Self-organized mobile networks IEEE Prof. JP Hubaux. Reminder on frequencies and wavelenghts. 1 µm 300 THz. 1 m 300 MHz.

Transcription

1 Self-organized mobile neworks IEEE Prof. JP Hubaux 1 Reminder on frequencies and wavelenghs wised pair coax cable opical ransmission 1 Mm 300 Hz 10 km 30 khz 100 m 3 MHz 1 m 300 MHz 10 mm 30 GHz 100 µm 3 THz 1 µm 300 THz VLF LF MF HF VHF UHF SHF EHF infrared visible ligh UV VLF = Very Low Frequency LF = Low Frequency MF = Medium Frequency HF = High Frequency VHF = Very High Frequency UHF = Ulra High Frequency SHF = Super High Frequency EHF = Exra High Frequency UV = Ulraviole Ligh Frequency and wave lengh: λ = c/f wave lengh λ, speed of ligh c 3x10 8 m/s, frequency f 2

2 Frequencies for mobile communicaion VHF-/UHF-ranges for mobile radio simple, small anenna for cars deerminisic propagaion characerisics, reliable connecions SHF and higher for direced radio links, saellie communicaion small anenna large bandwidh available Wireless LANs use frequencies in UHF o SHF specrum some sysems planned up o EHF limiaions due o absorpion by waer and oxygen molecules (resonance frequencies) weaher dependen fading, signal loss caused by heavy rainfall ec. 3 Frequency allocaion Europe USA Japan Mobile phones Cordless elephones Wireless LANs NMT MHz, MHz; GSM MHz, MHz; MHz, MHz CT MHz, MHz; CT MHz DECT MHz IEEE MHz HIPERLAN MHz AMPS, TDMA, CDMA MHz, MHz; TDMA, CDMA, GSM MHz, MHz; PACS MHz, MHz PACS-UB MHz IEEE MHz PDC MHz, MHz; MHz, MHz PHS MHz JCT MHz IEEE MHz 4

3 Characerisics of wireless LANs Advanages flexibiliy (almos) no wiring difficulies (e.g., hisoric buildings) more robus agains disasers like, e.g., earhquakes, fire - or users pulling a plug... Disadvanages lower birae compared o wired neworks (1-10 Mbi/s) More difficul o secure 5 Design goals for wireless LANs low power no special permissions or licenses needed o use he LAN robus ransmission echnology easy o use for everyone, simple managemen proecion of invesmen in wired neworks (inerneworking) securiy (no one should be able o read my daa), privacy (no one should be able o collec user profiles), safey (low radiaion) ransparency concerning applicaions and higher layer proocols, bu also locaion awareness if necessary 6

4 Comparison: infrared vs. radio ransmission Infrared uses IR diodes Advanages simple, cheap, available in many mobile devices no licenses needed simple shielding possible Disadvanages inerference by sunligh, hea sources ec. many hings shield or absorb IR ligh low bandwidh Example IrDA (Infrared Daa Associaion) inerface available everywhere Radio ypically using he license free ISM band a 2.4 GHz Advanages coverage of larger areas possible (radio can penerae walls, furniure ec.) Disadvanages very limied license free frequency bands shielding more difficul, inerference wih oher elecrical devices more difficul o secure Examples IEEE , Blueooh 7 Infrasrucure vs. ad hoc neworks infrasrucure nework AP: Access Poin AP AP wired nework AP Ad hoc nework 8

5 STA 1 ESS LAN BSS 1 IEEE Archiecure of an infrasrucure nework Access Poin BSS 2 Poral Disribuion Sysem Access Poin 802.x LAN STA LAN STA 3 Saion (STA) erminal wih access mechanisms o he wireless medium and radio conac o he access poin Basic Service Se (BSS) group of saions using he same radio frequency Access Poin saion inegraed ino he wireless LAN and he disribuion sysem Poral bridge o oher (wired) neworks Disribuion Sysem inerconnecion nework o form one logical nework (ESS: Exended Service Se) based on several BSS Archiecure of an ad-hoc nework LAN STA 1 BSS 1 STA 2 STA 3 Direc communicaion wihin a limied range Saion (STA): erminal wih access mechanisms o he wireless medium Basic Service Se (BSS): group of saions using he same radio frequency LAN BSS 2 STA 5 STA 4 10

