art of digital troubleshooting

7/28/2019 Art of Digital Troubleshooting

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www.sunrisetelecom.com

By: Jerry Green


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Confidential and Proprietary

Manager,TechnicalKnowledge

SunriseTelecom,GlobalServices

CellPhone:707-694-9672

Skype:jergreen6

[email protected]

JerryGreen

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CompletetrainingcoursesareavailablethroughSunriseTelecom

GlobalServices.

Formoreinforma:oncontactusat

(email)[email protected](web)hp//www.sunrisetelecom.com/support/web_req.php(phone)800-701-5208(interna:onal408-360-2200,op:on3)

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Digital basics

Levels Constellations

MER

BER Adaptive equalizer Frequency response Group delay

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Confidential and Proprietary 5


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Time Time

FrequencyDomain

TimeDomain

F1 F2

F1 F2

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ItstheBUS

Modula:onispuWngcontentontheBus

Demodula:onistakingcontentofftheBus

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Time

Amplitude

Phase

90

Ref.Point

0

Rate(Frequency)

TimetocompleteacycleUnitofmeasure=Hertz1cycle/sec=1Hz

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Digital Language If bits are the letters, then symbols are the words in the digital

language

Symbols = one or more bits Bit rate = number of bits sent per second Symbol Rate = number of symbols sent per second Symbol Rate = Bit Rate / Number of bits per Symbol Transmission bandwidth = symbol rate

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QAM: Quadrature Amplitude Modulation Two carriers at the same frequency, 90 out-of-phase,

transmitted at the same time

Carriers are called (I) In-phase & (Q) Quadrature carrier

Uses multiple levels of amplitude & phase modulation Each carrier transmits half of the symbol

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I & Q channels are AM & Phase modulated I & Q are combined to create the 64 QAM signal

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101 010Local Osc

8 Level AM

Modulator

8 Level AM

Modulator

Bit Stream

OscillatorShifted 90

Combiner64 QAMSignal

Q Component

I Component


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4 levels of data 16 symbols transmitted in one clock cycle Each symbol contains 4 bits 16QAM

8 levels per carrier 64 symbols transmitted Symbol contains 6 bits 64QAM

16 levels per carrier

256 symbols transmitted Symbol contains 8 bits 256QAM

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10

11

01

00

10

11

Q 90

I 0

1010 1110

1011

1111

0100

0001

0000

0101

0100

I 180

Q 270


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10

11

01

00

10

11Q 90

I 0

1010 1110

1011

1111

0100

0001

0000

0101

0100

I 180

Q 270


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6BitsperSymbol


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8BitsperSymbol


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For a system operator it is essential to know how close signalquality is to the 'cliff edge

For example, could climate changes push your system overthe edge?

WorseningSignal

Quality

Picture

Quality

Perfect

No

Picture

Digital

Analog

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Levels Constellation MER, EVM BER Frequency Response Group Delay

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6 MHZ

300 KHz

6 MHZ

Peak

Average

Power


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MER - single figure of merit for quality for RF digital carriers Sum of all evils

It includes distortions such as CCN, CSO, CTB, lasercompression, etc..

Averaged measurement A 256 QAM picture tiles at 28dB MER Minimum MER for 256QAM = 31 dB

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10

11

01

00

10

11

Q 90

I 0

1010 1110

1011

1111

0100

0001

0000

0101

0100

I 180

Q 270


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H


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H


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Output of QAM Modulator 40 dB Input to Lasers 39 dB

Output of Nodes 37 dB

Output of Subscriber Taps 35 dB At the input to the subscribers receiver 34 dB The absolute minimum is 31db.

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Looksgood

hereinthe

Headend!


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H


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H


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BER is defined as the ratio of the number of wrong bits overthe number of total bits.

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Sent Bits 1101101101

Received Bits 1100101101

BER = # of Wrong Bits

# of Total Bits= 1

10= 0.1

error


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Displayed in Scientific Notation. The more negative the exponent, the better the BER. 1.0E-6 is limit of the FEC (Forward Error Correction)

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Lower and

Better

BER


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Pre-BER errors Errors that have occurred before the FEC has had an opportunity

to correct any of the errors.

Post-BER errors Errors that could not be corrected

Limit of FEC = 1E-06 (one error in one million bits) Post-BER cause:

Retransmissions requests Slowdowns in a DOCSIS systems Tiling

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Every Digital Receiver has an Adaptive Equalizer Exception, CMTS: Pre-equalization is used on the upstream

Compensates for amplitude imperfections of the digital signal Compensates for group delay Rings at the symbol rate to only allow one symbol at a time

into the digital receiver

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Standard EQ Used in current

equipment

More taps Improved correction

Min EQ Less taps Mirrors performance of

old equipment

Disables Auto Diagnosis

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Effective spanequal to symbolrate

Measurementcalculated usingEqualizer data

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Definition: Group delay is a measure of how long it takes a signal to

traverse a network, or its transit time.

Strong function of the length of the network Weak function of frequency. Expressed in units of time, pico-seconds for short distances or

nanoseconds for longer distances.

Measured in units of time for frequency Typically nanoseconds (ns) over frequency Or, Delay per MHz.

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Ideal system, all frequencies are transmitted through thesystem, network or component with equal time delay

Frequency response problems cause group delay problems Group delay is worse near band edges and diplex filter roll-off

areas

Excessive group delay increases bit error rate due to inter-symbol interference

DOCSIS spec. no greater than 200nSecs per MHZ Spec should be less than 100 nSecs per MHz

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CompletetrainingcoursesareavailablethroughSunriseTelecom

GlobalServices.

Formoreinforma:oncontactusat

(email)[email protected](web)hp//www.sunrisetelecom.com/support/web_req.php(phone)800-701-5208(interna:onal408-360-2200,op:on3)

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