Fundamental of Wireless LANFundamental of Wireless LANocw.nctu.edu.tw/course/ws992/C2.pdf · Fundamental of Wireless LANFundamental of Wireless LAN Radio Waves IEEE 802 Families Appenidx

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    Fundamental of Wireless LANFundamental of Wireless LAN

    � Radio Waves� IEEE 802 Families� Appenidx (Infrared and Carrier Currents LANs)

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    Wireless LAN Configuration

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    Radio-Based Wireless LANs

    � Most widely used method� Adv: penetrating walls and other obstacles with little

    attenuation.� Disadv: security, interference, etc.

    � 2 approaches: � Narrow band� ISM band and Spread spectrum

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    Narrow Band Modulation

    � Concentrate all transmission power in a narrow range of frequency.

    � Efficient use of radio spectrum; save bandwidth.� ex: television, AM, FM� Adv: for long distance transmission (e.g. metropolitan

    area)� Disadv: noise can easily corrupt the signals� Need to obtain FCC licenses to coordinate the use.

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    ISM Bands� In 1985, FCC modified part 15 to stimulate the use of

    wireless networks.� ISM=Industrial, Scientific, and Medical� Unlicensed, you can freelyinstall and move.

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    ISM Spectrum Availability

    � Only 2.4 GHz is the world-accepted ISM band.� 902 MHz is easier in manufacturing.

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    Spread Spectrum Modulation

    � Definition: spread a signals power over a wider band of frequency.

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    Spread Spectrum

    � Disadv: This contradicts with the goal of conserving bandwidth.

    � Adv: less susceptible to electrical noise (especially from narrow band sources)� In World War II, US Army uses spread spectrum to avoid

    hostile jamming. (invented by Hedy Lamarr, an actress)

    � To spread a signal, there are two ways:� direct sequence (DSSS)� frequency hopping (FHSS)

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    Direct Sequence Spread Spectrum (DSSS)

    � Use bit sequence to represent zero and one (Fig. 2-5)

    � Also referred to as chipping code.

    � Longer chipping codes are more resilient to noise.

    � Minimum length = 10 (by FCC)� IEEE 802.11 uses 11 chips per

    data bit.

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    Frequency Hopping Spread Spectrum (FHSS)

    � Data is modulated by carrier signals that hop from frequency to frequencyas a function of time, over a wide band of frequencies.

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    FHSS (cont.)

    � Hopping Code: to determine the order of hopping frequencies

    � The receiver must listen to incoming signals at the right time at the right frequency.

    � FCC regulation: at least 75 frequencies, with max. dwell time 400ms.

    � Adv.: very resilient to noise.� Orthogonal hopping codes: a set of hopping codes that

    never use the same frequencies at the same time (can be on-line adjusted by software).

    � Allow multiple wireless LANs to co-exist.

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    Forms of Radio-based Wireless LAN

    � Wireless Local Bridge� Infrastructure� Ad Hoc

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    IEEE 802 FamiliesIEEE 802 Families

    � IEEE? � a non-profit professional org. founded in 1884; now has

    320,000 members in 150 countries;

    � Most notable standards: � IEEE 802 family (802.2 LLC, 802.3 Ethernet, 802.5 Token

    Ring, etc.)

    � IEEE membership info:� Open to anyone. Its FREE!! Must pay meeting fees.� Membership Category: voting member, nearly member,

    aspirant member, sleeping member.

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    � 802.11a: Specification enabling up to 54 Mb/s to be achieved in the 5 GHz unlicensed radio band by utilizing OFDM

    � 802.11b: Specification enabling up to 22 Mb/s to be achieved in the 2.4 GHz unlicensed radio band by utilizing DSSS

    � 802.11c: Provides required information to ensure proper bridge operations, which is required when developing access points

    � 802.11d: Covers additional regulatory domains, which is especially important for operation in the 5 GHz bands because the use of these frequencies differ widely from one country to another

    Task groups of 802.11

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    � 802.11e: Covers issues of MAC enhancements for QoS, such as EDCF service differentiation and HCF

    � 802.11f: Provides interoperability for users roaming from one access point to another of different vendor

    � 802.11g: Specification enabling up to 54 Mb/s to be achieved in the 2.4 GHz unlicensed radio band

    � 802.11h: Dynamic channel selection and transmission power control� 802.11i: Specification for WLAN security to replace the weak Wired

    Equivalent Privacy (WEP)� 802.11k: Radio resource measurement for 802.11 specifications so that a

    wireless network can be used more efficiently

    � 802.11p:�� WAVE � Wireless Access in the Vehicular Environment����� IEEE 802.11 �� ���������������� ��������������� ���������������� �������� IEEE 802.11��� ! "�#$ %&’()*+ , Intelligent Transportation Systems" ITS�-./� �

    Task groups of 802.11

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    Appendix:(for your reference)

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    Infrared Light-based Wireless LAN

    � Wavelength range: 0.75 ~ 1000 u� ultraviolet < visible lights < infrared < radio� typically operating at 0.82u

    � Adv: � higher security:does not propagate through opaque objects,

    keeping signals within rooms and building� no interference from microwave ovens and radio transmitters

    � Disadv: � directional, not suitable for mobile applications

    � Two types of infrared light LANs:� Diffused� Point-to-point

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    Diffused Infrared-based LANDiffused Infrared-based LAN

    � Example: TV remote controller

    � Example: three stations using a ceiling� Just like using a flashlight,

    and talking to people using Morse Code.

    � A person wants to send must look at the ceiling to make sure that no other people are sending.

    � The receiver looks at the packet header to check for incoming packets.

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    � Point-to-Point Infrared LAN System� Each link has a pair of transducers (uni-directional):

    � up to 75 ft.

    � connect as a token ring(what is token ring??)� Each transducer can talk with a IEEE 802.5 token ring board.

    � data rate: 4 and 16 Mbps); � Good security:

    � Resilient to electrical noise.

    � No mobility, little fault-tolerant capability

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    Carrier Current LANsCarrier Current LANs

    � Use power line as a medium to transmit data.� Fig. 2.15

    � The electrical power companies provide 110V, 60Hz alternating current; the signal can support 1~2 Mbps data signals.

    � Adv: adopt already-existing cables� Products are being seen/proposed in the market.

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    Power Line CommunicationPower Line Communication

    � Power line communicationor power line carrier (PLC)is a system for carrying data on a conductor also used for electric power transmission � Different types of power line communications use different frequency

    bands, depending on the signal transmission characteristics of the power wiring used

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    � Ultra-High-frequency communication (� 100 MHz)� High-frequency communication (� 1 MHz)

    � Home networking (broadband)� Used to interconnect home computers, peripherals or other

    networked consumer peripherals

    � Internet access (broadband over power lines)� The use of PLC technology to provide broadband Internet

    access through ordinary power lines

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    broadband over power lines(BLP)

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    � Medium frequency (� 1 kHz)� Home control (narrowband)

    � Use the household electrical power wiring as a transmission medium

    � Low-speed narrow-band communication� Control and telemetry of electrical equipment such as meters,

    switches, heaters and domestic appliances

    � Transmitting radio programs� Low frequency (< 1 kHz)

    � PLC is one of the technologies used in the automatic meter reading industry

    Remote control of lighting and appliances without installation of

    additional control wiring

    Automatic meter reading X10

    ZigBee