Vehicle Networks. Wireless Local Area Network (WLAN) Univ.-Prof. Dr. Thomas Strang, Dipl.-Inform. Matthias Röckl

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1 Vehicle Networks Wireless Local Area Network (WLAN) Univ.-Prof. Dr. Thomas Strang, Dipl.-Inform. Matthias Röckl

2 Outline Wireless LAN Overview History IEEE MAC implementations PHY implementations IEEE b/g/n IEEE a PHY implementation IEEE e MAC implementation No. of layer ISO/OSI ref model 7 Application 6 Presentation 5 Session 4 Transport 3 Network 2 Data Link 1 Physical WLAN protocol specification Not specified in the WLAN standards IEEE e IEEE IEEE a IEEE b IEEE g IEEE n IEEE p

3 Wireless Local Area Network IEEE

4 Wireless LAN WLAN standards WLAN PHY/MAC standards IEEE base standard IEEE a HDR: 5 GHz, OFDM IEEE b HDR: 2.4 GHz, CCK IEEE g HDR: 2.4 GHz, OFDM IEEE n HDR: MIMO IEEE p Wireless Access for Vehicular Environments WLAN MAC extensions IEEE e QoS WLAN security extensions IEEE i WPA2 Additional standards IEEE h European 5 GHz amendment No. of layer ISO/OSI ref model 7 Application 6 Presentation 5 Session 4 Transport 3 Network 2 Data Link 1 Physical WLAN protocol specification Not specified in the WLAN standards (IEEE 802.2) IEEE e IEEE IEEE a IEEE b IEEE g IEEE n IEEE p HDR = Higher Data Rate Extension MIMO = Multiple Input Multiple Output QoS = Quality of Service

5 Wireless LAN History 1987: first standardization activities under IEEE 802.4L (Token Ring) 1990: Project Authorization Request (PAR) under IEEE : first WLAN standard IEEE : Finalization of Higher Data Rate extensions IEEE a and IEEE b 2003: Extension of IEEE a for 5Ghz frequency usage in Europe (IEEE h) 2003: Finalization of HDR extension IEEE g 2004: Finalization of security extension WPA2 (IEEE i) 2005: QoS extension (IEEE e) ~2009/10: MIMO extension IEEE n ~2009/10: Extension for V2X communication IEEE p

6 IEEE

7 IEEE Operating Modes Infrastructure-based networks: Requires central Access Point (AP) AP may be connected to other APs or to the Internet via a Distribution System (DS), e.g. Ethernet Stations (STA) communicating with an AP set up a Basic Service Set (BSS) STAs of different BSSs communicating via inter-connected APs set up an Extended Basic Service Set (EBSS) Ad-hoc networks: No central infrastructure required Stations (STAs) communicate directly to each other STAs set up a Independent Basic Service Set (IBSS) IBSS requires authentication and association procedures

8 IEEE Layers Logical Link Control (LLC): based on IEEE (identical for the whole 802.x family) Medium Access Control (MAC): common basic MAC for all IEEE WLAN systems Physical Layer Convergence Protocol (PLCP): unique access point to PHY layer (PHY-SAP) independent of transmission medium Physical Medium Dependent (PMD): PHY layer implementation dependent on transmission medium Management Plane: Layer management functions No. of layer 2b 2a 1b 1a ISO/OSI ref model Data Link Physical Channel Coding Analog & Digital Modulation Data Plane Logical Link Control (LLC) Medium Access Control (MAC) Physical Layer Convergence Protocol (PLCP) Physical Medium Dependent (PMD) Multiple Access Prioritization Management Plane MAC Management PHY Management Station Management MAC-PHY mapping Synchronization Carrier sense signaling Clear Channel Assessment (CCA)

9 IEEE MAC

10 IEEE Medium Access Control Distributed Foundation Wireless Medium Access Control (DFWMAC) Distributed Coordination Function (DCF): CSMA (mandatory) CSMA/CA with RTS/CTS (optional) Point Coordination Function (PCF): Polling (optional) DFWMAC Point Coordination Function (PCF) Distributed Coordination Function (DCF) CSMA RTS/CTS

11 IEEE Medium Access Control DFWMAC uses time delays to prioritize messages and avoid collisions Every message is deferred according to a distributed time delay scheme Two types of delays: Fixed delay time: Prioritization of more important messages Fixed Inter-Frame Spaces (IFS) according to message type High priority messages have short delay times Low priority messages have longer delay times Random delay time: Collision avoidance Based on traffic adaptive backoff mechanism In high traffic conditions delay time tend to be longer In low traffic conditions delay time tend to be shorter Random delay times are zero in case of only one node being allowed to send (e.g. the recipient of the last message)

