ecen4533 data communications lecture #7 23 january 2013 dr. george scheets

Read 3.3 - 3.5Read 3.3 - 3.5 Problems Web 3-11, 2010 Quiz #1Problems Web 3-11, 2010 Quiz #1 Design #1 due 1 February (Live)Design #1 due 1 February (Live)

8 February (Async DL)8 February (Async DL) Late = -1 per working dayLate = -1 per working day

Quiz #1Quiz #1 Lecture 12, 4 February (Live)Lecture 12, 4 February (Live) << 11 February (Async Distance Learning) 11 February (Async Distance Learning)

ECEN4533 Data CommunicationsECEN4533 Data CommunicationsLecture #7 23 January 2013Lecture #7 23 January 2013Dr. George ScheetsDr. George Scheets


Read: 4.1 - 4.3Read: 4.1 - 4.3 Problems: Quiz #1 for '11 & '12Problems: Quiz #1 for '11 & '12 Design #1 due 1 February (Live)Design #1 due 1 February (Live)

8 February (Async DL)8 February (Async DL) Late = -1 per working dayLate = -1 per working day

Quiz #1Quiz #1 Lecture 12, 4 February (Live)Lecture 12, 4 February (Live) << 11 February (Async Distance Learning) 11 February (Async Distance Learning)



MultiplexingMultiplexing

How a chunk of frequency bandwidth gets How a chunk of frequency bandwidth gets broken up into smaller broken up into smaller channelschannels...... FDMFDM TDMTDM Stat MuxStat Mux CDMCDM

Each channel can support one Each channel can support one conversationconversation..

Direct Sequence Spread SpectrumDirect Sequence Spread Spectrum

The transmitter multiplies the bit stream The transmitter multiplies the bit stream with a high-speed pseudo-random with a high-speed pseudo-random spreading signal.spreading signal.

If the receiver has a replica of the If the receiver has a replica of the spreading signal, properly aligned, the spreading signal, properly aligned, the original message can be recovered.original message can be recovered.

+1

-1time

time

time+1

-1-1-1

+1 +1 +1

Traffic(9 Kbps)

SpreadingSignal27 Kcps

TransmittedSignal27 Kcps

+1 +1+1

-1 -1

DSSS - Transmit Side

Wireless Wireless

X

27 KcpsSquare Pulses

cos(2πfct)

BPSK output27 Kcps90% of power in 54 KHz BW

centered at fc Hertz

X

cos(2πfct)

BPSK input27 Kcps+ noise

27 KcpsSquare Pulses+ filtered noise

RCVR Front End

RF Transmitter

Low PassFilter

time

time+1

-1-1-1

+1 +1 +1DespreadingSignal27 Kcps

ReceivedSignal27 Kcps

+1 +1+1

-1 -1

+1

-1

timeRecoveredTraffic9 Kbps

DSSS-Receiver

Receiver Bit DetectionReceiver Bit Detection Following the despread operation, a Bit Detector will Following the despread operation, a Bit Detector will

examine the noisy & distorted waveform and decide examine the noisy & distorted waveform and decide whether a 1 or 0 was transmitted in each message bit whether a 1 or 0 was transmitted in each message bit interval T.interval T.

Single Sample DetectorSingle Sample Detector Samples each bit once, usually near the middle.Samples each bit once, usually near the middle. Compares sampled value to waveform's average (DC) valueCompares sampled value to waveform's average (DC) value If sample > DC value, decide Logic 1 If sample > DC value, decide Logic 1 If sample < DC value, decide Logic 0If sample < DC value, decide Logic 0 Susceptible to noise burst. Susceptible to noise burst.

Receiver Bit DetectionReceiver Bit Detection Multiple Sample DetectorMultiple Sample Detector

Samples each bit interval numerous times.Samples each bit interval numerous times. Compares Compares averageaverage of sampled values to entire of sampled values to entire

waveform's Mean (DC) value (a.k.a. threshold)waveform's Mean (DC) value (a.k.a. threshold) If sample average > Mean, decide Logic 1 If sample average > Mean, decide Logic 1 If sample average < Mean, decide Logic 0If sample average < Mean, decide Logic 0 Less susceptible to noise burst. Less susceptible to noise burst.

Best would be to sample each message bit interval Best would be to sample each message bit interval an infinite # of times- equivalent to integration. an infinite # of times- equivalent to integration.

