T Computer Networks II

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1 T Computer Networks II Wireless Local/Personal Area networks Matti Siekkinen

2 The big picture: Wireless standards IEEE (iburst) IEEE (WiMax) IEEE (Wi-Fi) WAN 3GPP/3GPP2: GSM, CDMA2000, UMTS, LTE MAN LAN PAN IEEE (Bluetooth, ZigBee,...) 2

3 The big picture: Frequency bands, technologies 1.8, 1.9, 2.1 GHz (licenced) 2.3, 2.5, 3.5 GHz (licenced) 2.4, 5 GHz (unlicenced) WAN 450MHz 3.5 GHz (licenced) MAN LAN PAN 2.4 GHz (unlicenced) Technologies vary Modulation o OFDM, GFSK, QPSK, QAM... Channel access method o random w/ contention, CDMA, OFDMA, FHSS, SC- FDMA, HC-SDMA Error correction o FEC, ARQ Smart antenna techniques getting popular o MIMO 3

4 What about the Internet? Application Transport (TCP/UDP) Network (IP) WLANs define protocols and techniques for these layers 4

5 What about the Internet? Bluetooth Application API Security 32- / 64- / 128-bit encryption Network Star / Mesh / Cluster-Tree MAC PHY 868MHz / 915MHz / 2.4GHz Silicon Stack App WPAN often define a whole protocol stack (PHY->App) Non-IP Closed networks, not access networks to Internet But IP is coming! 6LoWPAN Customer ZigBee Alliance IEEE ZigBee 5

6 Outline IEEE Wireless LAN, a.k.a. Wi-Fi Basics Selected advanced features Non-standard stuff and future WPAN Bluetooth ( ) o Basics ZigBee ( ) o Basics Comparison IP over WPAN o 6LoWPAN

7 What is IEEE ? Set of standards for Wireless Local Area Networks Really a standard with a set of amendments Created and maintained by IEEE Standards Committee IEEE (Institute of Electrical and Electronics Engineers) is non-profit professional association Defines protocols and mechanisms mainly for Physical layer Medium access control (MAC) 7

8 What is Wi-Fi? A certification mark developed by the Wi-Fi Alliance to indicate that wireless local area network (WLAN) products are based on the Institute of Electrical and Electronics Engineers' (IEEE) standards. Wi-Fi Alliance is a global non-profit industry association of hundreds of companies Wi-Fi certification Interoperability and quality Need to be member of the alliance to get your products reviewed for certification 8

9 802.11: Overview of the many amendments PHY (.11, a, b, g, j, n, p, y, ac, ad, af, ah) Change data rate options Change spectrum MAC Security (i, w) Measurement and Management (k, v) Flow control and QoS (e, aa, ae) Time required to establish connection (p, r, ai) Spectral Efficiency Regulatory behavior (d, h) Radio node connection topology (s, z) Connection with other networks (u) 9

10 IEEE Wireless LAN: Some history of PHY 1997: IEEE approved , which specified the characteristics of devices with a signal rate of 1 and 2 Mb/s Standard specifies the MAC and the physical layers for transmissions in the 2.4 GHz band 1999: IEEE ratified IEEE b, which works at additional signal rates of 5.5 and 11 Mb/s 1999: IEEE approved the specifications of a, which uses the 5 Ghz band. The signal rates are 6, 9, 12, 18, 24, 36, 48 and 54 Mb/s. 2003: IEEE approved g as a further evolution of the standard. Same performance as a but works in the 2.4 GHz band Compatible with b devices. 10

11 In practice today (PHY) b 2.4 GHz band up to 11 Mbps direct sequence spread spectrum (DSSS) in physical layer MAC: all use CSMA/CA for multiple access all have base-station and ad-hoc network versions a 5 GHz band up to 54 Mbps OFDM g OFDM for 2.4 GHz band up to 54 Mbps n: GHz range up to 600 Mbps multiple antennae (MIMO) 2 channel widths (20/40MHz)

