Wireless Mobile Communications Chapter 5 Wireless LANs Characteristics

Wireless & Mobile Communications Chapter 5: Wireless LANs Characteristics Ø IEEE 802. 11 Ø PHY q MAC q Roaming q ØBluetooth ICS 243 E - Ch 5 Wireless Lans Winter 2001

Characteristics of wireless LANs Ø Advantages very flexible within the reception area q Ad-hoc networks without previous planning possible q (almost) no wiring difficulties (e. g. historic buildings, firewalls) q more robust against disasters like, e. g. , earthquakes, fire - or users pulling a plug. . . q Ø Disadvantages typically very low bandwidth compared to wired networks (1 -10 Mbit/s) q many proprietary solutions, especially for higher bit-rates, standards take their time (e. g. IEEE 802. 11) q products have to follow many national restrictions if working wireless, it takes a vary long time to establish global solutions like, e. g. , IMT-2000 q ICS 243 E - Ch 5 Wireless Lans Winter 2001 2

Design goals for wireless LANs q q q q q 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 ICS 243 E - Ch 5 Wireless Lans Winter 2001 3

Comparison: infrared vs. radio transmission ØInfrared q ØRadio uses IR diodes, diffuse light, multiple reflections (walls, furniture etc. ) ØAdvantages q ØAdvantages experience from wireless WAN and mobile phones can be used q coverage of larger areas possible (radio can penetrate walls, furniture etc. ) q simple, cheap, available in many mobile devices q no licenses needed q simple shielding possible q ØDisadvantages interference by sunlight, heat sources etc. q many things shield or absorb IR light q low bandwidth q ØExample q Ir. DA (Infrared Data Association) interface available everywhere ICS 243 E - Ch 5 Wireless Lans typically using the license free ISM band at 2. 4 GHz ØDisadvantages very limited license frequency bands q shielding more difficult, interference with other electrical devices q ØExample q Winter 2001 Wave. LAN, HIPERLAN, Bluetooth 4

Comparison: infrastructure vs. ad-hoc networks infrastructure network AP: Access Point AP AP wired network AP ad-hoc network ICS 243 E - Ch 5 Wireless Lans Winter 2001 5

802. 11 - Architecture of an infrastructure network ØStation 802. 11 LAN STA 1 802. x LAN q Portal Access Point ESS q Service Set (BSS) group of stations using the same radio frequency ØAccess Distribution System q Point station integrated into the wireless LAN and the distribution system ØPortal q BSS 2 bridge to other (wired) networks ØDistribution q STA 2 terminal with access mechanisms to the wireless medium and radio contact to the access point ØBasic BSS 1 (STA) 802. 11 LAN ICS 243 E - Ch 5 Wireless Lans STA 3 Winter 2001 System interconnection network to form one logical network (EES: Extended Service Set) based on several BSS 6

802. 11 - Architecture of an ad-hoc network ØDirect communication within a limited range 802. 11 LAN Station (STA): terminal with access mechanisms to the wireless medium q Basic Service Set (BSS): group of stations using the same radio frequency q STA 1 STA 3 BSS 1 STA 2 BSS 2 STA 5 STA 4 802. 11 LAN ICS 243 E - Ch 5 Wireless Lans Winter 2001 7

IEEE standard 802. 11 fixed terminal mobile terminal server infrastructure network access point application TCP IP IP LLC LLC 802. 11 MAC 802. 3 MAC 802. 11 PHY 802. 3 PHY ICS 243 E - Ch 5 Wireless Lans Winter 2001 8

802. 11 - Layers and functions ØPLCP Physical Layer Convergence Protocol ØMAC q access mechanisms, fragmentation, encryption Management synchronization, roaming, MIB, power management q clear channel assessment signal (carrier sense) ØPMD Physical Medium Dependent q ØPHY q modulation, coding Management channel selection, MIB ØStation LLC MAC Management PLCP PHY Management PMD ICS 243 E - Ch 5 Wireless Lans Winter 2001 coordination of all management functions Station Management PHY DLC q Management 9

