RETI DI TELECOMUNICAZIONI

Transcript

1 Corso di Studi in Ing. delle Telecomunicazioni LAUREA in INGEGNERIA DELLE TELECOMUNICAZIONI RETI DI TELECOMUNICAZIONI Stefano Giordano Lezione n.16n Wireless LAN IEEE Gruppo di Ricerca in Reti di Telecomunicazioni Dipartimento di Ingegneria della Informazione: Elettronica, Informatica, Telecomunicazioni 1

2 Wireless LANs IEEE Other examples (ETSI HIPERLAN - 20 Mbps, Bluetooth 700 Kbps, IrDA 75 Kbps, IrDA-D 115Kbps-4Mbps) Bandwidth: 1 or 2 Mbps on the first release 5.5 or 11 Mbit/s on its second generation 54 Mbit/s on its third generation 108 Mbit/s on its fourth generation Physical Media spread spectrum radio (2.4GHz or 5 Ghz) diffused infrared (10m) 2

3 Specifica orig. ratificata da IEEE Evoluzione di Tecnologia e degli Standard Prodotti Bluetooth disponibili (802.15) a e b ratific. da IEEE Fond. WECA Nuovi servizi di rete aggiunti (QoS, IAPP, WEP2, etc.) a 5Ghz (fino a 54 Mpbs) Courtesy of 3COM Italia g 2,4Ghz (fino a 54 Mpbs) EUROPA h i f ? 3

4 Courtesy of 3COM Italia Panoramica dei Sistemi a b g Ratifica Standard Banda /Modulaz. 5GHz / OFDM 2.4GHz / DSSS 2.4GHz / OFDM Velocità Fino a 54Mbps Fino a 11Mbps Fino a 54Mbps Copertura Fino a 50 Metri Fino a 100 Metri Fino a 100 Metri Vantaggi Minori possibilità di interferenze con le frequenze radio Buon supporto x applic. multimediali e ambienti con alto n di utenti Compatibilità certificata Wi-Fi Il sistema oggi più largamente sviluppato; prezzi convenienti Compatibile con b Alta velocità ed elevata copertura Svantaggi Richiede modifica hw Minore area di copertura bassa velocità Si noti che: 22 Mbps è una soluzione PROPRIETARIA di alcuni produttori 4

5 802.11b/g vs a Courtesy of 3COM Italia /54 Mbps b/g 100 Metri Mbps 50 Metri a Maggior Consumo Elettrico 5

6 A proposito di TURBO... Courtesy of 3COM Italia teoria pratica b g a turbo a turbo 108 6

7 WLAN Standard in corso di definizione Courtesy of 3COM Italia d Definisce i requisiti del livello fisico che soddisfano le richieste di quei paesi che non hanno ancora legiferato in merito a WLANS e Opera su MAC per incrementare la QoS ed assicurare il supporto alle applicazioni audio/video f Migliore la gestione del roaming per mantenere stabile una connessione WLAN tra apparati diversi che afferiscono a segmenti di rete diversi (IAPP) h Incrementa il controllo sulla potenza trasmissiva e il suo dosaggio sui canali adibiti a portare a. Questo standard è espressamente dedicato a soddisfare le obiezioni avanzate da ETSI in merito alle interferenze su canali RADAR i Implementa 802.1x come base per tutti gli sviluppi in termini di autentificazione. Include la gestione delle chiavi di encryption dinamiche che rimpiazzano quelle statiche e configurate manualmente (WEP key) Consente l'uso degli algoritmi AES j Si preoccupa di armonizzare due standard diversi come a e HiperLAN2 (sviluppato totalmente in Europa sotto ETSI) in modo da renderli coesistenti. 7

8 Wi-Fi assicura Interoperabilità Courtesy of 3COM Italia Wireless Ethernet? Compatibility Alliance Oltre 50 membri tra cui: 3Com, Symbol, Enterasys, Avaya, Agere, Cisco Scopo: Assicurare la interoperabilità delle soluzioni basate su Laboratori indipendenti certificano per conto del consorzio se i prodotti rispettano le specifiche Wi-Fi Solo i prodotti che superano tutti i test possono stampare il logo Wi-Fi su prodotto e confezione. L interoperabilità viene controllata su: Connessione Crittografia WEP Roaming 8

