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Module B WLAN – Engineering Aspects

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  1. Mobile Networks Module BWLAN – Engineering Aspects Prof. JP Hubaux

  2. Reminder on frequencies and wavelenghts twisted pair • VLF = Very Low Frequency UHF = Ultra High Frequency • LF = Low Frequency SHF = Super High Frequency • MF = Medium Frequency EHF = Extra High Frequency • HF = High Frequency UV = Ultraviolet Light • VHF = Very High Frequency • Frequency and wave length: •  = c/f • wave length , speed of light c  3x108m/s, frequency f coax cable optical transmission 1 Mm 300 Hz 10 km 30 kHz 100 m 3 MHz 1 m 300 MHz 10 mm 30 GHz 100 m 3 THz 1 m 300 THz VLF LF MF HF VHF UHF SHF EHF infrared UV visible light

  3. Frequencies for mobile communication • VHF-/UHF-ranges for mobile radio • simple, small antenna for cars • deterministic propagation characteristics, reliable connections • SHF and higher for directed radio links, satellite communication • small antenna • large bandwidth available • Wireless LANs use frequencies in UHF to SHF spectrum • some systems planned up to EHF • limitations due to absorption by water and oxygen molecules (resonance frequencies) • weather dependent fading, signal loss caused by heavy rainfall etc.

  4. Frequency allocation

  5. Characteristics of wireless LANs • Advantages • flexibility • (almost) no wiring difficulties (e.g., historic buildings) • more robust against disasters like, e.g., earthquakes, fire - or users pulling a plug... • Disadvantages • lower bitrate compared to wired networks (1-50 Mbit/s) • More difficult to secure

  6. Design goals for wireless LANs • low power • no special permissions or licenses needed to use the LAN • robust transmission technology • easy to use for everyone, simple management • protection of investment in wired networks (internetworking) • security (no one should be able to read my data), privacy (no one should be able to collect user profiles), safety (low radiation) • transparency concerning applications and higher layer protocols, but also location awareness if necessary

  7. Infrared uses IR diodes Advantages simple, cheap, available in many mobile devices no licenses needed simple shielding possible Disadvantages interference by sunlight, heat sources etc. many things shield or absorb IR light low bandwidth Example IrDA (Infrared Data Association) interface used to be available on many devices Radio typically using the license free ISM band at 2.4 GHz Advantages coverage of larger areas possible (radio can penetrate walls, furniture etc.) Disadvantages very limited license free frequency bands shielding more difficult, interference with other electrical devices more difficult to secure Examples IEEE 802.11, Bluetooth Comparison: infrared vs. radio transmission

  8. Infrastructure vs. ad hoc networks infrastructure network AP: Access Point AP AP wired network AP Ad hoc network

  9. Portal Distribution System IEEE 802.11 - Architecture of an infrastructure network • Station (STA) • terminal with access mechanisms to the wireless medium and radio contact to the access point • Basic Service Set (BSS) • group of stations using the same radio frequency • Access Point • station integrated into the wireless LAN and the distribution system • Portal • bridge to other (wired) networks • Distribution System • interconnection network to form one logical network (ESS: Extended Service Set) based on several BSS 802.11 LAN 802.x LAN STA1 BSS1 Access Point Access Point ESS BSS2 STA2 STA3 802.11 LAN

  10. 802.11 - Architecture of an ad-hoc network • Direct communication within a limited range • Station (STA):terminal with access mechanisms to the wireless medium • Basic Service Set (BSS):group of stations using the same radio frequency 802.11 LAN STA3 STA1 BSS1 STA2 802.11 LAN BSS2 STA5 STA4

  11. Interconnection of IEEE 802.11 with Ethernet fixed terminal mobile station server infrastructure network access point application application TCP TCP IP IP 802.11 MAC 802.11 MAC 802.3 MAC 802.3 MAC 802.11 PHY 802.11 PHY 802.3 PHY 802.3 PHY

