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

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

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

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

5. Multiplexing r Multiplexing in 4 dimensions m space (s i ) m time (t) m frequency (f) m code (c) r Goal: multiple use of a shared medium r Important: guard spaces needed! s2s2 s3s3 s1s1 f t c k2k2 k3k3 k4k4 k5k5 k6k6 k1k1 f t c f t c channels k i

6. Frequency multiplex r Separation of the whole spectrum into smaller frequency bands r A channel gets a certain band of the spectrum for the whole time r Advantages m no dynamic coordination necessary m works also for analog signals r Disadvantages m waste of bandwidth if the traffic is distributed unevenly m inflexible k2k2 k3k3 k4k4 k5k5 k6k6 k1k1 f t c

7. f t c k2k2 k3k3 k4k4 k5k5 k6k6 k1k1 Time multiplex r A channel gets the whole spectrum for a certain amount of time r Advantages m only one carrier in the medium at any time m throughput high even for many users r Disadvantages m precise synchronization necessary

8. f Time and frequency multiplex r Combination of both methods r A channel gets a certain frequency band for a certain amount of time r Example: GSM r Advantages m better protection against tapping m protection against frequency selective interference r but: precise coordination required t c k2k2 k3k3 k4k4 k5k5 k6k6 k1k1

9. Code multiplex r Each channel has a unique code r All channels use the same spectrum at the same time r Advantages m bandwidth efficient m no coordination and synchronization necessary m good protection against interference and tapping r Disadvantages m varying user data rates m more complex signal regeneration r Implemented using spread spectrum technology k2k2 k3k3 k4k4 k5k5 k6k6 k1k1 f t c

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

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

12. DSSS (Direct Sequence Spread Spectrum) I r XOR of the signal with pseudo-random number (chipping sequence) m many chips per bit (e.g., 128) result in higher bandwidth of the signal r Advantages m reduces frequency selective fading m in cellular networks base stations can use the same frequency range several base stations can detect and recover the signal soft handover r Disadvantages m precise power control necessary user data chipping sequence resulting signal 01 01101010100111 XOR 01100101101001 = tbtb tctc t b : bit period t c : chip period

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

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

15. FHSS (Frequency Hopping Spread Spectrum) II user data slow hopping (3 bits/hop) fast hopping (3 hops/bit) 01 tbtb 011t f f1f1 f2f2 f3f3 t tdtd f f1f1 f2f2 f3f3 t tdtd t b : bit periodt d : dwell time

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

17. Cell structure r Implements space division multiplex m base station covers a certain transmission area (cell) r Mobile stations communicate only via the base station r Advantages of cell structures m higher capacity, higher number of users m less transmission power needed m more robust, decentralized m base station deals with interference, transmission area etc. locally r Problems m fixed network needed for the base stations m handover (changing from one cell to another) necessary m interference with other cells r 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 r Frequency reuse only with a certain distance between the base stations r Standard model using 7 frequencies: r Fixed frequency assignment: m certain frequencies are assigned to a certain cell m problem: different traffic load in different cells r Dynamic frequency assignment: m base station chooses frequencies depending on the frequencies already used in neighbor cells m more capacity in cells with more traffic m assignment can also be based on interference measurements f4f4 f5f5 f1f1 f3f3 f2f2 f6f6 f7f7 f3f3 f2f2 f4f4 f5f5 f1f1

19. Frequency planning II f1f1 f2f2 f3f3 f2f2 f1f1 f1f1 f2f2 f3f3 f2f2 f3f3 f1f1 f2f2 f1f1 f3f3 f3f3 f3f3 f3f3 f3f3 f4f4 f5f5 f1f1 f3f3 f2f2 f6f6 f7f7 f3f3 f2f2 f4f4 f5f5 f1f1 f3f3 f5f5 f6f6 f7f7 f2f2 f2f2 f1f1 f1f1 f1f1 f2f2 f3f3 f2f2 f3f3 f2f2 f3f3 h1h1 h2h2 h3h3 g1g1 g2g2 g3g3 h1h1 h2h2 h3h3 g1g1 g2g2 g3g3 g1g1 g2g2 g3g3 3 cell cluster 7 cell cluster 3 cell cluster with 3 sector antennas

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

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

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

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

24. Access methods SDMA/FDMA/TDMA r SDMA (Space Division Multiple Access) m segment space into sectors, use directed antennas m cell structure r FDMA (Frequency Division Multiple Access) m assign a certain frequency to a transmission channel between a sender and a receiver m permanent (e.g., radio broadcast), slow hopping (e.g., GSM), fast hopping (FHSS, Frequency Hopping Spread Spectrum) r TDMA (Time Division Multiple Access) m 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 t 124 1 1 20 MHz 200 kHz 890.2 MHz 935.2 MHz 915 MHz 960 MHz