9 PORTATA Tutti i vendor dichiarano coperture con raggio di 100 metri negli uffici (open space), oppure di 300 metri all aria aperta e senza ostacoli Nella vita REALE: mt. < 5,5Mbps (3Mbps) mt. 11 Mbps teorici (6Mbps) Fattori attenuanti della portata: Problemi temporanei Microonde, impianti e condizionatori Notebook in movimento Consistenza dei Muri Orientamento dell antenna Courtesy of 3COM Italia 11Mbps 5.5Mbps 2Mbps 1Mbps 9

10 Task Groups (TG) TGa PHY for UNII (US 5 GHz) TGb Higher rate PHY for 2.4 GHz ISM-band (5.5 or 11 Mbps) TGb_cor1 Corrections to b TGc bridging (802.1) TGd Operation in new regulatory domains, roaming TGe QoS (previously also security, authentication) TGf Inter-AP protocol, interoperability TGg Higher data rates for b, > 20 Mb/s ( 54 Mbps) TGh Enhance MAC and a PHY (for CEPT approval) TGi Enhance MAC for security and authentication TGj and a PHY 5 GHz operation in Japan TGk Radio resource measurements (for higher layers) TGl - TGm standard corrections maintenance TGn High throughput PHY 10

11 IEEE

12 Infrastructure network AP AP wired network AP: Access Point AP Ad-hoc network 12

13 Rete Wireless collegata a Internet Singolo Access Point Notebook Wireless LAN PC Cards ISP Provided Modem/Gateway ISP Access Point 13

14 IEEE Wireless LAN wireless LANs: untethered (i.e. mobile: better NOMADIC) networking IEEE standard: MAC protocol unlicensed frequency spectrum: 900Mhz, 2.4Ghz, 5 GHz Basic Service Set (BSS) (a.k.a. cell ) contains: wireless hosts access point (AP): base station BSS s combined to form distribution system (DS) Tens of meters 14

15 Ad Hoc Networks Ad hoc network: IEEE stations can dynamically form network without AP. A single indipendent BSS can be used to form an ad hoc network. Applications: laptop meeting in conference room, car, train interconnection of personal devices battlefield IETF MANET (Mobile Ad hoc Networks) working group 15

16 IEEE a High rate PHY, 6-54 Mb/s 5 GHz UNII-band OFDM (Orthogonal frequency division multiplexing) Almost same PHY as HiperLAN2 Approved

17 IEEE b High rate PHY, 5.5 and 11 Mb/s 2.4 GHz ISM-band (83 MHz, 22 MHz channel) CCK (Complementary code keying; the chipping code is complex) Approved

18 IEEE g Higher rate PHY for 2.4 GHz ISM-band > 20 Mb/s (max 54 Mb/s) Backward compatibility with b (through CCK and RTS/CTS) CCK and OFDM mandatory Other optional modulation schemes 18

19 Spread Spectrum Idea spread signal over wider frequency band than required originally designed to thwart jamming Frequency Hopping transmit over random sequence of frequencies sender and receiver share pseudorandom number generator seed uses 79 x 1 MHz-wide frequency bands 2.4 GHz ISM Band (Industrial Scientific Medical) MHz, MHz, MHz 19

20 DSSS Direct Sequence Spread Spectrum Direct Sequence for each bit, send XOR of that bit and n pseudo-random chips random sequence known to both sender and receiver The random sequence is called n-bit chipping code defines an 11-chip chipping code Data stream: 1010 Random sequence: XOR of the two: The periodicity of the chipping code is in reality exaclty equal to bit duration 20

