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Characteristics of Wireless Environment

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Characteristics of Wireless Environment

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  1. Characteristics of Wireless Environment

  2. Radio Propagation Mechanism

  3. Characteristics of Wireless Channel • Path loss • Pr/Pt = O(d-γ), where d: distance • γ: 2 (free space), 5 (strong attenuation) • Fading: fluctuation of signal strength • Fast fading: due to multipath propagation • Slow fading: occurs when objects absorb the transmission • May reduced by diversity or adaptive modulation • Interference • Adjacent channel interference guard band • Co-channel interference  cellular, directional antenna, dynamic channel allocation • Inter-symbol interference  adaptive equalization • Doppler shift

  4. Multiple Access Techniques • FDMA • OFDM • TDMA • Hard to compute good schedules in a distributed fashion. • Schedule needs to be traffic dependent. • Need synchronized clocks in hardware to implement slots • CDMA • FHSS • DSSS • SDMA • Duplexing • FDD • TDD

  5. CDMA DSSS • used in several wireless broadcast channels (cellular, satellite, etc) standards • unique “code” assigned to each user; i.e., code set partitioning • all users share same frequency, but each user has own “chipping” sequence (i.e., code) to encode data • encoded signal = (original data) X (chipping sequence) • decoding: inner-product of encoded signal and chipping sequence • allows multiple users to “coexist” and transmit simultaneously with minimal interference (if codes are “orthogonal”)

  6. d0 = 1 1 1 1 1 1 1 d1 = -1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 M Di = SZi,m.cm m=1 M d0 = 1 d1 = -1 CDMA Encode/Decode channel output Zi,m Zi,m= di.cm data bits sender slot 0 channel output slot 1 channel output code slot 1 slot 0 received input slot 0 channel output slot 1 channel output code receiver slot 1 slot 0

  7. CDMA: two-sender interference

  8. Wireless MAC • MAC (Medium Access Control) • Sharing a Single Broadcast Medium among Multiple Users • Contention : Most Widely Used, Suffer from Collision • Non-Contention : Reservation/Round-Robin, Collision Free • Wireless MAC vs. Ad Hoc MAC ? • Ad Hoc Network: Multi-Hop Wireless Network

  9. ALOHA and CSMA • ALOHA - University of Hawaii (1970) • Transmit whenever it has data to send. • Listen to the acknowledgement feedback from the receiver. • If a collision occurs (no ACK), retransmits after a random delay. • Utilization: pure Aloha = 18.5%, slotted Aloha = 37% • CSMA - Kleinrock (1975) • Listen (Carrier Sense) before transmission • 1. If channel is idle, transmit • 2. Otherwise, do one of the followings: • Wait until channel become idle and transmit  1 Persistent-CSMA • Wait until idle and transmit with probability p  p Persistent-CSMA • Defer transmission and try again after a random delay  NP-CSMA • Carrier sense not foolproof • Propagation delay (also a problem in wireline). • Can sense only at transmitter; but collision happens at receiver (a wireless problem).

  10. Contention Window A Frame Remaining Backoff B Frame Data Arrival C Frame Backoff = Uniform[0, CW] D Frame DIFS DIFS DIFS CSMA/CA (Collision Avoidance) • RTS/CTS Dialog before Data Transmission • RTS (Request To Send : Sender) / CTS (Clear To Send : Receiver) / DATA • Contention Window • How about CSMA/CD (Collision Detection) ? • Need the ability to Listen while transmitting to detect collision • The strength of its own transmission would mask all other signals on the air

  11. Hidden and Exposed Terminal Problem A B C A B C D C wants to transmit to D. It hears B’s transmission, and unnecessarily defers, although it could transmit in parallel as A can’t hear C’s transmission B transmits to A C wants to transmit to B. It does not hear A’s transmission, accesses the channel and collides A transmits to B Time Packet Transmission From B to A Packet Transmission From B to A Time Packet Transmission From A to B Packet Transmission From C to B Packet can be transmitted from C to D, But don’t it. Waste Resource Collision C and A are Hidden Terminals relative to each other – one can’t sense the other’s transmission C is an Exposed Terminal relative to B. B’s transmission inhibits C, although there would be no collision at the receiver (D). If C were to transmit.

