1. IT 6011 IT 601: Mobile Computing Wireless LANs (most of the slides are borrowed from Prof. Sridhar Iyer) ‏

2. IT 6012 Wireless LANs: Characteristics Advantages –Flexible deployment; Minimal wiring problems –More robust against disasters –Historic buildings, conferences, … Disadvantages –Low bandwidth compared to wired networks – Need to follow wireless spectrum regulations

3. IT 6013 Infrastructure and Adhoc Networks infrastructure network ad-hoc network AP wired network AP: Access Point Source: Schiller

4. IT 6014 Wireless LANs are different… Destination address does not equal destination location The media impact the design –wireless LANs intended to cover reasonable geographic distances must be built from basic coverage blocks Impact of handling mobile (portable) stations –Propagation effects –Mobility management –power management

5. IT 6015 Difference Between Wired and Wireless If both A and C sense the channel to be idle at the same time, they send at the same time. Collision can be detected at sender in Ethernet. A B C A B C Ethernet LAN Wireless LAN

6. IT 6016 Wireless PHY –Medium has neither absolute nor readily observable boundaries outside which stations are unable to receive frames –Are unprotected from outside signals and are significantly less reliable than wired PHYs –Have time varying and asymmetric propagation properties –Lack full connectivity the assumption that every station (STA) can hear every other STA in invalid

7. IT 6017 Wireless MAC: Motivation Can we apply media access methods from fixed networks? Example CSMA/CD – C arrier S ense M ultiple A ccess with C ollision D etection –send as soon as the medium is free, listen into the medium if a collision occurs (original method in IEEE 802.3) ‏

8. IT 6018 Wireless MAC –signal strength decreases inversely proportional to the square of the distance –sender would apply CS and CD, but the collisions happen at the receiver –sender may not “hear” the collision, i.e., CD does not work –CS might not work, e.g. if a terminal is “hidden”

9. IT 6019 –A and C cannot hear each other. –A sends to B, C cannot receive A. –C wants to send to B, C senses a “free” medium (CS fails) ‏ –Collision occurs at B. –A cannot receive the collision (CD fails). –A is “hidden” for C. Hidden Terminal Problem BAC

10. IT 60110 Exposed Terminal Problem –A starts sending to B. –C senses carrier, finds medium in use and has to wait for A->B to end. –D is outside the range of A, therefore waiting is not necessary. A B C D

11. IT 60111 Solution for Hidden Terminals A first sends a Request-to-Send (RTS) to B On receiving RTS, B responds Clear-to-Send (CTS) ‏ Hidden node C overhears CTS and keeps quiet –Transfer duration is included in both RTS and CTS Exposed node overhears a RTS but not the CTS –D’s transmission cannot interfere at B ABC RTS CTS DATA D RTS

12. IT 60112 IEEE 802.11 Wireless LAN standard defined in the unlicensed spectrum (2.4 GHz and 5 GHz U-NII bands) ‏

13. IT 60113 802.11 (contd.) ‏ Standards covers the MAC sublayer and PHY layers Three different physical layers in the 2.4 GHz band –FHSS, DSSS and IR OFDM based PHY layer in the 5 GHz band

14. IT 60114 802.11 architecture The basic service set (BSS) is the basic building block of an IEEE 802.11 LAN ad-hoc network BSS2 BSS1

15. IT 60115 802.11 architecture (contd.) ‏ The ovals can be thought of as the coverage area within which member stations can directly communicate The Independent BSS (IBSS) is the simplest LAN. It may consist of as few as two stations IBSS is also called the ad hoc mode or DCF mode in 802.11

16. IT 60116 802.11 - 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 BSS 2 802.11 LAN BSS 1 STA 1 STA 4 STA 5 STA 2 STA 3 Source: Schiller

17. IT 60117 802.11 - infrastructure Source: Schiller

18. IT 60118 PCF components 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

19. IT 60119 Distribution System (DS) concepts The Distribution system interconnects multiple BSSs 802.11 standard logically separates the wireless medium from the distribution system – it does not preclude, nor demand, that the multiple media be same or different

20. IT 60120 DS (contd.) ‏ An Access Point (AP) is a STA that provides access to the DS by providing DS services in addition to acting as a STA. Data moves between BSS and the DS via an AP The DS and BSSs allow 802.11 to create a wireless network of arbitrary size and complexity called the Extended Service Set network (ESS) ‏

