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Elements of a wireless network

wireless hosts laptop, PDA, IP phone run applications may be stationary (non-mobile) or mobile wireless does not always mean mobility. network infrastructure. Elements of a wireless network. base station typically connected to wired network

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Elements of a wireless network

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  1. wireless hosts • laptop, PDA, IP phone • run applications • may be stationary (non-mobile) or mobile • wireless does not always mean mobility network infrastructure Elements of a wireless network Csci 183/183W/232: Wireless LAN

  2. base station • typically connected to wired network • relay - responsible for sending packets between wired network and wireless host(s) in its “area” • e.g., cell towers for cellular networks, 802.11 access points network infrastructure Elements of a wireless network Csci 183/183W/232: Wireless LAN

  3. network infrastructure Elements of a wireless network wireless link • typically used to connect mobile(s) to base station • also used as backbone link (WiMAX) • multiple access protocol coordinates link access • various data rates, transmission distance Csci 183/183W/232: Wireless LAN

  4. infrastructure mode • base station connects mobiles into wired network • handoff: mobile changes base station providing connection into wired network network infrastructure Elements of a wireless network Csci 183/183W/232: Wireless LAN

  5. Elements of a wireless network Ad hoc mode • no base stations • nodes can only transmit to other nodes within link coverage • nodes organize themselves into a network: route among themselves Csci 183/183W/232: Wireless LAN

  6. Wireless Link Characteristics Differences from wired link …. • decreased signal strength: radio signal attenuates as it propagates through media (path loss) • interference from other sources: standardized wireless network frequencies (e.g., 2.4 GHz) shared by other devices (e.g., phone); devices (motors) interfere as well • multipath propagation: radio signal reflects off objects, ground, arriving at destination at slightly different times …. make communication across (even a point to point) wireless link much more “difficult” Csci 183/183W/232: Wireless LAN

  7. B A C C C’s signal strength A’s signal strength B A space Wireless network characteristics Multiple wireless senders and receivers create additional problems (beyond multiple access): Hidden terminal problem • B, A hear each other • B, C hear each other • A, C can not hear each other means A, C unaware of their interference at B Signal fading: • B, A hear each other • B, C hear each other • A, C can not hear each other interferring at B Csci 183/183W/232: Wireless LAN

  8. Special challenge for Wireless MAC • Collision detection is difficult • Transmission power is much higher than receiving power • Hidden station problem • A station is not able to detect a potential competitor for the medium because the competitor is too far away • Example: A is transmitting to B. C cannot hear the transmission, thus falsely concludes that it can transmit. • Exposed station problem • A station hears the on-going transmission, and falsely assume it cannot transmit • Example: B tis transmitting to A. C hears the transmission, thus falsely concludes that it cannot transmit o D. Csci 183/183W/232: Wireless LAN

  9. 802.11b 2.4-2.485 GHz unlicensed radio spectrum up to 11 Mbps direct sequence spread spectrum (DSSS) in physical layer all hosts use same chipping code (insensitive to multipath fading) widely deployed, using base stations 802.11a 5.1-5.8 GHz range up to 54 Mbps OFDM 802.11g 2.4-2.485 GHz range up to 54 Mbps DSSS All use CSMA/CA for multiple access All have base-station and ad-hoc network versions IEEE 802.11 Wireless LAN Csci 183/183W/232: Wireless LAN

  10. Spread Spectrum • Idea • spread signal over wider frequency band than required • originally deigned to thwart jamming • Frequency Hopping • transmit over random sequence of frequencies • sender and receiver share… • pseudorandom number generator • seed • 802.11 uses 79 x 1MHz-wide frequency bands Csci 183/183W/232: Wireless LAN

  11. 1 0 Data stream: 1010 1 0 Random sequence: 0100101101011001 1 0 XOR of the two: 1011101110101001 Spread Spectrum (cont) • Direct Sequence • for each bit, send XOR of that bit and n random bits • random sequence known to both sender and receiver • called n-bit chipping code • 802.11 defines an 11-bit chipping code Csci 183/183W/232: Wireless LAN

