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WCMC-MPR-A 1 Wireless Communications and Mobile Computing MAP-I Manuel P. Ricardo Faculdade de Engenharia da Universidade do Porto WCMC-MPR-A 2 Professors Adriano Moreira Universidade do Minho Manuel P. Ricardo (mricardo@fe.up.pt)

  1. WCMC-MPR-A 36 CSMA/CD – Not Used in Wireless CDMA/Collision Detection  Efficiency < 80% – station monitors de medium (carrier sense)  medium free  transmits the packet  medium busy  waits until medium is free  transmits packet  if, during a round trip time, detects a collision  station aborts transmission and stresses collision ( no ACK packet ) Problems of CDMA/CD in wireless networks Carrier sensing carrier sensing difficult for hidden terminal Collision detection near-end interference makes simultaneous transmission and reception difficult

  2. WCMC-MPR-A 37 To think about? How to minimize collision in a wireless medium?

  3. WCMC-MPR-A 38 CSMA with Collision Avoidance (CSMA/CA) DIFS DATA S1 DIFS S2-bo DATA S2 S3-bo DIFS DIFS S3-bo-r S3-bo-e S3-bo-r DATA S3 DATA DIFS S2-bo - Packet arrival - Transmission of DATA - Time interval DIFS - Backoff time, station 2 - Elapsed backoff time, station 3 - Remaining backoff time, station 3 S3-bo-e S3-bo-r

  4. WCMC-MPR-A 39 CSMA with Collision Avoidance (CSMA/CA) Station with a packet to transmit monitors the channel activity until an idle period equal to a Distributed Inter-Frame Space (DIFS) has been observed If the medium is sensed busy a random backoff interval is selected. The backoff time counter is decremented as long as the channel is sensed idle, stopped when a transmission is detected on the channel, and reactivated when the channel is sensed idle again for more than a DIFS. The station transmits when the backoff time reaches 0 To avoid channel capture, a station must wait a random backoff time between two consecutive packet transmissions, even if the medium is sensed idle in the DIFS time

  5. WCMC-MPR-A 40 CSMA/CA – ACK Required DIFS DATA S1 SIFS SIFS ACK ACK AP DIFS S2-Backoff DATA S2 DATA DIFS - Packet arrival - Transmission of DATA - Time interval DIFS

  6. WCMC-MPR-A 41 CSMA/CA – ACK Required CSMA/CA does not rely on the capability of the stations to detect a collision by hearing their own transmission A positive acknowledgement is transmitted by the destination station to signal the successful packet transmission In order to allow an immediate response, the acknowledgement is transmitted following the received packet, after a Short Inter-Frame Space (SIFS) If the transmitting station does not receive the acknowledge within a specified ACK timeout, or it detects the transmission of a different packet on the channel, it reschedules the packet transmission according to the previous backoff rules. Efficiency of CSMA/CA depends strongly of the number of competing stations. An efficiency of 60% is commonly found

  7. WCMC-MPR-A 42 To Think About How to enable hidden terminals to sense the carrier? D A B C Hidden node  C is hidden to A

  8. WCMC-MPR-A 43 RTS-CTS Mechanism DIFS SIFS RTS DATA S1 SIFS SIFS CTS ACK AP S2-bo DIFS DATA S2 DATA DIFS - Packet arrival - Transmission of DATA - Time interval DIFS

  9. WCMC-MPR-A 44 RTS-CTS Mechanism For some scenarios where long packets are used or the probability of hidden terminals is not irrelevant, the efficiency of CSMA/CA can be further improved with a Request To Send (RTS) - Clear to Send (CTS) mechanism The basic concept is that a sender station sends a short RTS message to the receiver station. When the receiver gets a RTS from the sender, it polls the sender by sending a short CTS message. The sender then sends its packet to the receiver. After correctly receiving the packet, the receiver sends a positive acknowledgement (ACK) to the sender This mechanism is particularly useful to transmit large packets. The listening of the RTS or the CTS messages enable the stations in range respectively of the sender or receiver that a big packet is about to be transmitted. Usually both the RTS and the CTS contain information about the number of slots required to transmit the 4 packets. Using this information the other stations refrain themselves to transmit packets, thus avoiding collisions and increasing the system efficiency. SIFS are used before the transmission of CTS, Data, and ACK In optimum conditions the RTS-CTS mechanism may add an efficiency gain of about 15%

  10. WCMC-MPR-A 45 Guaranteed Access Control Polling » AP manages stations access to the medium » Channel tested first using a control handshake

  11. WCMC-MPR-A 46 Fundamental Networking

  12. WCMC-MPR-A 47 Packet Switching Technologies: Ethernet, IP Path defined by packet destination address

  13. WCMC-MPR-A 48 To Think About Suppose terminal a moves from port 2 to port 1 » What needs to be done so that terminal a can continue receiving packets?

