EECS 4215 Mobile Communications - Wireless LANs PDF

EECS 4215 Mobile Communications W2025, Section Z Wireless LANs  Recap Physical layer Modulation (A-/F-/PSK, QAM, DS-/FHSS) Media Access Fixed/dynamic York University EECS 4215 Z (W2025) Wireless LANs 2 Wireless LANs I E E E 8 0 2. 11 _ _ ( P...

EECS 4215 Mobile Communications W2025, Section Z Wireless LANs  Recap Physical layer Modulation (A-/F-/PSK, QAM, DS-/FHSS) Media Access Fixed/dynamic York University EECS 4215 Z (W2025) Wireless LANs 2 Wireless LANs I E E E 8 0 2. 11 _ _ ( P HY, M AC, Ro a m i ng ,. 1 1 a, b , g , …) Bl u eto ot h / BL E / I E E E 8 0 2.15. x / Zi gBe e I E E E 8 0 2. 16/.19/. 20/.21/.2 2 Wireless LANs 3  Next Ch 7: Wireless LAN Will return to Ch 4–6 later MAC algorithms York University EECS 4215 Z (W2025) Wireless LANs 4  Mobile Communication Technology according to IEEE WiFi Local wireless networks 802.11a 802.11h WLAN 802.11 802.11i/e/…/n/…/z/aa 802.11b 802.11g ZigBee 802.15.4 802.15.4a/b/c/d/e/f/g Personal area wireless nw WPAN 802.15 802.15.5,.6 (WBAN) 802.15.2 802.15.3 802.15.3b/c 802.15.1 Bluetooth Wireless distribution networks WMAN 802.16 (Broadband Wireless Access) WiMAX + Mobility [802.20 (Mobile Broadband Wireless Access)] 802.16e (addition to.16 for mobile devices) Wireless LANs 5  Characteristics of wireless LANs Advantages Very flexible within the reception area Ad-hoc networks without previous planning possible (Almost) no wiring difficulties (e.g., historic buildings, firewalls) More robust against disasters like, e.g., earthquakes, fire - or users pulling a plug... Disadvantages Typically, lower user data rates/higher delays and delay jitter compared to wired networks due to shared medium, interference (depends on your neighbours!) Different/proprietary solutions, especially for higher bit-rates or low- power, standards take their time Products must follow many national restrictions if working wirelessly, it takes longer time to establish global solutions York University EECS 4215 Z (W2025) Wireless LANs 6  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 … York University EECS 4215 Z (W2025) Wireless LANs 7 Comparison: infrastructure vs. ad-hoc networks infrastructure network AP: Access Point AP AP wired network AP ad-hoc network Wireless LANs 8 Comparison: ad-hoc vs. mesh networks ad-hoc network mesh network Wireless LANs 9  802.11: [Classical] Architecture of an infrastructure network Station (STA) 802.11 LAN 802.x LAN ◦ terminal with access mechanisms to the wireless medium and radio contact to the access point STA1 Basic Service Set (BSS) BSS1 Portal ◦ group of stations using the same radio Access frequency Point Distribution System Access Point ◦ station integrated into the wireless LAN Access and the distribution system ESS Point Portal BSS2 ◦ bridge to other (wired) networks Distribution System ◦ interconnection network to form one logical network (EES: Extended Service STA2 802.11 LAN STA3 Set) based on several BSS Wireless LANs 10  802.11: Architecture of an ad-hoc network 802.11 LAN Direct communication within a limited range STA1 STA3 ◦ Station (STA): IBSS1 terminal with access mechanisms to the wireless STA2 medium ◦ Independent Basic Service Set (IBSS): IBSS2 group of stations using the same radio frequency STA5 STA4 802.11 LAN Wireless LANs 11  802.11: Architecture of a mesh network Mesh BSS forming a meshed network with 802.11 LAN possibly redundant paths using the Hybrid 802.x LAN Wireless Mesh Protocol (HWMP) STA1 BSS Access Portal Mesh Gate, AP and Point Distribution DS can be System Mesh