12 WLAN architecture BSS 1 AP Internet hub, switch or router wireless host communicates with base station base station = access point (AP) Basic Service Set (BSS) (aka cell ) in infrastructure mode contains: wireless hosts access point (AP): base station ad hoc mode: hosts only AP BSS 2

13 802.11: Channels, association g: 2.4GHz-2.485GHz spectrum divided into 13 channels at different frequencies AP admin chooses frequency for AP interference possible o Channel can be same as that chosen by neighboring AP o Channels overlap host: must associate with an AP scans channels, listening for beacon frames containing AP s name (SSID) and MAC address selects AP to associate with may perform authentication will typically run DHCP to get IP address in AP s subnet

14 802.11: passive/active scanning BBS 1 BBS 2 BBS 1 BBS 2 AP AP 2 AP AP H1 Passive Scanning: (1) beacon frames sent from APs (2) association Request frame sent: H1 to selected AP (3) association Response frame sent: H1 to selected AP H1 Active Scanning: (1) Probe Request frame broadcast from H1 (2) Probes response frame sent from APs (3) Association Request frame sent: H1 to selected AP (4) Association Response frame sent: H1 to selected AP

15 802.11: multiple access avoid collisions: 2 + nodes transmitting at same time : CSMA - sense before transmitting don t collide with ongoing transmission by other node : no collision detection! difficult to receive (sense collisions) when transmitting due to weak received signals (fading) can t sense all collisions in any case: hidden terminal, fading goal: avoid collisions: CSMA/C(ollision)A(voidance) C A B C A B A s signal strength C s signal strength space

16 MAC Protocol: CSMA/CA sender 1 if sense channel idle for DIFS then transmit entire frame (no CD) 2 if sense channel busy then start random backoff time timer counts down while channel idle transmit when timer expires if no ACK, increase random backoff interval, repeat receiver - if frame received OK return ACK after SIFS (ACK needed due to hidden terminal problem) DIFS sender data ACK receiver SIFS

17 CSMA/CA (cont.) Why DIFS/SIFS? SIFS allow priority to control frames (e.g. ACKS) or subsequent fragments (SIFS < DIFS) Why random backoff? Imagine two senders waiting and third one transmitting Without backoff, both would send right after third one shuts up o Undetected collision o Entire frames sent in vain Random backoff ensures different waiting times o Two waiting -> one starts before the other o The second one hears the first one (CA) and backs off 17

18 Avoiding collisions with RTS/CTS Sender reserves channel rather than random access of data frames Transmits small request-to-send (RTS) packets to BS using CSMA BS broadcasts clear-to-send CTS in response to RTS CTS heard by all nodes Pros Cons Avoid data frame collisions completely by using small reservation packets! sender transmits data frame other stations defer transmissions Avoid collisions of long data frames RTS frames small -> their collision is no problem Solves hidden terminal problem Adds delay and consumes resources Useful only with large data frames

19 Collision Avoidance: RTS-CTS exchange A AP B RTS(A) RTS(A) reservation collision RTS(B) CTS(A) CTS(A) DATA (A) defer time ACK(A) ACK(A)

20 frame: addressing frame control duration address 1 address 2 address 3 seq control address 4 payload CRC Address 1: MAC address of wireless host or AP to receive this frame Address 2: MAC address of wireless host or AP transmitting this frame Address 3: MAC address of router interface to which AP is attached Address 4: used only in ad hoc mode

21 frame: addressing H1 R1 router Internet AP R1 MAC addr H1 MAC addr dest. address source address frame AP MAC addr H1 MAC addr R1 MAC addr address 1 address 2 address frame

22 802.11: mobility within same subnet H1 remains in same IP subnet: IP address can remain same switch: which AP is associated with H1? self-learning: switch will see frame from H1 and remember which switch port can be used to reach H1 BBS 1 AP 1 router hub or switch AP 2 H1 BBS 2

