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Local Area Networks

Local Area Networks

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Local Area Networks

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  1. Local Area Networks Chapter 10 – Wireless LANs

  2. Wireless Communication • The proliferation of laptop computers and other mobile devices (PDAs and cell phones) created an obvious application level demand for wireless local area networking. • Companies jumped in, quickly developing incompatible wireless products in the 1990’s. • Industry decided to entrust standardization to the IEEE committee that dealt with wired LANS – namely, the IEEE 802 committee!! • Wireless communications compelling • Easy, low-cost deployment • Mobility & roaming: Access information anywhere • Supports personal devices • PDAs, laptops, data-cell-phones • Supports communicating devices • Cameras, location devices, wireless identification • Signal strength varies in space & time • Signal can be captured by snoopers • Spectrum is limited & usually regulated

  3. Wireless Links • Many end systems use wireless links: • Portable PCs within a wireless LAN • PDAs that connect to the Internet through wireless telephony infrastructure • Cameras, automobiles, etc. • Two standards for wireless networking: • IEEE 802.11b standard for wireless LANs (aka Wi-Fi) • Bluetooth standard that allows devices to communicate without being in line of sight • Wireless devices classified wrt power, range, and data rate • IEEE 802.11  high power, medium range, and high rate “access” technology • Bluetooth  low power, short range, low rate, “cable replacement” technology

  4. IEEE 802.11 Wireless LAN • Wireless LANs: mobile networking • IEEE 802.11 standard: • MAC protocol • Unlicensed frequency spectrum: 2.4Ghz (802.11b) or 5-6 Ghz (802.11a) • Provides wireless Ethernet access at 11 Mbps or54 Mbps (802.11a) • Basic Service Set (BSS) (a.k.a. “cell”) contains: • wireless hosts • access point (AP): base station • BSS’s combined to form distribution system (DS)

  5. IEEE 802.11 Wireless LAN

  6. IEEE 802.11 Wireless LAN

  7. The 802.11 Protocol Stack

  8. The 802.11 Protocol Stack

  9. Wireless Standards Frequency, Hopping Spread Spectrum (FHSS) Direct Sequence Spread Spectrum (FHSS) HR: High Rate Orthogonal Frequency Division Multiplexing (OFDM, VOFDM, COFDM)

  10. Wireless Physical Layer • Physical layer conforms to OSI (five options) • 1997: 802.11 infrared, FHSS, DHSS • 1999: 802.11a OFDM and 802.11b HR-DSSS • 2001: 802.11g OFDM • 802.11 Infrared • Two capacities 1 Mbps or 2 Mbps. • Range is 10 to 20 meters and cannot penetrate walls. • Does not work outdoors. • 802.11 FHSS (Frequency Hopping Spread Spectrum) • The main issue is multipath fading. • 79 non-overlapping channels, each 1 Mhz wide at low end of 2.4 GHz ISM band. • Same pseudo-random number generator used by all stations. • Dwell time: min. time on channel before hopping (400msec).

  11. Wireless Physical Layer Frequency Hopping Spread Spectrum

  12. Wireless Physical Layer • 802.11 DSSS (Direct Sequence Spread Spectrum) • Spreads signal over entire spectrum using pseudo-random sequence (similar to CDMA see Tanenbaum sec. 2.6.2). • Each bit transmitted using an 11 chips Barker sequence, PSK at 1Mbaud. • 1 or 2 Mbps. • 802.11a OFDM (Orthogonal Frequency Divisional Multiplexing) • CompatiblewithEuropean HiperLan2. • 54Mbps in wider 5.5 GHz band  transmission range is limited. • Uses 52 FDM channels (48 for data; 4 for synchronization). • Encoding is complex ( PSM (Power saving mode) up to 18 Mbps and QAM above this capacity). • E.g., at 54Mbps 216 data bits encoded into into 288-bit symbols. • More difficulty penetrating walls.

