example:csma & rts/cts - uni koblenz-landauunikorn/lehre/drako...Übersicht motivation für...

Example:CSMA & RTS/CTS

Drahtlose Kommunikation - MedienzugriffskontrolleWS 12/13 51

S1 T1

RTS

CTS

DataNAV indicatesbusy medium

Respect CTSresponse time

Drahtlose Kommunikation - Medienzugriffskontrolle

MACA-Variante: DFWMAC in IEEE802.11

Ruhe

Warte aufSenderecht

Warte aufQuittung

Sender Empfänger

Paket sendebereit; RTS

time-out; RTS

CTS; Daten

ACK

RxBusy

Ruhe

Warte aufDaten

RTS; RxBusy

RTS; CTS

Daten; ACK

time-out Daten; NAK

ACK: positive EmpfangsbestätigungNAK: negative Empfangsbestätigung

RxBusy: Empfänger beschäftigt

time-out NAK;RTS

WS 12/13 52

RTS/CTS and the HT Problem

Drahtlose Kommunikation - MedienzugriffskontrolleWS 12/13

S1 T1 S2 T2

CTS prevents S2 from sending

RTSCTS CTS

53

RTS/CTS and the ET Problem


S1T1 S2 T2

S2 does not hear CTS and is not blocked by NAV

RTSCTS

54

HT Problem Always Resolved?


S1 T1 S2T2

RTS

CTS

Data

Example 1: Data-CTS Collision

RTS

CTS

Data

S1 T1 S2 T2

Example 2: Data-Data Collision55

Übersicht

Motivation für spezielle MAC-VerfahrenMehrfachzugriff durch Raummultiplex (SDMA)Mehrfachzugriff durch Frequenzmultiplex (FDMA)Mehrfachzugriff durch Zeitmultiplex (TDMA)

Clock-Sync und IFS Statisches TDMA Carrier Sense Multiple Access (CSMA) Clear-Channel-Assessment (CCA) bei CSMA Klassisches und Slotted Aloha Demand Assigned Multiple Access (DAMA) Polling Inhibit Sense Multiple Access (ISMA) Multiple Access with Collision Avoidance (MACA) Backoff am Beispiel MACA MACA Durchsatzanalyse

Code Division Multiple Access (CDMA)


Problem: Sync of Deterministic RTS Retry


RTS RTS

RTS RTS

CTS CTSExpected CTS

Retry

First Round

s1 s2t

Collision

Collision

57

Solution: Add Random Component


RTS RTS

RTS

RTS

CTS CTSExpected CTS

Random Backoff

First Round

s1 s2t

Collision

Success

Random Backoff

CTS

58

Waiting how long?


RTS

RTS

RTSCTS

CTS RTS

CTS

CTS

CTS

CTS

CTS

CTSR

TS

RTS

RTS

RTS

RTS RTS

CTS

Waiting too short

Data

CTS

RTS

RTS

CTS

CTS RTS

RTS

CTS Data

CTS Data

Waiting too long

t

s1

s2

t

s1

s22 ¢ data waste

2 ¢ data waste

DataRTS

CTS

59

Binary Exponential Backoff


success?

if (b · bmax ) {b = b ¢ 2

}

b = b0

RTS/CTSwithin [0,b]

no

yes

Data transmission

60

Übersicht

Motivation für spezielle MAC-VerfahrenMehrfachzugriff durch Raummultiplex (SDMA)Mehrfachzugriff durch Frequenzmultiplex (FDMA)Mehrfachzugriff durch Zeitmultiplex (TDMA)

Clock-Sync und IFS Statisches TDMA Carrier Sense Multiple Access (CSMA) Clear-Channel-Assessment (CCA) bei CSMA Klassisches und Slotted Aloha Demand Assigned Multiple Access (DAMA) Polling Inhibit Sense Multiple Access (ISMA) Multiple Access with Collision Avoidance (MACA) Backoff am Beispiel MACA MACA Durchsatzanalyse

Code Division Multiple Access (CDMA)


Recall: Throughput over Offered Load

packetarrivals

successfulpackets (S)

one packetper packettime

one packetper packettime

real MAC protocols

ideal MAC protocol

WS 12/13 62Drahtlose Kommunikation - Medienzugriffskontrolle

Recall: Throughput of CSMA (1/2)

Infinite number of stations

Poisson aggregate packet arrival with mean

Station has at most one packet to send at once

Stations can hear each other

Packet size is seconds

Exact end-to-end propagation delay of seconds

Neglect packet retransmission due to collision

Retransmission delays much larger than

Switching between send and transmit in 0 time SimplifyingAssumptions


Recall: Throughput of CSMA (2/2)

Parameters

Normalized Parameters

Result:


Example: Small Data Packets (1/2)Signal propagation speed ¼ 2 ¢ 108 m/sMaximum node distance 300 mIEEE 802.11b: 11 Mbps data rateCompute a for 20 Byte data frame size …


Example: Small Data Packets (2/2)

Throughput over offered load forCSMA with RTS/CTS?


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