23 802.11: Rate adaptation base station is mobile SNR varies Dynamically change transmission rate Depending on SNR Change physical layer modulation technique BER SNR(dB) 1. SNR decreases, BER increase as node moves away from base station QAM256 (8 Mbps) QAM16 (4 Mbps) BPSK (1 Mbps) operating point 2. When BER becomes too high, switch to lower transmission rate but with lower BER

24 802.11: Power Saving Enables the device to enter sleep mode Coordinated with the AP Node-to-AP: I am going to sleep until next beacon frame AP knows not to transmit frames to this node, buffers them Node wakes up before next beacon frame Beacon frame: contains list of mobiles with AP-to-mobile frames waiting to be sent Node will stay awake and request frames buffered at AP if any; otherwise sleep again until next beacon frame Sleep mode consumes ten times less power than idle mode Adaptive version used in practice Use timer: if no frames for 100ms, then sleep Reduces delay 24

25 Quality of Service Different traffic has different requirements/tolerance Throughput, delay, jitter, packet loss New mechanisms Enhanced Distributed Channel Access (EDCA) o A.k.a. EDCF (Enhanced Distributed Coordination Function) o TXOP Power saving variant: APSD Block acks Direct Link Setup Originally mostly as e Wi-Fi certification: Wireless Multimedia Extensions (WME) a.k.a. Wi-Fi Multimedia (WMM) o Subset of e mechanisms 25

26 EDCA Divides traffic into four different priority levels, Access Categories (AC) Best effort Background Video Voice Channel access is controlled by using four parameters: Minimum contention window size (CWmin) Maximum contention window size (CWmax) Arbitration Interframe Space (AIFS) = variable DIFS Transmission Opportunity (TXOP) o Specifies the time during which a station can transmit a series of frames after winning ch access o Not necessarily a single frame 26

27 EDCA (cont.) ACs implemented using the parameters Parameter values specified in EDCA parameter set element of beacon frames Default values: AC Application CWmin CWmax AIFS TXOP limit (802.11g) 0 Best effort CWmin CWmax Background CWmin CWmax 1 1.5ms 2 Video (CWmin+1)/2-1 CWmin 1 3ms 3 Voice (CWmin+1)/4-1 (CWmin+1)/ ms 27

28 EDCA (cont.) Each station maintains four queues corresponding to ACs Internal collisions (between ACs) resolved by giving priority to higher-ac queues

29 Rate anomaly is well-known in WiFi networks Low-rate stations degrade throughput of high-rate stations Why does rate anomaly exist? Stations reduce data rates when signal strength is poor Low-rate stations packets consume more airtime Per-packet fairness in basic Higher-rate stations receive less airtime throughput degrades TXOP solves also this fairness issue by providing time slots Throughput (Mbps) AP Mbps A 54Mbps A B B 18Mbps A B A, B near AP A far from AP

30 Other mechanisms Power saving variant: APSD Unscheduled-APSD (U-APSD) Main novel idea: use data frames sent in the uplink (STA AP) as indications (triggers) of station being awake Upon trigger, AP delivers data frames buffered while STA was sleeping Especially suited for bidirectional traffic Block acks Ack an entire TXOP in a single frame Less protocol overhead with long TXOPs Direct Link Setup Allow direct station-to-station frame transfer within a BSS 30

31 Future of Wi-Fi Gigabit Wi-Fi and beyond ac o Wider channels (80 and 160 MHz) o MIMO using up to 8 antennas o Multi-user MIMO o Up to 1Gb/s ad and WiGig (Wireless Gigabit Alliance) o 60GHz band o Much wider channel o Beamforming o Short distance o Requires line of sight most likely o Up to 7Gb/s 31

32 Outline IEEE Wireless LAN, a.k.a. Wi-Fi Basics Selected advanced features Non-standard stuff and future WPAN Bluetooth ( ) o Basics ZigBee ( ) o Basics Comparison IP over WPAN o 6LoWPAN

