CCM 4300 Lecture 14Computer Networks, Wireless and Mobile

Communication Systems

1

Introduction to Wireless Networks - II

Dr S Rahman

Session Content� Recap of last session

� Lesson Objectives

� Frequency Hopping Spread Spectrum

� Cell based networks

2

� Frequency planning

�Wireless Media Access Control Methods

o Reservation ALOHA (R-ALOHA)

� TDMA, FDMA, CDMA

� Distribution Foundation Wireless MAC (DFWMAC)

Recap of Last Session

� Why use wireless networks instead of wired

� Basics of wireless networks

� Wireless network limitations and solutions

� Intro to existing wireless access technologies

3

� Radio Connectivity and Diffusion modes

� Intro to Spread Spectrum techniques

� Direct Sequence Spread Spectrum (DS-SS)

Lecture objectivesAt the completion of this lecture you should be able to

� Understand what is Frequency Hopping Spread Spectrum (FHSS) (Slow and Fast FHSS)

� Compare between FHSS and DS-SS (Direct Sequence Spread Spectrum)

� Understand the need for specific support for mobile and wireless scenarios

4

� Understand the need for specific support for mobile and wireless scenarios

�Understand and analyse the problems:

• with wireless networks compared to wired networks

• in running existing protocols in a mobile scenario

� Understand Frequency planning and Cell structure in mobile networks

Introduction

� Frequency-hopping spread spectrum (FHSS) – signal is

broadcast over a seemingly random series of radio

frequencies, hoping from frequency to frequency at fixed

intervals.

� Direct sequence spread spectrum (DSSS) – each bit in

the original signal is represented by multiple bits in the the original signal is represented by multiple bits in the

transmitted signal, using a spreading code.

� Code division multiple access (CDMA) – enable multiple

users to independently use the same bandwidth

5

Frequency Hopping Example

6

Stallings Figure 9.2: A number of channels are allocated for the FH signal. Typically, there are 2k carrier frequencies forming 2k channels. The spacing between carrier frequencies and hence the width of each channel usually corresponds to the bandwidth of the input signal. The transmitter operates in one channel at a time for a fixed interval. During that interval, some number of bits is transmitted using some encoding scheme. Both transmitter and receiver use the same code to tune into a sequence of channels in synchronization.

Frequency hopping spread spectrum

• Bandwidth split into:

•channels

• Hopping sequence:

•Tx hops between channels

•psuedorandom hop code

• chip period:

Frequency channel numbers

0 1 2 3 4 5 6

0236 A’s code

6320 B’s code

N-bits

N-bits

a

ba

ab

ab

ab

ab

b

ab

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• chip period:

•hold time on a channel

•chipping rate:

•hopping rate

•Good Tx/Rx sync required

time

a

N-bits

N-bits

N-bits

a

b

b

bb

ab

ab

ab

ab

ab

a

b

ab

a

802.11 uses 79 1MHz channels, it hops 400 ms or less (2.5 hops or more per second), min hop size 6MHz

Fast frequency hopping

• Multiple chips per bit,

e.g., 3 hops/bit

• Good noise immunity

• More expensive than slow

frequency hopping

Slow frequency hopping

• Multiple bits per chip, e.g., 3

bits/hop

• Easier to sync than fast

frequency hopping

• Not as good immunity to

Frequency hopping spread spectrum

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frequency hopping

• Hard to sync Tx and Rx

• Not as good immunity to

noise as fast frequency

hopping

- Input bit stream is encoded 2 bits at a time, with each of the four possible 2-bit combinations transmitted as a different frequency (p. 145, 4th ed., Stallings)- Noisy channel can be dropped from hopping sequence

� A common modulation technique used in conjunction with FHSS is

multiple FSK (MFSK), which uses M = 2L different frequencies to

encode the digital input L bits at a time.

� For FHSS, the MFSK signal is translated to a new frequency every Tc

seconds by modulating the MFSK signal with the FHSS carrier signal.

� The effect is to translate the MFSK signal into the appropriate FHSS

channel. For a data rate of R, the duration of a bit is T = 1/R seconds

and the duration of a signal element is Ts = LT seconds.

Multiple FSK (Stallings, 4th ed.: Data and Computer Communications, Ch. 9)

and the duration of a signal element is Ts = LT seconds.

� If Tc is greater than or equal to Ts, the spreading modulation is referred

to as slow-frequency-hop spread spectrum; otherwise it is known as

fast-frequency-hop spread spectrum.