  12. PLCP (Physical Layer Convergence Protocol) clear channel assessment signal (carrier sense) PMD (Physical Medium Dependent) modulation, coding PHY Management channel selection, MIB Station Management coordination of all management functions MAC access mechanisms, fragmentation, encryption MAC Management synchronization, roaming, MIB, power management 802.11 - Layers and functions Station Management IP MAC MAC Management PLCP PHY Management PHY PMD

  13. 802.11 - Physical layer • 3 versions: 2 radio: DSSS and FHSS (both typically at 2.4 GHz), 1 IR • data rates 1, 2, 5 or 11 Mbit/s • DSSS (Direct Sequence Spread Spectrum) • DBPSK modulation (Differential Binary Phase Shift Keying) or DQPSK (Differential Quadrature PSK) • chipping sequence: +1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1 (Barker code) • max. radiated power 1 W (USA), 100 mW (EU), min. 1mW • FHSS (Frequency Hopping Spread Spectrum) • spreading, despreading, signal strength • min. 2.5 frequency hops/s, two-level GFSK modulation (Gaussian Frequency Shift Keying) • Infrared • 850-950 nm, diffuse light, around 10 m range • carrier detection, energy detection, synchronization

  14. 802.11 - MAC layer principles (1/2) • Traffic services • Asynchronous Data Service (mandatory) • exchange of data packets based on “best-effort” • support of broadcast and multicast • Time-Bounded Service (optional) • implemented using PCF (Point Coordination Function) • Access methods (called DFWMAC: Distributed Foundation Wireless MAC) • DCF CSMA/CA (mandatory) • collision avoidance via randomized „back-off“ mechanism • minimum distance between consecutive packets • ACK packet for acknowledgements (not for broadcasts) • DCF with RTS/CTS (optional) • avoids hidden terminal problem • PCF (optional) • access point polls terminals according to a list • DCF: Distributed Coordination Function • PCF: Point Coordination Function

  15. 802.11 - MAC layer principles (2/2) • Priorities • defined through different inter frame spaces • no guaranteed, hard priorities • SIFS (Short Inter Frame Spacing) • highest priority, for ACK, CTS, polling response • PIFS (PCF IFS) • medium priority, for time-bounded service using PCF • DIFS (DCF, Distributed Coordination Function IFS) • lowest priority, for asynchronous data service DIFS DIFS PIFS SIFS medium busy contention next frame t direct access if medium is free  DIFS time slot Note : IFS durations are specific to each PHY

  16. 802.11 - CSMA/CA principles contention window (randomized back-offmechanism) • station ready to send starts sensing the medium (Carrier Sense based on CCA, Clear Channel Assessment) • if the medium is free for the duration of an Inter-Frame Space (IFS), the station can start sending (IFS depends on service type) • if the medium is busy, the station has to wait for a free IFS, then the station must additionally wait a random back-off time (collision avoidance, multiple of slot-time) • if another station occupies the medium during the back-off time of the station, the back-off timer stops (to increase fairness) DIFS DIFS medium busy next frame t direct access if medium has been free for at least DIFS time slot

  17. = 802.11 – CSMA/CA broadcast DIFS DIFS DIFS DIFS boe bor boe bor boe busy station1 boe busy station2 busy station3 (detection by upper layer) boe busy station4 boe bor boe busy (detection by upper layer) station5 t Here St4 and St5 happen to havethe same back-off time medium not idle (frame, ack etc.) busy boe elapsed backoff time packet arrival at MAC bor residual backoff time The size of the contention window can be adapted (if more collisions, then increase the size) Note: broadcast is not acknowledged

  18. 802.11 - CSMA/CA unicast • Sending unicast packets • station has to wait for DIFS before sending data • receiver acknowledges at once (after waiting for SIFS) if the packet was received correctly (CRC) • automatic retransmission of data packets in case of transmission errors DIFS data sender SIFS ACK receiver DIFS data other stations t waiting time Contentionwindow The ACK is sent right at the end of SIFS(no contention)