  12. Sense Carrier at Receiver • Busy Tone Multiple Access (BTMA) • Receiver sounds a tone when busy receiving. • Carrier sense on busy tone before transmission. • Perfect solution. But need a busy tone channel and extra interface. Channel gains on data and busy tone channels may be different. • “In band” solution • Use virtual carrier sensing. Used in 802.11

  13. IEEE 802.11 WLAN

  14. Wireless LAN Standards IEEE802.11 ETSI BRAN 802.11f : Inter Access Point Protocol UMTS Integration IEEE 802.11 802.11e : QoS Enhancements MAC 802.11i : Security Enhancements HiperLAN 802.11h DFS & TPC DFS &TPC 5GHz 54Mbps 802.11a 5GHz 54Mbps 802.11g 2.4GHz 20Mbps 802.11b 2.4GHz 11Mbps 802.11 2.4GHz 2Mbps PHY

  15. WLAN Standards 802.11 802.11b 802.11g 802.11a Hperlan2 Frequency 2.4~2.4835 GHz (83.5MHz) 2.4~2.4835 GHz (83.5MHz) 2.4~2.4835 GHz (83.5MHz) 5.150~5.350 GHz 5.725~5.825 GHz (455MHz) 5.150~5.350 GHz 5.470~5.725 GHz (300MHz) Modulation DBPSK, DQPSK DBPSK/CCK, DQPSK/CCK CCK, OFDM OFDM BPSK, QPSK 16QAM, 64QAM OFDM BPSK, QPSK 16QAM, 64QAM Max. PHY rate 1,2 Mbps 1,2,5.5,11Mbps 54 Mbps (1,2,5.5,6,9, 11,12,18,24, 36,48,54Mbps) 54 Mbps (6,9,12,18,24, 36,48,54Mbps) 54 Mbps (6,9,12,18,24, 36,48,54Mbps) Max. Data Rate(layer 3) 1.2 Mbps 5 Mbps 22~32 Mbps 32 Mbps 32 Mbps MAC CSMA/CA CSMA/CA CSMA/CA CSMA/CA TDMA/TDD Connectivity Connection-less Connection-less Connection-less Connection-less Connection -oriented Fixed Network support Ethernet Ethernet Ethernet Ethernet Ethernet, IP, ATM, UMTS, FireWire, PPP

  16. IEEE 802.11 Protocol Stack For centralized contention-free channel access For distributed contention-based channel access

  17. Possible Network Topologies BSS mode ESS mode

  18. 802.11: Channels, association • 802.11b: 2.4GHz-2.485GHz spectrum divided into 11 channels at different frequencies • AP admin chooses frequency for AP • interference possible: channel can be same as that chosen by neighboring AP! • host: must associate with an AP • scans channels, listening for beacon frames containing AP’s name (SSID) and MAC address • selects AP to associate with • may perform authentication • will typically run DHCP to get IP address in AP’s subnet

  19. B A C C C’s signal strength A’s signal strength B A space IEEE 802.11: multiple access • avoid collisions: 2+ nodes transmitting at same time • 802.11: CSMA - sense before transmitting • don’t collide with ongoing transmission by other node • 802.11: no collision detection! • difficult to receive (sense collisions) when transmitting due to weak received signals (fading) • can’t sense all collisions in any case: hidden terminal, fading • goal: avoid collisions: CSMA/C(ollision)A(voidance)

  20. 802.11 sender 1 if sense carrier idle for DIFSthen transmit entire frame (no CD) 2 ifsense (physical or virtual) carrier busy then Choose random backoff interval in [0, cw] counts down while medium is idle Count-down is supended if medium becomes busy transmit when backoff interval expires if no ACK, increase random backoff interval, repeat 2 802.11 receiver - if frame received OK return ACK after SIFS (ACK needed due to hidden terminal problem) DCF is a CSMA/CA protocol 802.11 DCF is suitable for multi-hop ad hoc networking sender receiver DIFS data SIFS ACK IEEE 802.11 MAC Protocol: DCF

  21. Distributed Coordination Function (DCF)

  22. Binary Exponential Backoff • Backoff Counter is randomly selected from [0,CW],where CW is contention window • For each unsuccessful frame transmission, CW doubles (from CWmin to CWmax) • CW  2 (CW+1)-1 • If successful transmission, • CW  CWmin • Reduces the collision probability

  23. A B AP RTS(B) RTS(A) reservation collision RTS(A) CTS(A) CTS(A) DATA (A) defer time ACK(A) ACK(A) Avoiding collisions (more): RTS/CTS idea: allow sender to “reserve” channel rather than random access of data frames: avoid collisions of long data frames • Sender first transmits small request-to-send (RTS) packets to AP using CSMA • RTSs may still collide with each other (but they’re short) • AP broadcasts clear-to-send CTS in response to RTS • RTS heard by all nodes • sender transmits data frame • other stations defer transmissions

  24. RTS/CTS Mechanism (Optional) • RTC/CTS solves HTP • But, non-negligible overhead • If frame size > RTSthreshhold, • RTS-CTS-DATA-ACK • Otherwise, • DATA-ACK

  25. Priorities in 802.11 • CTS and ACK have priority over RTS • After channel becomes idle • If a node wants to send CTS/ACK, it transmits SIFS duration after channel goes idle • If a node wants to send RTS, it waits for DIFS > SIFS