33 WPAN Centered around an individual person s workspace Wireless Connections Low cost Low power Short range IEEE

34 WPAN Technologies Bluetooth ZigBee Skinplex, Internalnet, Z-Wave, etc. IEEE Standard Max. Data Rate Traditionally Suitable Applications (Bluetooth) (ZigBee) 3 Mbps Cell phones, PDAs, PCs, Laptops, Printers, Speakers, Bar code readers, Microphones, Pagers 0.25 Mbps Industrial, vehicular, residential, medical applications, Sensors, Actuators, Remote Controls

35 Outline IEEE Wireless LAN, a.k.a. Wi-Fi Basics Selected advanced features Non-standard stuff and future WPAN Bluetooth ( ) o Basics ZigBee ( ) o Basics Comparison IP over WPAN o 6LoWPAN

36 Bluetooth Originally as cable replacement technology between traditional devices Main characteristics Low cost Low power Short range Main features Devices find and connect to each other via inquiry and paging processes Pairing for authenticated use of services Master and slave devices o Together form a piconet Different application profiles o Different stacks for each one o E.g. hands-free, streaming audio and video Secure data transfer

37 Bluetooth architecture Piconet consists of a master and up to 7 active slaves Up to 255 parked nodes in addition Two piconets can be connected to form a scatternet 37

38 Mainly three kinds of Bluetooth Version 2 + EDR A.k.a. Classic Enhanced Data Rate (EDR) adds 2 and 3 Mbps rates Basic rate is still 1 Mbps Version 3 + HS Adds Alternate MAC + PHY (Wi-Fi) to provide higher speed data channels Version 4 Adds Bluetooth low energy Runs up to two years on coin cell battery For e.g. sensors and watches

39 Bluetooth protocol stack Radio layer takes care of channel access and modulation Link control (or baseband) does framing and manages time slots Link manager establishes logical channels between devices L2CAP frames variable-length messages and can provide reliability Application profiles span almost whole stack 39

40 Radio layer License-free ISM band at GHz. 79 channels of 1 MHz Channel access GHz Adaptive Frequency-Hopping (AFH) spread spectrum o Up to 1600 hops/s o All nodes of piconet hop synchronously Master dictates timing and pseudorandom hop sequence o Dynamically exclude channels with interference Channel map update Master controls transmission schedule within a hop sequence Three kinds of modulation 1-bit symbol per µs -> 1Mbps 2/3-bit symbol per µs (EDR) -> 2/3Mbps GHz 40

41 More about the other layers Link control time slot mgmt E.g. TDM: master s tx starts with each even and slaves tx each odd 625µs slot Link manager establishes links Secure simple pairing SCO (Synchronous Connection Oriented) link o Master and slave set up a periodic schedule o Real time data, s.a. telephone conversation ACL (Asynchronous ConnectionLess) link o Master polls, slave responds o Packet data, best effort L2CAP Takes in packets -> outputs frames for link mgr Mux and demux data for upper layers 41

42 Frame structure basic data rate higher rate modulation only here enhanced data rate Access code specifies master Address specifies one of the 7 slaves 42

43 New connection establishment Two procedures: inquiry and paging Inquiry Discover which units are in range What are their device addresses and clocks Paging Establish an actual connection 43

44 Inquiry Inquiry Scan Done by device that wants to be discovered Periodically listen for inquiry packets on special inquiry hopping sequence of 32 frequencies Inquiry Device sends an Inquiry packet addressed using a specific Inquiry Access Code to inquiry hopping sequence o Code indicates who should respond o Either generic or dedicated to certain type of devices Inquiry Response Send response packet containing the responding device address after receiving inquiry msg in inquiry scan state Send to corresponding inquiry hopping response sequence o For each inquiry hop there is a corresponding inquiry response hop 44

45 Paging procedure Page Master sends a page message to slave s address Send to special page hopping sequence of 32 frequencies Master uses the clock information from slave to be paged o Estimate where in the hop sequence slave is listening in page scan mode o Send to the frequencies just before and after Page Scan Slave enters page scan state when it wishes to receive page packets Slave listens to packets addressed to its DAC Page Response Upon receiving page message, slave enters page response state Send back a page response containing its DAC Use frequencies from corresponding page response sequence o For each page hop there is a corresponding page response hop 45