  19. 802.11 – DCF with RTS/CTS • Sending unicast packets • station can send RTS with reservation parameter after waiting for DIFS (reservation determines amount of time the data packet needs the medium) • acknowledgement via CTS after SIFS by receiver (if ready to receive) • sender can now send data at once, acknowledgement via ACK • other stations store medium reservations distributed via RTS and CTS DIFS RTS data sender SIFS SIFS SIFS CTS ACK receiver DIFS NAV (RTS) data other stations NAV (CTS) t defer access Contentionwindow RTS/CTS can be present forsome packets and not for other NAV: Net Allocation Vector

  20. Fragmentation mode DIFS RTS frag1 frag2 sender SIFS SIFS SIFS SIFS SIFS CTS ACK1 ACK2 receiver NAV (RTS) NAV (CTS) DIFS NAV (frag1) data other stations NAV (ACK1) t contention • Fragmentation is used in case the size of the packets sent has to be reduced (e.g., to diminish the probability of erroneous frames) • Each fragi (except the last one) also contains a duration (as RTS does), which determines the duration of the NAV • By this mechanism, fragments are sent in a row • In this example, there are only 2 fragments

  21. 802.11 – Point Coordination Function (1/2) t0 t1 SuperFrame medium busy PIFS SIFS SIFS D1 D2 point coordinator SIFS SIFS U1 U2 wireless stations stations‘ NAV NAV contention free period • Purpose: provide a time-bounded service • Not usable for ad hoc networks • Di represents the polling of station i • Ui represents transmission of data from station i

  22. 802.11 – Point Coordination Function (2/2) t2 t3 t4 PIFS SIFS D3 D4 CFend point coordinator SIFS U4 wireless stations stations‘ NAV NAV contention free period t contention period • In this example, station 3 has no data to send

  23. 802.11 - MAC frame format • Types • control frames, management frames, data frames • Sequence numbers • important against duplicated frames due to lost ACKs • Addresses • receiver, transmitter (physical), BSS identifier, sender (logical) • Miscellaneous • sending time, checksum, frame control, data bytes 2 2 6 6 6 2 6 0-2312 4 Frame Control Duration ID Address 1 Address 2 Address 3 Sequence Control Address 4 Data CRC version, type, fragmentation, security, ... detection of duplication

  24. MAC address format DS: Distribution System AP: Access Point DA: Destination Address SA: Source Address BSSID: Basic Service Set Identifier - infrastructure BSS : MAC address of the Access Point - ad hoc BSS (IBSS): random number RA: Receiver Address TA: Transmitter Address

  25. 802.11 - MAC management • Synchronization • Purpose • for the physical layer (e.g., maintaining in sync the frequency hop sequence in the case of FHSS) • for power management • Principle: beacons with time stamps • Power management • sleep-mode without missing a message • periodic sleep, frame buffering, traffic measurements • Association/Reassociation • integration into a LAN • roaming, i.e. change networks by changing access points • scanning, i.e. active search for a network • MIB - Management Information Base • managing, read, write

  26. Synchronization (infrastructure case) beacon interval B B B B access point busy busy busy busy medium t B value of the timestamp beacon frame • The access point transmits the (quasi) periodic beacon signal • The beacon contains a timestamp and other management information used for power management and roaming • All other wireless nodes adjust their local timers to the timestamp

  27. Synchronization (ad-hoc case) beacon interval B1 B1 station1 B2 B2 station2 busy busy busy busy medium t B value of the timestamp beacon frame random delay • Each node maintains its own synchronization timer and starts the transmission of a beacon frame after the beacon interval • Contention  back-off mechanism  only 1 beacon wins • All other stations adjust their internal clock according to the received beacon and suppress their beacon for the current cycle

  28. Power management • Idea: switch the transceiver off if not needed • States of a station: sleep and awake • Timing Synchronization Function (TSF) • stations wake up at the same time • Infrastructure case • Traffic Indication Map (TIM) • list of unicast receivers transmitted by AP • Delivery Traffic Indication Map (DTIM) • list of broadcast/multicast receivers transmitted by AP • Ad-hoc case • Ad-hoc Traffic Indication Map (ATIM) • announcement of receivers by stations buffering frames • more complicated - no central AP • collision of ATIMs possible (scalability?)

