Networking Solution Presentation Archive0900aecd802c5261

7/30/2019 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


ZS ? ZT ? ZL


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



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Remember the

upstream slide at thebeginning of thispresentation?

Heres why 16-QAMwouldnt work



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This upstreamconstellation

shows a not-so-good 16-QAMsignal

UnequalizedMER is 21.3 dB,

close to thefailure thresholdfor 16-QAM



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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

Networking Solution Presentation Archive0900aecd802c5261

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