1 kyung hee university chapter 15 wireless lans. 2 kyung hee university ieee 802 standards working...

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Kyung Hee University

Chapter 15Chapter 15Wireless LANsWireless LANs

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IEEE 802 Standards Working GroupsIEEE 802 Standards Working Groups

WiMAX, an acronym that stands for Worldwide Interoperability for Microwave Access(802.16)

WiFi (Wireless Fidelity) : 802.11

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IntroductionIntroduction

Architecture

Physical Layer

MAC Layer

Addressing Mechanism

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15.1 IEEE 802.11 - Architecture15.1 IEEE 802.11 - Architecture

Basic Service Set (BSS)

Extended Service Set (ESS)

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Basic Service SetBasic Service Set

• made up of stationary or mobile wireless stations, and possibly a central base station (access point)

• BSS without an AP is stand-alone, cannot send data to other BSSs

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ESS – Extended Service Set

Made up of two or more BSSs with APs – connected through a distr

ibution system (usually a wired LAN)

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Station Types

No-transition mobility – either stationary or moving

only inside a BSS

BSS-transition mobility – can move from one BSS to

another, confined to one ESS

ESS-transition mobility – can move from one ESS to

another

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Physical layer specifications

802.11 defines the specs for the conversion of bits to

a signal

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FHSS – Frequency hopping spread spectrum

sender sends on one carrier frequency for a short amount of time, hops to another frequency for the same time, then another, and so on, eventually the cycle is repeated

makes it difficult for unauthorized person to make sense of data

2.4 GHz industrial, scientific, and medical (ISM) band – 79 subbands of 1MHz

modulation is FSK (two-level or four-level)

1010


Frequency-Hopping exampleFrequency-Hopping example

1111


DSSS – direct sequence spread spectrum

Each bit sent is replaced by a chip code (similar to CDMA, but this is at the physical layer, not a multiple-access data link layer method)

2.4 GHz ISM band

Modulation is PSK (BPSK or QPSK)

1212


802.11802.11

802.11 Infrared

Two capacities 1 Mbps or 2 Mbps.

Range is 10 to 20 meters and cannot penetrate walls.

Does not work outdoors.

802.11 FHSS (Frequence Hopping Spread Spectrum)

The main issue is multipath fading.

79 non-overlapping channels, each 1 Mhz wide at low end of 2.4 GHz ISM band.

Same pseudo-random number generator used by all stations.

Dwell time: min. time on channel before hopping (400msec).

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801.11a,b,g 802.11a OFDM – orthogonal frequency division multiplexing

5-GHz ISM band

48 subbands for sending 48 groups of bits at a time, 4 subbands for control information

PSK for 18Mbps and QAM for 54 Mbps

802.11b HR-DSSS – high-rate direct sequence spread spectrum

2.4 GHz band

similar to DSSS except for encoding, complementary code keying (CCK), encodes 4 or 8 bits to one CCK symbol

modulation BPSK and QPSK

4 data rates (1, 2, 5.5, and 11 Mbps) : DSSS for 1, 2 Mbps, BPSKllfor 5.5, QPSK for 11 Mbps

Range is 7 times greater than 11a.

802.11b and 11a are incompatible!!

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801.11a,b,g (cont’d)

802.11g OFDM

2.4 GHz band

54-Mbps data rate

Is backward compatible with 802.11b.

1515


MAC layers in IEEE 802.11 standard

PCF : Point Coordination Function DCF : Distributed Coordination Function

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CSMA/CA

Cannot implement CSMA/CD:

collision detection implies the ability to send and receive at the same time, which implies costly stations and increased bandwidth requirements

collision may not be detected because of a hidden terminal

distance could cause signal fading which could prevent a station from hearing a collision

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Hidden and Exposed TerminalsHidden and Exposed Terminals

(a)The hidden station problem (b) The exposed station problem

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CSMA/CA flowchart

1919


CSMA/CA and NAV

DIFS – distributed interframe spread NAV – network allocation vector

SIFS – short interframe spread collision only occurs during

RTS – request to send handshake period (RTS, CTS)

CTS – clear to send

2020


Frame format

2121


Subfields in FC(frame control) fieldSubfields in FC(frame control) field

Field Explanation

Version The current version is 0.