  14. WCMC-MPR-A 49 L2 Networking – Ethernet Format 7x 10101010 10101011 Protocolo=IP Ethernet

  15. WCMC-MPR-A 50 L2 Networking - Bridges Bridge builds forwarding tables automatically Address learning » Source Address of received frame is associated to a bridge input port  station reachable through that port Frame forwarding » When a frame is received, its Destination Address is analysed – If address is associated to a port  frame forwarded to that port – If not  frame transmitted through all the ports but the input port

  16. WCMC-MPR-A 51 L2 Networking - Single Tree Required • Ethernet frame – No hop-count – Could loop forever – Same for broadcast packet • Layer 2 network L2 Networking - Single Tree Required – Required to have tree topology – Single path between every pair of stations • Spanning Tree Protocol (STP) – Running in bridges – Helps building the spanning tree – Blocks ports

  17. WCMC-MPR-A 52 L3 Networking – Packet Formats 0 4 8 16 24 31 0 4 8 16 19 31 Version Traffic Class Flow Label Version HLen TOS Length Ident Flags Offset Payload Lengtht Next Header Hop Limit TTL Protocol Checksum SourceAddr (4 words) SourceAddr DestinationAddr DestinationAddr (4 words) Pad Options (variable) (variable) Options (variable number) Data Data IPv4 IPv6

  18. WCMC-MPR-A 53 L3 Networking – Multiple Trees … Every router » finds the shortest path to the other routers and their attached networks » Calculates its Shortest Path Tree (SPT) Routing protocol » Run in routers » Helps routers build their SPT » RIP, OSPF, BGP B’s routing view Destination Cost NextHop B A 1 A C A C 1 C D D 2 C E 2 A E F 2 A F G G 3 A

  19. WCMC-MPR-A 54 Traditional TCP/IP Communications Stack IETF IP address based switching APP APP TCP TCP IP IP IP IP T1 T1 | T2 T3 | T4 T4 | T5 T5 T2 | T3 host router bridge router bridge host IEEE MAC address based switching

  20. WCMC-MPR-A 55 Tunnel IP-in-IP APP APP TCP IP TCP IP IP IP IP IP T1 T1 | T2 T3 | T4 T4 | T5 T5 T2 | T3 H1 R2 bridge R1 bridge Server outer IP header inner IP header data ver. IHL TOS length IP identification flags fragment offset TTL IP-in-IP IP checksum SA= red IP address of H1 DA= red IP address of R2 ver. IHL TOS length IP identification flags fragment offset TTL lay. 4 prot. IP checksum SA=H1 DA= Server TCP/UDP/ ... payload

  21. WCMC-MPR-A 56 Tunnel PPP over IP (E.g PPTP) APP TCP IP IP PPP PPP APP GRE GRE TCP IP IP IP IP T1 T1 | T2 T3 | T4 T4 | T5 T5 T2 | T3 H1 R2 bridge R1 bridge Server » GRE – virtual point-to-point link – routers at remote points – over an IP network » PPP adequate for – Authentication – Transporting IP packets

  22. WCMC-MPR-A 57 IEEE 802.11

  23. WCMC-MPR-A 58 Infrastructure Networks vs Ad-Hoc Networks Infrastructure AP: Access Point AP AP wired network AP Ad-hoc

  24. WCMC-MPR-A 59 IEEE 802.11 – Infrastructure Network Station 802.11 LAN 802.x LAN » Terminal with radio access Basic Service Set (BSS) STA 1 » Set of stations in the same band BSS 1 Portal Access Point (AP) Access Point » Interconnects LAN to wired network Distribution System » Provides access to stations Access Stations communicate with AP ESS Point Portal  bridge to other networks BSS 2 Distribution System » Interconnection network » Logical network – EES, Extended Service Set STA 2 STA 3 802.11 LAN – Based on BSSs

  25. WCMC-MPR-A 60 IEEE 802.11 – Ad-Hoc Network Direct communication between 802.11 LAN stations STA 1 Independent Basic Service Set, IBSS STA 3 IBSS 1 » Set of stations working the the same carrier (radio channel) STA 2 IBSS 2 STA 5 STA 4 802.11 LAN

  26. WCMC-MPR-A 61 IEEE 802.11 – Protocol Stack fixed terminal mobile 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