802. 11 - Physical layer Ø 3 versions: 2 radio (typ. 2. 4 GHz), 1 IR q Ø data rates 1 or 2 Mbit/s FHSS (Frequency Hopping Spread Spectrum) spreading, despreading, signal strength, typ. 1 Mbit/s q min. 2. 5 frequency hops/s (USA), two-level GFSK modulation q Ø DSSS (Direct Sequence Spread Spectrum) DBPSK modulation for 1 Mbit/s (Differential Binary Phase Shift Keying), DQPSK for 2 Mbit/s (Differential Quadrature PSK) q preamble and header of a frame is always transmitted with 1 Mbit/s, rest of transmission 1 or 2 Mbit/s q chipping sequence: +1, -1, +1, +1, -1, -1 (Barker code) q max. radiated power 1 W (USA), 100 m. W (EU), min. 1 m. W q Ø Infrared 850 -950 nm, diffuse light, typ. 10 m range q carrier detection, energy detection, synchonization q ICS 243 E - Ch 5 Wireless Lans Winter 2001 10

FHSS PHY packet format Ø Synchronization q Ø SFD (Start Frame Delimiter) q Ø length of payload incl. 32 bit CRC of payload, PLW < 4096 PSF (PLCP Signaling Field) q Ø 0000110010111101 start pattern PLW (PLCP_PDU Length Word) q Ø synch with 010101. . . pattern data of payload (1 or 2 Mbit/s) HEC (Header Error Check) q CRC with x 16+x 12+x 5+1 80 synchronization 16 12 4 16 variable SFD PLW PSF HEC payload PLCP preamble ICS 243 E - Ch 5 Wireless Lans bits PLCP header Winter 2001 11

DSSS PHY packet format Ø Synchronization q Ø SFD (Start Frame Delimiter) q Ø data rate of the payload (0 A: 1 Mbit/s DBPSK; 14: 2 Mbit/s DQPSK) Service q Ø 1111001110100000 Signal q Ø synch. , gain setting, energy detection, frequency offset compensation Length future use, 00: 802. 11 compliant length of the payload HEC (Header Error Check) q protection of signal, service and length, x 16+x 12+x 5+1 128 synchronization 16 SFD 8 8 16 16 signal service length HEC PLCP preamble ICS 243 E - Ch 5 Wireless Lans variable bits payload PLCP header Winter 2001 12

802. 11 - MAC layer I - DFWMAC Ø Traffic services q Asynchronous Data Service (mandatory) exchange of data packets based on “best-effort” support of broadcast and multicast q Time-Bounded Service (optional) implemented using PCF (Point Coordination Function) Ø Access methods q DFWMAC-DCF CSMA/CA (mandatory) collision avoidance via randomized „back-off“ mechanism minimum distance between consecutive packets ACK packet for acknowledgements (not for broadcasts) q DFWMAC-DCF w/ RTS/CTS (optional) Distributed Foundation Wireless MAC avoids hidden terminal problem q DFWMAC- PCF (optional) access point polls terminals according to a list ICS 243 E - Ch 5 Wireless Lans Winter 2001 13

802. 11 - MAC layer II Ø Priorities defined through different inter frame spaces q no guaranteed, hard priorities q SIFS (Short Inter Frame Spacing) q highest priority, for ACK, CTS, polling response q PIFS (PCF IFS) medium priority, for time-bounded service using PCF q DIFS (DCF, Distributed Coordination Function IFS) lowest priority, for asynchronous data service DIFS medium busy DIFS PIFS SIFS direct access if medium is free DIFS ICS 243 E - Ch 5 Wireless Lans Winter 2001 contention next frame t 14

802. 11 - CSMA/CA access method I DIFS medium busy contention window (randomized back-off mechanism) next frame direct access if medium is free DIFS t slot time 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) q ICS 243 E - Ch 5 Wireless Lans Winter 2001 15

802. 11 - competing stations - simple version DIFS station 1 station 2 DIFS boe bor boe busy boe bor boe busy station 3 station 4 boe bor station 5 busy bor t busy medium not idle (frame, ack etc. ) boe elapsed backoff time packet arrival at MAC bor residual backoff time ICS 243 E - Ch 5 Wireless Lans Winter 2001 16