6 Inerconnecion of IEEE wih Eherne mobile saion fixed erminal server infrasrucure nework access poin applicaion applicaion TCP TCP IP IP MAC MAC MAC MAC PHY PHY PHY PHY Layers and funcions MAC access mechanisms, fragmenaion, encrypion MAC Managemen synchronizaion, roaming, MIB, power managemen PLCP (Physical Layer Convergence Proocol) clear channel assessmen signal (carrier sense) PMD (Physical Medium Dependen) modulaion, coding PHY Managemen channel selecion, MIB Saion Managemen coordinaion of all managemen funcions PHY IP MAC PLCP PMD MAC Managemen PHY Managemen Saion Managemen 12

7 Physical layer 3 versions: 2 radio: DSSS and FHSS (boh ypically a 2.4 GHz), 1 IR daa raes 1, 2, 5 or 11 Mbi/s DSSS (Direc Sequence Spread Specrum) DBPSK modulaion (Differenial Binary Phase Shif Keying) or DQPSK (Differenial Quadraure PSK) chipping sequence: +1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1 (Barker code) max. radiaed power 1 W (USA), 100 mw (EU), min. 1mW FHSS (Frequency Hopping Spread Specrum) spreading, despreading, signal srengh min. 2.5 frequency hops/s, wo-level GFSK modulaion (Gaussian Frequency Shif Keying) Infrared nm, diffuse ligh, around 10 m range carrier deecion, energy deecion, synchronizaion MAC layer principles (1/2) Traffic services Asynchronous Daa Service (mandaory) exchange of daa packes based on bes-effor suppor of broadcas and mulicas Time-Bounded Service (opional) implemened using PCF (Poin Coordinaion Funcion) Access mehods (called DFWMAC: Disribued Foundaion Wireless MAC) DCF CSMA/CA (mandaory) collision avoidance via randomized back-off mechanism minimum disance beween consecuive packes ACK packe for acknowledgemens (no for broadcass) DCF wih RTS/CTS (opional) avoids hidden erminal problem PCF (opional) access poin polls erminals according o a lis DCF: Disribued Coordinaion Funcion PCF: Poin Coordinaion Funcion 14

8 Prioriies MAC layer principles (2/2) defined hrough differen iner frame spaces no guaraneed, hard prioriies (Shor Iner Frame Spacing) highes prioriy, for ACK, CTS, polling response PIFS (PCF IFS) medium prioriy, for ime-bounded service using PCF (DCF, Disribued Coordinaion Funcion IFS) lowes prioriy, for asynchronous daa service medium busy PIFS conenion nex frame direc access if medium is free ime slo Noe Noe :: IFS IFS duraions duraions are are specific specific o o each each PHY PHY CSMA/CA principles conenion window (randomized back-off mechanism) medium busy direc access if medium has been free for a leas ime slo nex frame saion ready o send sars sensing he medium (Carrier Sense based on CCA, Clear Channel Assessmen) if he medium is free for he duraion of an Iner-Frame Space (IFS), he saion can sar sending (IFS depends on service ype) if he medium is busy, he saion has o wai for a free IFS, hen he saion mus addiionally wai a random back-off ime (collision avoidance, muliple of slo-ime) if anoher saion occupies he medium during he back-off ime of he saion, he back-off imer sops (o increase fairness) 16

9 saion CSMA/CA broadcas bo e bo r = bo e bo r bo e busy saion 2 bo e busy saion 3 busy bo e busy (deecion by upper layer) saion 4 bo e bo r bo e busy (deecion by upper layer) saion 5 Here S4 and S5 happen o have he same back-off ime busy medium no idle (frame, ack ec.) bo e elapsed backoff ime packe arrival a MAC bo r residual backoff ime The The size size of of he he conenion conenion window window can can be be adaped adaped (if (if more more collisions, collisions, hen hen increase increase he he size) size) Noe: Noe: broadcas broadcas is is no no acknowledged acknowledged 17 Sending unicas packes CSMA/CA unicas saion has o wai for before sending daa receiver acknowledges a once (afer waiing for ) if he packe was received correcly (CRC) auomaic reransmission of daa packes in case of ransmission errors sender receiver oher saions daa waiing ime ACK Conenion window daa The The ACK ACK is is sen sen righ righ a a he he end end of of (no (no conenion) conenion) 18

