introduction to wireless networking - tu berlin .ad hoc networks have limited application ......

Download Introduction to Wireless Networking - TU Berlin .Ad hoc networks have limited application ... Exception:

Post on 04-Jul-2018

215 views

Category:

Documents

0 download

Embed Size (px)

TRANSCRIPT

  • Introduction to Wireless Networking

    Communication without Wires

  • Overview Wireless and Mobile Network Architecture Wireless Link Characteristics IEEE 802.11 Wireless LAN (WLAN) Cellular Networks

  • Wireless and Mobile Network Architecture

  • Wireless Is Everyones Favorite Number of wireless (mobile) phone

    subscribers now exceeds number of wired phone subscribers (5-to-1)!

    Number of wireless Internet-connected devices exceeds number of people on Earth!

    Laptops, smartphones, tablets promise anytime untethered Internet access

    Two important (but different) challenges

    Wireless: Communication over wireless access link

    Mobility: Maintaining Internet connectivity for a mobile device that changes its point of attachment to the access network

  • Architecture of a wireless network

    network infrastructure

  • Wireless Hosts Laptop, smartphone,

    tablet Run applications Limited battery life if

    mobile May be stationary

    (non-mobile) or mobile Wireless does not

    always mean mobility

    Devices

    network infrastructure

  • Base Station Typically connected

    to wired network Or point to point

    wireless backhaul

    Relay - responsible for sending packets between wired network and wireless host(s) in its area e.g., cell towers,

    802.11 access points

    Infrastructure

    network infrastructure

  • Wireless Link Typically used to

    connect mobile(s) to base station Also used as

    backbone link Multiple access

    protocol coordinates link access

    Various data rates, transmission distance

    Radio Access Network Link

    network infrastructure

  • Infrastructure Mode

    Base station connects mobiles into wired network

    Handoff: mobile changes base station providing connection into wired network May or may not

    change IP subnet

    Infrastructure Mode

    network infrastructure

  • Ad hoc mode No base stations Nodes can only

    transmit to other nodes within link coverage

    Nodes organize themselves into a network: route among themselves

    Ad hoc Mode

  • A Word about Ad hoc Routing Latencies in mobile ad hoc

    networks can get very high Each hop adds more latency Movement means routing tables

    need continual adjustment Peculiarities of wireless link (next

    up) make maintaining connectivity challenging

    Ad hoc networks have limited application One or two hop sensor networks Military applications (drones, etc.) Small sized fixed ad hoc meshes

    Maximum size of ad hoc network: 3 8 hops depending on

    application Practical size is 2 hops for mobile Fixed ad hoc backbones can extend

    The Internet

  • Wireless Link Characteristics

  • Bandwidth of selected wireless links

    Indoor10-30m

    Outdoor50-200m

    Mid-rangeoutdoor

    200m 4 Km

    Long-rangeoutdoor

    5Km 20 Km

    .056

    .384

    1

    4

    5-11

    54

    2G: IS-95, CDMA, GSM

    2.5G: GSM-Edge, CDMA2000

    802.11b

    802.11a,g

    3G: UMTS/WCDMA/HSPDA, CDMA2000-1xEVDO

    4G: LTE

    802.11a,g point-to-point

    200 802.11n

    Dat

    a ra

    te (

    Mbp

    s)

    1000 802.11ac

    cellular protocol in regulated spectrumIEEE 802.11 WLANin unregulated spectrum

    5G peakRate: 10-20 Gbps!

  • Wireless Network Taxonomy

    single hop multiple hops

    infrastructure(e.g., APs)

    noinfrastructure

    host connects to base station (WiFi,

    cellular) which connects to

    larger Internet

    no base station, noconnection to larger Internet (Bluetooth,

    ad hoc nets)

    host may have torelay through several

    wireless nodes to connect to larger

    Internet: mesh net, cellularrelay

    no base station, noconnection to larger Internet. May have torelay to reach other a given wireless node

    Signal propagation characteristics of wireless links makes maintaining connectivity a significant

    challenge!

  • Basic Sources of Signal Loss on Wireless Link Signal power decreases in proportion to the

    distance from the sender: For free space propagation: True of wired signals as well but not as steep

    Interference from other sources Devices using the same band

    Other devices communicating Microwave ovens for WLAN on 2.4 GHz band Wireless phones, garage door openers, baby monitors

    Radio frequency interference (RFI) inadvertently radiated Solar panel inverters and power supplies AC motors and wiring

    Controlling interference is the Number One taskof radio protocols!