802. 11 - CSMA/CA access method II Ø Sending unicast packets 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 q DIFS sender data SIFS receiver ACK DIFS other stations ICS 243 E - Ch 5 Wireless Lans waiting time Winter 2001 data t contention 17

802. 11 - DFWMAC Ø 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) 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 q DIFS sender RTS data SIFS receiver CTS SIFS other stations ICS 243 E - Ch 5 Wireless Lans SIFS NAV (RTS) NAV (CTS) defer access Winter 2001 ACK DIFS data t contention 18

Fragmentation DIFS sender RTS frag 1 SIFS receiver CTS SIFS frag 2 SIFS ACK 1 SIFS ACK 2 NAV (RTS) NAV (CTS) NAV (frag 1) NAV (ACK 1) other stations ICS 243 E - Ch 5 Wireless Lans DIFS data t contention Winter 2001 19

DFWMAC-PCF I t 0 t 1 Super. Frame medium busy PIFS D 1 point SIFS coordinator wireless stations‘ NAV ICS 243 E - Ch 5 Wireless Lans SIFS D 2 SIFS U 1 U 2 NAV Winter 2001 20

DFWMAC-PCF II t 2 point coordinator D 3 PIFS D 4 t 4 CFend SIFS U 4 wireless stations‘ NAV SIFS t 3 NAV contention free period ICS 243 E - Ch 5 Wireless Lans Winter 2001 contention period t 21

802. 11 - Frame format Ø Types q Ø Sequence numbers q Ø important against duplicated frames due to lost ACKs Addresses q Ø control frames, management frames, data frames receiver, transmitter (physical), BSS identifier, sender (logical) Miscellaneous q sending time, checksum, frame control, data bytes 2 2 6 6 6 2 6 Frame Duration Address Sequence Address Control ID 1 2 3 Control 4 0 -2312 4 Data CRC version, type, fragmentation, security, . . . ICS 243 E - Ch 5 Wireless Lans Winter 2001 22

MAC address format DS: Distribution System AP: Access Point DA: Destination Address SA: Source Address BSSID: Basic Service Set Identifier RA: Receiver Address TA: Transmitter Address ICS 243 E - Ch 5 Wireless Lans Winter 2001 23

802. 11 - MAC management Ø Synchronization try to find a LAN, try to stay within a LAN q timer etc. q Ø Power management sleep-mode without missing a message q periodic sleep, frame buffering, traffic measurements q Ø Association/Reassociation integration into a LAN q roaming, i. e. change networks by changing access points q scanning, i. e. active search for a network q Ø MIB - Management Information Base q managing, read, write ICS 243 E - Ch 5 Wireless Lans Winter 2001 24

Synchronization using a Beacon (infrastructure) beacon interval access point medium B B busy t value of the timestamp ICS 243 E - Ch 5 Wireless Lans B Winter 2001 beacon frame 25

Synchronization using a Beacon (ad-hoc) beacon interval station 1 B 1 B 2 station 2 medium busy B 2 busy value of the timestamp ICS 243 E - Ch 5 Wireless Lans B busy beacon frame Winter 2001 t random delay 26

Power management Idea: switch the transceiver off if not needed Ø States of a station: sleep and awake Ø Timing Synchronization Function (TSF) Ø q Ø stations wake up at the same time Infrastructure q Traffic Indication Map (TIM) list of unicast receivers transmitted by AP q Delivery Traffic Indication Map (DTIM) list of broadcast/multicast receivers transmitted by AP Ø Ad-hoc q Ad-hoc Traffic Indication Map (ATIM) announcement of receivers by stations buffering frames more complicated - no central AP collision of ATIMs possible (scalability? ) ICS 243 E - Ch 5 Wireless Lans Winter 2001 27

Power saving with wake-up patterns (infrastructure) TIM interval access point DTIM interval D B T busy medium busy T d D B busy p station d t T TIM B broadcast/multicast ICS 243 E - Ch 5 Wireless Lans D DTIM awake p PS poll Winter 2001 d data transmission to/from the station 28