  27. WCMC-MPR-A 62 802.11 – Protocol Stack

  28. WCMC-MPR-A 63 802.11 – Layers and Functionalities Data plane » MAC  medium access, fragmentation, encryption » PLCP - Physical Layer Convergence Protocol  carrier detection » PMD - Physical Medium Dependent  modulation, codification Management plane » PHY Management  channel selection, MIB » MAC Management  synchronisation, mobility, power, MIB » Station Management  coordenation management functions Station Management LLC DLC MAC MAC Management PLCP PHY PHY Management PMD

  29. WCMC-MPR-A 64 MAC Layer – Access Methods DCF – Distributed Coordination Function PCF - Point Coordination Function MAC-DCF CSMA/CA – Carrier sense, collision avoidance using back-off mechanism – ACK packet required for confirmations (except for broadcast packets) – mandadory MAC-DCF with RTS+CTS – Used to avoid hidden terminal problem – Optional MAC- PCF – Access Point asks stations to transmit – Optional

  30. WCMC-MPR-A 65 MAC Layer – Guard Time Intervals DIFS DIFS PIFS SIFS medium busy contention next frame t direct access if medium is free DIFS » DIFS (DCF IFS) – Lowest priority, used for asynchronous data » PIFS (PCF IFS) – Medium priority, used for real time traffic /QoS » SIFS (Short Inter Frame Spacing) – Maximum priority  used for signalling: ACK, CTS, answers to polling

  31. WCMC-MPR-A 66 MAC-DCF CSMA/CA Sending a frame in unicast » Station waits DIFS before sending the packet » If packet is correctly received (no errors in CRC)  Receiver confirms reception immediatly, using ACK, after waiting SIFS » In case of errors, frame is re-transmitted » In case of retransmission  Maximum value for the contention window duplicates  Contetion window has minimum and maximum values (eg.: 7 and 255) DIFS data sender SIFS ACK receiver DIFS data other stations t waiting time contention

  32. WCMC-MPR-A 67 MAC- PCF t 0 t 1 SuperFrame medium busy PIFS SIFS SIFS D 1 D 2 point SIFS SIFS coordinator U 1 U 2 wireless stations NAV stations‘ NAV

  33. WCMC-MPR-A 68 MAC-PCF II t 2 t 3 t 4 PIFS SIFS D 3 D 4 CF end point SIFS coordinator U 4 wireless stations NAV stations‘ contention free period t NAV contention period

  34. WCMC-MPR-A 69 MAC – Frame Format Frame types » Data, control, management Sequence number Addresses » destination, source, BSS identifier, ... Others » Error control, frame control, data bytes 2 2 6 6 6 2 6 0-2312 4 Frame Duration/ Address Address Address Sequence Address Data CRC Control ID 1 2 3 Control 4 bits 1 1 1 1 1 1 2 2 4 1 1 Protocol Subtype To From More Retry Power More Type WEP Order version DS DS Frag Mgmt Data

  35. WCMC-MPR-A 70 To Think About STA 1 needs to send a frame to STA 2 . In the Infrastructure mode, the frame is sent via the AP. What MAC addresses are required in the frame sent by STA 1 to the AP? Access Point BSS 2 STA 1 STA 2 802.11 LAN

  36. WCMC-MPR-A 71 Addresses in MAC 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 1 1 RA TA DA SA network, within DS DS: Distribution System AP: Access Point DA: Destination Address SA: Source Address BSSID: Basic Service Set Identifier RA: Receiver Address TA: Transmitter Address

  37. WCMC-MPR-A 72 Special Frames- ACK, RTS, CTS 2 2 6 4 bytes Frame Duration Receiver Acknowledgement ACK CRC Control Address bytes 2 2 6 6 4 Frame Duration Receiver Transmitter Request To Send CRC RTS Control Address Address bytes 2 2 6 4 Frame Duration Receiver CTS CRC Clear To Send Control Address (Fig. 7.17 do livro está errada)

  38. WCMC-MPR-A 73 Station Management LLC DLC MAC MAC Management MAC Management PLCP PHY PHY Management PMD Synchronization – Station discovers a LAN; station associates to an AP – stations synchronize clocks; Beacon is generated by AP Power management – Save terminal’s power  terminal enters sleep mode  Periodically  No frame loss; frames are stored Roaming – Station looks for new access points – Station decides about best access point – Station (re-)associates to new AP MIB - Management Information Base

  39. WCMC-MPR-A 74 Synchronization by Beacon – Infrastructure Network Stations must be synchronised. E.g. – To preview PCF cycles – To change state: sleep  wake Infrastructure networks – Access Point sends (almost) periodically a Beacon with timestamp e BSSid sometimes medium is busy – Timestamp sent is the correct – Other stations adjust their clocks beacon interval B B B B access point busy busy busy busy medium t B value of the timestamp beacon frame

  40. WCMC-MPR-A 75 Power Management Objective » If transceiver not in use  sleep mode Station in 2 states: sleep , wake Infrastructure network » Stations wake periodically and simultaneously » They listen beacon to know if there are packets to receive » If a station has packets to receive  remains awake until it receives them – If not, go sleep; after sending its packets!

