Mobile & Wireless Networking. Lecture 7: Wireless LAN

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1 Mobile & Wireless Networking Lecture 7: Wireless LAN [Schiller, Section 7.3] [Reader, Part 6] [Optional: "IEEE n Development: History, Process, and Technology", Perahia, IEEE Communications Magazine, July 2008] Geert Heijenk

2 Outline of Lecture 9 q Wireless LAN q General Characteristics / IEEE standard q IEEE Physical Layers q Medium Access Control l CSMA/CA l CSMA with RTS/CTS l MAC Quality of Service Enhancements q MAC Frame Format q MAC Management q IEEE : Important Developments 2

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

4 Comparison: infrastructure vs. ad-hoc networks infrastructure network AP AP wired network AP: Access Point AP ad-hoc network 4

5 IEEE Architecture of an infrastructure network STA 1 ESS LAN BSS 1 Access Point BSS 2 Distribution System Access Point 802.x LAN Portal STA LAN STA 3 Station (STA) q terminal with access mechanisms to the wireless medium and radio contact to the access point Basic Service Set (BSS) q group of stations using the same radio frequency Access Point q station integrated into the wireless LAN and the distribution system Portal q bridge to other (wired) networks Distribution System q interconnection network to form one logical network (ESS: Extended Service Set) based on several BSS 5

6 IEEE Architecture of an ad-hoc network LAN Direct communication within a limited range STA 1 IBSS 1 STA 3 q Station (STA): terminal with access mechanisms to the wireless medium STA 2 q Independent Basic Service Set (IBSS): group of stations using the same radio frequency IBSS 2 STA 5 Exception: q IEEE p for Vehicular Networks: communication without setting up a basic service set. STA LAN 6

7 IEEE standard mobile terminal fixed terminal application TCP IP LLC access point LLC infrastructure network application TCP IP LLC MAC MAC MAC MAC PHY PHY PHY PHY 7

8 IEEE Layers and functions MAC q access mechanisms, fragmentation, encryption MAC Management q synchronization, roaming, authentication, MIB, power management PLCP Physical Layer Convergence Protocol q clear channel assessment signal (carrier sense) PMD Physical Medium Dependent q modulation, coding PHY Management q channel selection, MIB Station Management q coordination of all management functions PHY DLC LLC MAC PLCP PMD MAC Management PHY Management Station Management 8

9 The Physical Layer q IEEE l Direct Sequence Spread Spectrum (DSSS) PHY 2.4 GHz : RF : 1 2 Mbps l Frequency Hopping Spread Spectrum (FHSS) PHY 2.4 GHz : RF : 1-2 Mbps l Infrared (IR) PHY Indoor : IR : 1 and 2 Mbps q IEEE b l High Rate DSSS PHY 2.4 GHz : 5.5 Mbps 11 Mbps q IEEE a l OFDM PHY 5 GHz : 6-54 Mbps q IEEE g l OFDM PHY 2.4 GHz : 6-54 Mbps q IEEE n l OFDM PHY + MIMO (up to 4x4) + 40 MHz channel (instead of 20 MHz) Extension of a/g: Mbps q IEEE ac l OFDM PHY + MIMO (up to 8x8) + Multi-User MIMO (up to 4 simultaneous users) + 80 MHz QAM extension of n in 5GHz: up to 4 x 1.69 Gbit/s q IEEE ad l WiGig in 60 GHz, upto 7 Gbps

10 IEEE a Data rate q 6, 9, 12, 18, 24, 36, 48, 54 Mbit/s, depending on SNR q User throughput (1500 byte packets): 5.3 (6), 18 (24), 24 (36), 32 (54) q 6, 12, 24 Mbit/s mandatory Transmission range q 100m outdoor, 10m indoor Frequency l 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 q Free 5 GHz ISM-band Special Advantages/Disadvantages q Advantage: fits into 802.x standards, free ISM-band, available, simple system, uses less crowded 5 GHz band q Disadvantage: stronger shading due to higher frequency 10

