Advanced Wireless. Networks

Transcription

1 TC IST Semester Networks - Part 2 - Advanced Wireless Networks Département Télécommunications Services & Usages

2 Advanced Wireless Networks Speakers Razvan Stanica Alexandre Mouradian Lab CITI [Centre of Innovation in Telecommunications and Integration of services] INRIA UrbaNet team Structure WiFi (6h), MANET (4h), WSN (8h) Paper review work in autonomy Evaluation: paper presentation and Q&A session 2

3 IEEE : The beginnings In 1985, the US Federal Communications Commission (FCC) created the Industrial, Scientific and Medical band (ISM) for non-licensed applications (2,4GHz) In 1990 the IEEE establishes the committee The IEEE standard was finalized in 1997 and became the de-facto standard for WLAN 3

4 IEEE : The beginnings Multiple competitors Busy tone medium access control (HiperLan) CSMA/CA (remember NET1) A standard is the result of negotiations and politics not necessarily the best technical result 4

5 IEEE : The beginnings A CSMA/CA solution has been chosen Several thousand pages of specifications Compliance with the standard difficult to assess The birth of the WiFi alliance (2002, existing as WECA in 1999) 5

6 IEEE : Evolutions Higher data rate: 11 Mbps (b), 54 Mbps (g), 100+ Mbps (n) 500+ Mbps (ac) Use of different frequencies (a 5GHz, ad 60GHz) Use of multiple antennas (n, ac) Integrating Quality of Service (e) Dedicated environments: mesh (s), vehicular (p) Security enhancements (i) 6

7 Basic Service Set (BSS) Architecture Formed by an Access Point (AP) and all the associated stations (STA) Similar to a cell in 2G/3G/4G The BSSID is the MAC address of the AP and is broadcast periodically 7

8 Independent Basic Service Set (IBSS) No real AP, only synchronized STAs One STA acts as AP Architecture The BSSID is the MAC address of the master STA. As close as it gets from ad hoc networking 8

9 Architecture Distribution system (DS) A backbone (usually, but not necessarily wired) connecting the APs The DS may use any communication technology (even WiFi), with Ethernet being the most deployed 9

10 Architecture 10

11 Architecture Extended service set (ESS) An ESS is the union of multiple BSSs connected through a DS The ESS is equivalent to a single IBSS for the logical link control layer The BSSs forming an ESS can use different frequency channels No physical restrictions: BSSs can be collocated, overlapping, or connected through a long range DS 11

12 IEEE : Frame Format The frame Preamble PLCP MAC Data CRC The MAC Data Frame Control Sequence Duration Address1 Address2 Address3 Control Address4 Frame Body CRC 12

13 IEEE : Frame Format Why do we need 4 addresses? The Frame Control field contains (among others) two bits named To DS and From DS The value of To DS and From DS gives the meaning of the 4 addresses 13

14 Mobility WiFi not designed for mobility The usual WiFi user experience - nomadic Mobility is possible in IEEE networks The obvious scenario: moving within the area covered by the same AP Handovers between APs are possible inside an ESS 14

15 Mobility How mobility works The DS must implement a location service (not specified by the IEEE standard) A STA can be associated with no more than one AP at a given time (hard handover) The STA continuously measures the channel quality for the neighbouring APs The handover is initiated by the STA 15

16 Mobility Network entry Scanning STA chooses an AP nearby Passive: Just wait for the periodic AP beacon Active: Probe a known AP Authentication STA proves it has legit access to the AP Open: Authentication phase is skipped Secure: Challenge by the AP, the STA needs to know a shared key to answer correctly 16

17 Mobility Network entry Authentication STA proves it has legit access to the AP Open: Authentication phase is skipped Secure: Challenge by the AP, the STA needs to know a shared key to answer correctly Association STA enters the BSS STA -> AP: association request AP -> STA: association reply 17

18 Mobility Handover AP scanning and selection of target AP (by the STAs) Authentication (if needed) with the target AP Re-association with the target AP 18

19 Mobility Handover AP scanning and selection of target AP (by the STAs) Authentication (if needed) with the target AP Re-association with the target AP Pair-wise master key (PMK) negotiation IEEE 802.1X Pair-wise transient key (PTK) negotiation IEEE i Quality of Service admission control 19