  12. OFDM • A large number of closely-spaced orthogonalsub-carriers are used to carry data. The data are divided into several parallel data streams or channels, one for each sub-carrier. Each sub-carrier is modulated with a conventional modulation scheme (such as quadrature amplitude modulation or phase shift keying) at a low symbol rate, maintaining total data rates similar to conventional single-carrier modulation schemes in the same bandwidth. Csci 183/183W/232: Wireless LAN

  13. Base Station Approach • Wireless host communicates with a base station • base station = access point (AP) • Basic Service Set (BSS) (a.k.a. “cell”) contains: • wireless hosts • access point (AP): base station • BSS’s combined via a distribution system (DS) • The DS runs at layer 2 of the ISO architecture! Csci 183/183W/232: Wireless LAN

  14. Ad Hoc Network Approach • No access point (i.e., base station) • “peer-to-peer” mode • wireless hosts communicate with each other • to get packet from wireless host A to B may need to route through wireless hosts X,Y,Z • Applications: • “laptop” meeting in conference room, car • interconnection of “personal” devices • battlefield • IETF MANET (Mobile Ad hoc Networks) working group Csci 183/183W/232: Wireless LAN

  15. 802.11: Channels, association • 802.11b/g: 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 Csci 183/183W/232: Wireless LAN

  16. Definitions • Definitions • MAC-level Acknowledgement • Indicate the received frame is correct • The source should wait ACKTimeout amount of time for ACK • Inter-frame Space (IFS) • SIFS: short IFS • PIFS: PCF IFS • DIFS: DCF IFS • DIFS>PIFS>SIFS Csci 183/183W/232: Wireless LAN

  17. 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) Csci 183/183W/232: Wireless LAN

  18. DIFS data SIFS ACK IEEE 802.11 MAC Protocol: CSMA/CA 802.11 sender 1 if sense channel idle for DIFSthen transmit entire frame (no CD) 2 ifsense channel busy then start random backoff timer; timer counts down while channel idle; transmits when timer 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) sender receiver Csci 183/183W/232: Wireless LAN

  19. Avoiding collisions – Option I 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 BS using CSMA • RTSs may still collide with each other (but they’re short) • BS broadcasts clear-to-send CTS in response to RTS • RTS heard by all nodes • sender transmits data frame • other stations defer transmissions Avoid data frame collisions completely using small reservation packets! Csci 183/183W/232: Wireless LAN

  20. RTS(B) RTS(A) reservation collision RTS(A) CTS(A) CTS(A) DATA (A) ACK(A) ACK(A) Collision Avoidance: RTS-CTS exchange B A AP defer time Csci 183/183W/232: Wireless LAN

  21. IEEE 802.11 MAC Layer – Option II • Protocol Architecture • Distributed Coordination Function (DCF) • Point Coordination Function (PCF) Csci 183/183W/232: Wireless LAN

  22. 802.11 MAC Layer Carrier Sensing • Carrier sense at two levels • Physical carrier sense: done by physical layer • Virtual carrier sense at MAC layer using Network Allocation Vector (NAV) set while RTS/CTS/Data/Ack are overheard: partiallysolves problem of Hidden and Exposed terminal • The Duration field reserves the media! • Reduces collision by deferring transmission if any of the carrier sense mechanisms sense the channel busy Csci 183/183W/232: Wireless LAN

  23. DCF Basic Access • Basic Access • When a STA has data to send, it senses medium • The STA may transmit a MAC Protocol Data Unit (MPDA) when medium idle time is greater or equal to DIFS • If medium is busy, wait for a random backoff time Csci 183/183W/232: Wireless LAN

  24. DCF • Backoff Procedure • Backoff procedure is invoked for a STA to transfer a frame but the medium is busy • Set Backoff Timer to be random backoff time • Backoff Timer start decreasing after an idle time of DIFS following the medium busyness • Backoff Timer is suspended when medium is busy, and won’t resume until the medium is idle for DIFS • A frame may be transmitted immediately when Backoff Timer is 0 Csci 183/183W/232: Wireless LAN

