IEEE Characteristics System Architecture Protocol Architecture Physical Layer. Power Management Roaming Current Developments

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1 TECHNISCHE UNIVERSITÄT ILMENAU IEEE Integrated Hard- and Software Systems Characteristics System Architecture Protocol Architecture Physical Layer MAC Layer MAC Management Power Management Roaming Current Developments

2 Characteristics of Wireless LANs Advantages very flexible alternative to wired LANs (almost) no wiring difficulties (e.g. historic buildings, firewalls) ad-hoc networks without previous planning possible more robust against disasters, e.g. earthquakes, fire, or users pulling a plug... Disadvantages lower bandwidth compared to wired networks possible interference may reduce bandwidth no guaranteed service due to license-free spectrum need to consider security issues proprietary solutions, especially for higher bit-rates wireless products have to follow many national restrictions => long time to establish global standards like, e.g. IMT-2000 (UMTS) 2

3 Design goals for Wireless LANs global, seamless operation low power for battery use no special permissions or licenses needed to use the LAN robust transmission technology simplified spontaneous cooperation at meetings easy to use for everyone, simple management protection of investment in wired networks security (no one should be able to read my data), privacy (no one should be able to collect user profiles), safety (low radiation) transparency concerning applications and higher layer protocols, but also location awareness if necessary 3

4 Comparison: Infrared vs. Radio Transmission Infrared uses IR diodes, diffuse light, multiple reflections (walls, furniture etc.) Advantages simple, cheap, available in many mobile devices no licenses needed simple shielding possible Disadvantages interference by sunlight, heat sources etc. many things shield or absorb IR light low bandwidth Example IrDA (Infrared Data Association) interface available everywhere Radio typically using the license free ISM band at 2.4 GHz Advantages experience from wireless WAN and mobile phones can be used coverage of larger areas possible (radio can penetrate walls, furniture etc.) Disadvantages very limited license-free frequency bands shielding more difficult, interference with other electrical devices Examples a/b/g, HIPERLAN1/2, Bluetooth, ZigBee 4

5 Comparison: Infrastructure vs. Ad-hoc Networks infrastructure network AP AP wired network AP: Access Point AP ad-hoc network 5

6 802.11: Architecture of an Infrastructure Network ESS STA LAN BSS 1 Access Point BSS 2 Portal Distribution System Access Point 802.x LAN STA 2 STA LAN 3 Station (STA) terminal with access mechanisms to the wireless medium and radio contact to the access point Basic Service Set (BSS) group of stations using the same radio frequency Access Point station integrated into the wireless LAN and the distribution system Portal bridge to other (wired) networks Distribution System interconnection network to form one logical network (ESS: Extended Service Set) based on several BSS 6

7 802.11: Architecture of an Ad-hoc Network LAN STA 1 BSS 1 STA 3 Direct communication within a limited range Station (STA): terminal with access mechanisms to the wireless medium STA 2 Basic Service Set (BSS): group of stations using the same radio frequency BSS 2 STA 4 STA LAN 7

8 IEEE Standard mobile terminal fixed terminal server infrastructure network application TCP IP LLC access point LLC application TCP IP LLC MAC MAC MAC MAC PHY PHY PHY PHY 8

9 Layers and Functions MAC access mechanisms, fragmentation, encryption MAC Management synchronization, roaming, MIB, power management PLCP (Physical Layer Convergence Protocol) clear channel assessment signal (carrier sense) PMD (Physical Medium Dependent) modulation, coding PHY Management channel selection, MIB Station Management coordination of all management functions PHY DLC LLC MAC PLCP PMD MAC Management PHY Management Station Management 9

10 Physical Layer 3 versions: 2 radio (typ. 2.4 GHz), 1 IR data rates 1 or 2 Mbit/s FHSS (Frequency Hopping Spread Spectrum) spreading, despreading, signal strength, typ. 1 Mbit/s min. 2.5 frequency hops/s (USA), two-level GFSK modulation DSSS (Direct Sequence Spread Spectrum) DBPSK modulation for 1 Mbit/s (Differential Binary Phase Shift Keying), DQPSK for 2 Mbit/s (Differential Quadrature PSK) preamble and header of a frame is always transmitted with 1 Mbit/s, rest of transmission 1 or 2 Mbit/s chipping sequence: +1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1 (Barker code) max. radiated power 1 W (USA), 100 mw (EU), min. 1mW Infrared nm, diffuse light, typ. 10 m range carrier detection, energy detection, synchonization 10