  13. Wireless Physical Layer Direct Sequence Spread Spectrum

  14. Wireless Physical Layer • 802.11bHR-DSSS (High Rate Direct Sequence Spread Spectrum) • 11a and 11bshows a split in the standards committee. • 11b approved and hit the market before 11a. • Up to 11 Mbps in 2.4 GHz band using 11 million chips/sec. • Note in this bandwidth all these protocols have to deal with interference from microwave ovens, cordless phones and garage door openers. • Range is 7 times greater than 11a. • 11b and 11a are incompatible!! • 802.11g OFDM(Orthogonal Frequency Division Multiplexing) • An attempt to combine the best of both 802.11a and 802.11b. • Supports bandwidths up to 54 Mbps. • Uses 2.4 GHz frequency for greater range. • Is backward compatible with 802.11b.

  15. B1 A1 Gateway to the Internet Portal Server Distribution System Portal AP1 A2 B2 AP2 BSS A BSS B Infrastructure Network • Permanent Access Points provide access to Internet

  16. IEEE 802.11 Wireless LAN

  17. 802.11 Definitions • Basic Service Set (BSS) • Group of stations that coordinate their access using a given instance of MAC • Located in a Basic Service Area (BSA) • Stations in BSS can communicate with each other • Distinct collocated BSS’s can coexist • Extended Service Set (ESS) • Multiple BSSs interconnected by Distribution System (DS) • Each BSS is like a cell and stations in BSS communicate with an Access Point (AP) • Portals attached to DS provide access to Internet

  18. Ad Hoc Networks • Ad hoc network: IEEE 802.11 stations can dynamically form network without AP • Formed “on the fly” when mobile devices are in proximity • Applications: • “Laptop” meeting in conference room, car • Interconnection of “personal” devices • Battlefield • IETF MANET (Mobile Ad hoc Networks) working group

  19. Ad Hoc Networks

  20. C A B (b) Data Frame B C Data Frame A C transmits data frame & collides with A at B Hidden Terminal Problem (a) Data Frame A transmits data frame C senses medium, station A is hidden from C • New MAC: CSMA with Collision Avoidance

  21. 802.11 CSMA: sender if sense channel idle for Distributed Inter Frame Space (DIFS) sec. then transmit entire frame (no collision detection) if sense channel busy then binary backoff 802.11 CSMA receiver: if received OK return ACK after Short Inter Frame Spacing (SIFS) (DIFS = SIFS + 2 × slot time) Time slot= 20 micro s, SIFS=10 micro s, DIFS=50 micro s. IEEE 802.11 MAC Protocol: CSMA/CA (collision avoidance)

  22. IEEE 802.11 MAC Protocol 802.11 CSMA Protocol: others • Other stations wait for a random backoff period after DIFS after current transmission • Avoids collisions • Collisions  uses exponentially increasing backoff period • Collisions detection is difficult: • Hidden terminal problem • Fading • NAV: Network Allocation Vector: • 802.11 frame has transmission duration field • Others (hearing stations) defer access (to save power) for NAV time units

  23. IEEE 802.11 MAC Protocol

  24. Hidden Terminal effect • Hidden terminals: A, C cannot hear each other • Obstacles, signal attenuation • Collisions at B • Goal: avoid collisions at B • CSMA/CA: CSMA with Collision Avoidance Fading can also result in collisions

  25. Collision Avoidance: RTS-CTS exchange • CSMA/CA: explicit channel reservation • sender: send short RTS: request to send • receiver: reply with short CTS: clear to send • CTS reserves channel for sender, notifying (possibly hidden) stations • Benefit: RTC-CTS avoids hidden station collisions

  26. Collision Avoidance: RTS-CTS exchange • CA with RTS-CTS: • Collisions less likely, of shorter duration • End result similar to collision detection • IEEE 802.11 allows: • CSMA • CSMA/CA: reservations • polling from AP

  27. (a) B RTS C A requests to send (b) CTS B CTS A C B announces A ok to send (c) B Data Frame A sends C remains quiet CSMA with Collision Avoidance

  28. IEEE 802.11 Wireless LAN • Stimulated by availability of unlicensed spectrum • U.S. Industrial, Scientific, Medical (ISM) bands • 902-928 MHz, 2.400-2.4835 GHz, 5.725-5.850 GHz • Targeted wireless LANs @ 20 Mbps • MAC for high speed wireless LAN • Ad Hoc & Infrastructure networks • Variety of physical layers