12 Sender A Sender B Sender C IEEE MAC: CSMA 1. If a node wants to access the medium, it listens on the channel at least for the DCF Inter-Frame Space (DIFS) 2. If the channel remains idle for the whole DIFS, the node immediately accesses the channel short waiting times in low traffic conditions 3. If the channel gets busy, the node defers its operation until the channel gets idle and again listens on the channel for DIFS 4. If the channel remains idle, it starts its backoff counter and decrements it with every empty slot 5. If the channel gets busy, its freezes the backoff algorithm for the channel busy time 6. If the backoff counter eventually reaches zero the node accesses the channel DIFS DIFS DIFS DIFS Channel busy DIFS Channel busy Data to transmit Channel idle for DIFS yes Transmit no yes Wait until channel becomes idle Channel idle for DIFS yes (Re)start Backoff Ch. idle for Backoff time Channel busy no no

13 IEEE MAC: CSMA Acknowledgements (ACKs) are used to detect collisions in unicast communication ACKs require a timely delivery In order to prioritize ACKs, nodes that compete for the channel to send an ACK only have to wait for a shorter duration, the Short Inter-Frame Space (SIFS) < DIFS Sender A Sender B Sender C DIFS DIFS Data to B SIFS DIFS ACK DIFS 2 1

14 IEEE MAC: CSMA/CA Avoidance of Hidden-Terminal-Problem (HTP) and Exposed-Terminal-Problem (ETP) by explicit channel reservation with RTS/CTS messages RTS and CTS include channel reservation time Every node stores the channel reservation time in its Network Allocation Vector (NAV) 1. To send an RTS message, initiator has to use CSMA with DIFS 2. Responder acknowledges the RTS with a CTS after SIFS 3. Initiator is allowed to transmit after waiting another SIFS Sender A Sender B NAV of sender C NAV of sender D DIFS RTS SIFS CTS Channel Reserved SIFS Data to B Channel reserved Free to transmit SIFS ACK D A B C Avoids HTP Avoids ETP

15 IEEE MAC: Point Coordination Function Point Coordinator Station A Station C Applicable in infrastructure-based mode only Central coordinated MAC: Polling by access point acting as Point Coordinator (PC) Periodic super frames consisting of a Contention Free Period (CFP) and a Contention Period (CP) CFP is introduced by the PC at the beginning of each super frame with a PCF Inter-Frame Space (PIFS) SIFS < PIFS < DIFS ( PCF has higher priority than DCF) If a polled station does not reply, the PC polls the next station after waiting PIFS PIFS Poll A SIFS Super frame Contention Free Period Data SIFS Poll B PIFS Poll C SIFS Data Contention Period Super frame

16 IEEE Backoff algorithm Contention Window (CW) is exponentially increased in case of collisions CW = x 2 1, x=x+1 in case of collision Backoff time = Random(CW) * SlotTime CW CW Random(CW): Random number max from the interval [0;CW] SlotTime = PHY layer dependent FHSS: 50 μs DSSS: 20 μs CW is reset in case of successful 48 transmission (detected by ACKs) 35 Upper bounds (CW max ) and lower CW min 24. bounds (CW min ) for CW 15 depend on PHY layer: FHSS: CW min = 15, CW max = 1023 DSSS: CW min = 31, CW max = 1023 Initial Attempt (no backoff) 1023 Third Retransmit Second Retransmit First Retransmit

17 IEEE MAC frame structure Header Payload Trailer Frame Duration Address 1 Address 2 Address 3 Sequence Frame Address 4 FCS Control Control Body Byte Prot. Vers. To DS From DS More Frag. Retry Pwr Mgmt More Data WEP Bit To DS From DS Addr. 1 Addr. 2 Addr. 3 Addr. 4 physical Recv. physical Recv. physical Sender BSSID 1 0 BSSID physical Sender 1 1 physical Recv. physical Sender BSSID - logical Sender logical Recv. logical Recv. - - logical Sender Type Subtype Order

18 IEEE PHY

19 IEEE Physical Layer 3 basic implementations: Frequency Hopping Spread Spectrum (FHSS) 2.4 GHz ISM band (EU: 100mW EIRP, US: 1W EIRP) Frequency Spreading Direct Sequence Spread Spectrum (DSSS) 2.4 GHz ISM band (EU: 100mW EIRP, US: 1W EIRP) Code Spreading Diffused Infrared (DFIR) Infrared: 850nm 900nm Not used in practice