11 FHSS PHY Packet Format only, not a/b! Synchronization synch with pattern SFD (Start Frame Delimiter) start pattern PLW (PLCP_SDU Length Word) length of payload incl. 32 bit CRC of payload, PLW < 4096 (octets) PSF (PLCP Signaling Field) data rate of packet (1 or 2 Mbit/s) HEC (Header Error Check) CRC with x 16 +x 12 +x variable bits synchronization SFD PLW PSF HEC payload PLCP preamble PLCP header 11

12 DSSS PHY Packet Format (802.11b) Synchronization synch., gain setting, energy detection, frequency offset compensation SFD (Start Frame Delimiter) Signal data rate of the packet (0A: 1 Mbit/s DBPSK; 14: 2 Mbit/s DQPSK) Service Length future use, 00: compliant length of the payload HEC (Header Error Check) protection of signal, service and length, x 16 +x 12 +x variable bits synchronization SFD signal service length HEC payload PLCP preamble PLCP header 12

13 MAC Layer DFWMAC Traffic services Asynchronous Data Service (mandatory) exchange of data packets based on "best-effort" support of broadcast and multicast implemented using DCF (Distributed Coordination Function) Time-Bounded Service (optional) implemented using PCF (Point Coordination Function) Access methods DFWMAC-DCF CSMA/CA (mandatory) Distributed Foundation Wireless MAC collision avoidance via randomized "back-off" mechanism minimum distance between consecutive packets ACK packet for acknowledgements (not for broadcasts) DFWMAC-DCF with RTS/CTS Extension (optional) avoids hidden terminal problem DFWMAC- PCF (optional) access point polls terminals according to a list 13

14 MAC Priorities defined through different Inter-Frame Spaces (IFSs) no guaranteed, hard priorities SIFS (Short Inter-Frame Spacing) highest priority, for ACK, CTS, polling response PIFS (PCF IFS) medium priority, for time-bounded service using PCF DIFS (DCF IFS) lowest priority, for asynchronous data service DIFS DIFS medium busy PIFS SIFS contention next frame direct access if medium is free DIFS t 14

15 Timing Details Transmission and Processing Delays Times for a SlotTime 9 μs SIFS 16 μs PIFS 25 μs DIFS 34 μs 15

16 CSMA/CA Access Method DIFS medium busy DIFS PIFS SIFS contention window (randomized back-off mechanism) next frame direct access if medium is free DIFS slot time t station ready to send starts sensing the medium (Carrier Sense based on CCA, Clear Channel Assessment) if the medium is free for the duration of an Inter-Frame Space (IFS), the station can start sending (IFS depends on service type) if the medium is busy, the station has to wait for a free IFS, then the station must additionally wait a random back-off time (collision avoidance, multiple of slot-time) if another station occupies the medium during the back-off time of the station, the back-off timer stops (fairness) 16

17 Competing Stations Simple Version DIFS DIFS bo e bo r DIFS bo e bo r DIFS bo e busy station 1 bo e busy station 2 station 3 busy bo e busy bo e bo r station 4 bo e bo r bo e busy bo e bo r station 5 t busy medium not idle (frame, ack etc.) bo e elapsed backoff time packet arrival at MAC bo r residual backoff time 17

18 CSMA/CA Access Method Sending unicast packets station has to wait for DIFS before sending data receivers acknowledge at once (after waiting for SIFS) if the packet was received correctly (CRC) automatic retransmission of data packets in case of transmission errors sender DIFS data receiver SIFS ACK other stations waiting time DIFS contention data t 18

19 Hidden Terminal Problem A sends to B, C cannot receive A C wants to send to B, C senses a free medium -> CS fails collision at B, A cannot receive the collision -> CD fails A is hidden for C => Application of RTS/CTS A B C 19