20 Mobility IEEE r amendment (2008) Originally, only 4 messages were needed for intra-ess handover: 2xAuthentication and 2xAssociation Security and QoS admission control highly increased the number of messages and the handover delay The PTK negotiation needs 4 messages IEEE 802.1X authentication requires a time consuming key negotiation with an authentication server at every handover 20

21 Mobility IEEE r amendment (2008) Specification of Fast Basic Service Set transitions between APs The PMK is cached in the DS and reused for handovers, avoiding the negotiation process PTK negotiation and QoS admission control are piggybacked with the Authentication and Reassociation messages 21

22 Mobility IEEE p amendment (2010) Designed in the context of wireless access in vehicular environments High relative speeds between STAs (~200 km/h), resulting in small contact time PTK negotiation and QoS admission control are piggybacked with the Authentication and Reassociation messages 22

23 CSMA/CA: The implementation The IEEE standard implements CSMA/CA in the Distributed Coordination Function (DCF) Two types of carrier sense Physical Carrier Sense Clear Channel Assessment Virtual Carrier Sense Network Allocation Vector 23

24 CSMA/CA: The implementation Physical Carrier Sense How the magic happens CCA - A function of the Physical Layer Convergence Protocol Logical Link Control Data Link Layer Physical Layer CCA Medium Access Control Physical Layer Convergence Protocol Carrier Sense Physical Medium Dependent 24

25 Clear Channel Assessment Case 1 Header detection PLCP Preamble PLCP Header MAC Header LLC Network Data FCS PLCP Preamble Depends on the physical layer In OFDM (most popular PHY today): 12 symbols Used as a training sequence 25

26 Clear Channel Assessment Case 1 Header detection PLCP Preamble PLCP Header MAC Header LLC Network Data FCS RATE 4 bits Reserved 1 bit LENGTH 12 bits Parity 1 bit Tail 6 bits SERVICE 16 bits Most robust modulation (3 Mb/s today) Transmitted at RATE 26

27 Clear Channel Assessment Case 1 Header detection RATE 4 bits Reserved 1 bit LENGTH 12 bits Parity 1 bit Tail 6 bits SERVICE 16 bits PLCP Header Stations are required to decode any header arriving with 6 db over the interference level Decoding the RATE and LENGTH field gives the transmission duration Transmission sensed even if reception fails 27

28 Clear Channel Assessment Case 2 Energy detection Headers can not always be detected WiFi signals arriving simultaneously Other devices emit in the same band (Bluetooth, microwave ovens, other WiFi devices) CCA measures the energy level If the perceived energy level is higher than -65 dbm, the channel is considered busy Continuous measuring when station not idle or channel not busy 28

29 Clear Channel Assessment To summarize CCA blocks a transmission if another station is already using the channel CCA does not solve the problem of transmissions that start at the same time (see the back-off mechanisms in the next class) Through the energy detection mechanism, CCA can discover cross-network and even cross-technology interference 29

30 Network Allocation Vector Great, this solves everything! Not exactly: hidden terminals and cannot hear each other Common scenarios sends DATA to wants to access the channel but hears the DATA from waits the end of DATA, then senses the channel idle and transmits collision between DATA and ACK sends DATA to wants to access the channel and senses it idle and transmits collision between DATA and DATA 30

31 Network Allocation Vector The RTS/CTS handshake The collision probability between hidden nodes increases with the size of the messages The idea: use two short control messages to reserve the medium: Request-to-Send (RTS) and Clear-to- Send (CTS) RTS and CTS contain information about the duration of the following transmission (DATA+ACK) 31

32 Network Allocation Vector The RTS/CTS handshake and cannot hear each other sends RTS to sends CTS to hears the CTS and considers the channel busy for the announced duration sends DATA to sends ACK to The RTS/CTS handshake is also known as virtual carrier sense. 32

33 Network Allocation Vector And when you think everything works... exposed terminal a tremendous reduction in throughput sends RTS to sends CTS to receives RTS from and refrain from transmitting to...but transmission from to would not cause a collision! 33