21. IT 60121 802.11- in the TCP/IP stack mobile terminal access point server fixed terminal application TCP 802.11 PHY 802.11 MAC IP 802.3 MAC 802.3 PHY application TCP 802.3 PHY 802.3 MAC IP 802.11 MAC 802.11 PHY LLC infrastructure network LLC

22. IT 60122 802.11 - Layers and functions 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 PMD PLCP MAC LLC MAC Management PHY Management MAC –access mechanisms, fragmentation, encryption MAC Management –synchronization, roaming, MIB, power management PHY DLC Station Management 7.8.1

23. IT 60123 802.11 - Physical layer 3 versions: 2 radio (typically 2.4 GHz), 1 IR –data rates 1, 2, 5.5, or 11 Mbit/s Infrared –850-950 nm, diffuse light, typ. 10 m range –carrier detection, energy detection, synchonization FHSS (Frequency Hopping Spread Spectrum) ‏ –spreading, despreading, signal strength –typically 1 Mbit/s (mandatory), 2Mbits/s (optional) ‏ –min. 2.5 frequency hops/s (USA), two-level GFSK (Gaussian FSK) modulation

24. IT 60124 802.11 DSSS DSSS (Direct Sequence Spread Spectrum) ‏ –DBPSK modulation for 1 Mbit/s (Differential Binary Phase Shift Keying), –DQPSK (differential quadrature PSK) for 2 Mbit/s, CCK (complementary code keying) for 5.5 and 11 Mbits/s –preamble and header of a frame is always transmitted with 1 Mbit/s –chipping sequence: +1, -1, +1, +1, -1, +1, +1, +1, -1, -1, -1 (Barker code) (11 chip) ‏ –max. radiated power 1 W (USA), 100 mW (EU) ‏ –min. 1mW

25. IT 60125 Spread-spectrum communications Source: Intersil

26. IT 60126 DSSS Barker Code modulation Source: Intersil

27. IT 60127 802.11 - MAC layer Traffic services –Asynchronous Data Service (mandatory) – DCF –Time-Bounded Service (optional) - PCF Access methods –DCF CSMA/CA (mandatory) ‏ collision avoidance via randomized back-off mechanism ACK packet for acknowledgements (not for broadcasts)‏

28. IT 60128 802.11 access methods –DCF CSMA/CA (mandatory) ‏ –DCF with RTS/CTS (optional) ‏ avoids hidden terminal problem –PCF (optional) ‏ access point polls terminals according to a list

29. IT 60129 802.11 - Carrier Sensing In IEEE 802.11, carrier sensing is performed –at the air interface ( physical carrier sensing ), and –at the MAC layer ( virtual carrier sensing ) ‏ Physical carrier sensing –detects presence of other users by analyzing all detected packets –Detects activity in the channel via relative signal strength from other sources

30. IT 60130 802.11 virtual carrier sensing Virtual carrier sensing is done by sending MPDU duration information in the header of RTS/CTS and data frames Channel is busy if either mechanisms indicate it to be –Duration field indicates the amount of time (in microseconds) required to complete frame transmission –Stations in the BSS use the information in the duration field to adjust their network allocation vector (NAV) ‏

31. IT 60131 802.11 – Reliability: ACKs –When B receives DATA from A, B sends an ACK –If A fails to receive an ACK, A retransmits the DATA –Both C and D remain quiet until ACK (to prevent collision of ACK) ‏ –Expected duration of transmission+ACK is included in RTS/CTS packets ABC RTS CTS DATA D RTS ACK

32. IT 60132 802.11 - CSMA/CA –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) ‏ t medium busy DIFS next frame contention window (randomized back-off mechanism)‏ slot time direct access if medium is free  DIFS

33. IT 60133 802.11 – CSMA/CA –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 (fairness) ‏

34. IT 60134 802.11 –CSMA/CA example t busy bo e station 1 station 2 station 3 station 4 station 5 packet arrival at MAC DIFS bo e busy elapsed backoff time bo r residual backoff time busy medium not idle (frame, ack etc.) bo r DIFS bo e bo r DIFS busy DIFS bo e busy bo e bo r