20 DCF with RTS/CTS Extension Sending unicast packets station can send RTS with reservation parameter after waiting for DIFS (reservation determines amount of time the data packet needs the medium) acknowledgement via CTS after SIFS by receiver (if ready to receive) sender can now send data at once, acknowledgement via ACK other stations store medium reservations distributed via RTS and CTS sender DIFS RTS data receiver SIFS CTS SIFS SIFS ACK other stations NAV (RTS) NAV (CTS) defer access DIFS contention data t NAV: network allocation vector (implicit in RTS and CTS) 20

21 Fragmentation sender receiver DIFS RTS SIFS CTS SIFS frag 1 SIFS ACK SIFS 1 frag 2 SIFS ACK2 other stations NAV (RTS) NAV (CTS) NAV (frag 1 ) NAV (ACK 1 ) DIFS contention data t 21

22 PCF (Polling) SuperFrame defines time span for polling of (all) wireless stations by AP (including time to reply) t 0 t 1 SuperFrame medium busy point coordinator PIFS D 1 SIFS SIFS D 2 SIFS SIFS wireless stations U 1 U 2 stations NAV NAV D1: polling of wireless station 1 U1: station 1 responds to polling by sending its data D2: polling of wireless station 2 U2: station 2 responds to polling by sending its data 22

23 PCF (Polling) t 2 t 3 t 4 PIFS SIFS CFend point coordinator D 3 D 4 SIFS wireless stations U 4 stations NAV NAV contention free period contention period t D3: polling of wireless station 3 U3: no data -> no response by station 3 within SIFS D4 (after PIFS): polling of wireless station 4 U4: station 4 responds to polling by sending its data Discussion: unpredictable beacon delays unknown transmission duration of polled stations => no QoS guarantees 23

24 MAC Frame Format Types (frame control) control frames, management frames, data frames Sequence number important against duplicated frames due to lost ACKs Addresses receiver, transmitter (physical), BSS identifier, sender (logical) Miscellaneous duration (NAV), checksum, data bytes Duration/ ID Address 1 Address 2 Address 3 Sequence Control Frame Control Protocol version Type Subtype To DS More Frag Power Retry Mgmt Address Data 4 bits From DS More Data WEP Order CRC 24

25 MAC Address Format scenario to DS from DS address 1 address 2 address 3 address 4 ad-hoc network 0 0 DA SA BSSID - infrastructure 0 1 DA BSSID SA - network, from AP infrastructure 1 0 BSSID SA DA - network, to AP infrastructure network, within DS 1 1 RA TA DA SA DS: Distribution System DA: Destination Address BSSID: Basic Service Set Identifier TA: Transmitter Address AP: Access Point SA: Source Address RA: Receiver Address Management frame format bytes Frame Duration DA SA BSSID Sequence Frame Body CRC Control Control 25

26 Control Frames: ACK, RTS, CTS Acknowledgement Request To Send Clear To Send ACK RTS CTS bytes Frame Duration Receiver CRC Control Address bytes Frame Duration Receiver Transmitter CRC Control Address Address bytes Frame Duration Receiver CRC Control Address Duration is used to determine NAV value 26

27 MAC Management Synchronization try to find a LAN, try to stay within a LAN synchronization of internal clocks to coordinate access (e.g. SIFS, PIFS, etc.), send beacons, etc. Power management sleep-mode without missing a message periodic sleep, frame buffering, traffic measurements Association/Reassociation integration into a LAN roaming, i.e. change networks by changing access points scanning, i.e. active search for a network MIB - Management Information Base managing, read, write 27

28 Synchronization Using a Beacon (Infrastructure) beacon interval access point medium B B B B busy busy busy busy value of the timestamp B beacon frame t Beacon sent only by access point Beacon may be delayed due to busy medium; beacon interval is not influenced by this! 28

29 Synchronization Using a Beacon (Ad-hoc) beacon interval station 1 B 1 B 1 station 2 B 2 B 2 medium busy busy busy busy t value of the timestamp B beacon frame random delay Beacon may be sent by any station (sending of beacon employs a random delay to avoid collisions) 29