  41. WCMC-MPR-A 76 Power Management – Infrastructure Network Infrastructure network  traffic information sent in the beacon » Traffic Indication Map – TIM: list of unicast receivers » Delivery Traffic Indication Map - DTIM: list broadcast/multicast receivers TIM interval DTIM interval D B T T D D B access point busy busy busy busy medium P D station t T D DTIM TIM awake D data transmission B P PS poll broadcast/multicast to/from the station

  42. WCMC-MPR-A 77 802.11 – Physical Layer 3 versões: 2 rádio, 1 IR – Bitrates: 1, 2 Mbit/s FHSS (Frequency Hopping Spread Spectrum) – Spreading, despreading – 79 sequências de salto pseudo aleatórias. Para 1 Mbit/s, modulação de 2 níveis GFSK DSSS (Direct Sequence Spread Spectrum) – 1 Mbit/s  Modulation DBPSK (Differential Binary Phase Shift Keying) – 2 Mbit/s  Modulation DQPSK (Differential Quadrature PSK) – Preamble and header of frame transmitted at 1 Mbit/s (DBPSK)  Remainning transmitted at 1 (DBPSK) ou 2 Mbit/s (DQPSK) – Maximum radiated power  1 W (EUA), 100 mW (UE), min. 1mW Infravermelho – 850-950 nm, distância de 10 m – Detecção de portadora, detecção de energia, sincronização All versions provide Clear Channel Assessment (CCA) – Used by MAC to detect if medium is free

  43. WCMC-MPR-A 78 Frame FHSS PHY » Sincronization  010101... » SFD (Start Frame Delimiter  0000110010111101 » PLW (PLCP_PDU Length Word) – Payload length in bytes, including 2 CRC bytes. PLW < 4096 » PSF (PLCP Signaling Field) – Transmission bitrate of payload (1, 2 Mbit/s)  PLCP (preâmbulo and header) sent at 1 Mbit/s  Payload sent at 1 ou 2 Mbit/s » HEC (Header Error Check) – CRC with x 16 +x 12 +x 5 +1 » Data MAC  scrambled with z 7 +z 4 +1 bits 80 16 12 4 16 variable synchronization SFD PLW PSF HEC payload PLCP preamble PLCP header

  44. WCMC-MPR-A 79 Frame DSSS PHY – Barker sequence of 11 chips  +1,-1,+1,+1,-1,+1,+1,+1,-1,-1,-1 – Sincronization  Sincronization  Gain control, Clear Channel Assessement, compensate frequency deviation – SFD (Start Frame Delimiter  1111001110100000 – Signal  Payload bitrate (0A: 1 Mbit/s DBPSK; 14: 2 Mbit/s DQPSK) – Service  utilização futura, 00 = conforme 802.11 – Length  Payload length in us – HEC (Header Error Check)  Protection of sinal, service and length, using x 16 +x 12 +x 5 +1 – Data (payload) MAC  scrambled with z 7 +z 4 +1 bits 128 16 8 8 16 16 variable synchronization SFD signal service length HEC payload PLCP preamble PLCP header

  45. WCMC-MPR-A 80 IEEE 802.11b Bitrate (Mbit/s) – 1, 2, 5.5, 11 (depends on SNR) – Useful bitrate  6 Transmission range – 300m outdoor, 30m indoor Frequencies  open, ISM 2.4 GHz band Only physical layer is redefined » MAC and MAC management are the same

  46. WCMC-MPR-A 81 IEEE 802.11b – Trama PHY Payload bitrate Long PLCP PPDU format bits 128 16 8 8 16 16 variable synchronization SFD signal service length HEC payload PLCP preamble PLCP header 192 µs at 1 Mbit/s DBPSK 1, 2, 5.5 or11 Mbit/s Short PLCP PPDU format (optional) bits 56 16 8 8 16 16 variable 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

  47. WCMC-MPR-A 82 Channel Selection channel i = 2412MHz + (i-1)*5MHz There are 14 channels of 5MHz In 801.11b only 3 non-overlap channels can be used Europe (ETSI) channel 1 channel 7 channel 13 [MHz] 2400 2412 2442 2472 2483.5 22 MHz US (FCC)/Canada (IC) channel 1 channel 6 channel 11 2400 2412 2437 2462 2483.5 [MHz] 22 MHz

