Mobile Networks

Module B

WLAN – Protocol Aspects

Prof. JP Hubaux http://mobnet.epfl.ch

1

If you want your quiz grade to be registered… Please register your clicker today !!!

Go to Moodle (Mobile Networks class) and select Turning Technologies Device ID Search Tool

2

twisted pair

Reminder on frequencies and wavelenghts

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 VLF = Very Low Frequency LF = Low Frequency MF = Medium Frequency HF = High Frequency VHF = Very High Frequency VHF UHF SHF EHF infrared visible light UV UHF = Ultra High Frequency SHF = Super High Frequency EHF = Extra High Frequency UV = Ultraviolet Light Frequency and wave length:  = c/f wave length  , speed of light c  3x10 8 m/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 Europe USA Japan Mobile phones Cordless telephones Wireless LANs Dig. Dividend

800MHz

GSM

890-915 MHz, 935-960 MHz; 1710-1785 MHz, 1805-1880 MHz

UMTS

1920-1980 MHz 2110-2170 MHz

LTE

800 and 2600MHz

CT1+

885-887 MHz, 930-932 MHz;

CT2

864-868 MHz

DECT

1880-1900 MHz

IEEE 802.11

2400-2483 MHz 5725 –5875 MHz

AMPS

,

TDMA

,

CDMA

824-849 MHz, 869-894 MHz;

TDMA

,

CDMA

,

GSM

1850-1910 MHz, 1930-1990 MHz;

UMTS

1850-1910 MHz 1930-1990 MHz

PACS

1850-1910 MHz, 1930-1990 MHz

PACS-UB

1910-1930 MHz

IEEE 802.11

2400-2483 MHz 5725 –5875 MHz

PDC

810-826 MHz, 940-956 MHz; 1429-1465 MHz, 1477-1513 MHz

UMTS

1749.9-1784.9 1844.9-1879.9

PHS

1895-1918 MHz

JCT

254-380 MHz

IEEE 802.11

2471-2497 MHz 5725 –5875 MHz 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 802.11a

802.11n

802.11ac

802.11

802.11g

802.11b

WLAN 802.15.I

Bluetooth 802.15.4

WPAN WPAN: Wireless Personal Area Network Data rate 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

STA 1 ESS

IEEE 802.11 - Architecture of an infrastructure network

802.11 LAN 802.x LAN

BSS 1 Access Point Portal Distribution System Access Point STA 2 BSS 2

802.11 LAN

STA 3 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 10

STA 1

802.11 - Architecture of an ad-hoc network

BSS 1

802.11 LAN

STA 2 STA 3 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

BSS 2 STA 5 STA 4 11

Interconnection of IEEE 802.11 with Ethernet

mobile station server infrastructure network access point fixed terminal application TCP IP 802.11 MAC 802.11 PHY 802.11 MAC 802.11 PHY 802.3 MAC 802.3 PHY application TCP IP 802.3 MAC 802.3 PHY 12

802.11 - Layers and functions

MAC  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 IP MAC PLCP PMD MAC Management PHY Management 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 direct access if medium is free  DIFS contention time slot Note : IFS durations are specific to each PHY next frame t 16

DIFS

802.11 - CSMA/CA principles

DIFS contention window (randomized back-off mechanism) medium busy direct access if medium has been free for at least DIFS next frame time slot t     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) 17

station 1 DIFS

802.11 – CSMA/CA broadcast

= DIFS bo e bo r DIFS bo e bo r DIFS bo e busy bo e busy station 2 busy station 3 bo e busy (detection by upper layer) station 4 station 5 bo e bo r bo e busy (detection by upper layer) t Here St4 and St5 happen to have the same back-off time busy medium not idle (frame, ack etc.) packet arrival at MAC The size of the contention window can be adapted (if more collisions, then increase the size) bo e elapsed backoff time bo r residual backoff time 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 other stations DIFS data t waiting time Contention window The ACK is sent right at the end of SIFS (no contention) See file B1-802-11-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 sender receiver DIFS RTS SIFS CTS SIFS data SIFS ACK other stations NAV: Net Allocation Vector NAV (RTS) NAV (CTS) defer access DIFS data t Contention window RTS/CTS can be present for some packets and not for other 20