co-located in 802.11 LAN Gate one device Mesh BSS STA2 BSS Access Distribution Mesh System Mesh STA2 Point Gate Mesh STA1 Mesh STA3 Mesh STA5 Mesh STA4 York University EECS 4215 Z (W2025) Wireless LANs 12  IEEE standard 802.11 fixed mobile terminal terminal infrastructure network access point application application TCP TCP IP IP LLC LLC LLC 802.11 MAC 802.11 MAC 802.3 MAC 802.3 MAC 802.11 PHY 802.11 PHY 802.3 PHY 802.3 PHY Wireless LANs 13  802.11: Layers and functions MAC PLCP Physical Layer Convergence Protocol ◦ clear channel assessment signal (carrier ◦ access mechanisms, sense) fragmentation, encryption PMD Physical Medium Dependent MAC Management ◦ modulation, coding ◦ synchronization, roaming, MIB, PHY Management power management ◦ channel selection, MIB Station Management ◦ coordination of all management functions Station Management LLC DLC MAC MAC Management PLCP PHY PHY Management PMD Wireless LANs 14 802.11: Physical layer (legacy) 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 ◦ 850–950 nm, diffuse light, typ. 10 m range ◦ carrier detection, energy detection, synchronization Wireless LANs 15  See also https://mrncciew.com/2014/10/14/cwap-802-11-phy-ppdu/ DSSS PHY packet format (legacy) Synchronization ◦ synch., gain setting, energy detection, frequency offset compensation SFD (Start Frame Delimiter) ◦ 1111001110100000 Signal ◦ data rate of the payload (0A: 1 Mbit/s DBPSK; 14: 2 Mbit/s DQPSK) Service ◦ future use, 00: 802.11 compliant Length ◦ length of the payload HEC (Header Error Check) ◦ protection of signal, service and length, x16+x12+x5+1 128 16 8 8 16 16 variable bits synchronization SFD signal service length HEC payload PLCP preamble PLCP header Wireless LANs 16 FHSS PHY packet format (legacy) Synchronization ◦ synch with 010101... Pattern (80 bits) SFD (Start Frame Delimiter) 16 bits ◦ 0000110010111101 start pattern PLW (PLCP_PDU Length Word) ◦ length of payload incl. 32 bit CRC of payload, PLW < 4096 PSF (PLCP Signaling Field) ◦ data of payload (1 or 2 Mbit/s) HEC (Header Error Check) ◦ CRC with x16+x12+x5+1 80 16 12 4 16 variable bits synchronization SFD PLW PSF HEC payload PLCP preamble PLCP header Wireless LANs 17  Later 802.11 versions (b, g, n…) 11b uses CCK ∼DSSS with less spreading, 5.5 or 11 Mbps Practical throughput: ∼6 Mbps 11g (and 11a) uses OFDM 52 OFDM subcarriers (0.3125 MHz each), 48 are for data and 4 are pilot subcarriers, BPSK, QPSK, 16-QAM or 64-QAM in each 11n OFDM with slightly more subcarriers (64) + MIMO + 40 MHz + better/“tighter” coding/timing York University EECS 4215 Z (W2025) Wireless LANs 18  802.11b CCK/HR/DSSS Long PLCP PPDU format 128 16 8 8 16 16 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) 56 16 8 8 16 16 variable bits short synch. SFD signal service length HEC payload PLCP preamble PLCP header (1 Mbit/s, DBPSK) (2 Mbit/s, DQPSK) 96 µs 2, 5.5 or 11 Mbit/s York University EECS 4215 Z (W2025) Wireless LANs 19 Channel selection (non-overlapping) Europe (ETSI) channel 1 channel 7 channel 13 2400 2412 2442 2472 2483.5 22 MHz [MHz] US (FCC)/Canada (IC) channel 1 channel 6 channel 11 2400 2412 2437 2462 2483.5 22 MHz [MHz] Wireless LANs 20  IEEE 802.11 OFDM – PHY frame format (was 802.11a, then also.11g/.11n…) 4 1 12 1 6 16 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 York University EECS 4215 Z (W2025) Wireless LANs 21  OFDM in IEEE 802.11 OFDM with 52 used subcarriers (64 in total) pilot 312.5 kHz 48 data + 4 pilot (plus 12 virtual subcarriers) 312.5 kHz spacing -26 -21 -7 -1 1 7 21 26 subcarrier channel center frequency number York University