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Module B WLAN – Protocol Aspects

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1 Module B WLAN – Protocol Aspects
Mobile Networks Module B WLAN – Protocol Aspects Prof. JP Hubaux

2 Reminder on frequencies and wavelenghts
twisted pair 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 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

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, at least within frequencies allocated to mobile phones (first row of the above table)

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 Complexity I Bluetooth 802.11a 802.11g 802.11 WPAN 802.11b WLAN 802.11ac 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 Infrastructure vs. ad hoc networks
infrastructure network AP: Access Point AP AP wired network AP Ad hoc network

9 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 LAN 802.x LAN STA1 BSS1 Portal Access Point Distribution System Access Point ESS BSS2 STA2 STA3 LAN 9

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 LAN STA3 STA1 BSS1 STA2 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 MAC MAC 802.3 MAC 802.3 MAC PHY PHY 802.3 PHY 802.3 PHY

12 802.11 - Layers and functions MAC MAC Management
access mechanisms, fragmentation, encryption MAC Management synchronization, roaming, MIB, power management 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 Station Management IP MAC MAC Management PLCP PHY Management PHY PMD

13 802.11b - Physical layer 2 versions: DSSS and FHSS (both typically at 2.4 GHz) 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)

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 and rarely used in practice) 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 CSMA/CA principles contention window (randomized back-off mechanism) DIFS DIFS medium busy next frame direct access if medium has been free for at least DIFS t time slot 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) 12

17 802.11 – CSMA/CA broadcast = DIFS DIFS DIFS DIFS boe bor boe bor boe
busy station1 boe busy station2 busy station3 boe busy (detection by upper layer) station4 boe bor boe busy (detection by upper layer) station5 t Here St4 and St5 happen to have the same back-off time busy medium not idle (frame, ack etc.) 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 See file B1-802-11-Traces.pdf
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 Contention window The ACK is sent right at the end of SIFS (no contention) See file B Traces.pdf

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 Contention window RTS/CTS can be present for some 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 - MAC frame format Types Sequence numbers Addresses
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

22 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

23 802.11 - MAC management Synchronization Power management
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

24 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

25 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

26 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?)

27 Power saving (infrastructure case)
Here the access point announces data 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 T TIM D DTIM awake d data transmission to/from the station B broadcast/multicast p Power Saving poll: I am awake, please send the data

28 Power saving (ad-hoc case)
ATIM window beacon interval B1 A D B1 station1 B2 B2 a d station2 t B A transmit ATIM 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)

29 802.11 - 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

30 MIMO – Multiple Input Multiple Output
Both the transmitter and the receiver use multiple antennas SU-MIMO: Single-user MIMO: exploits the presence of multiple transmit and receive antennas to improve both the capacity and the reliability of a transmission MU-MIMO: Multi-user MIMO: stations having multiple antennas can simultaneously transmit or receive multiple information flows SU-MIMO: MU-MIMO: User 1 AP 1 3 2 4 AP 1 3 2 4 User 2 User User 3

31 Multi-User Beamforming (MUBF)
Transmitter sends multiple streams concurrently to different users Improves theoretical system capacity compared to SUBF Now standardized in IEEE ac Channel sounding for pre-coding and zero- forcing (to null multi-user interference signal) High spectral efficiency R1 SUBF TX R2 R3 R4 MUBF Courtesy Ed Knightly

32 Channel Sounding Timeline for 802.11ac
Tx Pilots Data CSI ack Rx A CSI ack Rx B CSI ack Rx C Transmission Procedure Select group and send channel sounding training sequence (Pilot Tones) Receive channel state feedback (CSI) from each receiver serially Construct steering weights and transmit data Acknowledge transmission Courtesy Ed Knightly

33 IEEE 802.11 – Standardization efforts
IEEE b 2.4 GHz band DSSS (Direct-sequence spread spectrum) Bitrates 1 – 11 Mbit/s IEEE a 5 GHz band Based on OFDM (orthogonal frequency-division multiplexing) transmission rates up to 54 Mbit/s Coverage is not as good as in b IEEE g 2.4 GHz band (same as b) Based on OFDM Bitrates up to 54Mb/s IEEE n 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 ac Extension of IEEE n; works 5GHz band; see recommended reading IEEE e Enhanced DCF: to support differentiated service IEEE i Security, makes use of IEEE 802.1x IEEE p For vehicular communications IEEE s For mesh networks

34 Conclusion on Wireless LANs
IEEE is the technology for wireless LANs Developed over the last 20 years Extremely widespread and successful Excellent complement of cellular networks, especially with the emergence of smart phones Found in most households and at almost all business buildings (with one major exception) Envisioned also for mobile ad hoc networks (see next slides) and vehicular ad hoc networks Interesting phenomenon: Fon

35 References J. Schiller: Mobile Communications, Addison-Wesley, Second Edition, 2004 Leon-Garcia & Widjaja: Communication Networks, McGrawHill, 2000 IEEE standards, available at

36 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.

37 AODV : Route discovery (1)

38 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)

39 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

40 AODV : Route discovery (4)

41 AODV : Route discovery (5)

42 AODV : Route discovery (6)

43 AODV : Route discovery (7)

44 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

45 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

46 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

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

48 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.

49 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

50 Hands-on exercises in room INF019
Next Week Hands-on exercises in room INF019 Please read and bring with you the description of the hands-on exercises available at:

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