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Wireless MAC-1

MotivationHidden Terminal ProblemCapture EffectNear/Far effect

SDMA, FDMA, TDMA

Aloha & S-Aloha

Reservation schemes

Collision avoidance, MACA

Polling

CDMA

SAMA

Comparison

Ack: Many slides are due to J. Schiller

Wireless Medium Access ControlWireless Medium Access Control

Wireless MAC-2

MotivationMotivation

Can we apply media access methods from fixed networks?

Example CSMA/CD

Carrier Sense Multiple Access with Collision Detection

send as soon as the medium is free, listen into the medium if a collision

occurs (original method in IEEE 802.3)

Problems in wireless networks

signal strength decreases proportional to the square of the distance

the sender would apply CS and CD, but the collisions happen at thereceiver

it might be the case that a sender cannot hear the collision, i.e., CD does

not work furthermore, CS might not work if, e.g., a terminal is hidden

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Wireless MAC-3

Hidden Terminal ProblemHidden Terminal Problem

A and B can be heard by BS butnot by each other

Therefore carrier sensing does notwork

Base Station

A

B

Wireless MAC-4

Hidden terminals

A 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 C

Exposed terminals

B sends to A, C wants to send to another terminal (not A or B) C has to wait, CS signals a medium in use

but A is outside the radio range of C, therefore waiting is notnecessary

C is exposed to B

MotivationMotivation -- hidden and exposed terminalshidden and exposed terminals

BA C

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Wireless MAC-5

Near/Far EffectNear/Far Effect

A and B both send at same power

B is much closer to the BS than A

Bs signal drowns out As signal at

the BS

=> BS cannot receive A

Power control is used to alleviatethis.

Base Station

A

B

Wireless MAC-6

Terminals A and B send, C receives

signal strength decreases proportional to the square of the distance

the signal of terminal B therefore drowns out As signal

C cannot receive A

If C for example was an arbiter for sending rights, terminal B woulddrown out terminal A already on the physical layer

Also severe problem for CDMA-networks - precise power control

needed!

MotivationMotivation -- near and far terminalsnear and far terminals

A B C

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Wireless MAC-7

Capture EffectCapture Effect

When 2 packets collide, all is notnecessarily lost

Stronger collider may be receivedcorrectly

Base Station

A

B

Wireless MAC-8

Access methods SDMA/FDMA/TDMAAccess methods SDMA/FDMA/TDMA

SDMA (Space Division Multiple Access)

segment space into sectors, use directed antennas

cell structure

FDMA (Frequency Division Multiple Access)

assign a certain frequency to a transmission channel between a sender

and a receiver

permanent (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 asender and a receiver for a certain amount of time

Multiplexing schemes are now used to control medium access!

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Wireless MAC-9

FDD/FDMAFDD/FDMA -- general schemegeneral scheme

example GSMexample GSM

f

t

124

1

124

1

20 MHz

200 kHz

890.2 MHz

935.2 MHz

915 MHz

960 MHz

Wireless MAC-10

TDD/TDMATDD/TDMA -- general scheme, example DECTgeneral scheme, example DECT

1 2 3 11 12 1 2 3 11 12

tdownlink uplink

417 s

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Wireless MAC-11

ALOHAALOHA

Station transmits a packet independently of other network users

Random, distributed (no central arbiter), time-multiplex

Maximum throughput ~18%

Why? Assume:

Large number of contending stations

Poisson arrival rate of packets packets per sec

Fixed packet transmission time hsec/packet

Colliding packets retransmitted after a reasonably long time

Rate of (fresh + retransmitted) packets is Poisson, rate packets persec

sender A

sender B

sender C

collision

t

Wireless MAC-12

ALOHAALOHA

he

'2}periodblein vulneraarrivalsnoPr{ =

h

Test Packet

2 h

Vulnerable period

( ) '2'2 '' === eheh h

'2' = e

0.20

1.0

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Wireless MAC-13

Slotted ALOHASlotted ALOHA

Slot length = packet length

Vulnerable period reduced from 2hto h

Maximum throughput increased to 37%

h

Test Packet

h

Vulnerable period

'' = e

sender A

sender B

sender C

collision

t

Constrain packet transmissions to begin at slot boundaries.

