DataCommChapter 5 Part3b

8/14/2019 DataCommChapter 5 Part3b

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Addressing Mechanism

The IEEE 802.11 addressing mechanism is complicated due to theinvolvement of intermediate stations (APs)

There are 4 cases defined by the value of To DS and From DSflags in the FC field:

Sourcestation

Destination

station

Sending APReceivingAP

11

N/ADestinationstation

Sourcestation

ReceivingAP

01

N/ASourcestation

Sending APDestinationstation

10

N/ABSS IDSource

station

Destination

station

00

Address 4Address 3Address 2Address 1From DSTo DS

Address 1 is always the address of the next deviceAddress 2 is always the address of the previous deviceAddress 3 is the address of the final destination station if it is not defined by Address 1

Address 4 is the address of the original source station if it is not the same as Address 2


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Case 1 (To DS = 0 & From DS = 0)

o The frame is not going to a distribution system and is not

coming from a distribution systemo Instead, it is going from one station in a BSS to another without

passing through the distribution system

o The ACK frame should be sent to the original sender

A B

BSS

AB

BSS-ID

ADDR

1

ADDR

2

ADDR

3

ADDR

4


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o The frame is coming from a distribution system (via an AP) and

going to a stationo The ACK frame should be sent to the AP

o Address 3 contains the original sender of the frame (in anotherBSS)

APA

APB

ADDR

1

ADDR

2

ADDR

3

ADDR

4

BDistributionsystem

A

BSS


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Case 3 (To DS = 1 & From DS = 0)o The frame is going to a distribution system (from a station to an

AP)o The ACK frame is sent to the original station

o Address 3 contains the final destination of the frame (in anotherBSS)

AP A

BAP

ADDR

1

ADDR

2

ADDR

3

ADDR

4

B Distribution

system

A

BSS


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o The frame is going from one AP to another AP in a wireless

distribution system

o Four addresses are required to define the original sender, thefinal destination, and 2 intermediate APs

AP1 AP2

Wirelessdistribution system

A B

AP1AP2

ADDR

1

ADDR

2

ADDR

3

ADDR

4

B A

BSS BSS


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IEEE 802.11 Implementation

This IEEE 802.11 standard defines the media access control (MAC)and physical (PHY) layers for a LAN with wireless connectivity.

IEEE 802.11 standards mapped to the OSI reference model


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Physical Medium

Infrared :o limited to a single room as infrared light cannot penetrate opaque

walls

Spread spectrum:o 802.11: frequency hopping spread spectrum (FHSS), direct

sequence spread spectrum(DSSS)

o Operate in the 2.4GHz ISM [Industrial, Scientific and Medical]bands, so that no licensing is required

o 802.11a: Using OFDM (orthogonal frequency-divisionmultiplexing) in 5GHz ISM band

o 802.11b: Using HR-DSSS (high rate DSSS) in 2.4GHz ISM bando 802.11g: Using OFDM in 2.4GHz ISM band


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Access Modes

DCF(Distributed coordination Function)

o In this mode there is no central control, and stations competefor air time, just as they do with Ethernet.

o DCF uses a contention algorithm (CSMA/CA) to provideaccess to all traffic

o In some circumstances, the DCF may use the CTS/RTS

clearing technique to further reduce the possibility of collisions PCF(Point Coordination Function)

o PCF provides contention free service and is built on top ofDCF and exploits features of DCF to assure access for its users(for time sensitive transmission).

o The base station polls the other stations (one after another),asking them if they have any frames to send


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Another interframe space has been defined: PIFS (PCF IFS)

PIFS is shorter than DIFS so that the base station using PCF hashigher priority over other station using DCF

For e.g., if a station wants to use DCF and an AP wants to usePCF, the AP has priority since its waiting time is shorter (PIFS

DataCommChapter 5 Part3b

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