20 IEEE FHSS: PMD Frequency spreading ISM band is separated in 79 non-overlapping channels with bandwidth of 1 MHz Channel Frequency c 1 c 7 c 25 c 37 c 55 c 61 c 79 Channel is changed with 2.5 Hz ( channel dwell period = 400 ms) according to a pseudo-random hopping sequence (e.g. c 1,c 7,c 25,c 55,c 37,c 61, c 1,c 7,c 25 ) Next channel has to be at least 6 MHz apart Channel width = 1 MHz Symbol rate = 1 Msps (million symbols per second) Modulation: 2-Level Gaussian FSK 1 Mbps data rate 4-Level Gaussian FSK 2 Mbps data rate f

21 IEEE FHSS: PLCP frame Preamble Header Data SYNC SFD PLW PSF CRC MAC-PDU Bit <=4095 Byte Data rate = 1 Mbps 1-2 Mbps Preamble (1 Mbps): SYNC (Synchronisation): alternating 0 and 1 SFD (Start of Frame Delimiter): Header (1 Mbps): PLW (Packet Length Width): Length of SDU in bits PSF (Packet Signaling Field): Data rate in 0.5 Mbps steps starting with 1 Mbps Cyclic Redundancy Check (CRC): G(x) = x 16 + x 12 + x Data (1-2 Mbps)

22 IEEE DSSS: PMD Code spreading 11-bit chipping sequence (Barker code) : Barker code has a very good autocorrelation good separation of superimposed signals in multipath situations ISM band is separated into 11 partially overlapping channels Channel width = 22 MHz Channel spacing = 5 MHz Symbol rate = 1 Msps Modulation: Differential BPSK 1 Mbps data rate Differential QPSK 2 Mbps data rate Transmitter Non-overlapping channels MHz 5 MHz Data to transmit XOR Chip Sequence Spreaded data

23 IEEE DSSS: PLCP frame Preamble Header Data SYNC SFD Signal Service Length CRC MAC-PDU Bit <=4095 Byte Data rate = 1 Mbps 1-2 Mbps Preamble (1 Mbps): SYNC (Synchronisation): alternating 0 and 1 SFD (Start of Frame Delimiter): Header (1 Mbps): Signal: Data rate in 0.1 Mbps steps Service: reserved Length: Length of SDU in microseconds Cyclic Redundancy Check (CRC): G(x) = x 16 + x 12 + x Data (1-2 Mbps)

24 IEEE Comparison FHSS vs. DSSS FHSS DSSS Spreading Frequency Code Modulation FSK PSK Signal-to-Noise Ratio (SNR) 18 db 12 db Frequency band GHz GHz Bandwidth Data rates 79 MHz (Short term: 1 MHz for a single hop) 1 Mbps (mandatory) 2 Mbps (optional) 22 MHz (for a single sub-band) 1 Mbps (mandatory) 2 Mbps (optional) Slot time 50 μs 20 μs SIFS 28 μs 10 μs Preamble length 96 bits (96 μs) 144 bits (144 μs) Header length 32 bits (32 μs) 48 bits (48 μs)

25 IEEE b/g/n

26 IEEE b/g/n IEEE b Data rate IEEE b extends IEEE DSSS with two additional PMDs: 5.5 Mbps: QPSK symbols spread by 8-chip Complementary Code Keying (CCK) encoding 2 bits 11 Mbps: QPSK symbols spread by 8-chip Complementary Code Keying (CCK) encoding 6 bits Code length Modulation 1 Mbps 11 (Barker-Code) 2 Mbps 11 (Barker-Code) 5.5 Mbps 8 (CCK) 11 Mbps 8 (CCK) Symbol rate Bits/ Symbol BPSK 1 Msps 1 QPSK 1 Msps 2 QPSK Msps QPSK Msps 4 (2+2) 8 (2+6)

27 IEEE b/g/n IEEE g 2.4 GHz ISM band Modulation: OFDM (similar to IEEE a) Data rates: 6, 9, 12, 18, 24, 36, 48, 54 Mbps (OFDM) + 1, 2, 5.5, 11 Mbps (CCK) Backward compatible to IEEE b

28 IEEE b/g/n IEEE n Designed for applications with very high data rate requirements (e.g. home entertainment, harddisk streaming, gaming) Several antennas used for parallel transmission: Multiple Input / Multiple Output (MIMO) e.g. 2x2 (= 2 transmitting antennas, 2 receiving antennas) Max. data rate: ~600 Mbps Frequency band: 2.4 GHz / 5 GHz OFDM modulation with 20/40 MHz channels and 4 spatial streams (4x4) Additional MAC enhancements for faster transmission

29 IEEE a

30 IEEE a PHY: OFDM Orthogonal sub-carriers f Each sub-carrier can have an individual modulation (e.g. QPSK or QAM) Guardband (aka Guardinterval) per symbol reduce Inter-Symbol Interference Synchronization by pilot signals in specific sub-carriers Channel estimation with training symbols Advantages of OFDM: High spectrum efficiency Resistance against narrow-band interferers and signal distortions Resistance against multipath errors RECAP