� Typically, a large number of frequencies is used in FHSS so that

bandwidth of the FHSS signal is much larger than that of the original

MFSK signal.

� One benefit of this is that a large value of k results in a system that is

quite resistant to jamming.

� If frequency hopping is used, the jammer must jam all 2k frequencies.

� In general, fast FHSS provides improved performance compared to

slow FHSS in the face of noise or jamming. 9

Slow and Fast FHSS

� MFSK uses M = 2L different frequencies to encode the

digital input L bits at a time.

� For FHSS, the MFSK signal is translated to a new

frequency every Tc seconds by modulating the MFSK

signal with the FHSS carrier signal.

� For a data rate of R, the duration of a bit is T = 1/R

seconds and the duration of a signal element is Ts = LT

seconds.

� Slow FHSS has Tc ≥ Ts

� Fast FHSS has Tc < Ts

� FHSS quite resistant to noise or jamming

� with fast FHSS giving better performance

Slow FHSS

� Stallings Figure 9.4 - shows an example of slow FHSS

� Here we have M = 4, which means that four different

frequencies are used to encode the data input 2 bits at a

time.

� Each signal element is a discrete frequency tone, and

the total MFSK bandwidth is Wd = Mfd. the total MFSK bandwidth is Wd = Mfd.

� We use an FHSS scheme with k = 2. That is, there are 4

= 2k different channels, each of width Wd.

� The total FHSS bandwidth is Ws = 2kWd. Each 2 bits of

the PN sequence is used to select one of the four

channels. That channel is held for a duration of two

signal elements, or four bits (Tc = 2Ts = 4T).

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Slow MFSK FHSS(Stallings: Data and Computer Communications)

Fast MFSK FHSS

� Stallings Figure 9.5 - shows an example of fast FHSS.

� Again, M = 4 and k = 2.

� In this case, however, each signal element is

represented by two frequency tones.

� Again, Wd = Mfd and Ws = 2kWd.

In this example T = 2T = 2T. � In this example Ts = 2Tc = 2T.

� In general, fast FHSS provides improved performance

compared to slow FHSS in the face of noise or jamming.

� For example, if three or more frequencies (chips) are

used for each signal element, the receiver can decide

which signal element was sent on the basis of a majority

of the chips being correct.

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Fast MFSK FHSS(Stallings: Data and Computer Communications)

Slow and fast FHSS

15

FHSS vs. DSSS� DSSS

� Ease of implementation

� High data rates 1, 2, 5.5 and 11 Mbps in 2.4 GHz ISM band

� has better immunity to noise

� has less latency, no pause while channel hops

� supplies a large per network bandwidth 11Mb/S

allows just 3 networks to coexist

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� allows just 3 networks to coexist

� FHSS� allows 26 networks to coexist

� has aggregate bandwidth of 52Mb/s, supplies 2Mb/s

� uses less power, better for portable devices

� cheaper to build

� degrades more gracefully under heavy load

Wireless LANs

• Infrastructure Wireless

•wireless connectivity to a fixed network, e.g., PDA

•fixed wire replacement e.g. laptops

•portable access unit (PAU) AP AP: Access Point

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•portable access unit (PAU)

•Ad hoc wireless

•totally wireless network

•communication only between portable devices

APAP

AP

wired network

AP: Access Point

Radio frequency usage.

•Infrastructure wireless

•LANs

•WANs

•Limited frequency use

•Limited frequency allocation

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•Limited frequency allocation

•LAN: ISM band

•WAN: regulatory controls

•How to support large number of users

•limited radio/(electrical bandwidth

•shared media? Bandwidth

Cell-based network•Radio-based mobile communication

•Digital mobile telephones:

•privacy

•data/voice/X•extendable network

•network topologyBS

BS

BS

BS

BS

BS

BS

BS

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•network topology

•cells

•base-stations•LAN/PAN vs. WAN:

•Connectionless, shared media vs. circuit switched•3G wireless – connectionless

•Base Stations – covers a certain area, a cell

•interconnected by terrestrial network

BS

BS

BS

BS

BS

BS

Frequency planning I

� Frequency reuse only with a certain distance between the base stations

� Standard model using 7 frequencies:f4

f5

f1

f3

f6

f7

f3

f2

f4

f5

f1

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� Fixed frequency assignment:� certain frequencies are assigned to a certain cell

� problem: different traffic load in different cells

� Dynamic frequency assignment:� base station chooses frequencies depending on the frequencies already used in neighbor

