Wireless

transcript

Corso di Studi in Corso di Studi in Ing.Ing. delle Telecomunicazionidelle Telecomunicazioni

LAUREA in INGEGNERIA DELLE TELECOMUNICAZIONILAUREA in INGEGNERIA DELLE TELECOMUNICAZIONI

RETI DI TELECOMUNICAZIONIRETI DI TELECOMUNICAZIONI

Stefano GiordanoStefano Giordano

Lezione nLezione n.16.16

““WirelessWireless LAN IEEE 802.11LAN IEEE 802.11””

Gruppo di Ricerca in Reti di TelecomunicazioniDipartimento di Ingegneria della Informazione:

Elettronica, Informatica, Telecomunicazioni

Wireless LANsWireless LANs

• IEEE 802.11• 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 generation54 Mbit/s on its third generation108 Mbit/s on its fourth generation

• Physical Media– spread spectrum radio (2.4GHz or 5 Ghz)– diffused infrared (10m)

Evoluzione di Tecnologia e degli Standard

Nuovi servizi di reteaggiunti (QoS, IAPP,

WEP2, etc.)

Prodotti Bluetoothdisponibili (802.15)

Specifica802.11 orig.ratificata da

• 802.11a e 802.11b ratific. da IEEE

• Fond. WECA

20032002

• 802.11a5Ghz (fino a 54 Mpbs)

• 802.11g2,4Ghz (fino a 54 Mpbs)EUROPA

2004 ?

• 802.11h802. 11i802.11f

Courtesy of 3COM Italia

Panoramica dei Sistemi 802.11

bassa velocitàRichiede modifica hwMinore area di

copertura

Svantaggi

• Compatibile con 802.11b

• Alta velocità ed elevatacopertura

Compatibilitàcertificata Wi-Fi Il sistema oggi più

largamente sviluppato; prezzi convenienti

Minori possibilità diinterferenze con le

frequenze radioBuon supporto x applic. multimediali e ambienti

con alto n° di utenti

Vantaggi

Fino a 100 MetriFino a 100 MetriFino a 50 MetriCoperturaFino a 54MbpsFino a 11MbpsFino a 54MbpsVelocità

2.4GHz / OFDM2.4GHz / DSSS5GHz / OFDMBanda /Modulaz.

200319992002Ratifica Standard

802.11g802.11b802.11a

Si noti che: 22 Mbps è una soluzione PROPRIETARIA di alcuni produttori

802.11a802.11a

802.11b/g vs. 802.11a100 M

50 Metri

11/54 Mbps

54 Mbps

Maggior Consumo Elettrico

802.11b/g802.11b/g

A proposito di TURBO...

2733 35

802.11b 802.11g 802.11a turbo 72 802.11a turbo 108

teoria pratica

WLAN Standard in corso di definizione• 802.11d

– Definisce i requisiti del livello fisico che soddisfano le richieste di quei paesi chenon hanno ancora legiferato in merito a 802.11 WLANS

• 802.11e– Opera su 802.11MAC per incrementare la QoS ed assicurare il supporto alle

applicazioni audio/video• 802.11f

– Migliore la gestione del roaming per mantenere stabile una connessione WLAN tra apparati diversi che afferiscono a segmenti di rete diversi (IAPP)

• 802.11h– Incrementa il controllo sulla potenza trasmissiva e il suo dosaggio sui canali

adibiti a portare 802.11a. Questo standard è espressamente dedicato a soddisfarele obiezioni avanzate da ETSI in merito alle interferenze su canali RADAR.

• 802.11i– 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

• 802.11j– Si preoccupa di armonizzare due standard diversi come 802.11a e HiperLAN2

(sviluppato totalmente in Europa sotto ETSI) in modo da renderli coesistenti.

Wi-Fi assicura Interoperabilità

• ¿Wireless Ethernet? Compatibility Alliance• Oltre 50 membri tra cui: 3Com, Symbol,

Enterasys, Avaya, Agere, Cisco• Scopo: Assicurare la interoperabilità delle soluzioni

basate su 802.11 • 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

1Mbps1Mbps

2Mbps2Mbps

• Tutti i vendor dichiarano “coperture con raggiodi 100 metri negli uffici (open space), oppure di300 metri all’aria aperta e senza ostacoli

• Nella vita REALE:– 30 - 50 mt. < 5,5Mbps (3Mbps) – 12 – 25 mt. 11 Mbps teorici (6Mbps)

• Fattori attenuanti della portata:– Problemi temporanei

• Microonde, impianti e condizionatori

• Notebook in movimento– Consistenza dei Muri– Orientamento dell’antenna

5.5Mbps5.5Mbps

11Mbps

PORTATACourtesy of 3COM Italia

802.11 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 802.11bTGc 802.11 bridging (802.1)TGd Operation in new regulatory domains, roamingTGe QoS (previously also security, authentication)TGf Inter-AP protocol, interoperabilityTGg Higher data rates for 802.11b, > 20 Mb/s (≤ 54 Mbps)TGh Enhance MAC and 802.11a PHY (for CEPT approval)TGi Enhance MAC for security and authenticationTGj 802.11 and 802.11a PHY 5 GHz operation in JapanTGk Radio resource measurements (for higher layers)TGl -TGm 802.11 standard corrections maintenanceTGn High throughput PHY

IEEE 802.11

Infrastructure network

Ad-hoc network

wired network

AP: Access Point

Rete Wireless 802.11 collegata a InternetSingolo Access Point

NotebookWireless LAN PC Cards

ISP Provided Modem/Gateway

Access Point

IEEE 802.11 Wireless LAN• wireless LANs: untethered (i.e. mobile: better NOMADIC)

networking• IEEE 802.11 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

Ad Hoc Networks

• Ad hoc network: IEEE 802.11 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