30 Power Management Idea: switch the transceiver off if not needed States of a station: sleep and awake Timing Synchronization Function (TSF) stations wake up at the same time Infrastructure Ad-hoc AP stores frames intended for sleeping stations AP transmits indication about stored frames in periodic beacons (Indication Maps) sent during awake interval Traffic Indication Map (TIM): List of unicast receivers Delivery Traffic Indication Map (DTIM): List of broadcast/multicast receivers Ad-hoc Traffic Indication Map (ATIM) announcement of receivers by stations buffering frames more complicated - no central AP collision of ATIMs possible (scalability?) 30

31 Power Saving with Wake-up Patterns (Infrastructure) TIM interval DTIM interval beacon indicates station that data are available station replies with PS (power save) poll and continues listening to the medium AP transmits data station acknowledges the data access point medium D B busy T T d D busy busy busy B station p d t T TIM D DTIM awake B broadcast/multicast p PS poll d data transmission to/from the station 31

32 Power Saving with Wake-up Patterns (Ad-hoc) ATIM window beacon interval B station 1 A D B 1 1 station 2 B 2 B 2 a d medium busy busy busy t B beacon frame random delay A transmit ATIM D transmit data awake a acknowledge ATIM d acknowledge data 32

33 Roaming No or bad connection? Then perform: Scanning scan the environment, i.e. listen into the medium for beacon signals or send probes into the medium and wait for an answer => time-consuming (scan all channels) Association Request station sends a request to an AP Association Response success: AP has answered, station can now participate failure: continue scanning AP accepts Reassociation Request signal the new station to the distribution system the distribution system updates its data base (i.e. location information) the distribution system may inform the old AP so it can release resources f standard (IAPP Inter AccessPoint Protocol) 33

34 WLAN: IEEE b Data rate 1, 2, 5.5, 11 Mbit/s, depending on SNR User data rate max. approx. 6 Mbit/s Transmission range 300m outdoor, 30m indoor Max. data rate ~10m indoor Frequency Free 2.4 GHz ISM-band Security Limited, WEP insecure, SSID Cost 25 adapter, 100 base station Availability Many products, many vendors Connection set-up time Connectionless/always on Quality of Service Typ. best effort, no guarantees (unless polling is used, limited support in products) Manageability Limited (no automated key distribution, sym. encryption) Advantages/Disadvantages Advantage: many installed systems, lot of experience, available worldwide, free ISM band, many vendors, integrated in laptops, simple system Disadvantage: heavy interference on ISM band, no service guarantees, slow relative speed only 34

35 IEEE b PHY Frame Formats Long PLCP PPDU format 8 (corresponds to ) variable bits synchronization SFD signal service length HEC payload PLCP preamble PLCP header 192 µs at 1 Mbit/s DBPSK 1, 2, 5.5 or 11 Mbit/s Short PLCP PPDU format (optional) variable bits short synch. SFD signal service length HEC payload PLCP preamble (1 Mbit/s, DBPSK) PLCP header (2 Mbit/s, DQPSK) 96 µs 2, 5.5 or 11 Mbit/s 35

36 Channel Selection (Non-overlapping) Europe (ETSI) channel 1 channel 7 channel MHz [MHz] US (FCC)/Canada (IC) channel 1 channel 6 channel MHz [MHz] 36

37 WLAN: IEEE g Data rate 6, 9, 12, 18, 24, 36, 48, 54 Mbit/s, depending on SNR User throughput (1500 byte packets): 5.3 (6), 18 (24), 24 (36), 32 (54) 6, 12, 24 Mbit/s mandatory Transmission range 150m outdoor, 20m indoor 54 Mbit/s up to 6 m Frequency 2.412~2.472GHz (Europe ETSI) 2.457~2.462GHz (Spain) 2.457~2.472GHz (France) Security Limited, WEP insecure, SSID Cost 50 adapter, 200 base station Availability Some products, some vendors Connection set-up time Connectionless/always on Quality of Service Typ. best effort, no guarantees (same as all products) Manageability Limited (no automated key distribution, sym. encryption) Advantages/Disadvantages Advantage: free ISM band, compatible with b standard Disadvantage: heavy interference on ISM band, no service guarantees 37