11 IEEE g Frequency q Free 2.4 GHz ISM-band q But shared with many legacy WLANs and other systems, e.g. Bluetooth Backward compatible with IEEE b Data rate (as a) q 6, 9, 12, 18, 24, 36, 48, 54 Mbit/s, depending on SNR q User throughput significantly lower in presence of b stations Transmission range q Somewhat higher than a Special Advantages/Disadvantages q Advantage: backward compatibility, better propagation conditions q Disadvantage: crowded band, lower speed due to backward compatibility 11

12 b/g Channel selection (non-overlapping) Europe (ETSI) channel 1 channel 7 channel MHz [MHz] US (FCC)/Canada (IC) channel 1 channel 6 channel MHz [MHz] 12

13 IEEE a/g 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 13

14 MAC layer I Access methods q DCF CSMA/CA (mandatory) l collision avoidance via randomized back-off mechanism l minimum distance between consecutive packets l ACK packet for acknowledgements (not for broadcast) q DCF w/ RTS/CTS (optional) l reduces hidden terminal problem q PCF (optional) l access point polls terminals according to a list q HCF EDCA (optional) l CSMA/CA with priority levels q HCF CCA (optional) l Improved polling 14

15 MAC layer II Priorities q defined through different inter frame spaces q no guaranteed, hard priorities q SIFS (Short Inter Frame Spacing) l highest priority, for ACK, CTS, polling response q PIFS (PCF IFS) l medium priority, for time-bounded service using PCF q DIFS (DCF, Distributed Coordination Function IFS) l lowest priority, for asynchronous data service DIFS DIFS medium busy PIFS SIFS contention next frame direct access if medium is free DIFS t 15

16 CSMA/CA access method (DCF) I DIFS DIFS contention window (randomized back-off mechanism) medium busy direct access if medium is free DIFS slot time next frame t q station ready to send starts sensing the medium (Carrier Sense based on CCA, Clear Channel Assessment) q if the medium is free for the duration of an Inter-Frame Space (IFS), the station can start sending (IFS depends on service type) q 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) q 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 Binary Exponential Backoff q Stations choose their backoff time randomly from contention window q Ideal contention window size is trade-of between acceptable load and experienced delay q Initial contention window size (CW min ) is 7 slots (backoff time between 0 and 7) q After collision (no ack), contention window is doubled until CW max = 255 is reached: 7 -> 15 -> 31 -> 63 -> 127 ->

19 CSMA/CA access method (DCF) II q Sending unicast packets q station has to wait for DIFS before sending data q receivers acknowledge at once (after waiting for SIFS) if the packet was received correctly (CRC) q automatic retransmission of data packets in case of transmission errors sender receiver other stations DIFS data SIFS waiting time ACK DIFS contention data t 19

20 CSMA/CA with RTS/CTS q Sending unicast packets q station can send RTS with reservation parameter after waiting for DIFS (reservation determines amount of time the data packet needs the medium) q acknowledgement via CTS after SIFS by receiver (if ready to receive) q sender can now send data at once, acknowledgement via ACK q 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 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

22 802.11e - MAC services (QoS support) Distributed Coordination Function Point Coordination Function (not implemented) + Hybrid Coordination Function (HCF) backward compatible with DCF HCF: HCF Controlled Channel Access (HCCA) Enhanced Distributed Channel Access (EDCA) Differentiation priority levels 4 access categories with separate queues in each STA 22

23 EDCA Multiple Access Categories with their own transmission queue Differentiation by means of: Arbitray Inter Frame Spacing (AIFS) Initial Contention Window (CW min ) Maximum Contention Window (CW max ) Transmission Opportunity Limit (TXOP Limit) 23

24 DCF Summary Immediate access when medium is free >=DIFS DIFS DIFS PIFS Contention Window SIFS Busy Medium Backoff-Window Next Frame Slot Time Defer Access Select Slot and Decrement Backoff as long as medium is idle 24

