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Link Layer; Media Access Control Lec 03

ECOM 6320. Link Layer; Media Access Control Lec 03. 07 /03/2010. Outline. Multipath propagation Multiplexing Modulation GSM IEEE 802.11. Multipath propagation. Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction

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Link Layer; Media Access Control Lec 03

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  1. ECOM 6320 Link Layer;Media Access ControlLec 03 07/03/2010

  2. Outline • Multipath propagation • Multiplexing • Modulation • GSM • IEEE 802.11

  3. Multipath propagation • Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction • Time dispersion: signal is dispersed over time • interference with “neighbor” symbols, Inter Symbol Interference (ISI) • The signal reaches a receiver directly and phase shifted • distorted signal depending on the phases of the different parts multipath pulses LOS pulses signal at sender signal at receiver

  4. Effects of mobility • Channel characteristics change over time and location • signal paths change • different delay variations of different signal parts • different phases of signal parts •  quick changes in the power received (short term fading) • Additional changes in • distance to sender • obstacles further away •  slow changes in the averagepower received (long term fading) long term fading power t short term fading

  5. Multiplexing channels ki • Multiplexing in 4 dimensions • space (si) • time (t) • frequency (f) • code (c) • Goal: multiple use of a shared medium • Important: guard spaces needed! k1 k2 k3 k4 k5 k6 c t c s1 t s2 f f c t s3 f

  6. Frequency multiplex • Separation of the whole spectrum into smaller frequency bands • A channel gets a certain band of the spectrum for the whole time • Advantages • no dynamic coordination necessary • works also for analog signals • Disadvantages • waste of bandwidth if the traffic is distributed unevenly • inflexible k1 k2 k3 k4 k5 k6 c f t

  7. Time multiplex • A channel gets the whole spectrum for a certain amount of time • Advantages • only one carrier in themedium at any time • throughput high even for many users • Disadvantages • precise synchronization necessary k1 k2 k3 k4 k5 k6 c f t

  8. Time and frequency multiplex • Combination of both methods • A channel gets a certain frequency band for a certain amount of time • Example: GSM • Advantages • better protection against tapping • protection against frequency selective interference • but: precise coordinationrequired k1 k2 k3 k4 k5 k6 c f t

  9. Code multiplex k1 k2 k3 k4 k5 k6 • Each channel has a unique code • All channels use the same spectrum at the same time • Advantages • bandwidth efficient • no coordination and synchronizationnecessary • good protection against interferenceand tapping • Disadvantages • varying user data rates • more complex signal regeneration • Implemented using spread spectrum technology c f t

  10. Spread spectrum technology • Problem of radio transmission: frequency dependent fading can wipe out narrow band signals for duration of the interference • Solution: spread the narrow band signal into a broad band signal using a special code • protection against narrow band interference • Side effects: • coexistence of several signals without dynamic coordination • tap-proof • Alternatives: Direct Sequence, Frequency Hopping signal power interference power spread signal spread interference detection at receiver f f

  11. Effects of spreading and interference dP/df dP/df user signal broadband interference narrowband interference i) ii) f f sender dP/df dP/df dP/df iii) iv) v) f f f receiver

  12. DSSS (Direct Sequence Spread Spectrum) I • XOR of the signal with pseudo-random number (chipping sequence) • many chips per bit (e.g., 128) result in higher bandwidth of the signal • Advantages • reduces frequency selective fading • in cellular networks • base stations can use the same frequency range • several base stations can detect and recover the signal • soft handover • Disadvantages • precise power control necessary tb user data 0 1 XOR tc chipping sequence 0 1 1 0 1 0 1 0 1 1 0 1 0 1 = resulting signal 0 1 1 0 1 0 1 1 0 0 1 0 1 0 tb: bit period tc: chip period

  13. DSSS (Direct Sequence Spread Spectrum) II spread spectrum signal transmit signal user data X modulator chipping sequence radio carrier transmitter correlator lowpass filtered signal sampled sums products received signal data demodulator X integrator decision radio carrier chipping sequence receiver

  14. FHSS (Frequency Hopping Spread Spectrum) I • Discrete changes of carrier frequency • sequence of frequency changes determined via pseudo random number sequence • Two versions • Fast Hopping: several frequencies per user bit • Slow Hopping: several user bits per frequency • Advantages • frequency selective fading and interference limited to short period • simple implementation • uses only small portion of spectrum at any time • Disadvantages • not as robust as DSSS • simpler to detect