10 Sending unicas packes DCF wih RTS/CTS saion can send RTS wih reservaion parameer afer waiing for (reservaion deermines amoun of ime he daa packe needs he medium) acknowledgemen via CTS afer by receiver (if ready o receive) sender can now send daa a once, acknowledgemen via ACK oher saions sore medium reservaions disribued via RTS and CTS sender receiver RTS CTS daa ACK oher saions NAV: NAV: Ne Ne Allocaion Allocaion Vecor Vecor NAV (RTS) NAV (CTS) defer access Conenion window daa RTS/CTS RTS/CTS can can be be presen presen for for some some packes packes and and no no for for oher oher 19 Fragmenaion mode sender receiver RTS CTS frag 1 ACK 1 frag 2 ACK2 oher saions NAV (RTS) NAV (CTS) NAV (frag 1 ) NAV (ACK 1 ) conenion daa Fragmenaion is used in case he size of he packes sen has o be reduced (e.g., o diminish he probabiliy of erroneous frames) Each frag i (excep he las one) also conains a duraion (as RTS does), which deermines he duraion of he NAV By his mechanism, fragmens are sen in a row In his example, here are only 2 fragmens 20

11 Poin Coordinaion Funcion (1/2) 0 1 SuperFrame medium busy poin coordinaor PIFS D 1 D 2 wireless saions U 1 U 2 saions NAV NAV conenion free period Purpose: provide a ime-bounded service No usable for ad hoc neworks D i represens he polling of saion i U i represens ransmission of daa from saion i Poin Coordinaion Funcion (2/2) poin coordinaor D 3 PIFS D 4 CFend wireless saions U 4 saions NAV NAV conenion free period conenion period In his example, saion 3 has no daa o send 22

12 MAC frame forma Types conrol frames, managemen frames, daa frames Sequence numbers imporan agains duplicaed frames due o los ACKs Addresses receiver, ransmier (physical), BSS idenifier, sender (logical) Miscellaneous sending ime, checksum, frame conrol, daa byes Duraion ID Address 1 Address 2 Address 3 Sequence Conrol Frame Conrol Address Daa 4 CRC version, ype, fragmenaion, securiy,... deecion of duplicaion 23 MAC address forma scenario o DS from address 1 address 2 address 3 address 4 DS ad-hoc nework 0 0 DA SA BSSID - infrasrucure 0 1 DA BSSID SA - nework, from AP infrasrucure 1 0 BSSID SA DA - nework, o AP infrasrucure nework, wihin DS 1 1 RA TA DA SA DS: Disribuion Sysem AP: Access Poin DA: Desinaion Address SA: Source Address BSSID: Basic Service Se Idenifier - infrasrucure BSS : MAC address of he Access Poin - ad hoc BSS (IBSS): random number RA: Receiver Address TA: Transmier Address 24

13 MAC managemen Synchronizaion Purpose for he physical layer (e.g., mainaining in sync he frequency hop sequence in he case of FHSS) for power managemen Principle: beacons wih ime samps Power managemen sleep-mode wihou missing a message periodic sleep, frame buffering, raffic measuremens Associaion/Reassociaion inegraion ino a LAN roaming, i.e. change neworks by changing access poins scanning, i.e. acive search for a nework MIB - Managemen Informaion Base managing, read, wrie 25 Synchronizaion (infrasrucure case) beacon inerval access poin medium B B B B busy busy busy busy value of he imesamp B beacon frame The access poin ransmis he (quasi) periodic beacon signal The beacon conains a imesamp and oher managemen informaion used for power managemen and roaming All oher wireless nodes adjus heir local imers o he imesamp 26

14 Synchronizaion (ad-hoc case) beacon inerval saion 1 B 1 B 1 saion 2 B 2 B 2 medium busy busy busy busy value of he imesamp B beacon frame random delay Each node mainains is own synchronizaion imer and sars he ransmission of a beacon frame afer he beacon inerval Conenion back-off mechanism only 1 beacon wins All oher saions adjus heir inernal clock according o he received beacon and suppress heir beacon for he curren cycle 27 Power managemen Idea: swich he ransceiver off if no needed Saes of a saion: sleep and awake Timing Synchronizaion Funcion (TSF) saions wake up a he same ime Infrasrucure case Traffic Indicaion Map (TIM) lis of unicas receivers ransmied by AP Delivery Traffic Indicaion Map (DTIM) lis of broadcas/mulicas receivers ransmied by AP Ad-hoc case Ad-hoc Traffic Indicaion Map (ATIM) announcemen of receivers by saions buffering frames more complicaed - no cenral AP collision of ATIMs possible (scalabiliy?) 28