21 Basic principles of CDMA (Code Division Multiplexing Access) 21

22 Spreading 22

23 Bande frequenziali b Utilizza le frequenze ISM (Industrial, Scientific, Medical) comprese fra 2,4 e 2,483 GHz modulazione DSSS Extremely Low Short-Wave FM AM Radio Broadcast Audio TV Very Low Low Medium High Very High Ultra High TLC 1 KHz 1 MHz 1 GHz Microwave Infrared 1 THz Visible Light 2.4 GHz. ISM (Industrial Scientific, Medical) Band Ultra Violet X-rays Gamma Rays Cosmic Rays 13 canali 3 non si sovrappongono

24 Roaming m. 100m. Roaming: passaggio da un AP a un altro senza perdere la connessione Gli access point possono essere programmati su 3 canali differenti per la migliore copertura 24

25 IEEE Frame Format A Source AP-1 C Dest B Distribution system Trasm AP-2 H D G AP-3 Rec E F Dimensions in bytes 25

26 Frame Structure AP-1 A Source C Dest Distribution system Trasm B AP-3 AP-2 H G Rec F D E Dimensions in bits 32 Frame Control Duration Three types of frames: Management frames Control frames Data frames Address1 Address2 Address3 Sequence Control Address4 Body CRC Management frames are used for station association and dissociation with AP, timing and synchronization, authentication and deauthentication. Control frames are used for handshaking and for positive ACK during the data exchange. Protocol Version Type Subtype To DS From DS More Frags Retry Pwr Mgt More WEP Rsvd Data Protocol Version (0) Type of Frame (Mgt=00, Control=01, Data=10) Subtype example (Type=Control, Subtype=ACK or Type=Mgt, Subtype= Assoc. Request) ToDS=1 indicates frames destined for DS FromDS=1 indicates frames exiting the distribution system More Frags=1 indicates Data Units that have another fragments which follows Retry Field=1 in Data or Mgt frames that are retransmissions of an earlier frame (this helps receiver) Pwr Mgt =1 Indicates that the station is in Power Management Mode 26 More Data=1 indicates to a station that in Power Save mode that more Data Units are buffered for it at the AP WEP=1 indicates that the Frame body contains information that has been processed by a cryptog. algorithm

27 Frame Structure AP-1 A Source C Dimensions in bits Distribution system Dest Trasm AP-3 AP-2 B H G Rec D E F Frame Control Duration Address1 Address2 Address3 Sequence Control Address4 Body CRC The duration/id field usually contains a frame duration value (in µsec) that is used for the MAC. In Control type frames of subtype PS-Poll it contains the ID of the Station The use of the Address Field is the following The sequence control field provide 4 bits to indicate the number of each fragment of a Data Unit which is identified by the other 12 bits 27

28 CSMA/CA: Why not Wireless Ethernet Carrier Sensing Multiple Access with Collisions Avoidance Ethernet uses CSMA/CD Problem: hidden and exposed nodes A B C D 28

29 The hidden stations problem A B C D Hidden terminals A and C cannot hear each other. A sends to B, C cannot receive A. C wants to send to B, C senses a free medium (CS fails) Collision occurs at B. A & C cannot receive the collision (CD fails). A is hidden for C (and vice-versa). Solution? Hidden terminal is peculiar to wireless (not found in wired) Need to sense carrier at receiver, not sender! virtual carrier sensing : Sender asks receiver whether it already hear something. If so, behave as if channel busy. 29

30 The exposed station problem A B C D B transmit to A C wants to transmit to D but find the medium busy: C cannot transmit its information although B does not have coverage of D. C is an exposed station which in this case cannot transmit! (CS fails) 30

31 Medium access control layer Asynchronous data service (DCF) - CSMA/CA - RTS/CTS Time bounded service (PCF) - Polling Inter-frame spacing (IFS) - DIFS - PIFS - SIFS 31

32 MACA: Multiple Access with Collision Avoidance MACA uses signaling packets for collision avoidance RTS (request to send) (20 bytes) sender request the right to send from a receiver with a short RTS packet before it sends a data packet CTS (clear to send) (14 bytes) receiver grants the right to send as soon as it is ready to receive Signaling (RTS/CTS) packets contain sender address receiver address packet size Variants of this method are used in IEEE