  26. Ranges and Zones • Transmission range • Frame can be successfully received • Carrier-sensing zone (C-Zone) • Signal can be detected, but not decoded. • Interfering range • Receiving node can be interfered by another transmission  collision

  27. Collisions are not completely avoidedin IEEE 802.11 !! • H does not sense any signal during D’s DATA tx • H may transmit • Collision in E’s reception

  28. Energy Conservation: Powercontrol • Power control has two potential benefit • Reduced interference & increased spatial reuse • Energy saving • If C reduces transmit power, it can still communicate with D • Reduces energy consumption at node C • Allows B to receive A’s transmission (spatial reuse)

  29. Point Coordination Function (PCF) • To provide real-time service • Poll-and-response MAC for nearly Isochronous service • In infrastructure BSS only – Point Coordinator (PC) resides in AP • Alternating Contention-Free Period (CFP) and Contention Period (CP)

  30. Contention Free Operation • Two consecutive frames are separated by SIFS • CFP lengths depend on traffic amount • Maximum length announced by AP; used for NAV set

  31. 6 4 2 2 6 6 6 2 0 - 2312 frame control duration address 1 address 2 address 3 address 4 payload CRC seq control 802.11 frame: addressing Address 4: used only in ad hoc mode Address 1: MAC address of wireless host or AP to receive this frame Address 3: MAC address of router interface to which AP is attached Address 2: MAC address of wireless host or AP transmitting this frame

  32. router AP Internet R1 MAC addr AP MAC addr source address dest. address 802.3frame AP MAC addr H1 MAC addr R1 MAC addr address 3 address 2 address 1 802.11 frame 802.11 frame: addressing H1 R1

  33. 6 4 2 2 6 6 6 2 0 - 2312 frame control duration address 1 address 2 address 3 address 4 payload CRC seq control 2 2 4 1 1 1 1 1 1 1 1 Protocol version Type Subtype To AP From AP More frag Retry Power mgt More data WEP Rsvd 802.11 frame: more frame seq # (for reliable ARQ) duration of reserved transmission time (RTS/CTS) frame type (RTS, CTS, ACK, data)

  34. H1 remains in same IP subnet: IP address can remain same switch: which AP is associated with H1? self-learning (Ch. 5): switch will see frame from H1 and “remember” which switch port can be used to reach H1 router 802.11: mobility within same subnet hub or switch BBS 1 AP 1 AP 2 H1 BBS 2

  35. WEB: Wired Equivalent Privacy • RSA RC4 algorithm with 40-bit secret key • Data encryption and integrity

  36. Other MAC Layer Functionalities • Synchronization • Quasi periodic beacon frame are transmitted by AP (may be deferred if medium is busy) • Beacon contains time stamp • Power management • Sleep and awake states • Sleeping stations wake up periodically • Sender has to buffer the data if receiver is on sleep state • Roaming • Active scanning: send a probe on each channel and waiting for response • Passive scanning: listen into medium to find other network

  37. The Other IEEE 802.11 Efforts • 802.11e • Provides QoS support by differentiating traffic streams • Applicable to 802.11 PHY a, b, and g • 802.11h • Supplementary to MAC layer so as to comply with European regulations for 5 GHz WLAN • 802.11i • Security enhancement • 802.11n • Enhancement for higher throughput (> 100 Mbps ) • Decrease overhead within 802.11 protocol • Packet preamble, CW, ACK, IFS parameters • 802.11r • Speed up handoff between APs (Fast BSS-Transition) • Important for VoWLAN • 802.11s • Support mesh networks

  38. HIPERLAN

  39. ETRI BRAN Project HIPERLAN/1 RLAN without a wired infrastructure Suited to both ad hoc and infra-based net 5.15 GHz, 17.1 GHz : ~23.5 Mbps HIPERLAN/2 Short range (~200m) wireless access to IP, ATM, other infra-based net To integrate WLANS into cellular systems 5 GHz: 6 ~ 54 Mbps HIPERACCESS (HIPERLAN/3) HIPERLINK (HIPERLAN/4) HIPERLAN Standards

  40. HIPERLAN/1 – EY-NPMA • Elimination Yield Non-Preemptive MA • Efficiency: 8 ~ 83% for packet size 50B ~ 2KB

  41. HIPERLAN/2 • IEEE 802.a (54Mbps) + QoS + handoff + data integrity • To integrate WLANs into cellular system (3G+) • ATM-compatible WLAN • CO • Fixed size packets • support QoS • MAC: based on TDMA/TDD • 2msec MAC frame consists of • BCH: broadcast control • FCH: frame control • ACH: access feedback control • DL: downlink data • UL: uplink data • DiL: direct link (for Ad Hoc)