Wireless Data Networks Wireless Data Networks

8/14/2019 Wireless Data Networks Wireless Data Networks

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Motivation


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Wireless Data Networks

Wireless WANs (Mobile Data Network) Voice-Oriented: GSM, CDMA

Data-Oriented: Mobitex, D-AMPS

Voice/Data Mixed: IMT-2000

Wireless LANS Data-Oriented: IEEE 802.11, Hyperlan

Wireless Local Loop (IEEE 802.16) Data/Voice-Oriented: WiMax

Wireless PANS (IEEE 802.15) Data/Voice-Oriented: Bluetooth


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Wireless Data Services


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Wireless Data NetworkCategories

Infrared (IR) LANs (1-10 Mbps) Direct beam and diffused

Spread spectrum LANs (1-54 Mbps) IEEE802.11, 802.15(PAN),Bluetooth

Narrowband microwave (100 Mbps) IEEE802.16 (WiMAX)

Applications: LAN Extension -WLAN Wireless Broadband Access Nomadic Access Ad hoc Networking


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Data vs. Voice

voice traffic changes into data traffic


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Wireless LAN Configuration


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IEEE802 Standard Series


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IEEE802 Standard Series 802.1: Higher level interface (HILI)

802.2: Logical Link Control 802.3: CSMA/CD Ethernet (10Mbps) 802.4: Token Bus 802.5: Token Ring 802.6: MAN 802.7: Broadband Technical Advisory Group 802.8: Fiber Optics Technical Advisory Group 802.9: Integrated Service LAN Interface 802.10: Standard for Interoperable LAN Security

802.11: Wireless LANs 802.12: Demand Priority 802.14: Cable TV Based Broadband Communication

Networks


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Challenges in WLANs

Multiple devices need to share the ether

channel efficiently Problems: interference, contention, access

control, channel quality varies over space andtime

Different service requirements Voice (real-time, reservation-based) Data (best effort, reliable deliver)

Different approaches and trade-offs Centralized vs. Distributed

Other challenges Mobility

Power conservation

Security considerations


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Challenges in WLANs

Intrinsic Problems Multi-path

Hidden terminals

Burst errors

Near-far: If two terminals at differentdistances from the receiver start transmission atthe same power simultaneously, the receiverwill get more power from the nearer transmitter.

The SNR of the farther transmitter may getbelow detection in some cases and as a result,the communication channel may be jammed.


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Random Access Methods

Fixed access techniques (FDMA,TDMA and

CDMA) are inefficient in transmittingbursty data! The random access techniques are used in

mobile data networks, which can bedivided into two groups:

ALOHA based access methods: The mobile terminals transmit their contention packet

without any coordination between them.

CSMA (Carrier-Sense Multiple Access): The mobile terminals senses the availability of the

channel before it transmits its packets.


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Pure ALOHA


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Slotted ALOHA


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Reservation Based ALOHA


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CSMA: Listen-before-talk


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CSMA/CA


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Performance of Random Access Methods


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Hidden Terminals


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History of IEEE802.11

1987: Started as 802.4L

1989: Moved to 802.11

1997: MAC & PHY for 1&2Mbps at2.4GHz

1999 (a, b): PHY for 11Mbps at 2.4GHz(3 Ch) and 54Mbps at 5GHz (12 ch)

2000 (c, d): Supplement to 802.1dbridges. Dynamic regulatory domain

update

Current (e, g, h, i, j, k, m, n, p, r, s)

IEEE 802 11 WLAN S d d


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IEEE 802.11 WLAN StandardActivities

802.11a: 5 GHz, 54 Mbps

802.11b: 2.4 GHz, 11 Mbps 802.11d: Multiple regulatory domains 802.11e: Quality of Service (QoS) 802.11f: Inter-Access Point Protocol (IAPP) 802.11g: 2.4 GHz, 54 Mbps

802.11h: Dynamic Frequency Selection (DFS) and Tran Power 802.11i: Security Ratified | WPAv2 Draft 9 802.11j: Japan 5 GHz Channels (4.9-5.1 GHz) 802.11k: Measurement 802.11m: Maintenance

802.11n: High Throughput 802.11p: Wireless Access for Vehicular Environment 802.11r: Public WLAN Fast Roaming 802.11s: Mesh Networking

O i f IEEE802 11


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Overview of IEEE802.11Architecture

