mc - ch03 media access jochen schiller

Mobile CommunicationsMobile CommunicationsChapter 3 : Media Access

Motivation Collision avoidance, MACASDMA, FDMA, TDMA Aloha and contention

based schemes

PollingCDMA (Lecture 6)

based schemesReservation schemes

Prof. Dr.-Ing. Jochen Schiller, http://www.jochenschiller.de/ MC SS02 3.1

FROM HAWAII TO CSMA/CD RANDOM ACCESS

MAC protocols: here we look at some so calledcontention protocols!

University of Hawaii aloha networkAloha data packetAloha data packetpure alohaslotted alohaCSMANon persistent--persistent CSMA

CSp-persistent CSMACSMA/CD


ALOHA NET

TCU TTYTCUGRAPHICS

PCUGRAPHICS TCU TTY

MINIPCUTTY

CONCENTR.TTYCONCENTR.

TTY

MINI-COMPUTER

ALOHANET

TTY TTYCENTRAL STATION

GATEWAY

ATSGROUNDSTATION

BCC 500 IBM 370/158TIP

ARPANETPACNETINTELSAT IV

GROUNDSTATION

ARPANETPACNET


ALOHA PACKET

32 bits longcontains user ID

and packet

Max 80 bytes -

bi

d D t

and packet length

640 bits

HeaderHeadercheck-sum Data

Datacheck-sumsum

16 bi 16 bi16 bits long

16 bits long

At 9,600 bps the maximum transmission time is 73 msec


BASIC ALOHA

Full duplexOriginally UHF communicationIncoming traffic> 9,600 bps (403.500 MHz)> Random access (pure aloha)

Outgoing traffic> 9,600 bps (413.474 MHz)> Broadcast


PURE ALOHA

Whenever a station has something to send, it sends.Whenever a station has something to send, it sends.When a packet is received by the central, an acknowledgement is sent back in broadcast. If the sending station does not receive an “ack” within a set time, a collision is assumed.When a collision, retransmit within a random time slot, 200-1500 ms.e a co s o , et a s t t a a do t e s ot, 00 500 s


SLOTTED ALOHA

Packets may just be transmitted within time slots.If a station has started to transmit in a time slot, other station who wish ,to transmit within this time slot can not interfere. This principle leads to a much better utilization of the channel.

BANG!!BANG!!


ALOHA PERFORMANCE

Performance of ALOHA, S-ALOHA with a=0

0.35

0.4

0 2

0.25

0.3

ughp

ut)

0.1

0.15

0.2

S (T

hrou

Slotted ALOHAPure ALOHA1/e

0

0.05

0 0 5 1 1 5 2 2 5 3 3 5 40 0.5 1 1.5 2 2.5 3 3.5 4G (Offered Channel Traffic)

1/2e


CSMA Carrier Sense Multiple Access

A station who wish to send starts listening

Carrier SenseIf the channel is free, it starts to sendIf the channel is busy the station waits to send, the station might be persistentAcknowledgementAcknowledgement

LBT: Listen Before Talk


CSMA

Wait a random time SendCarrier

Sense IdleBusyNon-persistent

CSMA

SendCarrier

SenseBusyp

1Idlep-Persistent

CSMASelfi

sh

SenseyDelay 1 time unit

1-pCSMA(after collision)

S

Carrier SendIdleBPersistent

CS A Sense SendIdleBusyCSMA


•CSMA/CD

Carrier Sense Multiple Access / Collision Detection

Carrier Sense, if the channel is free, sendThe station listens when sendingThe station listens when sendingWhen a station detects a collision, it stops sendingRetransmissions


Motivation

Can we apply media access methods from fixed networks?

E l CSMA/CDExample CSMA/CDCarrier Sense Multiple Access with Collision Detectionsend as soon as the medium is free, listen into the medium if a collision occurs (original method in IEEE 802.3)

Problems in wireless networkssignal strength decreases proportional to the square of the distancesignal strength decreases proportional to the square of the distancethe sender would apply CS and CD, but the collisions happen at the receiverit i ht b th th t d t “h ” th lli i i CD dit might be the case that a sender cannot “hear” the collision, i.e., CD does not workfurthermore, CS might not work if, e.g., a terminal is “hidden”


Motivation - hidden and exposed terminals

Hidden terminalsA sends to B, C cannot receive A C wants to send to B, C senses a “free” medium (CS fails)collision at B, A cannot receive the collision (CD fails)A is “hidden” for CA is hidden for C

Exposed terminals BA CExposed terminalsB sends to A, C wants to send to another terminal (not A or B)C has to wait, CS signals a medium in usebut A is outside the radio range of C, therefore waiting is not necessaryC is “exposed” to B


Motivation - near and far terminals

Terminals A and B send, C receivessignal strength decreases proportional to the square of the distancethe signal of terminal B therefore drowns out A’s signalC cannot receive A

A B C

Also severe problem for CDMA-networks - precise power control needed!