+1

-1

timeRecoveredTraffic9 Kbps

DSSS-Receiver

Detection (OSI Level 1)Detection (OSI Level 1)

Best Detector computes average value every TBest Detector computes average value every Tbitbit

seconds & compares to threshold. Here...seconds & compares to threshold. Here... If positive, says Logic 1If positive, says Logic 1 If negative, says Logic 0If negative, says Logic 0

Tbit Tbit

Someone else talking?Someone else talking? Would be using a different spreading code.Would be using a different spreading code. Output from our despread operation would be Output from our despread operation would be

random high speed chipsrandom high speed chips Bit Detector expecting lower speed bitsBit Detector expecting lower speed bits

Would output a random sequence of low speed bitsWould output a random sequence of low speed bits If a voice system, these random bits map to If a voice system, these random bits map to

random voltagesrandom voltages Would get static if played on loudspeakerWould get static if played on loudspeaker Real world system squelches this staticReal world system squelches this static

time

-1

+1

-1-1 -1

+1

Signal #227 Kcps

+ timeRecoveredGarbagefrom Signal #2 -1 -1 -1 -1

+1 +1

time

-1-1-1

+1 +1 +1OurDespreadingSignal27 Kcps

Someone else talking?Someone else talking?


+1 +1

Someone else talking?Someone else talking?

MessageBit Detectorlooks at message bitintervals.

-1 -1time

Wouldoutputrandomsequenceof 1's & 0's (2 logic 0's here).

Tbit Tbit

Two signals active?Two signals active? Receiver Detector sees Receiver Detector sees sumsum of our signal and of our signal and

unwanted signal.unwanted signal. Still able to detect message bits in example that Still able to detect message bits in example that

follows.follows. But bit errors may be more likely.But bit errors may be more likely.

Note average value of first bit (2/3) is now closer to the Note average value of first bit (2/3) is now closer to the threshold of 0 volts.threshold of 0 volts.

Unwanted signals have effect similar to noise. Unwanted signals have effect similar to noise. Message bit detection errors become more common. Message bit detection errors become more common.

Two signals active.Two signals active.

+1

-1

timeOurTraffic9 Kbps


+1 +1

time+2

-2sum

Our Traffic is Still RecoveredOur Traffic is Still Recovered

time+2

-2sum

Positive Area:Output a Logic 1

Negative Area:Output a Logic 0

+1

-1

timeDetectorOutput9 Kbps

Tbit Tbit

Switching: How long & in what manner will a user get to use a channel?

Switching: How long & in what manner will a user get to use a channel?

For the duration of the conversation?For the duration of the conversation?Circuit SwitchingCircuit Switching

For a tiny, variable length, portion of the For a tiny, variable length, portion of the conversation?conversation?

Packet SwitchingPacket Switching For a tiny, fixed length, portion of the For a tiny, fixed length, portion of the

conversation?conversation?Cell SwitchingCell Switching

StatMux TDM FDM

Circuit

Packet

Cell

MULTIPLEXINGS

WIT

CH

ING

CDM

PSTN

EthernetInternet

WiFi

ATM ATM

(GSM Mobile Phones)

Mobile

Any combo of Switching & Multiplexing is theoretically possible.Shown are some of the most common.

WiFi WiFi

TDM & Circuit SwitchingTDM & Circuit Switchingfrequency

tim

e

1

2

3

1etc.

1 byte

1/8000 thsecond

PSTN

StatMux & Packet SwitchingStatMux & Packet Switching

frequency

tim

e

1

3

1

2

InternetPackets 47 bytes

to1507 bytes

Data Networks

TDM or StatMux & Cell SwitchingTDM or StatMux & Cell Switching

frequency

tim

e

1

ATMCells

53 bytes

2

13

Empty

Empty

Empty

FDMFDMfrequency

tim

eDifferent channels use some of the frequency all of the time.

1 2 3 4 5

1st Generation Cell Phones

TDMA/FDMATDMA/FDMA

frequency

tim

eCombo of TDM & FDM(GSM)

1

2

3

1etc.

4

5

6

4

7

8

9

7

10

11

12

10

2nd Generation Cell Phones

CDMACDMAfrequency

tim

eDifferent channels use all of the bandwidth all of the time.Example: CDMA1 & CDMA2000

Channels use different codes. Other channels cause noise-like interference.

2nd & 3rd Generation Cell Phones

Wired Telecom Black BoxesWired Telecom Black Boxes OSI Layer 1OSI Layer 1

LAN Hubs (obsolete), a.k.a. Shared HubsLAN Hubs (obsolete), a.k.a. Shared Hubs WAN Regenerative RepeatersWAN Regenerative Repeaters

OSI Layer 1 & 2OSI Layer 1 & 2 LAN Bridges (obsolete)LAN Bridges (obsolete) LAN Switches, a.k.a. Switched HubsLAN Switches, a.k.a. Switched Hubs WAN SwitchesWAN Switches

OSI Layer 1, 2 & 3OSI Layer 1, 2 & 3 RouterRouter

Generally moving pulsesGenerally moving pulses

Repeater or HubRepeater or Hub Operates at OSI Level 1Operates at OSI Level 1

Bit AwareBit Aware Unaware that packets existUnaware that packets exist

Repeater: Single input, Single OutputRepeater: Single input, Single OutputUsually only used on WAN'sUsually only used on WAN's Will regenerate & retime symbols (a.k.a Regen)Will regenerate & retime symbols (a.k.a Regen)

Hubs: Multiple input, Multiple OutputHubs: Multiple input, Multiple OutputUsually only used on LAN'sUsually only used on LAN's May regenerate & retime symbolsMay regenerate & retime symbols ObsoleteObsolete

Black Box Performance...Black Box Performance...