Fragmentation mode

sender DIFS RTS SIFS CTS SIFS receiver frag 1 SIFS ACK 1 SIFS frag 2 SIFS ACK 2 NAV (RTS) NAV (CTS) other stations NAV (frag 1 ) NAV (ACK 1 ) DIFS contention data t • 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 frag i (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  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 Frame Control 2 Duration ID 6 Address 1 6 Address 2 6 Address 3 2 Sequence Control 6 Address 4 0-2312 Data 4 CRC version, type, fragmentation, security, ...

detection of duplication 22

MAC address format scenario

ad-hoc network infrastructure network, from AP infrastructure network, to AP infrastructure network, within DS

to DS

0 0

from DS

0 1

address 1 address 2 address 3 address 4

DA DA SA BSSID BSSID SA 1 1 0 1 BSSID RA SA TA DA DA SA 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   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 access point medium B B B busy busy busy value of the timestamp B beacon frame busy B t • 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 station 1 station 2 medium B 1 B 2 B 2 busy busy busy value of the timestamp B beacon frame B 1 busy t 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 access point medium station D B busy busy T T TIM D DTIM busy T p awake d d busy D B t B broadcast/multicast d data transmission to/from the station p Power Saving poll: I am awake, please send the data 28

Power saving (ad-hoc case)

ATIM window beacon interval station 1 B 1 A D B 1 station 2 B 2 B 2 a d B beacon frame awake random delay A transmit ATIM a acknowledge ATIM d acknowledge data t D transmit 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 MU-MIMO: SU-MIMO:

AP 1 2 3 4 AP 1 2 3 4

User 1 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 802.11ac Channel sounding for pre-coding and zero forcing (to null multi-user interference signal) High spectral efficiency

R4 MUBF SUBF R1 TX R3 R2

Courtesy Ed Knightly

Channel Sounding Timeline for 802.11ac

Data Tx Rx A Rx B Rx C Pilots CSI CSI CSI ack ack ack Transmission Procedure 1. Select group and send channel sounding training sequence (Pilot Tones) 2. Receive channel state feedback (CSI) from each receiver serially 3. Construct steering weights and transmit data 4. Acknowledge transmission Courtesy Ed Knightly

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; works 5GHz band; see recommended reading 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 34

Conclusion on Wireless LANs

      IEEE 802.11 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 https://corp.fon.com/en 35

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

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)

Q F H K A S B C E R G I J L D M N P 38

AODV : Route discovery (2)

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

AODV : Route discovery (3)

Q F H K A S E G B R C N : represents a

link

on the

reverse path

I J L D M P 40

AODV : Route discovery (4)

Q F H K A S B C E R G I J L D M N P 41

AODV : Route discovery (5)

Q F H K A S B C E R G I J L D M N P 42

AODV : Route discovery (6)

Q F K H A S B C E R G I J L D M N P 43

AODV : Route discovery (7)

Q F K H A S B C E R G I J L D M N P 44

AODV : Route reply and setup of the forward

path

Q F K H A S E G D J B R I C N : Link over which the RREP is transmitted : Forward path L M P 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

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

AODV : Route maintenance (1)

Q A S Data E F B R H G X J I K L C N M D P 48

AODV : Route maintenance (2)

Q S A B F K H C E RERR(G-J) G X J D P R M I L 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

If you want your quiz grade to be registered… Please register your clicker today !!!

Go to Moodle (Mobile Networks class) and select Turning Technologies Device ID Search Tool

51

Next Week

Hands-on exercises in room INF019

Please read and bring with you the description of the hands-on exercises available at: http://mobnet.epfl.ch/index.php?page=material 52