Uses CSMA/CD MAC, RTS/CTS with

optional PCF supporting all threePHYs

Supports three PHY layers DSSS FHSS DFIR

Supports WLAN with wiredinfrastructure as well as independentad-hoc WLANs


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Terminologies

Access Point (AP)

Provides access to distribution services via the wirelessmedium

Basic Service Area The coverage area of one access point

Basic Service Set (BSS) A set of stations controlled by one access point

Distribution System (DS) The fixed (wired) infrastructure used to connect a set of BSS to

create an extended service set (ESS)

Portal(s) The logical point(s) at which non-802.11 packets enter an ESS

MAC Protocol Data Unit (MPDU) Packets that describe protocol

MAC Service Data Unit (MSDU) Packets that describe service

A hit t /R f


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Architecture/ReferenceModel

IEEE802 11 Protocols in


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IEEE802.11 Protocols inContext


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IEEE 802.11 Protocol Layers


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Sub-layer Responsibilities

LLC: Provide an interface to higher layers

and performs flow and error control MAC Sublayer: access mechanism, data

format MAC Management: roaming in ESS,

power management, and security. PLCP: carrier sensing assessment, forming

packets for PHYs PMD: modulation and coding

PHY Layer Management: channel tuning Station Management: interacts with MAC

and PHY

D t il d Vi f P t l


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Detailed View of ProtocolArchitecture


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Physical Layer in 802.11

Three options Frequency Hopping Spread Spectrum (FHSS)

Direct Sequence Spread Spectrum (DSSS)

Diffused Infra Red (DFIR) not widely used.

Note, same MAC layer but all 802.11,802.11 a and 802.11 b all are incompatibleat the physical layer!

IEEE 802 11a and IEEE


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IEEE 802.11a and IEEE802.11b

IEEE 802.11a

Makes use of 5-GHz band Provides rates of 6, 9 , 12, 18, 24, 36, 48, 54 Mbps Uses orthogonal frequency division multiplexing

(OFDM) Sub-carrier modulated using BPSK, QPSK, 16-QAM or

64-QAM

IEEE 802.11b Provides data rates of 5.5 and 11 Mbps Complementary code keying (CCK) modulation

scheme based on DSSS


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IEEE 802.11g

IEEE 802.11g Up to 54 or 108 Mpbs at 2.4GHz Mandatory backward compatibility with IEEE802.11b

Mandatory OFDM and CCK

Optional CCK- OFDM and PBCC (Packet Binary

Convolutional Code)


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MAC Layer Functionality

MAC Sublayer Format of messages (data and control) Access control/mechanisms

1. Contention Mode For access to the channel by multiple

contending devices

1. Contention-free schemes RTS (Ready To-Send)/CTS (Clear-To-Send)

tackle the hidden terminal problem DATA and PCF for time bounded access

MAC Management Roaming support in the ESS, power

management and security

MAC Timing: Basic Access


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MAC Timing: Basic AccessMethod

For CSMA/CA, we have inter-frame spacing IFS.

As per the priority level of the packet, the IFS isdivided into three categories DCF-IFS (DIFS): used for contention, lowest priority,

longest delay. PCF-IFS (PIFS): for medium priority and medium

delay. Short-IFS (SIFS) used for high priority such as ACKs,

CTS, etc. has the lowest duration time and delay.

Distributed Coordination


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Distributed CoordinationFunction (DCF)

Use CSMA/CA Algorithm based on Inter-frameSpace (IFS):

If the medium is idle, the station waits to see if themedium is idle for a time equal to IFS. If so, it maytransmit immediately.

If the medium is busy, the station defers transmissionand continues to monitor the medium.

Once the current transmission is over, the station delaysanother IFS. If the medium remains idle for this period,

then it backs off the random amount of time and againsenses the medium. If the medium is still idle, it maytransmit. During the back-off time, if the mediumbecomes busy, the back-off timer is halted and resumeswhen the medium is idle.

CSMA/CA with ACK in


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CSMA/CA with ACK inan Infrastructure Network


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RTS /CTS Mechanism


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Wired Equivalent Privacy


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Wired Equivalent Privacy

WiFi Protected Access (WPA) /


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WiFi Protected Access (WPA) /IEEE802.11i

Wireless Data Networks Wireless Data Networks

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