34 Back-off: A reminder Role When a station senses the channel as busy, it needs to wait for a certain time This time interval is randomly selected in order to avoid collisions The mechanism is tuned based on the properties of the medium Short back-off in CSMA/CD on wired media Long(er) back-off in CSMA/CA on wireless 34

35 IEEE Implementation Distributed Coordination Function (DCF) 4 types of Inter-Frame Spaces (IFS) Short Inter-Frame Space (SIFS) used to separate transmissions belonging to a same dialogue (before a CTS or an ACK) Point coordination Inter-Frame Space (PIFS) for data in the contention-free period (not discussed here), to preempt any contention-based traffic Distributed Inter-Frame Space (DIFS) standard IFS, used to separate transmissions of different dialogues Extended InterFrame Space (EIFS) used by a station that received an erroneous frame SIFS < PIFS < DIFS < EIFS 35

36 DIFS DIFS SIFS DIFS SIFS Scenarios Distributed Coordination Function A B DATA ACK ACK C b=4 Freeze DATA 36

37 DIFS EIFS DIFS DIFS DIFS SIFS DIFS DIFS SIFS Scenarios Distributed Coordination Function A B C b=2 b=4 DATA Freeze Freeze ACK DATA Freeze ACK DATA Error on ACK reception 37

38 EIFS DIFS DIFS Tout Tout SIFS DIFS DIFS Scenarios Distributed Coordination Function A B C b=4 DATA DATA Freeze b=7 b=8 Freeze Freeze DATA DIFS ACK DATA Freeze... Error on DATA reception 38

39 SIFS SIFS DIFS SIFS Scenarios Distributed Coordination Function A B C RTS CTS DATA NAV ACK NAV = Network Allocation Vector = Virtual Carrier Sense The RTS/CTS handshake is optional in IEEE

40 Distributed Coordination Function Binary Exponential Back-off The back-off interval is randomly selected between 0 and the contention window (CW) Initially, CW= CW min (= 31 in IEEE , 15 in IEEE a) For every missing ACK, CW doubles and the procedure restarts, until CW= CW max (= 1023) Following a received ACK, CW= CW min 40

41 Distributed Coordination Function Broadcast messages Broadcast= one transmitter, multiple receivers If all receivers transmit CTS or ACK after SIFS, collisions are unavoidable Broadcast messages are transmitted only once using the minimum CW, and their transmission is unreliable (no ACK) 41

42 IEEE e Enhanced Distributed Channel Access Multimedia communications with specific delay constraints Important properties: can tolerate packet loss, can not tolerate jitter Quality of Service enhancements in the IEEE e standard (2005) 42

43 IEEE e Enhanced Distributed Channel Access Introduction of 4 traffic classes: Background (AC_BK) Best Effort (AC_BE) Video (AC_VI) Voice (AC_VO) Introduction of a new inter-frame space: Arbitration Inter-Frame Space (AIFS) 43

44 IEEE e Enhanced Distributed Channel Access AIFS replaces DIFS and is defined per traffic class: SIFS < AIFS[AC_VO] < AIFS[AC_VI] < AIFS[AC_BE] < AIFS[AC_BK] CW min and CW max are also defined per traffic class: Class CW min CW max AC_BK acw min acw max AC_BE acw min acw max AC_VI (acw min +1)/2-1 acw min AC_VO (acw min +1)/4-1 (acw min +1)/2-1 44

45 IEEE e Enhanced Distributed Channel Access Functioning: 45

46 IEEE : PCF There is more than DCF in IEEE Point Coordination Function (PCF) Contention-Free frame transfer protocol Based on polling made by the access point Coexists with DCF Contention free period (CFP) - PCF Contention period (CP) - DCF Contention free period (CFP) - PCF Contention period (CP) - DCF CFP Repetition Interval CFP Repetition Interval 46

47 How does PCF work? IEEE : PCF A B C D PIFS Beacon SIFS DATA + Poll SIFS ACK SIFS Poll Channel busy SIFS DATA + ACK SIFS CF-End CFP 47

48 PHY layer basics The usual way of imagining information Carrier wave We are not very good at making square signals But we are great at making sine waves A (usually sinusoidal) waveform modified with an input signal for the purpose of conveying information 48

49 PHY layer basics Modulation A mapping of the information on changes in the carrier wave phase, frequency or amplitude (or a combination of these) Amplitude Frequency Phase 49