  29. T D awake TIM DTIM data transmission to/from the station B d broadcast/multicast Power saving (infrastructure case) Here the access point announcesdata addressed to the station TIM interval DTIM interval D B T T d D B access point busy busy busy busy medium p d station t p Power Saving poll: I am awake, please send the data

  30. A transmit ATIM Power saving (ad-hoc case) ATIM window beacon interval B1 A D B1 station1 B2 B2 a d station2 t B D beacon frame random delay transmit data a d awake acknowledge ATIM acknowledge data • ATIM: Ad hoc Traffic Indication Map (a station announces the list of buffered frames) • Potential problem: scalability (high number of collisions)

  31. 802.11 - Roaming • No or bad connection? Then perform: • Scanning • scan the environment, i.e., listen into the medium for beacon signals or send probes into the medium and wait for an answer • Reassociation Request • station sends a request to one or several AP(s) • Reassociation Response • success: AP has answered, station can now participate • failure: continue scanning • AP accepts Reassociation Request • signal the new station to the distribution system • the distribution system updates its data base (i.e., location information) • typically, the distribution system now informs the old AP so it can release resources

  32. Security of 802.11 • WEP: Wired Equivalent Privacy • Objectives: • Confidentiality • Access control • Data integrity k k M Integritychecksum RC4 IV RC4 IV C(M) P = M C(M) P = M C(M) Note: several security weaknesses have been identified and WEP should not be used anymore.

  33. The new solution for 802.11 security: standard 802.1x Encapsulated EAP, Typically on RADIUS EAPOL(over Ethernet or 802.11) Authenticator Authentication Server Supplicant • EAP: Extensible Authentication Protocol (RFC 2284, 1998) • EAPOL: EAP over LAN • RADIUS: Remote authentication dial in user service (RFC 2138, 1997) • Features: • - Supports a wide range of authentication schemes, thanks to the usage of EAP • One-way authentication • Optional encryption and data integrity

  34. More on IEEE 802.1x Example of authentication, using one-time passwords (OTP): Supplicant Authenticator Authentication server EAP-request/identity EAP-response/identiy (MYID) EAP-request/OTP,OTP challenge EAP-response/OTP, OTPpassword EAP-success Authenticationsuccessfully completed Port authorized : exchange of EAPOL frame : exchange of EAP frames in a higher layer protocol (e.g., RADIUS) • Notes : • Weaknesses have been found in 802.1x as well, but are corrected in thevarious implementations. • New standard in the making : IEEE 802.11i

  35. IEEE 802.11 – Standardization efforts • IEEE 802.11b • 2.4 GHz band • Bitrates 1 – 11 Mbit/s • IEEE 802.11a • 5 GHz band • transmission rates up to 54 Mbit/s • close cooperation with BRAN (ETSI Broadband Radio Access Network) • Coverage is not as good as in 802.11b • IEEE 802.11g • Available since 2003, highly popular • 2.4 GHz band (same as 802.11b) • Bitrates up to 54Mb/s • IEEE 802.11i • Security, makes use of IEEE 802.1x • IEEE 802.11p • For vehicular communications • IEEE 802.11s • For mesh networks • + many other…

  36. Conclusion of Wireless LANs • IEEE 802.11 • Very widespread • Often considered as the system underlying larger scale ad hoc networks (although far from optimal, not designed for this purpose) • Tremendous potential as a competitor of 3G cellular networks in hot spots • Bluetooth • Security perceived as a major obstacle; initial solutions were flawed in both IEEE 802.11 (WEP) and Bluetooth • Future developments • Ultra Wide Band?

  37. References • J. Schiller: Mobile Communications, Addison-Wesley, Second Edition, 2004 • Leon-Garcia & Widjaja: Communication Networks, McGrawHill, 2000 • IEEE 802.11 standards, available at • • J. Edney and W. Arbaugh: Real 802.11 Security, Addison-Wesley, 2003