Power saving with wake-up patterns (ad-hoc) ATIM window station 1 beacon interval B 1 A B 2 station 2 B beacon frame awake random delay B 2 D a B 1 d A transmit ATIM t D transmit data a acknowledge ATIM d acknowledge data ICS 243 E - Ch 5 Wireless Lans Winter 2001 29

802. 11 - Roaming No or bad connection? Then perform: Ø Scanning Ø q Ø Reassociation Request q Ø scan the environment, i. e. , listen into the medium for beacon signals or send probes into the medium and wait for an answer station sends a request to one or several AP(s) Reassociation Response success: AP has answered, station can now participate q failure: continue scanning q Ø AP accepts Reassociation Request 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 q ICS 243 E - Ch 5 Wireless Lans Winter 2001 30

Future developments Ø IEEE 802. 11 a compatible MAC, but now 5 GHz band q transmission rates up to 20 Mbit/s q close cooperation with BRAN (ETSI Broadband Radio Access Network) q Ø IEEE 802. 11 b higher data rates at 2. 4 GHz q proprietary solutions already offer 10 Mbit/s q Ø IEEE WPAN (Wireless Personal Area Networks) market potential q compatibility q low cost/power, small form factor q technical/economic feasibility Bluetooth q ICS 243 E - Ch 5 Wireless Lans Winter 2001 31

Bluetooth ICS 243 E - Ch 5 Wireless Lans Winter 2001 32

History ICS 243 E - Ch 5 Wireless Lans Winter 2001 33

Usage Modes Personal Ad-hoc Networks Cable Replacement Data/Voice Access Points ICS 243 E - Ch 5 Wireless Lans Winter 2001 34

Bluetooth Consortium: Ericsson, Intel, IBM, Nokia, Toshiba - many members Ø Scenarios Ø q connection of peripheral devices loudspeaker, joystick, headset q support of ad-hoc networking small devices, low-cost q bridging of networks e. g. , GSM via mobile phone - Bluetooth - laptop Ø Simple, cheap, replacement of Ir. DA, low range, lower data rates q 2. 4 GHz, FHSS (CDMA), TDD ICS 243 E - Ch 5 Wireless Lans Winter 2001 35

Technology Characteristics Ø Low-cost, Ø Low-power, Ø Small-sized, Ø Short-range, Ø Robust wireless technology ICS 243 E - Ch 5 Wireless Lans Winter 2001 36

General Characteristics Ø Universal wireless interface Ø Ad-hoc networking architecture Ø 80 Mhz in unlicenced ISM band at 2. 45 Ghz Ø Gross bitrate 1 Mbps Ø Simultaneous voice and high speed data support Ø Evolves from cable replacement - > networking solution ICS 243 E - Ch 5 Wireless Lans Winter 2001 37

Protocol Architecture ICS 243 E - Ch 5 Wireless Lans Winter 2001 38

Baseband Ø 2. 4 GHz, FHSS (CDMA), TDD ICS 243 E - Ch 5 Wireless Lans Winter 2001 39

FHSS Pattern: Synchronization The slaves synchronize themselves to the Master Clock Slave Master Slave Piconet ICS 243 E - Ch 5 Wireless Lans Winter 2001 40

Frequencsy Selection Native clock (slave) Phase Hop frequency + Offset (master) ICS 243 E - Ch 5 Wireless Lans Winter 2001 Bleutooth address (master) 41

Piconets and Scatternets S 4 S 1 M 1 S 5 S 3 S 2 S 7 M 2 S 6 Piconet A Piconet B Scatternet ICS 243 E - Ch 5 Wireless Lans Winter 2001 42

FHSS 78 Frequency 0 Time ICS 243 E - Ch 5 Wireless Lans Winter 2001 43

Interference Ø Ø Sources: q Other equipment in ISM band e. g. WLAN, micro-wave oven, etc. q Frequency hopping Remedies: q Frequency hopping q Short range q Power control q FEC and ARQN q Short packets and fast acknowledgements ICS 243 E - Ch 5 Wireless Lans Winter 2001 44