25 EDCA Operation (AIFS) AIFS[j] Immediate access when Medium is free >= DIFS/AIFS[i] DIFS/AIFS PIFS AIFS[i] DIFS Contention Window SIFS Busy Medium Backoff Slots Next Frame Slot Time Defer Access Select Slot and Decrement Backoff as long as medium is idle 25

26 MAC Frame format q q q q q Types q control frames, management frames, data frames Duration /ID q for NAV during RTS/CTS and fragmentation, and CFP Sequence control q important against duplicated frames due to lost ACKs Addresses q receiver, transmitter (physical), BSS identifier, sender (logical) Miscellaneous q checksum, frame control, data bytes Duration/ Address Address Address Sequence ID Control Frame Control Protocol version Type Subtype To DS More Frag Retry Power Mgmt Address Data 4 bits From DS More Data WEP Order CRC 26

27 MAC address format scenario to DS from address 1 address 2 address 3 address 4 DS 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 AP: Access Point DA: Destination Address SA: Source Address BSSID: Basic Service Set Identifier RA: Receiver Address TA: Transmitter Address 27

28 Special Frames: ACK, RTS, CTS Acknowledgement ACK bytes Frame Duration Receiver CRC Control Address Request To Send RTS bytes Frame Duration Receiver Transmitter CRC Control Address Address Clear To Send CTS bytes Frame Duration Receiver CRC Control Address 28

29 MAC management Synchronization q try to find a LAN, try to stay within a LAN q timer etc. Power management q sleep-mode without missing a message q periodic sleep, frame buffering, traffic measurements Association/Reassociation q integration into a LAN q roaming, i.e. change networks by changing access points q scanning, i.e. active search for a network MIB - Management Information Base q managing, read, write 29

30 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 30

31 Synchronization using a Beacon (ad-hoc) beacon interval station 1 B 1 B 1 station 2 medium B 2 B 2 busy busy busy busy t value of the timestamp B beacon frame random delay 31

32 Power management Idea: switch the transceiver off if not needed States of a station: sleep and awake Timing Synchronization Function (TSF) q stations wake up at the same time Infrastructure Ad-hoc q Traffic Indication Map (TIM) l list of unicast receivers transmitted by AP q Delivery Traffic Indication Map (DTIM) l list of broadcast/multicast receivers transmitted by AP q Ad-hoc Traffic Indication Map (ATIM) l announcement of receivers by stations buffering frames l more complicated - no central AP l collision of ATIMs possible (scalability?) 32

33 Power saving with wake-up patterns (infrastructure) TIM interval DTIM interval access point medium D B busy T T d D busy busy busy B station T TIM D DTIM p awake d t B broadcast/multicast p PS poll d data transmission to/from the station 33

34 Power saving with wake-up patterns (ad-hoc) ATIM window beacon interval station 1 B 1 A D B 1 station 2 B 2 B 2 a d B beacon frame random delay A transmit ATIM t D transmit data awake a acknowledge ATIM d acknowledge data 34

35 Roaming No or bad connection? Then perform: Scanning q scan the environment, i.e., listen into the medium for beacon signals or send probes into the medium and wait for an answer Reassociation Request q station sends a request to one or several AP(s) Reassociation Response q success: AP has answered, station can now participate q failure: continue scanning AP accepts Reassociation Request q signal the new station to the distribution system q the distribution system updates its data base (i.e., location information) q typically, the distribution system now informs the old AP so it can release resources 35

36 IEEE standards and amendments e: MAC Enhancements QoS h: Spectrum Managed a (DCS, TPC) i: Enhanced Security Mechanisms IEEE : new release of the standard that includes a, b, d, e, g, h, i & j n: > 100 Mbps (MIMO) p: inter-vehicle communications s: Mesh Networking IEEE : new release of the standard that includes k z ac: 1 Gbps in 5 GHz band (> bw, > mimo-channels, 256 QAM) ad: 7 Gbps in 60 GHz band (a.o. beamforming) 36

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