  48. WCMC-MPR-A 83 IEEE 802.11a Bitrate (Mbit/s) » 6, 9, 12, 18, 24, 36, 48, 54 (depends on SNR) » Mandatory  6, 12, 24 Useful bit rate (frames 1500 bytes, Mbit/s) » 5.3 (6), 18 (24), 24 (36), 32 (54) Transmission range » 100m outdoor, 10 m indoor – 54 Mbit/s até 5 m, 48 até 12 m, 36 até 25 m, 24 até 30m, 18 até 40 m, 12 até 60 m Frequencies » Free, band ISM » 5.15-5.35, 5.47-5.725 GHz (Europa) Only the physical layer changes

  49. WCMC-MPR-A 84 Operating channels for 802.11a / US U-NII channel 36 40 44 48 52 56 60 64 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

  50. WCMC-MPR-A 85 OFDM in IEEE 802.11a OFDM with 52 used subcarriers (64 in total) 48 data + 4 pilot (plus 12 virtual subcarriers) 312.5 kHz pilot 312.5 kHz spacing -26 -21 -7 -1 1 7 21 26 subcarrier number channel center frequency

  51. WCMC-MPR-A 86 802.11a – Rate Dependent Parameters 250 kSymbol/s % of useful information

  52. WCMC-MPR-A 87 IEEE 802.16

  53. WCMC-MPR-A 88 IEEE 802.16 - Commonly used terms BS – Base Station SS – Subscriber Station, (i.e., CPE) DL – Downlink, i.e. from BS to SS UL – Uplink, i.e. from SS to BS FDD – Frequency Division Duplex TDD – Time Division Duplex TDMA – Time Division Multiple Access TDM – Time Division Multiplexing OFDM – Orthogonal Frequency Division Multiplexing OFDMA - Orthogonal Frequency Division Multiple Access QoS – Quality of Service

  54. WCMC-MPR-A 89 IEEE 802.16 - Introduction Source: WiMAX, making ubiquitous high-speed data services a reality, White Paper, Alcatel.

  55. WCMC-MPR-A 90 Reference Model

  56. WCMC-MPR-A 91 Adaptive PHY Source: Understanding WiMAX and 3G for Portable/Mobile Broadband Wireless, Technical White Paper, Intel.

  57. WCMC-MPR-A 92 Adaptive Burst Profiles Burst profile - Modulation and FEC On DL » multiple SSs can associate the same DL burst On UL » SS transmits in an given time slot with a specific burst Dynamically assigned according to link conditions » Burst by burst » Trade-off capacity vs. robustness in real time

  58. WCMC-MPR-A 93 OFDM PHY TDD Frame Structure Time Frame n-1 Frame n Frame n+1 Adaptive DL Subframe UL subframe DL TDM UL TDMA DL DL DL ... ... pre. FCH burst 1 burst 2 burst n UL burst 1 UL burst m pre. pre. DL UL DCD UCD MAP MAP opt. opt. Broadcast Conrol msgs

  59. WCMC-MPR-A 94 OFDM PHY FDD Frame Structure Time Frame n-1 Frame n Frame n+1 DL Subframe DL TDM DL TDMA DL DL DL DL DL ... ... pre. FCH pre. pre. burst 1 burst 2 burst k burst k+1 burst n Broadcast Control Msgs DL UL DCD UCD MAP MAP opt. opt. UL subframe UL TDMA UL MAP for next MAC frame UL bursts ... UL burst 1 UL burst m pre. pre.

  60. WCMC-MPR-A 95 FDD MAPs Time Relevance DL UL DL UL MAP MAP MAP MAP DOWNLINK UPLINK frame Broadcast Half Duplex T erminal #1 Full Duplex Capable User Half Duplex T erminal #2

  61. WCMC-MPR-A 96 OFDMA

  62. WCMC-MPR-A 97 OFDMA

  63. WCMC-MPR-A 98 OFDMA, TDD

  64. WCMC-MPR-A 99 IEEE 802.16 MAC Addressing and Identifiers SS has 48-bit IEEE MAC address BS has 48-bit base station ID » Not a MAC address; 24-bit operator indicator 16-bit connection ID (CID) 32-bit service flow ID (SFID) 16-bit security association ID (SAID)

  65. WCMC-MPR-A 100 Convergence Sub-Layer (CS) ATM Convergence Sub- Layer » Support for VP/VC connections » Support for end-to-end signaling of dynamically created connections » ATM header suppression » Full QoS support Packet Convergence Sub- Layer » Initial support for Ethernet, VLAN, IPv4, and IPv6 » Payload header suppression » Full QoS support

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