Error Control Ø 1/3 rate Forward Error Correction (FEC) Ø 2/3 rate Forward Error Correction (FEC) Ø Automatic Repeat re. Quest (ARQ) ICS 243 E - Ch 5 Wireless Lans Winter 2001 45

Link Types Ø Ø Synchronous Connection Oriented (SCO) q Circuit switched typically used for voice q Symmetric, synchronous service q Slot reservation at fixed intervals q Point-to-point Asynchronous Connectionless Link (ACL) q Packet switched q Symmetric or asymmetric, asynchronous service q Polling mechanism between master and slave(s) q Point-to-point and point-to-multipoint ICS 243 E - Ch 5 Wireless Lans Winter 2001 46

Flexibility: Multislot Packets No frequency hopping during multislots, sequence continues after transmission fn fn+1 fn+2 fn+3 fn+4 fn+5 Single slot fn fn+3 Three slot fn fn+5 Five slot ICS 243 E - Ch 5 Wireless Lans Winter 2001 47

ACL data rates ICS 243 E - Ch 5 Wireless Lans Winter 2001 48

ACL Polling Scheme: Master ICS 243 E - Ch 5 Wireless Lans Winter 2001 49

SCO data rates ICS 243 E - Ch 5 Wireless Lans Winter 2001 50

Voice and Data Transmission: SCO ACL ACL SCO ACL Master Slave ICS 243 E - Ch 5 Wireless Lans Winter 2001 51

Capacity of Piconet One ACL link (432 kbps symmetric or 721/56 kbps asymetric) or Three simultaneous SCO links (64 kbps) or A combination of voice/data S 4 S 1 M 1 S 3 S 2 ICS 243 E - Ch 5 Wireless Lans Winter 2001 52

States of a Bluetooth device (PHY layer) unconnected STANDBY inquiry page transmit PARK ICS 243 E - Ch 5 Wireless Lans connected HOLD Winter 2001 SNIFF connecting active low power 53

Bluetooth MAC layer Ø Synchronous Connection-Oriented link (SCO) q Ø Asynchronous Connectionless Link (ACL) q Ø packet switched, point-to-multipoint, master polls Access code q Ø symmetrical, circuit switched, point-to-point synchronization, derived from master, unique per channel Packet header q 1/3 -FEC, MAC address (1 master, 7 slaves), link type, alternating bit ARQ/SEQ, checksum 72 54 0 -2745 access code packet header 3 MAC address bits payload 4 1 1 1 8 type flow ARQN SEQN HEC ICS 243 E - Ch 5 Wireless Lans Winter 2001 bits 54

Error Control Details Ø Packet header: q Ø 1/3 rate FEC SCO payload: 1/3 rate FEC q 2/3 FEC q no FEC q Ø ACL payload: ARQN using payload CRC (except AUX packet) q 2/3 rate FEC optional q ICS 243 E - Ch 5 Wireless Lans Winter 2001 55

Connection Set up in an existing Piconet Master Resp. FHS data Page Slave ICS 243 E - Ch 5 Wireless Lans Winter 2001 56

Scatternets Each piconet has one master and up to 7 slaves Ø Master determines hopping sequence, slaves have to synchronize Ø Participation in a piconet = synchronization to hopping sequence Ø Communication between piconets = devices jumping back and forth between the piconets Ø piconets ICS 243 E - Ch 5 Wireless Lans Winter 2001 57

Summary Ø Bluetooth: q Ø Ir. DA: q Ø Wirelss data cable replacement for devices in line of sight Home. RF: q Ø Wireless voice and data for mobile devices Networking mobile data and voice devices to a PC anywhere in the home IEEE 802. 11 and Hiperlan 2: q Wireless enterprise networking in the office ICS 243 E - Ch 5 Wireless Lans Winter 2001 58

Where Bluetooth fits in Bandwidth Bluetooth GSM GPRS UMTS DECT WLAN Ir. DA IR Bluetooth WLAN UMTS GPRS Dect ICS 243 E - Ch 5 Wireless Lans GSM Winter 2001 Coverage 59

Bluetooth functionality ICS 243 E - Ch 5 Wireless Lans Winter 2001 60