35. IT 60135 802.11 - Collision Avoidance Collision avoidance: Once channel becomes idle, the node waits for a randomly chosen duration before attempting to transmit –When transmitting a packet, choose a backoff interval in the range [0,cw]; cw is contention window –Count down the backoff interval when medium is idle –Count-down is suspended if medium becomes busy –When backoff interval reaches 0, transmit RTS

36. IT 60136 DCF Example data wait B1 = 5 B2 = 15 B1 = 25 B2 = 20 data wait B1 and B2 are backoff intervals at nodes 1 and 2 cw = 31 B2 = 10

37. IT 60137 802.11 - Congestion Control Contention window (cw) in DCF: Congestion control achieved by dynamically choosing cw large cw leads to larger backoff intervals small cw leads to larger number of collisions

38. IT 60138 Congestion control (contd.) ‏ Binary Exponential Backoff in DCF: –When a node fails to receive CTS in response to its RTS, it increases the contention window cw is doubled (up to a bound CWmax)‏ –Upon successful completion data transfer, restore cw to CW min

39. IT 60139 802.11 - Priorities defined through different inter frame spaces – mandatory idle time intervals between the transmission of frames SIFS (Short Inter Frame Spacing) ‏ –highest priority, for ACK, CTS, polling response –SIFSTime and SlotTime are fixed per PHY layer –(10  s and 20  s respectively in DSSS) ‏

40. IT 60140 802.11 – Priorities (contd.) ‏ PIFS (PCF IFS) ‏ –medium priority, for time-bounded service using PCF –PIFSTime = SIFSTime + SlotTime DIFS (DCF IFS) ‏ –lowest priority, for asynchronous data service –DCF-IFS (DIFS): DIFSTime = SIFSTime + 2xSlotTime

41. IT 60141 802.11 - CSMA/CA II station has to wait for DIFS before sending data receivers acknowledge at once (after waiting for SIFS) if the packet was received correctly (CRC) ‏ automatic retransmission of data packets in case of transmission errors t SIFS DIFS data ACK waiting time other stations receiver sender data DIFS contention

42. IT 60142 802.11 –RTS/CTS t SIFS DIFS data ACK defer access other stations receiver sender data DIFS contention RTS CTS SIFS NAV (RTS)‏ NAV (CTS)‏

43. IT 60143 802.11 –RTS/CTS 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 (NAV) distributed via RTS and CTS

44. IT 60144 Fragmentation t SIFS DIFS data ACK 1 other stations receiver sender frag 1 DIFS contention RTS CTS SIFS NAV (RTS)‏ NAV (CTS)‏ NAV (frag 1 )‏ NAV (ACK 1 )‏ SIFS ACK 2 frag 2 SIFS

45. IT 60145 802.11 - Point Coordination Function

46. IT 60146 802.11 - PCF I PIFS stations‘ NAV wireless stations point coordinator D1D1 U1U1 SIFS NAV SIFS D2D2 U2U2 SuperFrame t0t0 medium busy t1t1 t0 = time when the superframe should have started t1 = time when it actually started due to contention in the prev period

47. IT 60147 802.11 - PCF II t stations‘ NAV wireless stations point coordinator D3D3 NAV PIFS D4D4 U4U4 SIFS CF end contention period contention free period t2t2 t3t3 t4t4 t2 = time when CFP actually finished t3 = initial planned CFP (but PCF finished polling earlier than expected)‏

48. IT 60148 CFP structure and Timing CFP is greater than beacon interval DTIM – Delivery Traffic Indication Message Source: 802.11 spec

49. IT 60149 CFP Then length of CFP is controlled by PC –CFPMaxDuration field is used for this When CFP is more than beacon interval –CFP_Dur_Remaining is included in beacons –CFP_Dur_Remaining is set to 0 for beacons in CP

50. IT 60150 PCF- Data transmission Source: 802.11 standard

51. IT 60151 Polling Mechanisms With DCF, there is no mechanism to guarantee minimum delay for time-bound services PCF wastes bandwidth (control overhead) when network load is light, but delays are bounded Implicit signaling mechanism for STAs to indicate when they have data to send improves performance

52. IT 60152 Coexistence of PCF and DCF PC controls frame transfers during a Contention Free Period (CFP). –CF-Poll control frame is used by the PC to invite a station to send data –CF-End is used to signal the end of the CFP CFPs are generated at the CFP repetition rate and each CFP begins with a beacon frame

53. IT 60153 PCF and DCF (contd.) ‏ The CFP alternates with a CP, when DCF controls frame transfers –The CP must be large enough to send at least one maximum-sized MPDU including RTS/CTS/ACK Superframe: One CFP + One CP. It repeats according to the CFP repetition rate and each CFP begins with a beacon frame

54. IT 60154 802.11 - Frame format bytes Frame Control Duration ID Address 1 Address 2 Address 3 Sequence Control Address 4 DataCRC 2 26666240-2312 version, type, fragmentation, security,...

