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Module B WLAN – Engineering Aspects
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  1. Mobile Networks Module BWLAN – Engineering Aspects Prof. JP Hubaux http://mobnet.epfl.ch

  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 handset • 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 Note: in the coming years, frequencies will become technology-neutral

  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 • More difficult to secure

  6. Scope of Various WLAN and WPAN Standards Power consumption 802.11n Complexity 802.11a 802.11g 802.11b 802.11 WLAN 802.15.I Bluetooth 802.15.4 WPAN Data rate WPAN: Wireless Personal Area Network

  7. 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, privacy, safety (low radiation) • transparency concerning applications and higher layer protocols • location awareness if necessary

  8. 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 materials 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 and 5 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

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

  10. 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

  11. 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

  12. 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

  13. 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

  14. 802.11b - 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 (rarely used in practice) • 850-950 nm, diffuse light, around 10 m range • carrier detection, energy detection, synchronization

  15. 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 and rarely used in practice) • access point polls terminals according to a list DCF: Distributed Coordination Function PCF: Point Coordination Function

  16. 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

  17. 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

  18. = 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

  19. 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)

  20. 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

  21. 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

  22. 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

  23. 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

  24. 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

  25. 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

  26. 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 (back-off) • 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

  27. 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?)

  28. 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

  29. 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)

  30. 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

  31. 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.

  32. 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

  33. 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

  34. IEEE 802.11 – Standardization efforts IEEE 802.11b • 2.4 GHz band • DSSS (Direct-sequence spread spectrum) • Bitrates 1 – 11 Mbit/s IEEE 802.11a • 5 GHz band • Based on OFDM (orthogonal frequency-division multiplexing) • transmission rates up to 54 Mbit/s • Coverage is not as good as in 802.11b IEEE 802.11g • 2.4 GHz band (same as 802.11b) • Based on OFDM • Bitrates up to 54Mb/s IEEE 802.11n • MIMO (multiple-input multiple-output) • 40MHz channel (instead of 20MHz) • Can operate in the 5GHz or 2.4Ghz (risk of interference with other systems, however) • Bitrates up to 600Mb/s IEEE 802.11ac • Extension of IEEE 802.11n, under development IEEE 802.11e • Enhanced DCF: to support differentiated service IEEE 802.11i • Security, makes use of IEEE 802.1x IEEE 802.11p • For vehicular communications IEEE 802.11s • For mesh networks

  35. 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?

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

  37. Ad Hoc On-Demand Distance Vector Routing (AODV) Note: this and the following slides are provided here because AODV is used in the hands-on exercises. We will come back to this topic in a later module of the course.

  38. AODV : Route discovery (1) K F H Q A E P S G D J B M R I L C N

  39. AODV : Route discovery (2) K F H Q A E P S G D J B M R I L C N Note: if one of the intermediate nodes (e.g., A)knows a route to D, it responds immediately to S : Route Request (RREQ)

  40. AODV : Route discovery (3) K F H Q A E P S G D J B M R I L C N : represents a link on the reverse path

  41. AODV : Route discovery (4) K F H Q A E P S G D J B M R I L C N

  42. AODV : Route discovery (5) K F H Q A E P S G D J B M R I L C N

  43. AODV : Route discovery (6) K F H Q A E P S G D J B M R I L C N

  44. AODV : Route discovery (7) K F H Q A E P S G D J B M R I L C N

  45. AODV : Route reply and setup of the forward path K F H Q A E P S G D J B M R I L C N : Link over which the RREP is transmitted : Forward path

  46. Route reply in AODV In case it knows a path more recent than the one previously known to sender S, an intermediate node may also send a route reply (RREP) The freshness of a path is assessed by means of destination sequence numbers Both reverse and forward paths are purged at the expiration of appropriately chosen timeout intervals

  47. AODV : Data delivery K F H Q A Data E P S G D J B M R I L C N The route is not included in the packet header

  48. AODV : Route maintenance (1) K F H Q A Data E P S G D X J B M R I L C N

  49. AODV : Route maintenance (2) K F H Q A RERR(G-J) E P S G D X J B M R I L C N When receiving the Route Error message (RERR), S removes the broken link from its cache. It then initializes a new route discovery.

  50. AODV (unicast) : Conclusion Nodes maintain routing information only for routes that are in active use Unused routes expire even when the topology does not change Each node maintains at most one next-hop per destination