Access methods SDMA/FDMA/TDMA

SDMA (Space Division Multiple Access)segment space into sectors, use directed antennas

ll t tcell structure

FDMA (Frequency Division Multiple Access)assign a certain frequency to a transmission channel between a sender and a receiverpermanent (e.g., radio broadcast), slow hopping (e.g., GSM), fast hopping (FHSS, Frequency Hopping Spread Spectrum)

TDMA (Time Division Multiple Access)assign the fixed sending frequency to a transmission channel between a sender and a receiver for a certain amount of time

The multiplexing schemes presented in chapter 2 are now used to control medium access!


FDD/FDMA - general scheme, example GSM

f

124960 MHz

1 200 kHz935.2 MHz

124

20 MHz

915 MHz

t

1890.2 MHz


TDD/TDMA - general scheme, example DECT

1 2 3 11 12 1 2 3 11 12

417 µs

1 2 3 11 12 1 2 3 11 12

tdownlink uplink


DAMA - Demand Assigned Multiple Access

Channel efficiency only 18% for Aloha, 36% for Slotted Aloha (assuming y y ( gPoisson distribution for packet arrival and packet length)

Reservation can increase efficiency to 80%a sender reserves a future time slota sender reserves a future time-slotsending within this reserved time-slot is possible without collisionreservation also causes higher delaystypical scheme for satellite links

Examples for reservation algorithms:Explicit Reservation according to Roberts (Reservation-ALOHA)Explicit Reservation according to Roberts (Reservation-ALOHA)Implicit Reservation (PRMA)Reservation-TDMA


Access method DAMA: Explicit Reservation

Explicit Reservation (Reservation Aloha):two modes:

ALOHA mode for reservation:ALOHA mode for reservation:competition for small reservation slots, collisions possible reserved mode for data transmission within successful reserved slots (no collisions possible)p )

it is important for all stations to keep the reservation list consistent at any point in time and, therefore, all stations have to synchronize from time to time

collision

Aloha reserved Aloha reserved Aloha reserved Alohat


Access method DAMA: PRMA

Implicit reservation (PRMA - Packet Reservation MA):a certain number of slots form a frame, frames are repeatedstations compete for empty slots according to the slotted alohastations compete for empty slots according to the slotted aloha principleonce a station reserves a slot successfully, this slot is automatically assigned to this station in all following frames as long as the stationassigned to this station in all following frames as long as the station has data to sendcompetition for this slots starts again as soon as the slot was empty in the last framein the last frame

frame

1 2 3 4 5 6 7 8 time-slot

A C D A B A FACDABA-F

reservation

frame1

frame2

frame3collision at

i

A C D A B A F

A C A B A

A B A F

ACDABA-F

AC-ABAF-

A BAFD 3

frame4

frame5

reservation attemptsA B A F D

A C E E B A F Dt

A---BAFD

ACEEBAFD


t

Access method DAMA: Reservation-TDMA

Reservation Time Division Multiple Access every frame consists of N mini-slots and x data-slotsevery station has its own mini-slot and can reserve up to k data-slots using this mini-slot (i.e. x = N * k).other stations can send data in unused data-slots according to a ground-robin sending scheme (best-effort traffic)

N * k data slotse.g. N=6, k=2

N mini-slots N * k data-slots

reservations other stations can use free data-slotsfor data-slots

other stations can use free data slotsbased on a round-robin scheme


MACA - collision avoidance

MACA (Multiple Access with Collision Avoidance) uses short signaling packets for collision avoidance

RTS (request to send): a sender request the right to send from a receiver with a short RTS packet before it sends a data packetCTS (clear to send): the receiver grants the right to send as soon as it is ready to receive

Signaling packets containsender addressreceiver addresspacket sizepacket size

Variants of this method can be found in IEEE802.11 as DFWMAC (Distributed Foundation Wireless MAC)


MACA examples

MACA avoids the problem of hidden terminalsA and C want to send to Bsend to BA sends RTS firstC waits after receiving CTS from B

RTS

CTS from B

A B CCTSCTS

MACA avoids the problem of exposed terminalsB wants to send to A, C to another terminalto another terminalnow C does not have to wait for it cannot receive CTS from A

RTS RTS

receive CTS from A

A B CCTS


mc - ch03 media access jochen schiller

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