OSI Level 1LAN Hub

Two packets simultaneously show up at input...

From Node A

Node B

Node C

To Node A

Node B

Node C


... will overwrite each other, i.e. garbage out.

a.k.a. Shared Hub

OSI Level 1LAN Hub

From Node A

Node B

Node C

To Node A

Node B

Node C

WAN SwitchLAN Switched Hub or Bridge

WAN SwitchLAN Switched Hub or Bridge

Operates at OSI Layers 1 & 2Operates at OSI Layers 1 & 2 Frame awareFrame aware Media Access Control (MAC) awareMedia Access Control (MAC) aware Bridge: single input, single output HubBridge: single input, single output Hub

Two packets arrive simultaneously for same Two packets arrive simultaneously for same output?output? Momentarily store one, ship the other.Momentarily store one, ship the other.

Requires CPU & MemoryRequires CPU & Memory


Two packets simultaneously show up at input...

OSI Layer 1-2Switch


... one will be transmitted, the other momentarily buffered and then transmitted.

OSI Layer 1-2Switch

OSI Layer 1-3Router

Internet RouterInternet Router

Operates at OSI Layers 1 - 3Operates at OSI Layers 1 - 3 Packet awarePacket aware Media Access Control (MAC) awareMedia Access Control (MAC) aware More "network aware" than a switchMore "network aware" than a switch

Exchange connectivity info with peersExchange connectivity info with peers

Two packets arrive simultaneously for same Two packets arrive simultaneously for same output?output? Momentarily store one, ship the other.Momentarily store one, ship the other.

Requires CPU & MemoryRequires CPU & Memory

802.3LAN

LAN

OSU Campus Network (> 2001)OSU Campus Network (> 2001)

RoutersRouters

1 & 10 GbpsEthernet

OneNetEthernetEthernetSwitchSwitch

802.3LAN

802.3LAN

LAN

LAN

Wireless Telecom Black BoxesWireless Telecom Black Boxes

OSI Layer 1 & 2OSI Layer 1 & 2 Access PointAccess Point

Generally transmitting RF EM sinusoidsGenerally transmitting RF EM sinusoids Between user & Access PointBetween user & Access Point

Wired or Wireless TransportWired or Wireless Transport Between Access Point & WorldBetween Access Point & World

Limited Deployment To Date & R&DLimited Deployment To Date & R&D Wireless Radio Protocols (Layers 1-3)Wireless Radio Protocols (Layers 1-3)

0

1.5

-1.50 .00001

1 MHz

Binary ASKBinary ASK

Two different Amplitudes are transmittedShown is a 1 MHz center frequency.5 cycles/symbol = 200 K bits/second

X

cos(2πfct)

t

in(t)

0

1.5

-1.50 .00001

Binary FSKBinary FSK

Two different frequencies are transmittedSymbol #1) 5 cycles/.000005 seconds = 1 MHzSymbol #2) 10 cycles/.000005 seconds = 2 MHz1.5 MHz Average (center) Frequency2 symbols in .00001 seconds = 200 K bits/second

tin(t) VCO

fc = 1.5 MHz

0

1.5

-1.50 .00001

1 MHz

Binary PSKBinary PSK

Two different phases are transmitted10 cycles/.00001 seconds = 1 MHz5 cycles/symbol = 200 K bits/second

X

cos(2πfct)

t

in(t)

Binary PSKBinary PSK

X

cos(2πfct)

t

in(t) in(t)cos(2πfct)

cos(2πfct)

X Low PassFilter

y(t)

y(t) = 0.5in(t)[cos (0) +cos(2π2fct)]

Wiped out by LPF.

At receiver...

POTS Connectivity (1920)POTS Connectivity (1920)

Phone Phone

CO CO

CopperLocalLoop

CopperLocalLoop

Analog

CopperLongHaul


Phone Phone

CO CO

CopperLocalLoop

CopperLocalLoop

CopperLongHaul

Analog Analog Digital TDM64 Kbps


Phone Phone

CO CO

Fiber OpticTrunk

CopperLocalLoop

CopperLocalLoop

Analog Analog Digital TDM64 Kbps

ecen4533 data communications lecture #7 23 january 2013 dr. george scheets

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