Did You Know?
Ron Hranac
Did You Know?
© 2010 Cisco Systems, Inc. All rights reserved.
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1
Terminology
 Did you know MSO is an abbreviation—it’s not even
an acronym—for multiple system operator?
“MSO” is a corporate entity such as Comcast or Time Warner
Cable that owns and/or operates more than one cable system.
It’s not a generic abbreviation in the same sense that, say, LEC
(local exchange carrier) is.
A local cable system is not an MSO—although it might be
owned by one—it’s just a cable system
All MSOs are cable operators, but not all cable operators are
MSOs
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Terminology
 Did you know BER is an abbreviation for bit error
ratio?
BER is the ratio of errored bits to the total number of bits transmitted,
received, or processed over a defined amount of time
 Mathematically, two formulas are commonly used to
describe BER:
( num berof errored bits )
BER 
(total num berof bits)
( error countin m easurem ent period )
BER 
(bit rate x m easurem ent period)
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BER Example
 Let’s say that 1,000,000 bits are transmitted,
and 3 bits out of the 1,000,000 bits received are
errored because of some kind interference
between the transmitter and receiver
 BER in this example is calculated by dividing the
number of errored bits received by the total
number of bits transmitted:
BER = 3/1,000,000 = 0.000003
 Most BER measurements are expressed in
scientific notation format, so 0.000003 = 3 x 10-6
 3 x 10-6 also can be written as 3 x 10^-6 or
3.0E-06
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2
 N  2
 