46 Security: pairing Using some services may require some level of security Prevent eavesdropping, MITM Pairing is used to establish a shared secret link key PIN code pairing (legacy pairing) Secure Simple Pairing Authentication based on shared secret 5478 Encryption of data based on shared secret 5478 Based on SAFER+ block cipher

47 Bluetooth Low Energy History Nokia initiated project First Bluetooth Low End Extension (2004) Then, WiBree (2006) Now, part of Bluetooth v4.0 (2009) Characteristics Very low power consumption Cheap For transmitting small amounts of data Two implementations o Single mode for sensors etc. low energy devices o Dual mode for less constrained, including BT Classic 47

48 Bluetooth Low Energy Technical details Same frequency band as Classic but only 40 2MHz channels AFH similar to Classic Simpler stack: L2CAP/link layer/phy Simpler protocols o States: Standby, Advertising, Scanning, Initiating, and Connection o Discovery using three dedicated channels Low power achieved through low duty cycles o Periodically wake up to send/receive (connection events), sleep rest of time Market availability Devkits available No smart phones etc. yet but expected very soon 48

49 Contents IEEE Wireless LAN, a.k.a. Wi-Fi Basics Selected advanced features Non-standard stuff and future WPAN Bluetooth ( ) o Basics ZigBee ( ) o Basics Comparison IP over WPAN o 6LoWPAN

50 ZigBee features Low rate <250Kbps Low power consumption 6 Months 2 years with AA batteries Very low cost Transmission device cost $2 High Capacity 255 devices per network Low latency Wake up time a few ms TelosB mote (TPR2400) - IEEE compliant - Rate:250Kbps - Range:20m- 30m 50

51 IEEE & ZigBee Application API Security 32- / 64- / 128-bit encryption Network Star / Mesh / Cluster-Tree MAC PHY 868MHz / 915MHz / 2.4GHz Customer ZigBee Alliance IEEE the software Network, Security & Application layers Brand management IEEE the hardware Physical & MAC layers Silicon Stack App

52 Physical Layer Direct Sequence Spread Spectrum Robust against noise Requires more bandwidth Energy Detection (ED) Scan channels for energy o Other or noise, no matter Value Range: 0x00-0xff Link Quality Indication (LQI) Reported upon reception of packet Implemented using ED and/or SNR estimation Usage unspecified, up to upper layers Clear Channel Assessment (CCA) Carrier sense ( detection) and/or energy (ED) above threshold Used by MAC layer /250 kbps /100/250 kbps 52

53 Devices, modes, and topologies Two device types Full-function devices (FFD) o Can serve as PAN coordinator Reduced-function devices (RFD) o Simple, very resource constrained devices o Cannot serve as coordinator Three modes PAN Coordinator o Transmits Beacons o Sets up a Network o Manages Network Nodes o Receives Constantly Local coordinator (router) o Also relays messages End device (RFD) o Battery Powered o Can Sleep for Long Periods 53

54 Channel access Optional use of superframe structure (below) Otherwise, unslotted CSMA/CA Contention Access Period Contention Free Period Inactive Period Slot Network beacon Beacon extension period Contention period Guaranteed Time Slot Transmitted by PAN coordinator. Contains network information, frame structure and notification of pending node messages. Space reserved for beacon growth due to pending node messages Access by any node using slotted CSMA-CA Reserved for nodes requiring guaranteed bandwidth [n = 0]. 54

55 CSMA/CA CW=0, BE=MIN, NB=0 NB: # of times to perform backoff BE: Backoff Exponent CW: Contention Window N BE++, NB++, CW=2 Wait for random (2^BE 1) Perform CCA Idle? Y CW=CW-1 N NB>MAX Y Failure only for slotted CSMA/CA CW=0? Y Success N 55