38 WLAN: IEEE a Data rate 6, 9, 12, 18, 24, 36, 48, 54 Mbit/s, depending on SNR User throughput (1500 byte packets): 5.3 (6), 18 (24), 24 (36), 32 (54) 6, 12, 24 Mbit/s mandatory Transmission range 100m outdoor, 10m indoor e.g. 54 Mbit/s up to 5 m, 48 up to 12 m, 36 up to 25 m, 24 up to 30m, 18 up to 40 m, 12 up to 60 m Frequency Free , , GHz ISM-band Security Limited, WEP insecure, SSID Cost 100 adapter, 200 base station Availability Some products, some vendors Connection set-up time Connectionless/always on Quality of Service Typ. best effort, no guarantees (same as all products) Manageability Limited (no automated key distribution, sym. encryption) Advantages/Disadvantages Advantage: fits into 802.x standards, free ISM band, available, simple system, uses less crowded 5 GHz band Disadvantage: stronger shading due to higher frequency, no service guarantees 38

39 WLAN: IEEE h Regulatory Details Spectrum Managed a power control dyn. channel/frequency selection 4 frequency bands: GHz 4 usable channels (100 MHz) indoor only max. 30mW EIRP (.11a) TPC (Transmit Power Control) max. 60mW EIRP combined TPC and DCS/DFS (Dynamic Channel/Frequency Selection) max. 200mW EIRP Turbo Mode: combination of two carriers to reach 108 Mbps GHz 4 usable channels TPC, DCS/DFS mandatory GHz indoor and outdoor max. 1W EIRP disallowed in US not supported by all chipsets bis GHz disallowed in Germany EIRP (Equivalent Isotropic Radiated Power) Usage in Germany according to the German Federal Regulation (Vorschrift der Regulierungsbehörde für das Telekommunikations- und Postwesen, RegTP, 35/2002) 39

40 IEEE a PHY Frame Format variable 6 variable bits rate reserved length parity tail service payload tail pad PLCP header PLCP preamble signal data 12 1 variable symbols 6 Mbit/s 6, 9, 12, 18, 24, 36, 48, 54 Mbit/s 40

41 Operating Channels for a / US U-NII channel [MHz] 16.6 MHz channel center frequency = *channel number [MHz] [MHz] 16.6 MHz 41

42 OFDM in IEEE a (and HiperLAN2) OFDM with 52 used subcarriers (64 in total) 48 data + 4 pilot (plus 12 virtual subcarriers) khz spacing pilot khz channel center frequency subcarrier number 42

43 IEEE a/b/g Data Rate Barker: 11-bits Barker coding sequence (DPSK/DQPSK modulation, fixed code) CCK: Complementary Code Keying (DPSK/DQPSK modulation, switching of the spreading sequence) PBCC: Packet Binary Convolutional Code (8PSK, complex version of CCK)) OFDM: Orthogonal Frequency Division Multiplexing (BPSK, QPSK, 16QAM, 64QAM) 43

44 WLAN: IEEE Extensions and developments (05/2007) d: Regulatory Domain Update completed e: MAC Enhancements QoS completed Enhance the current MAC to expand support for applications with Quality of Service requirements, and in the capabilities and efficiency of the protocol f: Inter-Access Point Protocol (IAPP) withdrawn 2006 Establish an Inter-Access Point Protocol for data exchange via the distribution system h: Spectrum Managed a (DCS, TPC) completed i: Enhanced Security Mechanisms completed Enhance the current MAC to provide improvements in security j: MAC and PHY Specifications for Operation in GHz Band in Japan completed n: Throughput enhancement to Mbps draft Study Groups 5 GHz (harmonization ETSI/IEEE) closed Radio Resource Measurements started High Throughput started See for an update 44