Type Type of information: management (00), control (01), or data (10).

Subtype Defines the subtype of each type (see ).

To DS Defined later.

From DS Defined later.

More flag When set to 1, means more fragments.

Retry When set to 1, means retransmitted frame.

Pwr mgt When set to 1, means station is in power management mode.

More data When set to 1, means station has more data to send.

WEP Wired equivalent privacy. When set to 1, means encryption implemented.

Rsvd Reserved.

2222


Control frames

2323


Values of subfields in control frames

type field : 01

Subtype Meaning

1011 Request to send (RTS)Request to send (RTS)

1100 Clear to send (CTS)Clear to send (CTS)

1101 Acknowledgment (ACK)Acknowledgment (ACK)

2424


Subfields in FC field

ToTo

DSDSFromFrom

DSDSAddressAddress

11AddressAddress

22AddressAddress

33AddressAddress

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00 00DestinationDestination

stationstationSourceSource

stationstationBSS IDBSS ID N/AN/A

00 11DestinationDestination

stationstationSendingSending

APAPSourceSource

stationstationN/AN/A

11 00ReceivingReceiving

APAPSourceSource

stationstationDestinationDestination

stationstationN/AN/A

11 11ReceivingReceiving

APAPSendingSending

APAPDestinationDestination

stationstationSourceSource

stationstation

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Addressing mechanism: case 1

•To DS = 0 and From DS = 0• Frame is not going to a distribution system• Frame is going from one station in a BSS to another• ACK sent to original sender

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To DS = 0 and From DS = 1

Frame is coming from a distribution system

Frame is coming from an AP and going to a station

ACK sent to AP

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Frame is going to a distribution system

Frame is coming from a station to an AP

ACK sent to original station

address 3 contains final destination of frame

•

2828




Distribution system is wireless

Frame is coming from an AP going to another AP

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15.2 Bluetooth15.2 Bluetooth

• Architecture

• Radio Layer

• Baseband Layer

• Other Upper Layers

• L2CAP Layer

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Architecture - - Two Types of Networks

Piconets

Scatternet

3131


Piconet

up to eight stations, one is the master, rest are slaves

slaves synchronize their clocks and hopping sequence with master

only one master

communication between master and slave can be 1-1 or 1-many

can have up to eight additional slaves in parked state

slave in parked state is synchronized, but cannot communicate

3232


Scatternet

scatternet – combined piconets

slave in one piconet can be master in another

3333


Bluetooth layers

3434


Radio LayersRadio Layers

roughly equivalent to the physical layer of the Internet

model

2.4-GHz ISM band

FHSS – frequency hopping spread spectrum

modulation – sophisticated version of FSK, called

GFSK (FSK w/ Gaussian bandwidth filtering)

3535


Baseband LayerBaseband Layer

roughly equivalent to MAC sublayer in LANs

access method is TDMA

TDD-TDMA (time division duplexing TDMA) – a kind of half duplex communication

single slave communication – master uses even number slots, slave uses odd numbered

multiple slave communication – master uses even number slots, slave sends in the next odd-numbered slot if the packet in the previous slot was addressed to it

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Single-slave CommunicationSingle-slave Communication

3737


Multiple-slave communication

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Physical Links

SCO – synchronous connection oriented link –

avoiding latency is more important than integrity –

damaged packet is never retransmitted

ACL – asynchronous connectionless link – data

integrity is more important than avoiding latency – if

packet is damaged, it is retransmitted

3939


Frame Format TypesFrame Format Types

4040


L2CAP data packet format

Logical link control and adaptation protocol – roughly equivalent to the LL

C sublayer in LANs

used for data exchange on an ACL link

specific duties: multiplexing, segmentation and reassembly, quality of servi

ce, and group management

4141


Questions !Questions !

1 kyung hee university chapter 15 wireless lans. 2 kyung hee university ieee 802 standards working...

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