31 IEEE a PHY IEEE a uses OFDM with 64 sub-carriers: 48 data sub-carriers 4 pilot sub-carriers 12 guard sub-carriers (at spectrum edges) Channel coding: Scrambling w/ LFSR and its G(x) = x 7 + x Forward Error Correction: Convolution Coder (2,1,7), (4,3,7), (3,2,7) Inter-carrier interleaving: Block interleaver (12x16) or (18x16) Data rates: 6-54 Mbps Symbol rate: 250 ksps Channel bandwidth: 20 MHz Sub-carrier spacing: khz (= 20 MHz/64) Symbol duration: 4 μs Guard period between symbols: 0.8 μs 250 ksps * 48 sub-carriers * 6 coded bits/sub-carrier * ¾ coding rate = 54 Mbps

32 IEEE a Physical Layer Convergence Protocol (PLCP) frame t 1 t 2 t 3 t 4 t 5 t 6 t 7 t 8 t 9 t 10 T 1 T 2 Preamble: 10 short training symbols t 1 -t 10 : used for timing and coarse frequency synchronization 2 long training symbols T 1 -T 2 : used for channel estimation and fine frequency acquisition

33 IEEE a Frequency band IEEE a is designed for the 5 GHz band Higher frequency higher signal attenuation (see unit wireless communication basics ) IEEE a needs higher power output to achieve the same range as in the 2.4 GHz band Advantages of 5 GHz: Less crowded less co-channel interference and adjacent channel interference Higher bandwidth more channels available (19 ch. in Europe) High power usage allowed in certain areas and frequencies

34 IEEE a European regulation In Europe the 5 GHz band was exclusively assigned to HIPERLAN/2, satellite and radar systems IEEE a was not allowed in Europe till 2005 Additional functionality required to use IEEE a in Europe is defined in IEEE h: Dynamic Frequency Selection (DFS) Avoidance of interference with radar systems Station has to switch the channel, if it detects an active radar system In infrastructure-based mode, AP decides on the channel switch Transmit Power Control (TPC) Reduction of interference with satellite systems (and possibly other systems in the same frequency band) Station has to reduce the transmit power, if it detects an active satellite communication

35 IEEE a Frequency bands DFS Dynamic Frequency Selection TPC Transmit Power Control Regulatory domain Frequency Band Channel Number Frequency Max. output power (CEPT) Indoor/ Outdoor United States (FCC) Europe (CEPT) United States (FCC) Europe (CEPT) Europe (CEPT) U-NII lower band GHz U-NII middle band GHz GHz GHz GHz GHz GHz GHz GHz GHz GHz GHz GHz GHz GHz GHz GHz GHz GHz GHz GHz GHz DFS No DFS TPC No TPC 200 mw (23 dbm) 60 mw (18 dbm) With TPC: 200 mw (23 dbm) Without TPC: 100 mw (20 dbm) With TPC: 1 W (30 dbm) Without TPC: 500 W (27 dbm) 30 mw (15 dbm) Only Indoor Only Indoor Indoor & Outdoor

36 WLAN IEEE Summary of basic standards Standard Standard approved Spectrum IEEE GHz nm Max. data rate Modulation 2 Mbps FHSS/DSSS IEEE a GHz 54 Mbps OFDM IEEE b GHz 11 Mbps DSSS (CCK) IEEE g GHz 54 Mbps OFDM+CCK IEEE n ~2009/ /5 GHz ~600 Mbps OFDM + MIMO IEEE p ~2009/ GHz 27 Mbps OFDM

37 IEEE e

38 IEEE e Quality of Service (QoS) Standard IEEE does not allow to prioritize different kinds of data packets IEEE e defines a MAC enhancement to support QoS Standard IEEE DCF/PCF extended by Hybrid Coordination Function (HCF): Enhanced Distributed Channel Access (EDCA) DCF with additional priority classes HCF Controlled Channel Access (HCCA) PCF with additional priority classes Point Coordination Function (PCF) CSMA HCCA HCF Distributed Coordination Function (DCF) EDCA RTS/CTS

39 IEEE e Access Categories 4 different traffic categories (Access Categories): AC0: Background traffic AC1: Best-Effort traffic AC2: Video traffic AC3: Voice traffic Two control mechanisms: IFS: Arbitrate Inter-Frame Space (AIFS) with different lengths instead of a fixed-length DIFS Contention Window: Different values for CW min and CW max High priority traffic has a higher probability to get access to the medium first AC0 AC1 AC2 AC3 Backoff AIFS[0] CW[0] Backoff AIFS[1] CW[1] Backoff AIFS[2] CW[2] Virtual Collision Handler Transmission Attempt Backoff AIFS[3] CW[3]

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