  • Signal Loss from Motion and the Urban Environment Multipath Interference

    Radio signal reflects off objects

    Arrives at a slightly different time

    Reduces signal strength

    Doppler Shift As the mobile device

    moves, the frequency of the signal changes slightly

    Changes channel response

  • Other Problems with Wireless ChannelsMultiple wireless senders and receivers create

    additional problems (beyond multiple access):

    AB

    C

    Hidden terminal problem B, A hear each other B, C hear each other A, C can not hear each

    other means A, C unaware of their interference at B

    A B C

    As signalstrength

    space

    Cs signalstrength

    Signal attenuation: B, A hear each other B, C hear each other A, C can not hear each

    other interfering at B

  • Signal to Noise Ratio (SNR) and Bit Error Ratio (BER) SNR: signal-to-noise ratio

    larger SNR easier to extract signal from noise (a good thing)

    SNR versus BER tradeoffs Given physical layer: increase

    power -> increase SNR->decrease BER

    Given SNR: choose physical layer that meets BER requirement, giving highest thruput SNR may change with

    mobility: dynamically adapt physical layer (modulation technique, rate)

    10 20 30 40

    QAM256 (8 Mbps)

    QAM16 (4 Mbps)

    BPSK (1 Mbps)

    SNR(dB)BE

    R

    10-1

    10-2

    10-3

    10-5

    10-6

    10-7

    10-4

    HighLevelModulationScheme

  • Techniques for Controlling Interference Geographical Separation

    Divide the geographic area into cells and use some property of radio to isolate devices in a cell Frequency Spreading Code

    Channel Multiplexing Devices share the channel by:

    Either: Each device gets a fine grained access identifier

    (timeslot, spreading code, etc.) Or:

    Each device is responsible for arranging its transmission such that the channel is contention-free

  • Geographical Separation Break geographic coverage area into cells

    Power falls off quickly as distance Use channel multiplexing within and between cells to allow

    devices within a cell to share frequency Within a cell, devices are distinguished by their multiplexing identifier When the mobile moves to a new cell, change multiplexing identifier

    Handover When signal strength goes below a certain point, switch over new cells

    base stationFine grained technique

    distinguishes

  • Access and Frequency Most cellular wireless protocols use different frequencies for uplink

    and downlink GSM, 3G, FD-LTE Exception: TD-LTE uses time division duplex to split the same channel

    between uplink and downlink For operators who dont have licenses for paired spectrum

    Wireless LAN protocols use different frequencies to reduce interference between cells 802.11, 802.11a, 802.11b, 802.11n, 802.11ac CSMA/CA uses to share channel between uplink and downlink

    Source: http://gnuradio.org/redmine/attachments/download/146/gsm900_arfcn1.jpg

    Source: http://alpha.tmit.bme.hu/meresek/2_4_ghz_wi-fi_channels_802_11bg_wlan.gif

    European 802.11g channels, 2.4 GHz Band (22 MHz channels, 5 MHz center separation)

    GSM900 Uplink and Downlink

  • Channel Multiplexing Time Division Multiple Access (TDMA)

    Each device gets a specific time slot Code Division Multiple Access (CDMA)

    Each device gets a specific spreading code Carrier Sense Multiple Access with Collision Avoidance

    (CSMA/CA) Check if someone is using channel first, random backoff if so

    Frequency Hopping Spread Spectrum (FHSS) Device changes frequency according to a prenegotiated random

    sequence Likelihood of interference is low Also slow frequency hopping

    Secondary technique Orthogonal Frequency Division Multiple Access (OFDMA)

    Break bandwidth down into separate subcarriers Assign each device a particular collection of subcarriers Particularly well suited to MIMO (Multiple Input Multiple Output) Best Bits per Hertz

  • MIMO Efficient Utilization of Multiple Antennas MIMO: Multiple Input Multiple Output Around 2005, realization that multiple antennas could increase

    throughput Use multipath propagation to your advantage! Maybe even signal steering to follow mobile

    Base station has lots of antennas Device only has a couple

    Use of MIMO approximately doubles throughput! MIMO incorporated into all recent wireless standards

    HSPA, LTE, 802.11n, 802.11ab

    Source: http://www.proxim.com/images/technology/Mimo_01.png

  • IEEE 802.11 Wireless LAN (WLAN)

  • **

    *

    Notes: * indicates uses MIMO DSSS = Direct Sequence Spread Spectrum, SC = Single Carrier all use Carrier Sense Multiple Access/Collision Avoidance (CSMA/CA) for multiple

    access all have base-station and ad-hoc network versions

    802.11 Wireless LAN Standard

    Source: http://www.mpdigest.com/issue/Articles/2012/Aug/Aeroflex/Default.asp

  • 802.11 LAN architecture wireless host

    communicates with base

Recommended

View more >