45 WLAN: IEEE Details d aims to produce versions of b that work at other frequencies, making it suitable for parts of the world where the 2.4GHz band isn't available. Most countries have now released this band, thanks to an ITU recommendation and extensive lobbying by equipment manufacturers. The only holdout is Spain, which may follow soon e add QoS capabilities to networks. It replaces the Ethernet-like MAC layer with a coordinated Time Division Multiple Access (TDMA) scheme, and adds extra error-correction to important traffic. The technology is similar to Whitecap, a proprietary protocol developed by Sharewave and used in Cisco's a prototypes. A standard was supposed to be finalized by the end of 2001, but has run into delays thanks to arguments over how many classes of service should be provided and exactly how they should be implemented f tries to improve the handover mechanism in so that users can maintain a connection while roaming between two different switched segments (radio channels), or between access points attached to two different networks. This is vital if wireless LANs are to offer the same mobility that cell phone users take for granted h attempts to add better control over transmission power and radio channel selection to a. Along with e, this could make the standard acceptable to European regulators i deals with 's most obvious weakness: security. Rather than WEP, this is an entirely new standard based on the Advanced Encryption Standard (AES), the U.S. government's "official" encryption algorithm. 45

46 WLAN: IEEE More Details IEEE THE WLAN STANDARD was original 1 Mbit/s and 2 Mb/s, 2.4 GHz RF and IR standard (1997), all the others listed below are Amendments to this standard, except for Recommended Practices F and T. IEEE a - 54 Mbit/s, 5 GHz standard (1999, shipping products in 2001) IEEE b - Enhancements to to support 5.5 and 11 Mb/s (1999) IEEE c - Bridge operation procedures; included in the IEEE 802.1D standard (2001) IEEE d - International (country-to-country) roaming extensions (2001) IEEE e - Enhancements: QoS, including packet bursting (2005) IEEE F - Inter-Access Point Protocol (2003) Withdrawn February 2006 IEEE g - 54 Mb/s, 2.4 GHz standard (backwards compatible with b) (2003) IEEE h - Spectrum Managed a (5 GHz) for European compatibility (2004) IEEE i - Enhanced security (2004) IEEE j - Extensions for Japan (2004) IEEE k - Radio resource measurement enhancements (proposed ?) IEEE m - Maintenance of the standard; odds and ends. (ongoing) IEEE n - Higher throughput improvements using MIMO (multiple input, multiple output antennas) (pre-draft ?) IEEE p - WAVE - Wireless Access for the Vehicular Environment (such as ambulances and passenger cars) (working ?) IEEE r -Fast roaming Working "Task Group r" ? IEEE s -ESS Extended Service Set Mesh Networking (working ?) IEEE T - Wireless Performance Prediction (WPP) - test methods and metrics Recommendation (working ?) IEEE u - Interworking with non-802 networks (for example, cellular) (proposal evaluation -?) IEEE v - Wireless network management (early proposal stages -?) IEEE w - Protected Management Frames (early proposal stages ?) IEEE y Operation in the U.S. (early proposal stages -?) 46

47 Reference Books on : Franz-Joachim Kauffels: Wireless LANs: Drahtlose Netze planen und verwirklichen, der Standard IEEE im Detail, WLAN-Design und Sicherheitsrichtlinien. 1. Aufl., mitp-verl., Bonn, 2002 Frank Ohrtman: WiFi-Handbook Building b wireless networks. McGraw-Hill, 2003 Jochen Schiller: Mobile Communications (German and English), Kap 7.3, Addison-Wesley, 2002 Details on e: Anders Lindgren, Andreas Almquist, Olov Schelén. Quality of service schemes for IEEE wireless LANs: an evaluation. Mobile Networks and Applications, Volume 8 Issue 3, June 2003 Daqing Gu; Jinyun Zhang. QoS enhancement in IEEE wireless local area networks. Communications Magazine, IEEE, Volume: 41 Issue: 6, June 2003 Qiu Qiang; Jacob, L., Radhakrishna Pillai, R., Prabhakaran, B.. MAC protocol enhancements for QoS guarantee and fairness over the IEEE wireless LANs. 11th Intl. Conf. on Computer Communications and Networks, Oct Mangold S, Choi S, May P, Klein O, Hiertz G, and Stibor L. IEEE e wireless LAN for quality of service. Proc. Of European Wireless (EW2002), Feb Web Links: The IEEE Wireless LAN Standards Introduction to the IEEE Wireless LAN Standard 47

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