50 PHY layer basics Spread spectrum The first IEEE standards were based on spread spectrum modulation techniques (DSSS, FHSS) = a signal generated with a particular bandwidth is deliberately spread in the frequency domain. Use of pseudo-random sequences known by both the receiver and transmitter. Increased resistance to interference, noise and jamming. 50

51 PHY layer basics Multiplexing Method of sharing a given bandwidth between different independent data channels Sharing can happen in time, frequency, code, or a combination of these. Frequency division multiplexing 51

52 OFDM Orthogonal Frequency Division Multiplexing A special case of FDM A combination of modulation and multiplexing The multiplexing is applied to multiple independent signals (just like in normal FDM), but these signals are a subset of the same main signal Analogy by C.Langton (complextoreal.com) Regular FDM Same amount of water coming from a lot of small streams Orthogonal FDM 52

53 OFDM OFDM advantages The faucet hole can be much easily blocked than the shower If something (water, information) is transported in a single stream, losing the stream means losing everything. Regular FDM Same amount of water coming from a lot of small streams Orthogonal FDM 53

54 OFDM OFDM in a nutshell The signal is divided into independent channels The independent carriers are modulated by data The results are recombined (multiplexed) to obtain the OFDM carrier 54

55 OFDM The importance of being orthogonal The area under a sine/cosine wave over a period is 0 55

56 OFDM The importance of being orthogonal Harmonic of a wave = a component of the main signal that is an integer multiple of the fundamental frequency (i.e. 2f, 3f, 4f ) The area under a sine wave multiplied by one of its harmonics is 0 56

57 OFDM The importance of being orthogonal Orthogonality allows simultaneous transmissions on multiple sub-carriers in a tight frequency space without interference from each other. 57

58 OFDM An example (simple but not exactly real) Bit stream to be transmitted: 1, 1, -1, -1, 1, 1, 1, -1, 1, - 1, -1, -1, -1, 1, -1, -1, -1, 1, -1, -1, -1, 1, 1, -1, -1, -1, 1, 1 4 carriers: c1, c2, c3, c4 Serial to parallel conversion: c1: 1, 1, 1, -1, -1, -1 c2: 1, 1, -1, 1, 1, -1 c3: -1, 1, -1, -1, 1, 1 c4: -1, -1, -1, -1, -1, 1 58

59 OFDM An example (simple but not exactly real) Taking the example of a PSK modulation c1: 1, 1, 1, -1, -1, -1 59

60 OFDM An example (simple but not exactly real) c2: 1, 1, -1, 1, 1, -1 c3: -1, 1, -1, -1, 1, 1 60

61 OFDM An example (simple but not exactly real) c4: -1, -1, -1, -1, -1, 1 61

62 OFDM An example (simple but not exactly real) 62

63 OFDM An example (simple but not exactly real) Resulting OFDM signal Constant amplitude sub-carriers, but amplitude varying OFDM signal. 63

64 OFDM Problems Frequency shift of the transmitted signal (because of Doppler effect): inter-carrier interference. Large delay spread (because of multipath): intersymbol interference OFDM solutions Guard time inserted between consecutive OFDM symbols. Cyclic prefix a part of the signal is copied at the beginning of the symbol (maintains orthogonality). Pilot carriers used to detect phase shift and intercarrier interference. 64

65 OFDM IEEE pilot sub-carriers, 48 data sub-carriers 65

66 OFDM IEEE a values 66

67 Back to PHY basics Multipath channel Wireless signal propagates on different paths because of scattering on different obstacles 67

68 MIMO Multiple Input Multiple Output Exploiting multipath rather than mitigating it Taking advantage of independent propagation on the different paths Use multiple antennas to send multiple parallel signals Multiple receiver radios collect multipath signals and process them to recover the transmitted data 2 x 3 MIMO 68

69 MIMO Types of MIMO Transmission diversity same data is encoded and transmitted through different antennas. This improves Signal-to-Noise ratio, especially for stations situated at the cell s edge. Spatial multiplexing parallel stream of data by exploiting independent multiple paths. It results in a theoretically linear increase (with the number of antenna elements) of channel capacity. 69