15 Power saving (infrasrucure case) Here he access poin announces daa addressed o he saion TIM inerval DTIM inerval access poin medium D B busy T T d D busy busy busy B saion T TIM D DTIM p awake d B broadcas/mulicas d daa ransmission o/from he saion p Power Saving poll: I am awake, please send he daa 29 Power saving (ad-hoc case) ATIM window beacon inerval B saion 1 A D B 1 1 saion 2 B 2 B 2 a d B beacon frame random delay A ransmi ATIM D ransmi daa awake a acknowledge ATIM d acknowledge daa ATIM: Ad hoc Traffic Indicaion Map (a saion announces he lis of buffered frames) Poenial problem: scalabiliy (high number of collisions) 30

16 Roaming No or bad connecion? Then perform: Scanning scan he environmen, i.e., lisen ino he medium for beacon signals or send probes ino he medium and wai for an answer Reassociaion Reques saion sends a reques o one or several AP(s) Reassociaion Response success: AP has answered, saion can now paricipae failure: coninue scanning AP acceps Reassociaion Reques signal he new saion o he disribuion sysem he disribuion sysem updaes is daa base (i.e., locaion informaion) ypically, he disribuion sysem now informs he old AP so i can release resources 31 Securiy of WEP: Wired Equivalen Privacy Objecives: Confidenialiy Access conrol Daa inegriy Inegriy checksum M C(M) IV k RC4 IV k RC4 P = M C(M) P = M C(M) Noe: several securiy weaknesses have been idenified 32

17 The new soluion for securiy: sandard 802.1x EAPOL (over Eherne or ) Encapsulaed EAP, Typically on RADIUS Supplican Auhenicaor Auhenicaion Server EAP: Exensible Auhenicaion Proocol (RFC 2284, 1998) EAPOL: EAP over LAN RADIUS: Remoe auhenicaion dial in user service (RFC 2138, 1997) Feaures: - Suppors a wide range of auhenicaion schemes, hanks o he usage of EAP - One-way auhenicaion - Opional encrypion and daa inegriy 33 More on IEEE 802.1x Example of auhenicaion, using one-ime passwords (OTP): Supplican Auhenicaor Auhenicaion server EAP-reques/ideniy EAP-response/ideniy (MYID) EAP-response/OTP, OTPpassword Auhenicaion successfully compleed Por auhorized EAP-reques/OTP, OTP challenge EAP-success : exchange of EAPOL frame : exchange of EAP frames in a higher layer proocol (e.g., RADIUS) Noes Noes :: Weaknesses Weaknesses have have been been found found in in 802.1x 802.1x as as well, well, bu bu are are correced correced in in he he various various implemenaions. implemenaions New New sandard sandard in in he he making making :: IEEE IEEE i i 34

18 Fuure developmens IEEE b 2.4 GHz band Biraes 1 11 Mbi/s IEEE a 5 GHz band compaible MAC ransmission raes up o 54 Mbi/s close cooperaion wih BRAN (ETSI Broadband Radio Access Nework) 35 ETSI - HIPERLAN ETSI sandard European sandard, cf. GSM, DECT,... Enhancemen of local Neworks and inerworking wih fixed neworks inegraion of ime-sensiive services from he early beginning HIPERLAN family HIPERLAN 1 sandardized since 1996 HIPERLAN 2 under sandardizaion Very uncerain fuure: no producs available so far 36

19 IEEE Very widespread Conclusion of Wireless LANs Ofen considered as he sysem underlying larger scale ad hoc neworks (alhough far from opimal, no designed for his purpose) Tremendous poenial as a compeior of 3G cellular neworks in ho spos Hiperlan Hiperlan I probably dead, Hiperlan II in jeopardy Too ambiious sandard? Blueooh Producs available No as successful as iniially hough Securiy perceived as a major obsacle; iniial soluions were flawed in boh IEEE (WEP) and Blueooh Fuure developmens Ulra Wide Band? 37 References J. Schiller: Mobile Communicaions, Addison-Wesley, 2000 Leon-Garcia & Widjaja: Communicaion Neworks, McGrawHill, 2000 IEEE and IEEE 802.1x sandards, available a J. Edney and W. Arbaugh: Real Securiy, Addison-Wesley,