Q

I j j 

j 1 

MER  10 log10  N
2
  2
 



Q


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  j
j
 j 1

Terminology
 Did you know MER is an abbreviation for modulation
error ratio?
It’s not modulation error rate
MER is the ratio of average signal constellation power to average
constellation error power — that is, digital complex baseband
signal-to-noise ratio (SNR). Indeed, MER is often called SNR.
MER = 10log(average symbol power/average error power)
Average error power
Average symbol
power
I
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In effect, MER is
a measure of how
“fuzzy” the symbol
points in a
constellation are.
7
Terminology
 Did you know the “coax” in coaxial cable refers to the
fact that the center and outer conductors—as viewed
from the end of the cable—share a common axis?
Jacket
Outer conductor (shield)
Dielectric
Outer conductor
Center conductor
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+
Center conductor
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Terminology
Department of Redundancy Department
 Did you know the following are redundant?
“Cable MSO” (if the entity is an MSO, it’s a cable operator)
“AM modulation” (amplitude modulation modulation)
“FM modulation” (frequency modulation modulation)
“QAM modulation” (quadrature amplitude modulation
modulation?)
“RF frequency” (radio frequency frequency?)
“NIC card” (network interface card card?)
“DOCSIS specification” (Data Over Cable Service Interface
Specification specification?)
“PIN number” (personal identification number number?)
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The Decibel
 Did you know dB is an abbreviation for decibel?
Decibels express the logarithmic ratio of two power levels:
dB = 10log10(P1/P2)
 Here’s an example of the decibel at work. Let’s say you
have a 50 watts-output stereo, and your neighbor has a
100 watts-output stereo. How much more power does
your neighbor’s stereo have than yours?
dB = 10 * [log10(100 watts/50 watts)]
dB = 10 * [log10(2)]
dB = 10 * [0.301]
dB = 3.01 dB power difference
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The Decibel
 Did you know that by itself, the decibel cannot be used
to express absolute signal levels?
For instance, one can correctly say that an amplifier has 20 dB
of gain, or a splitter has 4 dB of loss
It’s incorrect to say that the RF signal level at the input to a TV
set is -2 dB, or the RF signal level at a line extender output is 48
dB. For that, the decibel must be appended with a reference,
such as dBmV, or decibel millivolt. The two examples here are
correctly stated as -2 dBmV and +48 dBmV respectively.
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The Decibel
 Did you know dBmV expresses power in terms of
voltage?
0 dBmV defines the power produced when a voltage of 1
millivolt (mV) rms is applied across a defined impedance—75
ohms in the case of the cable industry. That is, 1 mV in a 75
ohm impedance is 13.33 nanowatts (nW), which we call 0
dBmV.
Other signal levels in dBmV are
technically ratios of those levels’
voltages to the 0 dBmV 1 mV
“reference”:
dBmV = 20log10(level in mV/1 mV)
Graphic source: Sunrise Telecom
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13
Analog TV Channel Signal Levels
 Did you know that when we measure the RF level of
analog TV channel visual carriers, we don’t measure
peak power? We measure peak envelope power (PEP),
which is the average power of one cycle during the
modulation crest. A visual carrier’s modulation crest
occurs during sync pulses.
When video modulation is present,
the visual carrier’s amplitude is
measured just during the sync peaks.
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14
Digital Signal Levels
 Did you know that when we measure the RF level of
digital signals carried on cable networks, we measure
the entire signal’s average power, also known as digital
channel power?
Graphics source: Sunrise Telecom and Agilent
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15
Digital or Analog?
 Did you know the “digital” signals we carry on our
networks aren’t “digital,” they’re analog?
Our networks can’t carry baseband digital data—for the purists,
a length of coaxial cable can, but that digital data won’t make it
past the first active—so we have to convert the digital data we
want to transmit to and from subscribers into analog RF signals.
…and so are
these!
These are
analog…
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What’s a QAM?
 Did you know digital data is converted into an analog
RF signal using quadrature amplitude modulation
(QAM), which results in a double-sideband,
suppressed-carrier analog RF signal?
The digital information to be transmitted is represented by
variations in the RF signal’s phase and amplitude. There are no
zeros and ones per se in what we call “digital” signals.
QAM
Modulator
QAM signal graphic source: Trilithic
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What’s a QAM?
 Did you know QAM is a type of modulation, like
amplitude modulation (AM) or frequency modulation
(FM)?
Technically it’s incorrect to call a QAM signal or a QAM modulator
a “QAM.” Doing so is no different than calling an FM signal or an
FM broadcast transmitter a “FM.” Let’s see—would we pronounce
that “foom”?
QAM in the time domain
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AM (top) and FM (bottom) in the time domain
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What’s a QAM?
 Did you know the number that often accompanies “QAM”—as
in 256-QAM—designates the number of states in the signal? In
the RF domain, each state (which represents a symbol) is a
specific value of phase and amplitude at a given instant.
4-QAM is 4-state quadrature amplitude modulation (more commonly
known as quadrature phase shift keying, or QPSK)
16-QAM is 16-state quadrature amplitude modulation
64-QAM is 64-state quadrature amplitude modulation, and so on
Graphic sources: Filtronic Sigtek and Sunrise Telecom
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19
Digital Proofs or Not?
 Did you know the FCC has required digital signals on
most cable networks to meet certain specs, and that
this requirement has been on the books for several
years?
Graphics source: Sunrise Telecom
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Digital Proofs or Not?
 §76.640(b)(1)(i) is where
you’ll find the rules for digital
signals:
(1) Digital cable systems with an activated
channel capacity of 750 MHz or greater
shall comply with the following technical
standards and requirements:
(i) SCTE 40 2003 (formerly DVS 313):
“Digital Cable Network Interface Standard”
(incorporated by reference, see §76.602),
provided however that with respect to
Table B.11, the Phase Noise requirement
shall be −86 dB/Hz, and also provided that
the “transit delay for most distant customer”
requirement in Table B.3 is not mandatory.
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21
Distortions in an All-Digital Network
 Did you know that distortions such as composite triple
beat (CTB) distortion, composite second order (CSO)