� more capacity in cells with more traffic

f2

Frequency planning II

f1

f2

f3f2

f1

f1

f2

f3f2

f3

f1

f2f1

f3f3

f3f3

f3

f4

f5

f1f3

f2

f6

f7

f3f2

f4

f5

f1f3

f5f6

f7f2

f2

3 cell cluster

Cells are combined in

clusters – three cells form a

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f1f1 f1f2f3

f2f3

f2f3

h1

h2

h3g1

g2

g3

h1

h2

h3g1

g2

g3g1

g2

g3

7 cell cluster

3 cell cluster

with 3 sector antennas (three sectors per cell

in a cluster with three cells)

clusters – three cells form a

cluster. Similarly seven

cells form a cluster

Cell-based networks

•Problems - fading:(shadowing,

multipath)

• interference due to

scattering of signal

• BER:

• ~10 -3 possible

• Some “fading factors”:

• free space loss

• street orientation•Variations of up to20dB

• foliage

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• ~10 possible

• FEC for data

• Network planning:

• surveys of propagation

characteristics

• foliage•Variations of 18dB between summer and winter

• tunnels•signal attenuation of up to 30dBAny solution to signal fading?

increase the transmitter power, is not available in mobile communication where transmitter power is limited.

Cell-based networks

•LAN/PAN technology:

• usually ISM (IR possible)

• a handful of high bandwidth channels

• media-access control

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• Smaller cell-size:

• micro-cells

• pico-cells

• use power detection to select “best” base-station

Media access control in WLANs

•Distributed and centralised MACs

•MAC – wireless LANs

•Hidden terminal and exposed terminal problems

•chapters 2 and 3 from Schiller 3rd edition

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•chapters 2 and 3 from Schiller 3rd edition•Mobile Communications by Jochen Schiller Softcover, Pearson Education, Limited, ISBN 0321123816 (0-321-12381-6)

•Key Questions:

•How to deal with connection in wireless LANs?

•How can you ensure that a terminal can receive a

transmission?

Centralised vs Distributed

Centralised

•Central controller:

•Signalling channel

•Connection based system

� Coordination

Distributed

� low latency

•general data application

•ad-hoc networks

� Better network utilisation

•MAC schemes can be centralised or decentralised.

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� Coordination

•synchronisation

•relay: full connectivity

� Resource control:

•allocation of capacity

χ Additional latency

χ Single point of failure

�Recovery protocol possible

� Better network utilisation

� Reliability

•no single point of failure

χ Increased complexity

•coordination mechanisms

•connectivity handshakes

•QoS ?

Wireless MAC methods•ALOHA:

•Pure Aloha (Covered in Lecture 4)

•Slotted Aloha (Covered in Lecture 4)

•R-ALOHA – Portable Access Unit (PAU) controls reservations (NEW)

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•CSMA/CA: (Covered in Lecture 5)

•Non-persistent, persistent and p-persistent

•CSMA/CD: (Covered in Lecture 5)

•Modification – collision detection comb (NEW)

•TDMA, FDMA, CDMA (Covered in Lecture 4)

•DFWMAC

Reservation ALOHASlot user

�R-Aloha:

•slots arranged in frames

•TDM channels: reservation

•unused slots up for grabs

•80% efficiency

A

A

C

D

A

BD

Slot allocation

A

B

C

D

A

Bcollisions

Unused

slots

time

WHY Collision?

Because B has not used and more than one other claimed the slot

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•80% efficiency

•Simple

•Possibility of sending without

collisions

χ R-Aloha:

•high latency

BD

C

D

A

B

C

D

Slots

reclaimed

B

C

D

A

B

C

D

collisions

Now after the backoff

B has something to send and it can reclaim the reserved slot

Reservation ALOHA

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CSMA•Carrier Sense Multiple Access (CSMA):

•if channel is free, transmit

•persistent

•At the receiver:

•checksum detects collision

•Non-persistent CSMA:

S1 S2

D

bit1

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•Non-persistent CSMA:

•random time back-off

•increased delay

•CSMA/CA

•P-persistent CSMA:

•transmit with probability P

•increased delay (1 - p)

time

bit1

bit1Tp

TF

Tp = D/V

CSMA/CD: comb•Pseudo-random bit pattern

•Comb

•Station(s) to transmit:

•First transmits comb

•For a 1, transmit

1000

1100

1110

A

A, B, C in

B, C, in A out

C in A, B out

BC

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•For a 1, transmit

•For a 0, listen

•Stations in contention

•“drop out” as they listen during a 0

C in A, B out

C can transmit

Protocol: generate a short pseudo-random sequence and put at front of preamble wait for medium to be quiet if sequence bit a ONE then transmit if sequencet bit a ZERO listen if receive a bit while listening

drop out of competition

TDMA: Time Division Multiple Access•Channel allocation:

• time-frame with fixed

number of time-slots• signalling time-slot

• source requests a time-slot

• Portable access unit :

• listens on signaling time-slot (0)

Time

frame

0

1

2

3

4

5

6

0

Time slot

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• listens on signaling time-slot (0)

for requests

• assigns channel to source

• source uses time-slot for a

single frame

• S-ALOHA with demandassignment time-slot

0

1

2

3

4

5

6

time

FDMA: Frequency Division Multiple Access

• Channel allocation:

• fixed number of frequency

channels

• signalling channel

• source requests a channel

• Portable access unit :

• listens on signalling

Frequency channel numbers

0 1 2 3 4 5

32

• listens on signalling

channel (0) for requests

• assigns channel to source

• source uses channel for a

single frame

Note: can use CSMA/CA or Aloha For signalling channel

CDMA: Code division Multiple access• Frequency hopping:

• multiple frequency

channels

• part of message transmitted

on each channel

• channel hopping sequence

is a code

Frequency channel numbers

0 1 2 3 4 5 6

a

ab

b

0236 A’s code

3542 B’s code

N-bits

N-bits

33

is a code

• each station has a different

code

• Slow frequency-hopping:

• transmit N bits then hop

a

ab

b

b

time

N-bits

N-bits

N-bits

CDMA continues

•DS CDMA also possible:

• code is pseudorandom number (PN)

• controller allocates station allocates PN

• Rx and Tx use same PN for a transmission

� Good noise immunity

� Soft hand-off using two codes

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� Soft hand-off using two codes

χNeeds very good synchronisation:

• large overhead to synchronisation mechanism

χ Complex to use than FDMA and TDMA

Hidden terminal and exposed terminal• A ⇔ B: OK

• A ⇔ C: OK

• B ⇔ C: not OK

• If C transmits to A, B

could also transmit

• A ⇔ B: OK

• C ⇔ D: OK

• C can “overhear” B:

• C will not transmit

when B transmits

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A

CB

A

B C D

B is hidden to CC is exposed to B

Near and far terminal

� Signal drowning!

� single strength decreases proportional to the square of the distance

� Consider terminals A, B send and C receive

� the signal of terminal B therefore drowns out A’s

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� the signal of terminal B therefore drowns out A’s signal as a consequence

� C cannot receive A

A B C

Multiple Access with 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 packet

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

� Signaling packets contain

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� 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)

Distribution Foundation Wireless MAC• Source and destination in

contact?

• DFWMAC:

• four-way handshake

• src: RTS

• dst: RxBUSY or CTS

• src: DATA

PAUPAURTS

CTS

ACK

data

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• src: DATA

• dst: ACK

• Used with any MAC

transmission method

• Also called RTS-CTS

Time-out

Portable device

or PAUPAU

RTS

RTSCTS

ACK

Rx busy

data

RTS: request to sendCTS: clear to send

MACA variant: DFWMAC in IEEE802.11

idle

wait for the

sender receiver

packet ready to send; RTS

time-out;

RTS

RxBusy

idle

RTS;

data;

ACK

time-out ∨

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wait for the

right to send

wait for ACK

RTS

CTS; data

ACK

wait for

data

RTS; RxBusy

RTS;

CTStime-out ∨

data;

NAK

ACK: positive acknowledgement

NAK: negative acknowledgementRxBusy: receiver busy

RTS: request to send

CTS: clear to send

time-out ∨

NAK;

RTS

Can MACA avoid hidden/exposed trmnl?

� 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

RTS

CTSCTS

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� MACA avoids the problem of exposed terminals� B wants to send to A, C to another terminal

� now C does not have to wait for it cannot receive CTS from A

A B C

CTSCTS

A B C

RTS

CTS

RTS

Summary

• Centralised wireless MACs:

• TDMA

• FDMA

• CDMA

• R-ALOHA

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• Distributed wireless MACs:

• ALOHA

• CSMA/CA

• CSMA/CD comb

• DFWMAC

• Frequency planning and cell- structure in a mobile network