  29. B1 A1 Gateway to the Internet Portal Distribution System Server Portal AP1 A2 B2 AP2 BSS A BSS B Infrastructure Network

  30. Distribution Services • Stations within BSS can communicate directly with each other • DS provides distribution services: • Transfer MAC SDUs between APs in ESS • Transfer MSDUs between portals & BSSs in ESS • Transfer MSDUs between stations in same BSS • Multicast, broadcast, or stations’s preference • ESS looks like single BSS to LLC layer

  31. Infrastructure Services • Select AP and establish association with AP • Then can send/receive frames via AP & DS • Reassociation service to move from one AP to another AP • Dissociation service to terminate association • Authentication service to establish identity of other stations • Privacy service to keep contents secret

  32. IEEE 802.11 MAC • MAC sublayer responsibilities • Channel access • PDU addressing, formatting, error checking • Fragmentation & reassembly of MAC SDUs • MAC security service options • Authentication & privacy • MAC management services • Roaming within ESS • Power management

  33. MSDUs MSDUs Contention-free service Contention service Point coordination function MAC Distribution coordination function (DCF) (CSMA-CA) Physical MAC Services • Contention Service: Best effort • Contention-Free Service: time-bounded transfer • MAC can alternate between Contention Periods (CPs) & Contention-Free Periods (CFPs). MAC Service Data Unit (MSDU)

  34. Contention window DIFS PIFS DIFS SIFS Busy medium Next frame Time Wait for reattempt time Defer access Distributed Coordination Function (DCF) • DCF provides basic access service • Asynchronous best-effort data transfer • All stations contend for access to medium • CSMA-CA • Ready stations wait for completion of transmission • All stations must wait Interframe Space (IFS)

  35. Contention window DIFS PIFS DIFS SIFS Busy medium Next frame Time Wait for reattempt time Defer access Priorities through Interframe Spacing • High-Priority frames wait Short IFS (SIFS) • Typically to complete exchange in progress • ACKs, CTS, data frames of segmented MSDU, etc. • PCF IFS (PIFS) to initiate Contention-Free Periods • DCF IFS (DIFS) to transmit data & MPDUs

  36. Contention & Backoff Behavior • If channel is still idle after DIFS period, ready station can transmit an initial MPDU • If channel becomes busy before DIFS, then station must schedule backoff time for reattempt • Backoff period is integer # of idle contention time slots • Waiting station monitors medium & decrements backoff timer each time an idle contention slot transpires • Station can contend when backoff timer expires • A station that completes a frame transmission is not allowed to transmit immediately • Must first perform a backoff procedure

  37. (a) B RTS C A requests to send (b) CTS B CTS A C B announces A ok to send (c) B Data Frame A sends C remains quiet (d) B ACK ACK B sends ACK

  38. Carrier Sensing in 802.11 • Physical Carrier Sensing • Analyze all detected frames • Monitor relative signal strength from other sources • Virtual Carrier Sensing at MAC sublayer • Source stations informs other stations of transmission time (in msec) for an MPDU • Carried in Duration field of RTS & CTS • Stations adjust Network Allocation Vector to indicate when channel will become idle • Channel busy if either sensing is busy

  39. DIFS Data Source SIFS ACK Destination DIFS NAV Other Wait for Reattempt Time Defer Access Transmission of MPDU without RTS/CTS

  40. DIFS RTS Data Source SIFS SIFS SIFS CTS Ack Destination DIFS NAV (RTS) NAV (CTS) Other NAV (Data) Defer access Transmission of MPDU with RTS/CTS

  41. Collisions, Losses & Errors • Collision Avoidance • When station senses channel busy, it waits until channel becomes idle for DIFS period & then begins random backoff time (in units of idle slots) • Station transmits frame when backoff timer expires • If collision occurs, recompute backoff over interval that is twice as long • Receiving stations of error-free frames send ACK • Sending station interprets non-arrival of ACK as loss • Executes backoff and then retransmits • Receiving stations use sequence numbers to identify duplicate frames

  42. Point Coordination Function • PCF provides connection-oriented, contention-free service through polling • Point coordinator (PC) in AP performs PCF • Polling table up to implementor • CFP repetition interval • Determines frequency with which CFP occurs • Initiated by beacon frame transmitted by PC in AP • Contains CFP and CP • During CFP stations may only transmit to respond to a poll from PC or to send ACK