distortion, and common path distortion (CPD) don’t go
away in an all-digital network?
Rather than clusters of discrete beats that occur in a network
carrying large numbers of analog TV channels, the digital
distortions are noise-like!
Those noise-like distortion products are variously known as
composite intermodulation noise (CIN), composite
intermodulation distortion (CID) or intermodulation noise
(IMN)—none of which should be confused with thermal noise.
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Signal Leakage in an All-Digital Network
 Did you know leaking digital signals can cause harmful
interference to over-the-air services under the right
conditions?
Despite the fact that a QAM signal’s power is spread across
most of the 6 MHz channel bandwidth, moderate to high field
strength leaks involving those noise-like QAM signals can indeed
cause harmful interference.
6 MHz bandwidth
Communications transceiver’s
S9+15 dB S-meter reading caused
by 400 µV/m digital leak at 10 ft.
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Signal Leakage in an All-Digital Network
 And did you know that the tens of thousands of
existing leakage detectors out in the field today can’t be
used to measure leaking digital signals?
The good news is that manufacturers are working on digitalcompatible leakage-detector technology, and one manufacturer
recently introduced a digital-compatible leakage detection
product.
Until new digital-compatible leakage detection gear becomes
widely available, the only way to comply with the FCC’s existing
leakage rules and maintain compatibility with existing leakage
detectors is to use an analog TV channel or continuous wave
(CW) carrier when measuring leakage.
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UHF TV Ingress
 Did you know that when an over-the-air UHF TV
channel—whether analog or digital—leaks into a cable
network it can interfere with two cable channels?
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UHF TV Ingress
 Did you know that when an over-the-air UHF TV
channel—whether analog or digital—leaks into a cable
network it can interfere with two cable channels?
There is a 2 MHz overlap between North American over-theair UHF channel slots and North American STD and IRC cable
channel slots
476 MHz
470 MHz
14
UHF
Cable
65
468 MHz
Did You Know?
482 MHz
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15
67
66
474 MHz
488 MHz
480 MHz
Cisco Public
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68
486 MHz
500 MHz
494 MHz
18
69
492 MHz
498 MHz
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UHF TV Ingress
 Did you know that when an over-the-air UHF TV
channel—whether analog or digital—leaks into a cable
network it can interfere with two cable channels?
There is a 2 MHz overlap between North American over-theair UHF channel slots and North American STD and IRC cable
channel slots
Over-the-air and cable VHF channel slots use the same
allocations (e.g, 174-180 MHz for Ch. 7, 180-186 MHz for Ch.
8, and so on)
180 MHz
174 MHz
186 MHz
192 MHz
204 MHz
198 MHz
OTA VHF
7
8
9
10
11
Cable VHF
7
8
9
10
11
174 MHz
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180 MHz
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186 MHz
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192 MHz
198 MHz
204 MHz
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Digital Signals and Amplifier AGC
 Did you know most older amplifier automatic gain
control (AGC) circuits don’t play nicely with a digital
signal on the AGC pilot frequency?
Those AGC circuits originally were designed for analog TV
channels or CW carriers, not for noise-like digital signals.
Most amplifiers manufactured during the last few years have
digital-compatible AGC, but hundreds of thousands (or more!)
of older amplifiers do not.
In most instances, it will be necessary to use an analog TV
channel or CW carrier on the AGC pilot frequency if you want
those older amplifiers’ AGC circuits to work properly.
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QAM Signal CNR
 Did you know when using a spectrum analyzer to
measure the carrier-to-noise ratio of a QAM signal, the
CNR is simply the signal’s height above the noise floor
in dB?
Make certain that the spectrum analyzer is displaying the cable
system’s noise floor, and not the test equipment’s noise floor!
CNR ≈ 15 dB
Graphic source: Agilent
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Solar Transit Outages
 Did you know that what are sometimes called “sunspot
outages” have nothing to do with sunspots?
Graphic source: www.oneminuteastronomer.com
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Solar Transit Outages
 Those outages actually are solar transit
outages—also called sun fade or sun
outages—which are twice-yearly
satellite reception outages that happen
when the sun lines up behind
geostationary satellites. The sun emits
electromagnetic radiation across a wide
range of frequencies, including those
used by communications satellites.
When the sun is behind a satellite from
the perspective of a given earth station
antenna, the RF energy from the sun is
strong enough to exceed the desired
signal(s) from that satellite (on sunny
days the heat can get pretty intense at
the antenna focal point if the dish is
solid/shiny). Solar-transit outages occur
for a few minutes on each of several
days near the spring and autumn
equinoxes.
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Geostationary satellite
Sun
Sun
Antenna beamwidth
Sun
Sun
Earth station antenna
40
Reliability vs. Availability
 Did you know that reliability and availability are not the
same thing?
Availability: The ratio of time that a service, device, or network
is available for use to total time, usually expressed as percent of
the total time.
Reliability: Probability that a system or device will not fail during
some specified period.
 One can say “four-nines availability,” but it is incorrect
to say “four-nines reliability”!
For example, four-nines availability—expressed as 99.99%—
means that a service is available 8759.12 hours out of 8760
total hours in a year. Another way to look at it is the service will
be unavailable no more than about 53 minutes per year!
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CMTS “Upstream SNR”
 Did you know that a CMTS’s reported upstream SNR
actually is modulation error ratio (MER)?
Factors that can affect the reported MER value include
transmitted phase noise, CNR, linear distortions (microreflections, amplitude ripple/tilt, group delay), nonlinear
distortions (common path distortion, etc.), in-channel ingress,
laser clipping, improper modulation profiles, upstream data
collisions…
Cable3/0 Upstream 0 is up
Frequency 25.392 MHz, Channel Width 3.200 MHz, QPSK Symbol Rate 2.560 Msps
Spectrum Group is overridden
BroadCom SNR_estimate for good packets - 26.8480 dB
Nominal Input Power Level 0 dBmV, Tx Timing Offset 2035
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Q and A
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