Wireless MAC-14

Carrier SensingCarrier SensingCSMA and CSMA/CD

Sense (i.e. listen) before transmission

Maximum throughput depends on normalised one-way propagationdelay a(ratio of propagation delay to time to transmit one packet)

0.00

0.20

0.40

0.60

0.80

1.00

Maximum

Throughput

0.001 0.01 0.1 1

a

CSMA/CD

Non-persistent CSMA

Persistent CSMA

S-ALOHA

ALOHA

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Wireless MAC-15

DAMADAMA -- Demand Assigned Multiple AccessDemand Assigned Multiple Access

Channel efficiency only 18% for Aloha, 36% for Slotted Aloha (assuming

Poisson distribution for packet arrival and packet length)Reservation can increase efficiency to 80%

a sender reservesa future time-slot

sending within this reserved time-slot is possible without collision

reservation also causes higher delays

typical scheme for satellite links

Examples for reservation algorithms:

Explicit Reservation according to Roberts (Reservation-ALOHA)

Implicit Reservation (PRMA)

Reservation-TDMA

Minislots used for

reservationReservable

slots

Wireless MAC-16

Access method DAMA: Explicit ReservationAccess method DAMA: Explicit Reservation

Explicit Reservation (Reservation Aloha):

two modes:

ALOHA modefor reservation:competition for small reservation slots, collisions possible

reserved modefor data transmission within successful reserved slots (nocollisions possible)

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

Aloha reserved Aloha reserved Aloha reserved Aloha

collision

t

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Wireless MAC-17

Access method DAMA: PRMAAccess method DAMA: PRMA

Implicit reservation (PRMA - Packet Reservation MA):

a certain number of slots form a frame, frames are repeated stations compete for empty slots according to the slotted aloha

principle

once a station reserves a slot successfully, this slot is automatically

assigned to this station in all following frames as long as the stationhas data to send

competition for this slots starts again as soon as the slot was emptyin the last frame

frame1

frame2

frame3

frame4

frame5

1 2 3 4 5 6 7 8 time-slot

collision at

reservationattempts

A C D A B A F

A C A B A

A B A F

A B A F D

A C E E B A F Dt

ACDABA-F

ACDABA-F

AC-ABAF-

A---BAFD

ACEEBAFD

reservation

Wireless MAC-18

Access method DAMA: ReservationAccess method DAMA: Reservation--TDMATDMA

Reservation Time Division Multiple Access

every frame consists of N mini-slots and x data-slots

every 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

round-robin sending scheme (best-effort traffic)

N mini-slots N * k data-slots

reservationsfor data-slots

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

e.g. N=6, k=2

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Wireless MAC-19

MACAMACA -- collision avoidancecollision 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 packet

CTS (clear to send): the receiver grants the right to send as soon as it is

ready to receive

Signaling packets contain

sender address

receiver address

packet size

Variants of this method can be found in IEEE802.11 as DFWMAC

(Distributed Foundation Wireless MAC)

Wireless MAC-20

MACA avoids the problem of hidden terminals

A and C want to

send to B

A sends RTS first

C waits after receiving

CTS from B

MACA avoids the problem of exposed terminals

B wants to send to A, C

to another terminal

now C does not haveto wait for it cannot

receive CTS from A

MACA examplesMACA examples

A B C

RTS

CTSCTS

A B C

RTS

CTS

RTS

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Wireless MAC-21

MACA variant: DFWMAC in IEEE802.11MACA variant: DFWMAC in IEEE802.11

idle

wait for theright to send

wait for ACK

sender receiver

packet ready to send; RTS

time-out;RTS

CTS; data

ACK

RxBusy

idle

wait fordata

RTS; RxBusy

RTS;CTS

data;ACK

time-out data;NAK

ACK: positive acknowledgement

NAK: negative acknowledgement

RxBusy: receiver busy

time-out NAK;RTS

Wireless MAC-22

Polling mechanismsPolling mechanisms

If one terminal can be heard by all others, this central terminal

(a.k.a. base station) can poll all other terminals according to acertain scheme

now all schemes known from fixed networks can be used (typical

mainframe - terminal scenario)