  15. FHSS (Frequency Hopping Spread Spectrum) II tb user data 0 1 0 1 1 t f td f3 slow hopping (3 bits/hop) f2 f1 t td f f3 fast hopping (3 hops/bit) f2 f1 t tb: bit period td: dwell time

  16. FHSS (Frequency Hopping Spread Spectrum) III narrowband signal spread transmit signal user data modulator modulator frequency synthesizer hopping sequence transmitter narrowband signal received signal data demodulator demodulator hopping sequence frequency synthesizer receiver

  17. Cell structure • Implements space division multiplex • base station covers a certain transmission area (cell) • Mobile stations communicate only via the base station • Advantages of cell structures • higher capacity, higher number of users • less transmission power needed • more robust, decentralized • base station deals with interference, transmission area etc. locally • Problems • fixed network needed for the base stations • handover (changing from one cell to another) necessary • interference with other cells • Cell sizes from some 100 m in cities to, e.g., 35 km on the country side (GSM) - even less for higher frequencies

  18. Frequency planning I f3 f5 f2 f4 f6 f5 f1 f4 f3 f7 f1 f2 • Frequency reuse only with a certain distance between the base stations • Standard model using 7 frequencies: • Fixed frequency assignment: • certain frequencies are assigned to a certain cell • problem: different traffic load in different cells • Dynamic frequency assignment: • base station chooses frequencies depending on the frequencies already used in neighbor cells • more capacity in cells with more traffic • assignment can also be based on interference measurements

  19. Frequency planning II f3 f3 f3 f2 f2 f1 f1 f1 f3 f3 f2 f2 f2 f1 f1 f3 f3 f3 f2 f2 f2 f1 f1 f1 f3 f3 f3 h2 h2 h1 h1 g2 g2 h3 h3 g2 g1 g1 g1 g3 g3 g3 f2 f3 f7 f5 f2 3 cell cluster f4 f6 f5 f1 f4 f3 f7 f1 f3 f2 f6 f2 f5 7 cell cluster 3 cell cluster with 3 sector antennas

  20. Cell breathing • CDM systems: cell size depends on current load • Additional traffic appears as noise to other users • If the noise level is too high users drop out of cells

  21. media access • Can we apply media access methods from fixed networks? • Example CSMA/CD • Carrier Sense Multiple Access with Collision Detection • send as soon as the medium is free, listen into the medium if a collision occurs (legacy method in IEEE 802.3) • Problems in wireless networks • signal strength decreases proportional to the square of the distance • the sender would apply CS and CD, but the collisions happen at the receiver • it might be the case that a sender cannot “hear” the collision, i.e., CD does not work • furthermore, CS might not work if, e.g., a terminal is “hidden”

  22. hidden and exposed terminals • Hidden terminals • A sends to B, C cannot receive A • C wants to send to B, C senses a “free” medium (CS fails) • collision at B, A cannot receive the collision (CD fails) • A is “hidden” for C • Exposed terminals • B sends to A, C wants to send to another terminal (not A or B) • C has to wait, CS signals a medium in use • but A is outside the radio range of C, therefore waiting is not necessary • C is “exposed” to B A B C

  23. near and far terminals • Terminals A and B send, C receives • signal strength decreases proportional to the square of the distance • the signal of terminal B therefore drowns out A’s signal • C cannot receive A • If C for example was an arbiter for sending rights, terminal B would drown out terminal A already on the physical layer • Also severe problem for CDMA-networks - precise power control needed! A B C

  24. Access methods SDMA/FDMA/TDMA • SDMA (Space Division Multiple Access) • segment space into sectors, use directed antennas • cell structure • FDMA (Frequency Division Multiple Access) • assign a certain frequency to a transmission channel between a sender and a receiver • permanent (e.g., radio broadcast), slow hopping (e.g., GSM), fast hopping (FHSS, Frequency Hopping Spread Spectrum) • TDMA (Time Division Multiple Access) • assign the fixed sending frequency to a transmission channel between a sender and a receiver for a certain amount of time

  25. FDD/FDMA - general scheme, example GSM f 960 MHz 124 200 kHz 935.2 MHz 1 20 MHz 915 MHz 124 890.2 MHz 1 t

  26. TDD/TDMA - general scheme 417 µs 1 2 3 11 12 1 2 3 11 12 t downlink uplink

  27. Aloha/slotted aloha • Mechanism • random, distributed (no central arbiter), time-multiplex • Slotted Aloha additionally uses time-slots, sending must always start at slot boundaries • Aloha • Slotted Aloha collision sender A sender B sender C t collision sender A sender B sender C t