  43. TBTT Contention-free repetition interval SIFS SIFS SIFS SIFS SIFS Contention period CF End B D2+Ack+Poll D1 + Poll U 2 + ACK U 1 + ACK PIFS Reset NAV NAV CF_Max_duration D1, D2 = frame sent by point coordinator U1, U2 = frame sent by polled station TBTT = target beacon transmission time B = beacon frame PCF Frame Transfer

  44. DCF, PCF, and Frame Format

  45. Distributed Coordination Function (DCF) • DCF is the access method used to support asynchronous data transfer on a best effort basis • All stations must support the DCF (DCF operates solely in the ad hoc network) • Operates solely or coexists with the PCF in an infrastructure network • DCF sits directly on top of the physical layer and supports contention services: • Each station with an MSDU queued for transmission must contend for access to the channel • Once the MSDU is transmitted, must recontend for access to the channel for all subsequent frames • Contention services promote fair access to the channel for all stations. • The DCF is carrier sense multiple access with collision avoidance (CSMA/CA). • CSMA/CD is not used because a station is unable to listen to the channel for collisions while transmitting • In IEEE 802.11, carrier sensing is performed at both the air interface, referred to as physical carrier sensing, and at the MAC sublayer, referred to as virtual carrier sensing • Physical carrier sensing detects the presence of other IEEE 802.11 WLAN users by analyzing all detected packets, and also detects activity in the channel via relative signal strength from other sources • Virtual carrier sensing • Stations include MPDU duration in the header of request to send (RTS), clear to send (CTS), and data frames • An MPDU is a complete data unit that is passed from the MAC sublayer • to the physical layer • The MPDU contains header information, information, payload, and a 32-bit CRC • The duration field indicates the time (in microseconds) after the end of the present frame the channel will be utilized tocomplete the successful transmission of the data or management frame. • Stations in the BSS use the duration field to adjust their network allocation vector (NAV) • NAV indicates the amount of time that must elapse until the current transmission session is complete

  46. Distributed Coordination Function (DCF) • DCF operates under the Contention Period (CP) • Three types of frames: management, control, and data • Management F: station association dis-association with AP • Control F: handshaking in CP, ACK data in CP, and end CFP • Basic DCF Access Method (no RTS-CTS): • When ST finds chaneel idle, it waits for DIFS and checks it again • If it is still idle, it transmits MPDU with medium busy time (including SIFS and ACK times) • Receiving st computes Checksum, if correct sends an ACK to source • All other STs in BSS hearing above messages adjust their NAV timers

  47. Distributed Coordination Function (DCF) • RTS-CTS Data Mode • Priority Accsess: SIFS, PIFS (SIFS+1), and DIFS (SIFS+2) • In BSS, STs hearing RTS, CTS, F0, and ACK adjust their NAV • Sts: Basic mode, RTS/CTS mode if MPDU exceeds L, or always use RTS/CTS mode • Fairness: BEB starts with (1,8) and end at some maximum

  48. Distributed Coordination Function (DCF) • MPDU (2300 bytes): collision lead to bandwidth loss • RTS is 20 bytes and CTS is 14 bytes • Fragmentation increases transmission reliability • Fragment MPDU, transmit Frag, receive ACK to completion • If no ACK, re-contend for medium and stat al last Frag. • In RTS-CTS mode, RTS-CTS used only in first frag.

  49. Point Coordination Function (PCF on top of DCF) • PCF (optional) operates under the Contention-Free Period (CFP) • Medium access contr. by Point Coordinator PC (AP/BSS, polling) • Polled Sts can transmit (No CSMA) • CFP Repetition Interval (Manag duration): (1) PCF, and (2) DCF

  50. Point Coordination Function (PCF on top of DCF) • Light traffic: shorter CFP if previous DCF traffic is not complete • PC: PIFS, Beacon, (CF-poll/data/Data+CF-poll), CF-end. • CF-aware st: • Gets CF-poll, • Responds: CF-ACK, Data+CF-ACK, • Then PC responds by Data+CF-ACK+CF-poll