55. IT 60155 802.11 - 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

56. IT 60156 Frame Control Field

57. IT 60157 Types of Frames Control Frames –RTS/CTS/ACK –CF-Poll/CF-End Data Frames Management Frames – Beacons – Probe Request – Probe Response – Association Request – Association Response – Dis/Reassociation – Authentication – Deauthentication – ATIM (Announcement TIM) ‏

58. IT 60158 MAC address format DS: Distribution System AP: Access Point DA: Destination Address SA: Source Address BSSID: Basic Service Set Identifier RA: Receiver Address TA: Transmitter Address

59. IT 60159 802.11 - MAC management Synchronization –try to find a LAN, try to stay within a LAN; timer etc. Power management –sleep-mode without missing a message Association/Reassociation –scanning, i.e. active search for a network –roaming, i.e. change networks by changing APs MIB - Management Information Base –managing, read, write

60. IT 60160 Synchronization using a Beacon (infrastructure) ‏ Synchronized clocks are needed for PCF, Power saving and for frequency hopping Within a BSS timing is conveyed by a periodic beacon STAs use the timestamp in beacon to adjust its internal local clock AP always tries to send beacon at scheduled period (even if the prev beacon was delayed) ‏

61. IT 60161 Synchronization using a Beacon (infrastructure) ‏ beacon interval t medium access point busy B BBB value of the timestamp B beacon frame

62. IT 60162 Synchronization using a Beacon (ad-hoc) ‏ Synchronization in ad hoc mode is more difficult, since there is no AP for beacon transmission Each STA maintains its synchronization timer and starts transmission of a beacon periodically Standard random back off is applied to beacon frames so that only one STA wins transmitting beacon

63. IT 60163 Synchronization using a Beacon (ad-hoc) ‏ t medium station 1 busy B1B1 beacon interval busy B1B1 value of the timestamp B beacon frame station 2 B2B2 B2B2 random delay

64. IT 60164 Power saving in 802.11 Basic idea is to switch off transceiver when there is no communication Easy for sender since they know when to send data Receivers should wakeup periodically to check if it has to receive anything

65. IT 60165 Power saving with wake-up patterns (infrastructure) ‏ All stations (one station shown) wake up prior to TIM or DTIM –With every beacon the AP sends TIM (Traffic Indication Map) ‏ TIM contains a list of stations for which unicast data frames are waiting DTIM (Deliverry TIM) is for sending broadcast frames –PS (Power Saving) poll is sent by STA in response to TIM destined to the STA

66. IT 60166 Power saving with wake-up patterns (infrastructure) ‏ TIM interval t medium access point busy D TTD T TIM D DTIM DTIM interval BB B broadcast/multicast station awake p PS poll p d d d data transmission to/from the station

67. IT 60167 Power saving with wake-up patterns (ad- hoc) ‏ PS in ad hoc mode is more complex (no centralized AP) ‏ All stations announce a list of buffered frames during a period when all of them are awake –Destinations are announced using ATIM (Adhoc TIM) ‏

68. IT 60168 Power saving with wake-up patterns (ad- hoc) ‏ awake A transmit ATIM D transmit data t station 1 B1B1 B1B1 B beacon frame station 2 B2B2 B2B2 random delay A a D d ATIM window beacon interval a acknowledge ATIM d acknowledge data

69. IT 60169 802.11 - Roaming Scanning –scan the environment, i.e., –passive scanning listen into the medium for beacon signals (to detect other network)‏ –active scanning send probes into the medium on each channel and wait for an answer Station then selects the best AP (e.g. based on signal strength) ‏ –sends association Request to the AP association Response –success: AP has answered, station is now associated with the new AP –failure: continue scanning