  28. DAMA - Demand Assigned Multiple Access • Channel efficiency only 18% for Aloha, 36% for Slotted Aloha (assuming Poisson distribution for packet arrival and packet length) • Reservation can increase efficiency to 80% • a sender reserves a future time-slot • sending within this reserved time-slot is possible without collision • reservation also causes higher delays • typical scheme for satellite links • Examples for reservation algorithms: • Explicit Reservation according to Roberts (Reservation-ALOHA) • Implicit Reservation (PRMA) • Reservation-TDMA

  29. Access method DAMA: Explicit Reservation • Explicit Reservation (Reservation Aloha): • two modes: • ALOHA mode for reservation:competition for small reservation slots, collisions possible • reserved mode for data transmission within successful reserved slots (no collisions possible) • it is important for all stations to keep the reservation list consistent at any point in time and, therefore, all stations have to synchronize from time to time collision t Aloha reserved Aloha reserved Aloha reserved Aloha

  30. Access method DAMA: PRMA • Implicit reservation (PRMA - Packet Reservation MA): • a certain number of slots form a frame, frames are repeated • stations compete for empty slots according to the slotted aloha principle • once a station reserves a slot successfully, this slot is automatically assigned to this station in all following frames as long as the station has data to send • competition for this slots starts again as soon as the slot was empty in the last frame reservation 1 2 3 4 5 6 7 8 time-slot ACDABA-F frame1 A C D A B A F ACDABA-F frame2 A C A B A AC-ABAF- collision at reservation attempts frame3 A B A F A---BAFD frame4 A B A F D ACEEBAFD frame5 A C E E B A F D t

  31. Access method DAMA: Reservation-TDMA • Reservation Time Division Multiple Access • every frame consists of N mini-slots and x data-slots • every station has its own mini-slot and can reserve up to k data-slots using this mini-slot (i.e. x = N * k). • other stations can send data in unused data-slots according to a round-robin sending scheme (best-effort traffic) e.g. N=6, k=2 N * k data-slots N mini-slots reservationsfor data-slots other stations can use free data-slots based on a round-robin scheme

  32. MACA - collision avoidance • MACA (Multiple Access with Collision Avoidance) uses short signaling packets for collision avoidance • RTS (request to send): a sender request the right to send from a receiver with a short RTS packet before it sends a data packet • CTS (clear to send): the receiver grants the right to send as soon as it is ready to receive • Signaling packets contain • sender address • receiver address • packet size • Variants of this method can be found in IEEE802.11 as DFWMAC (Distributed Foundation Wireless MAC)

  33. MACA examples A A C C • MACA avoids the problem of hidden terminals • A and C want to send to B • A sends RTS first • C waits after receiving CTS from B • MACA avoids the problem of exposed terminals • B wants to send to A, C to another terminal • now C does not have to wait for it cannot receive CTS from A RTS CTS CTS B RTS RTS CTS B

  34. MACA variant: DFWMAC in IEEE802.11 sender receiver idle idle packet ready to send; RTS data; ACK RxBusy time-out; RTS wait for the right to send RTS; CTS time-out  data; NAK ACK time-out  NAK; RTS CTS; data wait for data wait for ACK RTS; RxBusy ACK: positive acknowledgement NAK: negative acknowledgement RxBusy: receiver busy

  35. Polling mechanisms • If one terminal can be heard by all others, this “central” terminal (a.k.a. base station) can poll all other terminals according to a certain scheme • now all schemes known from fixed networks can be used (typical mainframe - terminal scenario) • Example: Randomly Addressed Polling • base station signals readiness to all mobile terminals • terminals ready to send can now transmit a random number without collision with the help of CDMA or FDMA (the random number can be seen as dynamic address) • the base station now chooses one address for polling from the list of all random numbers (collision if two terminals choose the same address) • the base station acknowledges correct packets and continues polling the next terminal • this cycle starts again after polling all terminals of the list

  36. ISMA (Inhibit Sense Multiple Access) • Current state of the medium is signaled via a “busy tone” • the base station signals on the downlink (base station to terminals) if the medium is free or not • terminals must not send if the medium is busy • terminals can access the medium as soon as the busy tone stops • the base station signals collisions and successful transmissions via the busy tone and acknowledgements, respectively (media access is not coordinated within this approach) • mechanism used, e.g., for CDPD (USA, integrated into AMPS)