56 Data transmission Node to Coordinator No-beacons: Simply transmits data frame using unslotted CSMA-CA Beacons enabled: o Node listens for network beacon o When found, synchronizes to superframe structure o Transmits frame in CAP using slotted CSMA-CA o Optional acknowledgements at end of slot no superframe (no beacons) Coordinator Node Coordinator superframe enabled Node Data Acknowledgment (optional) Beacon Data Acknowledgment (Optional)

57 Data transmission (cont.) Coordinator to Node No-beacons: o Coordinator stores pending data and waits for request o Node requests data using unslotted CSMA-CA o Coordinator acknowledges request o Data sent from coordinator to device o Node acknowledges data sent Coordinator Node Beacons enabled: o Coordinator indicates in beacon message that data pending o Node requests data using slotted CSMA-CA o Coordinator acknowledges request o Data sent from coordinator to device o Node acknowledges data sent Coordinator Beacon Node Data Request Acknowledgment Data Acknowledgment Data Request Acknowledgment Data Acknowledgment 57

58 ZigBee upper layers Network Addressing Routing uses AODV (Ad-hoc On-Demand Distance Vector) o MANET routing protocol Management o E.g. join and leave network Application Application Support (APS) sublayer o Maintaining tables for binding o Forward messages between bound devices ZDO: ZigBee Device Object o Define role of the device within network (coordinator or end device) o Discover devices on the network and determine which appl. services they provide o Initiating and/or respond to binding requests o Establishing a secure relationship between network devices Manufacturer-defined application objects 58

59 Outline IEEE Wireless LAN, a.k.a. Wi-Fi Basics Selected advanced features Non-standard stuff and future WPAN Bluetooth ( ) o Basics ZigBee ( ) o Basics Comparison IP over WPAN o 6LoWPAN

60 ZigBee vs. Bluetooth ZigBee / Bluetooth / Capacity active Rate 250Kbps 3Mbps (EDR) Latency <15ms depends on version, can be several sec for Classic PHY Layer DSSS FSSS MAC Layer Applications Energy consumption CSMA/CA, Coordinator/Node Controls, switches, monitoring, multi-hop sensor nw Asymmetric: low for nodes, high for coordinator Master/Slave, Master dictates scheduling Depends on version: hand set, multimedia, images, simple sensors Higher for classic, ultra low for BLE (symmetric)

61 Outline IEEE Wireless LAN, a.k.a. Wi-Fi Basics Selected advanced features Non-standard stuff and future WPAN Bluetooth ( ) o Basics ZigBee ( ) o Basics Comparison IP over WPAN o 6LoWPAN

62 Internet of Things (IoT) Scale of the IoT How many nodes are we talking about? 62

63 6LoWPAN 6LoWPAN = IPv6 over Low power Wireless Personal Area Networks IETF working group Goal: enable IPv6 based Internet of Things (IoT) Why IP? To enable IoT, need a way to interconnect smart objects with the Internet IP is the glue Why IPv6? IPv6 provides enough addresses (3.4x10^38) IPv6 over BLE also proposed as Internet draft 63

64 6LoWPAN architecture 64

65 Implementing IPv6 over Fragmentation needed IPv6 MTU over is max PHY packet is 127 octets Encapsulation using header compression Make IPv6 overhead for small packets tolerable TCP/IP ZigBee 6LoWPAN IPv6 with 6LoWPAN 65

66 Summary IEEE Wireless LAN, a.k.a. Wi-Fi The main Wireless LAN out there Infrastructure: access to Internet Also ad-hoc available Data rates up to 600Mbps now, Gigabit is coming Standard by IEEE, lots of amendments over the years WPAN: Personal area networking over the air Different solutions exist o Different stacks and radio technology o Usually no IP Different range, rates, power consumption, price Bluetooth o v4.0 includes Classic, High speed (using Wi-Fi), and Low Energy ZigBee / o IETF specifies PHY and MAC -> o ZigBee alliance specifies network and application layers 6LoWPAN: IPv6 over WPAN o For IoT o IPv6 over ZigBee specified (RFC) o IPv6 over BLE being drafted 66

67 Questions?

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