  25. DCF • Recovery procedures • Collision may happen during contention • When collision happens, retransmission • STAs maintain a station short retry counter (SSRC) and long retry counter (SLRC) for each MSDU and MMPDU • SSRC increases by one for each failed RTS or MPDU whose length is <= dot11RTSThreshold • SLRC increases by one for each failed MPDU whose length is > dot11RTSThreshold • Both counter is reset upon a successful MPDU • Retry is aborted when SSRC>=dot11ShortRetryLimt =7 or SLRC>=aLongRetryLimit Csci 183/183W/232: Wireless LAN

  26. DCF • Random backoff time=random()xaSlotTime • aSlotTime: the value of the correspondingly named PHY characteristic (20s for DSSS) • Random(): a random integer uniformly distributed over [0, CW] • CW (contention window) • Increases exponentially after each retry fails (so does average backoff time. Why to do this?) • Keep constant after reaching the maximum • Reset after a successful transmission Csci 183/183W/232: Wireless LAN

  27. DCF RTS/CTS Scheme • RTS/CTS Scheme • Four way handshake: RTS-CTS-DATA-ACK • NAV (Network Allocation Vector) • An indicator, maintained at each STA, for the period that transmission will not be initiated • Setting and resetting NAV according to “Duration” in MAC header when receiving a valid frame Csci 183/183W/232: Wireless LAN

  28. DCF -- Fragmentation • Control of the channel • Once the STA has contented for the channel, it shall continue to send fragments until • All fragments of a MSDU or MMPDU have been sent • An ACK is not received • STA is restricted from sending additional fragments by PHY layer • Duration field • RTS/CTS: time till the end of ACK0 • Fragments/ACK: time till the end of the ACK for the next fragment • Last fragment/ACK: length of ACK/0 Csci 183/183W/232: Wireless LAN

  29. DCF • Directed (Unicast) MPDU • STA uses RTS/CTS for directed MPDU only when the length of a MPDU >= dot11RTSThreshold • Always use RTS/CTS: set dot11RTSThreshold=0 • Don’t use RTS/CTS: set dot11RTSThreshold=maximum MPDU length=2304 octets • Broadcast and multicast • Regardless of the length of frame, no RTS/CTS • No ACK • No MAC layer recovery Csci 183/183W/232: Wireless LAN

  30. PCF • Point Coordinator • PCF provides contention free frame transfer • PC resides in AP; It is an option of AP to become PC • Fundamental Access • PC senses the medium • When medium is idle for PIFS, PC transmit a Beacon frame • After beacon, PC shall wait for SIFS, and then transmit • Data frame • CF-Poll (contention free poll) frame • Data + CF-Poll frame Csci 183/183W/232: Wireless LAN

  31. PCF • Polling list • STA indicates its CF-Pollability via Association and Reassociation • PC shall send a CF-Poll to at least one STA when there are entries in the polling list • NAV • Each STA set the NAV to CFPMaxDuration • PC shall transmit a CF-End frame at the end of CFP Csci 183/183W/232: Wireless LAN

  32. PCF • Only available for infrastructured architecture, why? • PCF is on top of DCF • Super frame contains a contention-free period and a contention period • Question: how time-bounded service is provided? Csci 183/183W/232: Wireless LAN

  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 802.11 frame: addressing Address 4: for Intra DS communication 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 Csci 183/183W/232: Wireless LAN

  34. 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 subtype (RTS, CTS, ACK, Beacon, etc.) frame type (management, control, data) Csci 183/183W/232: Wireless LAN

  35. Interpretation of the MAC Addrs Csci 183/183W/232: Wireless LAN

  36. router AP Internet R1 MAC addr H1 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 Csci 183/183W/232: Wireless LAN

  37. Supporting Mobility • Case 1: ad hoc net working • Case 2: access points (AP) • tethered • each mobile node associates with an AP Csci 183/183W/232: Wireless LAN

  38. Mobility (cont) • Scanning (selecting an AP) • node sends Probe frame • all AP’s w/in reach reply with ProbeResponse frame • node selects one AP; sends it AssociateRequest frame • AP replies with AssociationResponse frame • new AP informs old AP via tethered network • When • active: when join or move • passive: AP periodically sends Beacon frame Csci 183/183W/232: Wireless LAN

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