  37. Access method CDMA • CDMA (Code Division Multiple Access) • all terminals send on the same frequency probably at the same time and can use the whole bandwidth of the transmission channel • each sender has a unique random number, the sender XORs the signal with this random number • the receiver can “tune” into this signal if it knows the pseudo random number, tuning is done via a correlation function • Disadvantages: • higher complexity of a receiver (receiver cannot just listen into the medium and start receiving if there is a signal) • all signals should have the same strength at a receiver • Advantages: • all terminals can use the same frequency, no planning needed • huge code space (e.g. 232) compared to frequency space • interferences (e.g. white noise) is not coded • forward error correction and encryption can be easily integrated

  38. IEEE 802.11 Requirements • Design for small coverage (e.g. office, home) • Low/no mobility • High data-rate applications • Ability to integrate real time applications and non-real-time applications • Use un-licensed spectrum

  39. Portal Distribution System 802.11: Infrastructure Mode • Architecture similar to cellular • networks station (STA) • terminal with access mechanisms to the wireless medium and radio contact to the access point • access point (AP) • station integrated into the wireless LAN and the distribution system • basic service set (BSS) • group of stations using the same AP • portal • bridge to other (wired) networks • distribution system • interconnection network to form one logical network (EES: Extended Service Set) based on several BSS 802.11 LAN 802.x LAN STA1 BSS1 Access Point Access Point ESS BSS2 STA2 STA3 802.11 LAN

  40. IEEE 802.11 Physical Layer • Family of IEEE 802.11 standards: • unlicensed frequency spectrum: 900Mhz, 2.4Ghz, 5.1Ghz, 5.7Ghz 300 MHz 5.15-5.35 GHz 5.725-5.825 GHz and 802.11b/g 802.11a

  41. 802.11a Physical Channels channel# 36 40 44 48 52 56 60 64 5150 5180 5200 5220 5240 5260 5280 5300 5320 5350 [MHz] center frequency = 5000 + 5*channel number [MHz] 149 153 157 161 channel# 5725 5745 5765 5785 5805 5825 [MHz]

  42. The IEEE 802.11 Family

  43. 802.11 - MAC Layer • Traffic services • Asynchronous Data Service (mandatory) • exchange of data packets based on “best-effort” • support of broadcast and multicast • Time-Bounded Service (optional) • exchange of bounded delay service

  44. 802.11 MAC Layer: Access Methods • DFWMAC-DCF CSMA/CA (mandatory) • collision avoidance via randomized “back-off“ • ACK packet for acknowledgements • DFWMAC-DCF w/ RTS/CTS (optional) • additional virtual “carrier sensing: to avoid hidden terminal problem • DFWMAC- PCF (optional) • access point polls terminals according to a list

  45. 802.11 CSMA/CA • CSMA: Listen before transmit • Collision avoidance • when transmitting a packet, choose a backoff interval in the range [0, CW] • CW is contention window • Count down the backoff interval when medium is idle • count-down is suspended if medium becomes busy • Transmit when backoff interval reaches 0

  46. 802.11 Backoff • IEEE 802.11 contention window CWis adapted dynamically depending on collision occurrence • after each collision, CW is doubled • thus CW varies from CWmin to CWmax

  47. 802.11 – RTS/CTS + ACK • Sender sends RTS with NAV (Network allocation Vector, i.e. reservation parameter that determines amount of time the data packet needs the medium) • Receiver acknowledges via CTS (if ready to receive) • CTS reserves channel for sender, notifying possibly hidden stations • Sender can now send data at once, acknowledgement via ACK • Other stations store NAV distributed via RTS and CTS DIFS RTS data sender SIFS SIFS SIFS CTS ACK receiver DIFS NAV (RTS) data other stations NAV (CTS) t defer access new contention

  48. DIFS DIFS PIFS SIFS medium busy contention next frame t 802.11 – Inter Frame Spacing • Defined different inter frame spacing • SIFS (Short Inter Frame Spacing); 10 us in 802.11b • highest priority, for ACK, CTS, polling response • PIFS (PCF IFS); 30 us in 802.11b • medium priority, for time-bounded service using PCF • DIFS (DCF, Distributed Coordination Function IFS); 50 us in 802.11b • lowest priority, for asynchronous data service direct access if medium is free  DIFS

  49. 802.11 – Inter Frame Spacing

  50. 802.11: PCF for Polling (Infrastructure Mode) SIFS PIFS D D point coordinator SIFS U polled wireless stations NAV NAV contention free period t medium busy contention period D: downstream poll, or data from point coordinator U: data from polled wireless station

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