26. TDD/TDMA - general scheme 12311121231112 t downlinkuplink 417 µs

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

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

29. Access method DAMA: Explicit Reservation r Explicit Reservation (Reservation Aloha): m 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) m 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 AlohareservedAlohareservedAlohareservedAloha collision t

30. Access method DAMA: PRMA r Implicit reservation (PRMA - Packet Reservation MA): m a certain number of slots form a frame, frames are repeated m stations compete for empty slots according to the slotted aloha principle m 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 m competition for this slots starts again as soon as the slot was empty in the last frame frame 1 frame 2 frame 3 frame 4 frame 5 12345678time-slot collision at reservation attempts ACDABA F AC ABA A BAF A BAF D ACEEBAFD t ACDABA-F AC-ABAF- A---BAFD ACEEBAFD reservation

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

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

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

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

35. Polling mechanisms r 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 m now all schemes known from fixed networks can be used (typical mainframe - terminal scenario) r Example: Randomly Addressed Polling m base station signals readiness to all mobile terminals m 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) m the base station now chooses one address for polling from the list of all random numbers (collision if two terminals choose the same address) m the base station acknowledges correct packets and continues polling the next terminal m this cycle starts again after polling all terminals of the list

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

37. Access method CDMA r CDMA (Code Division Multiple Access) m all terminals send on the same frequency probably at the same time and can use the whole bandwidth of the transmission channel m each sender has a unique random number, the sender XORs the signal with this random number m the receiver can “tune” into this signal if it knows the pseudo random number, tuning is done via a correlation function r Disadvantages: m higher complexity of a receiver (receiver cannot just listen into the medium and start receiving if there is a signal) m all signals should have the same strength at a receiver r Advantages: m all terminals can use the same frequency, no planning needed m huge code space (e.g. 2 32 ) compared to frequency space m interferences (e.g. white noise) is not coded m forward error correction and encryption can be easily integrated

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

39. 39 802.11: Infrastructure Mode r Architecture similar to cellular m networks station (STA) terminal with access mechanisms to the wireless medium and radio contact to the access point m access point (AP) station integrated into the wireless LAN and the distribution system m basic service set (BSS) group of stations using the same AP m portal bridge to other (wired) networks m distribution system interconnection network to form one logical network (EES: Extended Service Set) based on several BSS Distribution System Portal 802.x LAN Access Point 802.11 LAN BSS 2 802.11 LAN BSS 1 Access Point STA 1 STA 2 STA 3 ESS

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

41. 41 802.11a Physical Channels 5150 [MHz] 5180 5350 5200 3644 center frequency = 5000 + 5*channel number [MHz] channel# 404852566064 149153157161 522052405260528053005320 5725 [MHz] 5745 5825 5765 channel# 57855805

42. 42 The IEEE 802.11 Family ProtocolRelease Data Freq.Rate (typical) Rate (max) Range (indoor) Legacy19972.4 GHz1 Mbps2Mbps? 802.11a19995 GHz25 Mbps54 Mbps ~30 m 802.11b19992.4 GHz6.5 Mbps11 Mbps ~30 m 802.11g20032.4 GHz25 Mbps54 Mbps ~30 m 802.11n20082.4/5 GHz 200 Mbps540 Mbps ~50 m

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

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

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

46. 46 802.11 Backoff r IEEE 802.11 contention window CW is adapted dynamically depending on collision occurrence m after each collision, CW is doubled m thus CW varies from CWmin to CWmax 802.11b802.11a802.11g aSlotTime20 usec9 usec20 usec (mixed); 9 usec (g-only) aCWmin31 slots15 slots

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

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

49. 49 802.11b802.11a802.11g aSIFSTime10 usec16 usec10 usec aSlotTime20 usec9 usec20 usec (mixed); 9 usec (g only) aDIFTime (2xSlot+SIFS) 50 usec34 usec50 usec; 28 usec 802.11 – Inter Frame Spacing

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

51. 802.11b Frame Format 51 SyncSFDPLCP headerCRC 2 Preamble (192 usec; or optional 96 short version) - Sync: alternating 0s and 1s (DSSS 128 bits) - SFD: Start Frame delimiter: 0000 1100 1011 1101 PLCH (Phsical Layer Convergence Procedure) Header - payload length - signaling field: the rate info. - CRC: 16 bit protection of header MAC Data preamble

52. 52 802.11 – MAC Data Format r Types m control frames, management frames, data frames r Sequence numbers m important against duplicated frames due to lost ACKs r Addresses m receiver, transmitter (physical), BSS identifier, sender (logical) r Miscellaneous m sending time, checksum, frame control, data Frame Control Duration/ ID Address 1 Address 2 Address 3 Sequence number Address 4 DataCRC 2 26666240-2312 bytes Protocol version TypeSubtype To DS More Frag Retry Power Mgmt More Data WEP 2241 From DS 1 Order bits111111