70. IT 60170 Roaming (contd.) ‏ AP accepts Association 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

71. IT 60171 Hardware Original WaveLAN card (NCR) ‏ –914 MHz Radio Frequency –Transmit power 281.8 mW –Transmission Range ~250 m (outdoors) at 2Mbps –SNRT 10 dB WaveLAN II (Lucent) ‏ –2.4 GHz radio frequency range –Transmit Power 30mW –Transmission range 376 m (outdoors) at 2 Mbps (60m indoors) ‏ –Receive Threshold = –81dBm –Carrier Sense Threshold = -111dBm

72. IT 60172 802.11 status MAC MIB DSSS FH IR PHY WEP LLC MAC Mgmt 802.11b 5,11 Mbps 802.11g 20+ Mbps 802.11a 6,9,12,18,24 36,48,54 Mbps OFDM 802.11i security 802.11f Inter Access Point Protocol 802.11e QoS enhancements

73. IT 60173 IEEE 802.11 Summary Infrastructure (PCF) and adhoc (DCF) modes Signaling packets for collision avoidance –Medium is reserved for the duration of the transmission –Beacons in PCF –RTS-CTS in DCF Acknowledgements for reliability Binary exponential backoff for congestion control Power save mode for energy conservation

74. IT 60174 HIPERLAN Wireless LAN ratified by ETSI HIPERLAN1 –First of the series of spec –Supports five different priorities –Data rate of 23.5 Mbps –Forwards packets using several relays Extends communication beyond the radio range –Power conservation by specific sleep and wakeup pattern –MSDU lifetime can be set to have time bound services –MAC layer uses residual lifetime and user priority to choose the next MSDU to be transmitted

75. IT 60175 HIPERLAN2 Operates at 5 GHz Data rates up to 54 Mbps OFDM in the physical layer and a dynamic TDMA/TDD based MAC QoS support –Each connection has its QoS parameters (delay, jitter, bit error) ‏ Connection oriented –Negotiation of QoS parameter during connection establishment Dynamic frequency selection –Best frequency chosen based on interference level and usage of radio channels Power save –Mobile devices can negotiate certain sleep and wakeup pattern for power save Access Points can have multiple transceivers APs can have sectorized antenna

76. IT 60176 HIPERLAN2 Two modes of operation Centralized Mode (CM) ‏ –Like the infrastructure mode in 802.11, APs are connected to a core network and Mobile Stations (MS) are associated with APs. Direct Mode (DM) ‏ –This is the optional ad hoc mode of HiperLAN2 –Data is directly exchanged between MS But the network is still controlled Done via an AP that has the central controller (CC) functionality or via an MS that has CC functionality This ensures QoS support in ad hoc mode also

77. IT 60177 HIPERLAN2 AP MS data control Centralized mode AP/CC MS data control Direct mode Different modes of operation of HiperLAN2

78. IT 60178 Bluetooth Design goal was to set up short range ad hoc network (called piconets ) ‏ 79 channels in the 2.4 GHz band with 1 MHz carrier spacing Devices perform frequency hopping at 1600 hops/s Maximum data rate of 1Mbps Range of about 10m

79. IT 60179 Bluetooth M S S S SB P P M – Master S - Slave SB – Standby P - Park Bluetooth Piconet

80. IT 60180 Bluetooth Piconet –A collection of bluetooth devices which are synchronized to the same hopping sequence –One of the devices is the master, all others are slaves –Master determines the hopping pattern in the piconet and the slaves have to synchronize to this pattern –Each piconet has a unique hopping pattern –Parked devices Cannot participate in the piconet, but are known and can be activated within few msec Devices in standby do not participate in piconet.

81. IT 60181 Bluetooth Piconet –Active members assigned a 3-bit active member address (AMA) ‏ Upto 8 devices can be active in a piconet –Parked devices use 8-bit parked member address (PMA). –Standby devices do not need address

82. IT 60182 Bluetooth Scatternet –Only having 1 piconet within 80 MHz in total is not very efficient –Many piconets with overlapping coverage can exist simulatenously A device may participate in two different piconets –Bluetooth uses FH-CDMA for separation of piconet –A slave first syncs to one piconet and communicates, then leaves that piconet and enters the other piconet (of scatternet) by syncing to its FH sequence. –A master cannot be shared between two piconets of a scatternet –Master can leave one piconet and enter the other as a slave All traffic in the former piconet is suspended until the master returns