ecen4533 data communications lecture #7 23 january 2013 dr. george scheets
DESCRIPTION
ECEN4533 Data Communications Lecture #7 23 January 2013 Dr. George Scheets. Read 3.3 - 3.5 Problems Web 3-11, 2010 Quiz #1 Design #1 due 1 February (Live) 8 February ( Async DL) Late = -1 per working day Quiz #1 Lecture 12, 4 February (Live) - PowerPoint PPT PresentationTRANSCRIPT
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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
ECEN4533 Data CommunicationsECEN4533 Data CommunicationsLecture #7 23 January 2013Lecture #7 23 January 2013Dr. George ScheetsDr. George Scheets
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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)
ECEN4533 Data CommunicationsECEN4533 Data CommunicationsLecture #8 25 January 2013Lecture #8 25 January 2013Dr. George ScheetsDr. George Scheets
ECEN4533 Data CommunicationsECEN4533 Data CommunicationsLecture #8 25 January 2013Lecture #8 25 January 2013Dr. George ScheetsDr. George Scheets
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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..
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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.
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+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
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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
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time
time+1
-1-1-1
+1 +1 +1DespreadingSignal27 Kcps
ReceivedSignal27 Kcps
+1 +1+1
-1 -1
+1
-1
timeRecoveredTraffic9 Kbps
DSSS-Receiver
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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.
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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.
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+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
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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
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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?
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+ timeRecoveredGarbagefrom Signal #2 -1 -1 -1 -1
+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
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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.
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Two signals active.Two signals active.
+1
-1
timeOurTraffic9 Kbps
+ timeRecoveredGarbagefrom Signal #2 -1 -1 -1 -1
+1 +1
time+2
-2sum
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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
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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
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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
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TDM & Circuit SwitchingTDM & Circuit Switchingfrequency
tim
e
1
2
3
1etc.
1 byte
1/8000 thsecond
PSTN
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StatMux & Packet SwitchingStatMux & Packet Switching
frequency
tim
e
1
3
1
2
InternetPackets 47 bytes
to1507 bytes
Data Networks
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TDM or StatMux & Cell SwitchingTDM or StatMux & Cell Switching
frequency
tim
e
1
ATMCells
53 bytes
2
13
Empty
Empty
Empty
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FDMFDMfrequency
tim
eDifferent channels use some of the frequency all of the time.
1 2 3 4 5
1st Generation Cell Phones
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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
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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
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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
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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
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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
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Black Box Performance...Black Box Performance...
... 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
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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
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Black Box Performance...Black Box Performance...
Two packets simultaneously show up at input...
OSI Layer 1-2Switch
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Black Box Performance...Black Box Performance...
... one will be transmitted, the other momentarily buffered and then transmitted.
OSI Layer 1-2Switch
OSI Layer 1-3Router
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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
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802.3LAN
LAN
OSU Campus Network (> 2001)OSU Campus Network (> 2001)
RoutersRouters
1 & 10 GbpsEthernet
OneNetEthernetEthernetSwitchSwitch
802.3LAN
802.3LAN
LAN
LAN
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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)
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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)
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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
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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)
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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...
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POTS Connectivity (1920)POTS Connectivity (1920)
Phone Phone
CO CO
CopperLocalLoop
CopperLocalLoop
Analog
CopperLongHaul
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POTS Connectivity (1970)POTS Connectivity (1970)
Phone Phone
CO CO
CopperLocalLoop
CopperLocalLoop
CopperLongHaul
Analog Analog Digital TDM64 Kbps
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POTS Connectivity (1990)POTS Connectivity (1990)
Phone Phone
CO CO
Fiber OpticTrunk
CopperLocalLoop
CopperLocalLoop
Analog Analog Digital TDM64 Kbps
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