer packets for sleeping stations. AP announces which stations have frames buffered Traffic Indication Map (TIM) sent with every Beacon Power Saving stations wake up periodically listen for Beacons TSF assures AP and Power Save stations are synchronized stations will wake up to hear a Beacon TSF timer keeps running when stations are sleeping synchronization allows extreme low power operation Independent BSS also have Power Management similar in concept, distributed approach wlan-tutorial.ppt-23 ( )

24 Infrastructure Power Management TIM Time-axis AP activity TIM-Interval DTIM interval TIM Busy Medium DTIM TIM TIM DTIM Broadcast Broadcast PS Station Broadcast frames are also buffered in AP. all broadcasts/multicasts are buffered PS-Poll broadcasts/multicasts are only sent after Delivery Traffic Indication Message (DTIM) DTIM interval is a multiple of TIM interval Stations wake up prior to an expected DTIM. If TIM indicates frame buffered station sends PS-Poll and stays awake to receive data else station sleeps again Tx operation wlan-tutorial.ppt-24 ( )

25 Scanning Scanning required for many functions. finding and joining a network finding a new AP while roaming initializing an Independent BSS (ad hoc) network MAC uses a common mechanism for all PHY. single or multi channel passive or active scanning Passive Scanning Find networks simply by listening for Beacons Active Scanning On each channel Send a Probe, Wait for a Probe Response Beacon or Probe Response contains information necessary to join new network. wlan-tutorial.ppt-25 ( )

26 Active Scanning Example Access Point A Access Point C Steps to Association: Station sends Probe. APs send Probe Response. Station selects best AP. Station sends Association Request to selected AP. AP sends Association Response. Initial connection to an Access Point - ReAssociation follows a similar process wlan-tutorial.ppt-26 ( )

27 Roaming Access Point B Station 2 Station 5 Station 6 Access Point A Access Point C Station 3 Station 4 Station 7 Mobile stations may move beyond the coverage area of their Access Point but within range of another Access Point Station 1 Reassociation allows station to continue operation wlan-tutorial.ppt-27 ( )

28 Roaming Approach Station decides that link to its current AP is poor Station uses scanning function to find another AP or uses information from previous scans Station sends Reassociation Request to new AP If Reassociation Response is successful then station has roamed to the new AP else station scans for another AP If AP accepts Reassociation Request AP indicates Reassociation to the Distribution System Distribution System information is updated normally old AP is notified through Distribution System wlan-tutorial.ppt-28 ( )

29 Privacy and Access Control Goal of is to provide Wired Equivalent Privacy (WEP) Usable worldwide provides for an Authentication mechanism To aid in access control. Has provisions for OPEN, Shared Key or proprietary authentication extensions. Optional (WEP) Privacy mechanism defined by Limited for Station-to-Station traffic, so not end to end. Only implements Confidentiality function. Uses RC4 algorithm based on: a 40 bit secret key (No Key distribution standardized) and a 24 bit IV that is send with the data. includes an ICV to allow integrity check. Only payload of Data frames are encrypted. Encryption on per MPDU basis. wlan-tutorial.ppt-29 ( )

30 IEEE Architecture Overview One MAC supporting multiple PHYs currently Frequency Hopping, Direct Sequence and Infrared PHYs Two configurations Independent (ad hoc) and Infrastructure CSMA/CA (collision avoidance) with optional point coordination Connectionless Service Transfer data on a shared medium without reservation data comes in bursts user waits for response, so transmit at highest speed possible is the same service as used by Internet Isochronous Service reserve the medium for a single connection and provide a continues stream of bits, even when not used works only when cells (using the same frequencies) are not overlapping. Robust against noise and interference (ACK) Hidden Node Problem (RTS/CTS) Mobility (Hand-over mechanism) Security (WEP) Power savings (Sleep intervals) wlan-tutorial.ppt-30 ( )

31 IEEE Current and future work TGd Regulatory updates TGe Enhancements of MAC MAC TGf Inter Access Point Protocol TGg b >20 Mbit/s Data Rate 2.4 GHz radio Freq. Hopping Spread Spectrum 2.4 GHz radio Direct Sequence Spread Spectrum Infra- Red 2.4 GHz Higher data rate extension b 5 GHz High data rate extension a 1 Mbit/s 2 Mbit/s 2 Mbit/s 1 Mbit/s 1 Mbit/s 2 Mbit/s 5.5 Mbit/s 11 Mbit/s 6,12,24 Mbit/s 9-54 Mbit/s Legend: italic (and red) = optional wlan-tutorial.ppt-31 ( )

32 WECA Mission Statement WECA s mission is to certify interoperability of Wi-Fi (IEEE b High Rate) products and to promote Wi-Fi as the global wireless LAN standard across all market segments. Current Activities: Promote IEEE b HR technology in enterprise, home, and education spaces One standard ---- everywhere Consortium of Over 40 companies Leading vendors WLAN equipment, PC companies, chip companies, service Published compliance matrix Independent test lab (SVNL) Wi-Fi seal of certified interoperability wlan-tutorial.ppt-32 ( )

33 2.4 GHz Standards Efforts 10M 8M Data Rate (Mbps) b By Data Rate and Range For 2.4GHz WLAN Applications 6M 4M 2M 0 Bluetooth HomeRF m 30m 60m 100m Range (meters) wlan-tutorial.ppt-33 ( )

34 Bluetooth Backed by cellular industry Ericsson, Nokia, Intel, IBM, Toshiba Not a network solution Simple point-to-point link Low data rate (sub 1Mbps) 10cm to 10m range Low power and low cost Under standard Applications Wireless desktop (replaces infrared) Cell phone, cordless phone, pager Internet bridge For more data: wlan-tutorial.ppt-34 ( )