IEEE 802.11a

• High rate PHY, 6 - 54 Mb/s

• 5 GHz UNII-band

• OFDM (Orthogonal frequency division multiplexing)

• Almost same PHY as HiperLAN2

• Approved 1999

IEEE 802.11b

• 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 1999

• Higher rate PHY for 2.4 GHz ISM-band• > 20 Mb/s (max 54 Mb/s)• Backward compatibility with 802.11b (through CCK and RTS/CTS)• CCK and OFDM mandatory• Other optional modulation schemes

IEEE 802.11g

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

– 802.11 uses 79 x 1 MHz-wide frequency bands

2.4 GHz ISM Band (Industrial Scientific Medical)902÷928 MHz, 2400÷2483.5 MHz, 5725÷5850 MHz

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 – 802.11 defines an “11-chip” chipping code

Random sequence: 0100101101011001

Data stream: 1010

XOR of the two: 1011101110101001

The periodicity of the chipping code is in reality exaclty equal to bit duration

Basic principles of CDMA (Code Division Multiplexing Access)

Spreading

Bande frequenziali 802.11b

2.422 2.432 2.442 2.452 2.462 2.472 2.4842.412

2.417 2.427 2.437 2.447 2.457 2.467

6 1112

1 KHz 1 MHz 1 GHz

Short-Wave RadioAM

Broadcast

TV TLCTLC

ExtremelyLow

Very Low

Medium

Very H

Ultra H

InfraredV

isible Light

Ultra V

X-rays

ic Rays

Microwave

2.4 GHz. ISM (Industrial Scientific, Medical) Band

13 canali3 non si

sovrappongono

• Utilizza le frequenze ISM (Industrial, Scientific, Medical) comprese fra2,4 e 2,483 GHz modulazioneDSSS

Roaming

• Roaming: passaggio da un AP a un altro senza perdere la connessione

• Gli access point possono essere programmati su 3 canalidifferenti per la migliore copertura

IEEE 802.11 Frame Format B

AP-2AP-3AP-1

Distribution system

Source

Dest Trasm

Dimensions in bytes

Frame Structure

CRCBody

320-18496

DurationFrameControl Address1 Address2 Address3 Sequence

Control Address4

16 16 48 48 48 16 48Dimensions in bits

Three types of frames:• Management frames• Control frames• Data frames

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.

FromDS

MoreFrags Retry Pwr

MgtMoreData WEP RsvdProtocol

Version Type Subtype

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 DSFromDS=1 indicates frames exiting the distribution systemMore Frags=1 indicates Data Units that have another fragments which followsRetry 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 ModeMore Data=1 indicates to a station that in Power Save mode that more Data Units are buffered for it at the APWEP=1 indicates that the Frame body contains information that has been processed by a cryptog. algorithm

B HA FG

AP-2 AP-3AP-1

Distribution system

Source

Dest Trasm

Frame Structure

CRCBody

320-18496

DurationFrameControl Address1 Address2 Address3 Sequence

Control Address4

16 16 48 48 48 16 48Dimensions in bits

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

B HA FG

AP-2 AP-3AP-1

Distribution system

Source

Dest Trasm

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

CSMA/CA: Why not Wireless EthernetCarrier Sensing Multiple Access with

Collisions Avoidance

• Ethernet uses CSMA/CD• Problem: hidden and exposed nodes

A B C D

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.

The exposed station problem

A B C D

B transmit to AC 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)

Medium access control layer

• Asynchronous data service (DCF)- CSMA/CA- RTS/CTS

• Time bounded service (PCF)- Polling

• Inter-frame spacing (IFS)- DIFS- PIFS- SIFS

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 802.11

• 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

MACA Solutions

A B CRTS

CTSCTS

A B CRTS

Use of RTS/CTS in IEEE 802.11 WLAN

DISF (Distributed Inter Frame Space)SISF (Short Inter Frame Space)

NAV (Network Allocation Vector)

IEEE 802.11 MAC Protocol: the simplest CSMA/CA

802.11 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)

802.11 CSMA receiver:if received OK

return ACK after SIFS

DISF (Distributed Inter Frame Space)

SISF (Short Inter Frame Space)

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 randombackoff time and counts down this time while the channelis sensed idle (if sensed busy stop contdown).

• When the timer expires then transmit.

IEEE 802.11 stations explicitly indicate the length of time thattheir frame will be transmitting on the channel. This value allowsother stations to determine the minimum amount of time (the so-called network allocation vector, NAV) for which they shoulddefer their access

Backoff in IEEE 802.11 WLAN

The exact numbers are different for

different PHY schemes

In DSSS PHY(CWmin=31, CWmax=1023)

SISF (Short Inter FrameSpace) = 10 µsec

TIME SLOT= 20 µsec

DIFS (Distributed InterFrame Space) = 50 µsec

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

IEEE 802.11 MAC PROTOCOL(details)

Il protocollo MAC della rete Wireless LAN IEEE 802.11 èspecificato 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)

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/ CollisionAvoidance) protocol

Station 1

Station 4

Station 3

Station 2

SIFS Ack

Timer = 3

Timer = 5Station defers, but keeps timer =2 RTS

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

Access Point(PC)

Station 3

Station 2

Station 1

Contention Free PeriodCP with DCF

Beacon

SIFSSIFS Data+

CF-Ack

CF- Poll

CF- ACK

Sets NAV and doesn’t transmit until the end of CFPif it doesn’t receive a CF-Poll frame

CF- Poll CF- Poll

CF- Ack

CF- End

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 èdeterministica ma dovuta alla fine delle trasmissioni nel precedente DCF (delayed Beacon)

Notevoli e imprevedibili ritardi del beacon frame

Incerta durata delletrasmissioni delle stazioni elette dal PC

Wireless

Business