33 MACA Solutions MACA avoids the problem of hidden terminals A (first) and C want to send to B A sends RTS to B B sends CTS to A C overhears CTS from B C waits for duration of A s transmission MACA avoids the problem of exposed terminals B wants to send to A, C to D C hears RTS from B->A C does not hear CTS from A C sends RTS to D RTS A B C CTS CTS RTS RTS A B C CTS RTS D D CTS 33

34 Use of RTS/CTS in IEEE WLAN DISF (Distributed Inter Frame Space) SISF (Short Inter Frame Space) NAV (Network Allocation Vector) 34

35 IEEE MAC Protocol: CSMA: sender - if sense channel idle for DISF sec. then transmit entire frame (no collision detection) - if sense channel busy (CS) then binary backoff (explained later) the simplest CSMA/CA DISF (Distributed Inter Frame Space) CSMA receiver: if received OK return ACK after SIFS SISF (Short Inter Frame Space) 35

36 Binary backoff - if sense channel busy then defer transmission: when the channel is sensed IDLE for an amount of time equal to DIFS the station camputes an additional random backoff time and counts down this time while the channel is sensed idle (if sensed busy stop contdown). When the timer expires then transmit. IEEE stations explicitly indicate the length of time that their frame will be transmitting on the channel. This value allows other stations to determine the minimum amount of time (the socalled network allocation vector, NAV) for which they should defer their access 36

37 Backoff in IEEE WLAN The exact numbers are different for different PHY schemes In DSSS PHY (CWmin=31, CWmax=1023) SISF (Short Inter Frame Space) = 10 µsec TIME SLOT= 20 µsec DIFS (Distributed Inter Frame Space) = 50 µsec 37

38 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 avoid hidden station collisions RTS and CTS short: collisions less likely, of shorter duration 38

39 IEEE MAC PROTOCOL (details) Il protocollo MAC della rete Wireless LAN IEEE è specificato in termini di FUNZIONI DI COORDINAMENTO che determinano quando è concessa la trasmissione ad una stazione all interno di una BSS. DCF (Distributed Coordination Function) PCF (Point Coordination Function) 39

40 DCF (Distributed Coordination Function) Quello descritto in precedenza è il meccanismo denominato DCF che rappresenta il metodo base di accesso alla rete, in quanto gestisce la contesa tra i diversi utenti e trasmettere i propri dati. CSMA/CA (Carrier Sense multiple access/ Collision Avoidance) protocol 40

41 DCF SIFS SIFS Station 1 Frame CTS Ack Station 2 SIFS Ack Timer = 3 SIFS Station 3 Station 4 RTS Timer = 5 Frame Station defers, but keeps timer =2 DIFS RTS DIFS Time 41

42 PCF (Point Coordination Function) Tale metodo di accesso è stato definito per poter supportare servizi time-bounded, come la trasmissione di video o voce. I nodi della rete sono coordinati da un PC (Point Coordinator) Ogni stazione può accedere al mezzo solo se viene eletta dal PC Il PC invia un CF-Poll frame alle diverse stazioni 42

43 PCF CP with DCF Contention Free Period Access Point (PC) Beacon CF- Poll CF- Poll CF- Poll CF- End Station 1 PIFS SIFS SIFS Data+ CF-Ack Station 2 Station 3 CF- ACK Sets NAV and doesn t transmit until the end of CFP if it doesn t receive a CF-Poll frame CF- Ack Time 43

44 Considerazioni Il DCF offre un servizio best-effort Il PCF permette la trasmissione di un traffico time-bounded, ma non fornisce comunque nessuna garanzia di qualità del servizio poichè la periodicità dei beacon non è deterministica ma dovuta alla fine delle trasmissioni nel precedente DCF (delayed Beacon) Notevoli e imprevedibili ritardi del beacon frame Incerta durata delle trasmissioni delle stazioni elette dal PC 44