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© 2010 Cisco Systems, Inc. All rights reserved. Cisco PublicDid You Know? 1

Did You Know?

Ron Hranac


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

https://ww2.cox.com/

http://www.cableone.net/

http://www.buytimewarnercable.com/


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:

bits) of number (total)bits errored of number(BER

period) tmeasuremen x rate (bit)period tmeasuremen in count error(BER


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


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 symbolpower

I

Average error power

N

jjj

N

jjj

QI

QIMER

1

22

1

22

10log10

In effect, MER is a measure of how “fuzzy” the symbol

points in a constellation are.


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)

Center conductor

Dielectric

Center conductor

Outer conductor


Terminology

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?)

Department of Redundancy Department


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


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.


Graphic source: Sunrise Telecom

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)


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.


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


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.

These are analog…

…and so are these!


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.

QAMModulator

QAM signal graphic source: Trilithic


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 AM (top) and FM (bottom) in the time domain


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


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


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.


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.


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.


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 digital-compatible 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.


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?


UHF TV Ingress


There is a 2 MHz overlap between North American over-the-air UHF channel slots and North American STD and IRC cable channel slots

14 15 16 17 18

65 66 67 68 69468 MHz

470 MHz

474 MHz 480 MHz 486 MHz 492 MHz 498 MHz


UHF

Cable


UHF TV Ingress


There is a 2 MHz overlap between North American over-the-air 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)

7 8 9 10 11

7 8 9 10 11174 MHz

174 MHz



OTA VHF

Cable VHF


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.


Graphic source: Agilent

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: www.oneminuteastronomer.com

Solar Transit Outages

Did you know that what are sometimes called “sunspot outages” have nothing to do with sunspots?

Ø


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.

Earth station antenna

Geostationary satellite

Antenna beamwidth

Day 1

Day 2

Day 3

Sun

Sun

Sun

Sun Day 4


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!


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 (micro-reflections, 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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