Example: Randomly Addressed Polling

base station signals readiness to all mobile terminals

terminals ready to send can now transmit a random number without

collision with the help of CDMA or FDMA (the random number canbe seen as dynamic address)

the base station now chooses one address for polling from the list of

all random numbers (collision if two terminals choose the sameaddress)

the base station acknowledges correct packets and continues polling

the next terminal

this cycle starts again after polling all terminals of the list

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Wireless MAC-23

ISMA (Inhibit Sense Multiple Access)ISMA (Inhibit Sense Multiple Access)

Current state of the medium is signaled via a busy tone

the base station signals on the downlink (base station to terminals) if themedium is free or not

terminals must not send if the medium is busy

terminals can access the medium as soon as the busy tone stops

the base station signals collisions and successful transmissions via the

busy tone and acknowledgements, respectively (media access is not

coordinated within this approach)

mechanism used, e.g.,

for CDPD(USA, integrated

into AMPS)

Wireless MAC-24

Access method CDMAAccess method CDMA

CDMA (Code Division Multiple Access)

all terminals send on the same frequency probably at the same time and

can use the whole bandwidth of the transmission channel

each sender has a unique random number, the sender XORs the signalwith this random number

the receiver can tune into this signal if it knows the pseudo randomnumber, tuning is done via a correlation function

Disadvantages:

higher complexity of a receiver (receiver cannot just listen into themedium and start receiving if there is a signal)

all signals should have the same strength at a receiver

Advantages:

all terminals can use the same frequency, no planning needed

huge code space (e.g. 232) compared to frequency space

interferences (e.g. white noise) is not coded

forward error correction and encryption can be easily integrated

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Wireless MAC-25

Aloha has only a very low efficiency, CDMA needs complex receivers to

be able to receive different senders with individual codes at the sametime

Idea: use spread spectrum with only one single code (chipping sequence)for spreading for all senders accessing according to aloha

SAMASAMA -- Spread Aloha Multiple AccessSpread Aloha Multiple Access

1sender A0sender B

0

1

t

narrowband

send for ashorter periodwith higher power

spread the signal e.g. using the chip sequence 110101 (CDMA without CD)

Problem: find a chip sequence with good characteristics

1

1

collision

Wireless MAC-26

Comparison SDMA/TDMA/FDMA/CDMAComparison SDMA/TDMA/FDMA/CDMA

Approach SDMA TDMA FDMA CDMA

Idea segment space intocells/sectors

segment sendingtime into disjointtime-slots, demanddriven or fixedpatterns

segment thefrequency band intodisjoint sub-bands

spread the spectrumusing orthogonal codes

Terminals only one terminal canbe active in onecell/one sector

all terminals areactive for shortperiods of time onthe same frequency

every terminal has itsown frequency,uninterrupted

all terminals can be activeat the same place at thesame moment,uninterrupted

Signalseparation

cell structure, directedantennas

synchronization inthe time domain

filtering in thefrequency domain

code plus specialreceivers

Advantages very simple, increasescapacity per km

established, fullydigital, flexible

simple, established,robust

flexible, less frequencyplanning needed, softhandover

Dis-advantages

inflexible, antennastypically fixed

guard spaceneeded (multipathpropagation),

synchronizationdifficult

inflexible,frequencies are ascarce resource

complex receivers, needsmore complicated powercontrol for senders

Comment only in combinationwith TDMA, FDMA orCDMA useful

standard in fixednetworks, togetherwith FDMA/SDMAused in manymobile networks

typically combinedwith TDMA(frequency hoppingpatterns) and SDMA(frequency reuse)

still faces some problems,higher complexity,lowered expectations; willbe integrated withTDMA/FDMA