networking solution presentation archive0900aecd802c5261
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1 2005 Cisco Systems, Inc. All rights reserved.SCTE Cable-Tec Expo2005
Understanding and TroubleshootingLinear Distortions: Micro-reflections,Amplitude Ripple/Tilt and GroupDelay
RON HRANAC
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A Clean Upstream: Or Is It?
Graphic courtesy of Sunrise Telecom
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Transmission Line Theory 101
Source LoadTransmissionMedium
ZS = ZT = ZL
Source LoadTransmissionMedium
ZS ? ZT ? ZL
Source LoadTransmissionMedium
ZS ? ZT ? ZL
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Impedance Mismatches
Source(7 dB return loss)
Load(7 dB return loss)
Transmission Medium(1 dB attenuation)
+31 dBmVat time T0
+30 dBmVat time T1
+22 dBmV
+15 dBmV
+23 dBmV
+14 dBmV(-16 dBc)at time T2
+30
+25
+20
+15
+10
+5
Amplitud
e
(dBmV)
Time (T)
Incident signal (T1) Echo (T2)
+14 dBmV
+30 dBmV
234 ns
100 ft23 20
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Micro-reflectionsThe Big Picture
Time (T)
Amplitude
Peak (+31.28 dBmV)
Null (+28.5 dBmV).
.
Frequency (F)
Amplitude
(dBmV)
~4.27 MHz+30+25
+20
+15
+10
+5
Amplitude
(dBmV)
Time (T)
Incident signal (T1) Echo (T2)
+30 dBmV
+14 dBmV (-16 dBc)234 ns
Echo calculation courtesy of Holtzman, Inc.
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Micro-reflections
Damaged or missing end-of-line terminators
Damaged or missing chassis terminators on directional coupler, splitter, or multiple-output amplifier unused ports
Loose center conductor seizure screws
Unused tap ports not terminated; this is especially critical on low value taps, but allunused tap ports should be terminated with 75-ohm terminations (locking terminatorswithout resistors or stingers do not terminate the tap port)
Poor isolation in splitters, taps and directional couplers
Unused customer premises splitter and directional coupler ports not terminated
Use of so-called self-terminating taps at feeder ends-of-line; these are the equivalentof splitters, and do not terminate the feeder cable unless all tap ports are terminated
Kinked or damaged cable (includes cracked cable, which causes a reflection and
ingress) Defective or damaged actives or passives (water-damaged, water-filled, cold solder
joint, corrosion, loose circuit board screws, etc.)
Cable-ready TVs and VCRs connected directly to the drop (return loss on most cable-ready devices is poor)
Some traps and filters have been found to have poor return loss in the upstream,especially those used for data-only service
Causes:
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Amplitude Ripple/Tilt
Amplitude ripple and tilt are known in cable industry vernacular asfrequency response, and can refer to in-channel frequency response orthe frequency response across a specified RF bandwidth such as 5-42MHz
The causes include gremlins such as improper network alignment andimpedance mismatches (micro-reflections!)
Graphics courtesy of Acterna and Sunrise Telecom
Combination ofamplitude ripple and tilt
Amplitude tilt, slightamount of ripple
Amplitude tilt
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Amplitude Ripple/Tilt
This 6.4 MHzbandwidth A-TDMAdigitally modulated
signal exhibitssevere in-channelamplitude tilt at theCMTS upstreaminput
Adaptiveequalization (pre-equalization) in thecable modem is ableto compensate fornearly all of theamplitude tilt
Adaptive equalization
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Group Delay
Group delay issaid to exist whensignals at somefrequencies takelonger to travelthrough a circuit,amplifier ornetwork thansignals at otherfrequencies
Group delay, likeother linearimpairments,causes inter-symbolinterference,which degradesMER
Graphic courtesy of Holtzman, Inc.
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Linear Distortions in the Real World
In this example, anecho at ~485 ns
causes visibleamplitude rippleacross the 5-42MHz spectrum
Group delay ripplealso is present
Graphic courtesy of Holtzman, Inc.
Echo
Amplitude ripple
Group delay ripple
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Linear Distortions in the Real World
Heres anotherexample: An approx.-33 dBc echo at just
over 1 s This echo meets the
DOCSISupstream -30 dBc at >1.0 secparameter
Here, too, the echo is
sufficient to causesome amplitude andgroup delay ripple
Graphic courtesy of Sunrise Telecom
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A Clean Upstream: Or Is It?
Remember the
upstream slide at thebeginning of thispresentation?
Heres why 16-QAMwouldnt work
Graphic courtesy of Sunrise Telecom
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A Clean Upstream: Or Is It?
This upstreamconstellation
shows a not-so-good 16-QAMsignal
UnequalizedMER is 21.3 dB,
close to thefailure thresholdfor 16-QAM
Graphic courtesy of Sunrise Telecom
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A Clean Upstream
Graphics courtesy of Sunrise Telecom
From a linear distortion
perspective, this is whata relatively unimpairedupstream looks like
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Wrapping Up
Linear distortions are real problems in cable networks, andcan seriously affect downstream and upstream datatransmission
Among the tools available to troubleshoot linear distortionsare
Forward and reverse sweep, set to the maximum supportedresolution
Adaptive equalization (DOCSIS 1.1 and 2.0 modems)
CMTS tools such as per-modem SNR (MER), FEC errorinformation
Avoid upstream frequencies above about 35 MHz to minimizediplex filter-related group delay
Use of specialized test equipment to characterize andtroubleshoot micro-reflections, amplitude and group delay ripple
An understanding of linear distortions is critical to achievingthe reliability necessary for new services being deployed ontodays cable networks
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