EECS 4215 Z (W2025) Wireless LANs 22  Operating channels for 802.11a in US/Canada 36 40 44 48 52 56 60 64 channel 5150 5180 5200 5220 5240 5260 5280 5300 5320 5350 [MHz] 16.6 MHz center frequency = 5000 + 5 × channel number [MHz] 149 153 157 161 channel 5725 5745 5765 5785 5805 5825 [MHz] 16.6 MHz York University EECS 4215 Z (W2025) Wireless LANs 23  How to get faster? OFDM becomes more accessible with newer technology and introduced @2.4 GHz (802.11g) 802.11n Use [slightly] more subcarriers Use multiple spatial streams Make packets shorter (legacy compatibility?) York University EECS 4215 Z (W2025) Wireless LANs 24  IEEE 802.11 ERP – PHY frame formats (was 802.11g) Extended Rate PHY @ 2.4GHz Data rates Builds on classical 1, 2 Mbit/s (DSSS) and 1, 2, 5.5, 11 Mbit/s (HR DSSS) Uses additionally OFDM for 6, 9, 12, 18, 24, 36, 48, and 54 Mbit/s (thus check 802.11 OFDM for frame formats) More options and modulation modes standardized Now obsolete or deprecated Summary: older 802.11a adapted to 2.4 GHz York University EECS 4215 Z (W2025) Wireless LANs 25  IEEE 802.11 HT – PHY frame formats (was 802.11n) – marketed as WiFi 4 High Throughput (HT) Orthogonal Frequency Division Multiplexing (OFDM) system @2.4 and 5 GHz Based on the OFDM system, but now using up to 4 spatial stream operating in 20 MHz bandwidth (additionally, 40 MHz bandwidth specified offering up to 600 Mbit/s) York University EECS 4215 Z (W2025) Wireless LANs 26  IEEE 802.11 HT – PHY frame formats (was 802.11n) York University EECS 4215 Z (W2025) Wireless LANs 27  Very High Throughput (VHT) PHY – uses OFDM (was 802.11ac) Source: IEEE Std 802.11-2016 York University EECS 4215 Z (W2025) Wireless LANs 28  IEEE 802.11 VHT – High-speed for WLANs at 5 GHz – marketed as WiFi 5 Single link throughput > 500Mbit/s, multi-station > 1 Gbit/s Bandwidth up to 160 MHz (80 MHz mandatory, and the highest available in Canada), up to 8 × MIMO, up to 256-QAM, beamforming, SDMA via MIMO Example home configuration: 8-antenna access point, 160 MHz bandwidth, 6.77 Gbps 4-antenna digital TV, 3.39 Gbps 2-antenna tablet, 1.69 Gbps Two 1-antenna smartphones, 867 Mbit/s each (@160 MHz ch.) Redefinition of many protocol fields and procedures! York University EECS 4215 Z (W2025) Wireless LANs 29 802.11 MAC Wireless LANs 30  802.11 - MAC layer architecture Source: IEEE Std 802.11-2016 York University EECS 4215 Z (W2025) Wireless LANs 31  How to access the medium in 802.11 Distributed Coordination Function (DCF) Fundamental access method in 802.11, mandatory Also known as CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance) Random backoff, certain fairness, refinement with RTS/CTS possible Point Coordination Function (not really used, will be kicked out of the standard in the future) Contention free access, reservation of the medium Hybrid Coordination Function (HCF) QoS support by combining DCF and PCF Contention-based channel access (Enhanced Distributed Channel Access, EDCA) and controlled channel access (HCF Controlled Channel Access, HCCA) Support of different priorities for, e.g., background, best effort, video, voice traffic (WiFi WMM Designations) Mesh Coordination Function (MCF) Only in a MBSS, EDCA for contention-based access, MCCA (MCS Controlled Channel Access) for contention-free access York University EECS 4215 Z (W2025) Wireless LANs 32  802.11 - MAC Inter Frame Space Priorities of packets defined through different inter frame spaces (not always guaranteed) RIFS (Reduced IFS) shortest IFS, reduced overhead, only if no SIFS expected, for higher throughput SIFS (Short IFS) for ACK, CTS, polling response PIFS (PCF IFS) used to gain priority access (PCF, TIM, …) DIFS (DCF IFS) EIFS for “normal” asynchronous data service AIFSi AIFS (Arbitration IFS) … variable depending on QoS AIFSi RIFS EIFS (Extended IFS) IFS e.g. after an incorrect FCS DIFS DIFS Additional “beamforming” IFSs PIFS medium busy SIFS contention next frame t direct access if medium is free  DIFS York University EECS 4215 Z (W2025) Wireless LANs 33 802.11 - CSMA/CA access method I 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) contention window DIFS DIFS (randomized back-off mechanism) medium busy next frame direct access if t medium is free  DIFS slot time (20µs) Wireless LANs 34  802.11 - competing stations - simple version DIFS DIFS DIFS DIFS boe bor boe bor boe busy station1 boe busy station2 busy station3 boe busy boe bor station4 boe bor boe busy boe bor station5 t busy medium not idle (frame, ack etc.) boe elapsed backoff time packet arrival at MAC bor residual backoff time Wireless LANs 35 802.11 - CSMA/CA access method II Sending unicast packets ◦ station must 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, with exponential increase of contention window DIFS data sender SIFS ACK receiver DIFS other data stations t waiting time contention Wireless LANs 36  802.11 – DCF with RTS/CTS 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 DIFS RTS data sender SIFS SIFS CTS SIFS ACK receiver NAV (RTS) DIFS other NAV (CTS) data stations t defer access contention Wireless LANs 37  Fragmentation DIFS RTS frag1 frag2 sender SIFS SIFS SIFS CTS SIFS ACK1 SIFS ACK2 receiver NAV (RTS) NAV (CTS) NAV (frag1) DIFS other NAV (ACK1) data stations t contention 38  802.11 – MAC Frame format Types Only the first three and the ◦ control frames, management frames, data frames last field are present in all frames! Sequence numbers ◦ important against duplicated frames due to lost ACKs Addresses ◦ receiver, transmitter (physical), BSS identifier, sender (logical) Miscellaneous ◦ sending time, checksum, frame control, data bytes 2 2 6 6 6 2 6 2 4 0-7951 4 Frame Duration/ Address Address Address Sequence Address QoS HT Frame FCS Control ID 1 2 3 Control 4 Control Control Body bits 2 2 4 1 1 1 1 1 1 1 1 Protec- Protocol To From More Power More +HTC/ Type Subtype Retry ted version DS DS Frag Mgmt Data Frame Order MAC address format (examples) Example scenario to DS from address 1 address 2 address 3 address 4 DS ad-hoc network 0 0 RA=DA TA=SA BSSID - infrastructure 0 1 RA=DA TA=BSSID SA - network, from AP infrastructure 1 0 RA=BSSID TA=SA DA - network, to AP within mesh BSS 1 1 RA TA DA SA AP: Access Point DA: Destination Address SA: Source Address BSSID: Basic Service Set Identifier RA: Receiver Address TA: Transmitter Address Special Frames: ACK, RTS, CTS Acknowledgement bytes 2 2 6 4 Frame Receiver ACK Duration FCS Control Address bytes 2 2 6 6 4 Request To Send RTS Frame Control Duration Receiver Address Transmitter Address FCS bytes 2 2 6 4 Frame Receiver Clear To Send CTS Control Duration Address FCS  802.11 - MAC management Synchronization try to find a LAN, try to stay within a LAN Clock synchronization 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 Accessible through SNMP York University EECS 4215 Z (W2025) 42  Synchronization TSF: timing synchronization function Needed for polling, frequency hopping Use of beacons for timestamps, other information Not periodic – not sent when medium is busy Who transmits beacons in ad hoc mode? York University EECS 4215 Z (W2025) 43  Synchronization using a Beacon (infrastructure) beacon interval (20ms – 1s) B B B B access point busy busy busy busy medium t value of the timestamp B beacon frame York University EECS 4215 Z (W2025) Synchronization using a Beacon (ad-hoc) beacon interval B1 B1 station1 B2 B2 station2 busy busy busy busy medium t value of the timestamp B beacon frame random delay  Security issues? Can a malicious node provide incorrect timing information? York University EECS 4215 Z (W2025) 46 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 (nodes run this at all times) Infrastructure ◦ Traffic Indication Map (TIM) sent with beacons ◦ list of unicast receivers transmitted by AP ◦ Delivery Traffic Indication Map (DTIM) ◦ list of broadcast/multicast receivers transmitted by AP Ad-hoc ◦ Ad-hoc Traffic Indication Map (ATIM) ◦ announcement of receivers by stations buffering frames ◦ more complicated - no central AP ◦ collision of ATIMs possible (scalability?) APSD (Automatic Power Save Delivery) ◦ new method in 802.11e replacing above schemes (part of WiFi WMM) 47 Power saving with wake-up patterns (infrastructure) TIM interval DTIM interval D B T T d D B access point busy busy busy busy medium p d station t T TIM D DTIM awake B broadcast/multicast p PS poll d data transmission to/from the station 48 802.11 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 Reassociation Request ◦ station sends a request to one or several AP(s) Reassociation 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) ◦ typically, the distribution system now informs the old AP so it can release resources Fast roaming – 802.11r,.11k,.11v ◦ Fewer steps to transition; share possible networks ahead of time; guide transitions… 50 WLAN: IEEE 802.11 – timeline of some developments 802.11e: MAC Enhancements – QoS ◦ Enhance the current 802.11 MAC to expand support for applications with Quality of Service requirements, and in the capabilities and efficiency of the protocol ◦ Definition of a data flow (“connection”) with parameters like rate, burst, period… supported by HCCA (HCF (Hybrid Coordinator Function) Controlled Channel Access, optional) ◦ Additional energy saving mechanisms and more efficient retransmission ◦ EDCA (Enhanced Distributed Channel Access): high priority traffic waits less for channel access 802.11g: Data Rates > 20 Mbit/s at 2.4 GHz; 54 Mbit/s, OFDM ◦ Successful successor of 802.11b, performance loss during mixed operation with.11b 802.11i: Security Mechanisms ◦ TKIP enhances the insecure WEP, but remains compatible to older WEP systems ◦ AES provides a secure encryption method and is based on new hardware 51 IEEE 802.11– other developments 802.11n: Higher data rates above 100 Mbit/s ◦ Changes of PHY and MAC with the goal of 100 Mbit/s at MAC SAP ◦ MIMO antennas (Multiple Input Multiple Output), up to 600 Mbit/s feasible ◦ Still a large overhead due to protocol headers and inefficient mechanisms 802.11p: Inter car communications ◦ Communication between cars/road side and cars/cars ◦ Usage of 5.850-5.925 GHz band in North America 802.11r: Faster Handover between BSS ◦ Secure, fast handover of a station from one AP to another within an ESS ◦ Current mechanisms (even newer standards like 802.11i) plus incompatible devices from different vendors are massive problems for the use of, e.g., VoIP in WLANs 802.11s: Mesh Networking ◦ Design of a self-configuring Wireless Distribution System (WDS) based on 802.11 ◦ Support of point-to-point and broadcast communication across several hops 802.11y: Extensions for the 3650-3700 MHz band in the USA 802.11ac: Very High Throughput