1. Did You Know?
Ron Hranac

2. 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

3. Terminology Did you know headend is one word?
It’s not “head end” or “head-end”

4. Terminology
Did you know modem is derived from a combination of the first part of the words modulator and demodulator?
A cable modem is an electronic interface between a cable network and personal computer or other device, converting RF signals on the cable network into baseband digital data and vice versa.
Graphic source: Cisco

5. 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:

6. 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 =
Most BER measurements are expressed in scientific notation format, so = 3 x 10-6
3 x 10-6 also can be written as 3 x 10^-6 or E-06
Here’s an example using the first formula. Let’s say that 1 million bits are transmitted, and three bits out of the 1 million bits received are errored because of some kind of interference between the transmitter and receiver. The BER is calculated by dividing the number of errored bits received by the total number of bits transmitted: 3/1,000,000 or We can further express in scientific notation format—the way most BER measurements are shown.

7. 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 symbol
power I
Average error power
In effect, MER is a measure of how “fuzzy” the symbol points in a constellation are.

8. Terminology
Did you know CLI is an abbreviation for cumulative leakage index?
CLI is NOT the same thing as signal leakage! CLI is a mathematical snapshot of a cable system’s overall signal leakage performance at a given point in time.
One cannot detect, test, or measure CLI
One must measure signal leakage in order to calculate CLI
Photo source: JDSU

9. Outer conductor (shield)
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
Outer conductor

10. 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?)

11. 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

12. 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.

13. 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 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

14. 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.

15. 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

16. 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!

17. 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

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

19. 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

20. 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

21. 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 (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.

22. 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.

23. Distortions in an All-Digital Network
Confusion does occur, though. We know that raising RF levels in the plant improves the carrier-to-noise ratio (CNR), where “N” is thermal noise. But in a system with a lot of digital signals, did you know raising levels improves CNR to a point, then the noise floor starts to increase and the CNR appears to get worse?
That seems counterintuitive, but the now-elevated noise floor no longer is just thermal noise. It’s a combination of thermal noise and the previously mentioned noise-like distortions. When characterizing plant performance in the presence of CIN, the term “carrier-to-composite noise (CCN) ratio” commonly is used. Indeed, CCN is a much more appropriate measurement metric than is CNR under these circumstances, because there is no practical way to differentiate thermal noise from CIN.
The following examples illustrate this

24. Distortions in an All-Analog Network
Visual carriers
Aural carriers
Thermal noise
CTB
CSO
For each 1 dB increase in system carrier levels:
CTB ratio degrades by 2 dB
CSO ratio degrades by 1 dB
CNR improves by 1 dB

25. Distortions in an Analog + Digital Network
Visual carriers
QAM signals
Composite noise
Thermal noise
Composite
intermodulation noise
CTB
CSO
For each 1 dB increase in system carrier levels:
CNR, CTB, & CSO ratios behave as before with all-analog operation
CIN ratio degrades by 1 to 2 dB (mix of 2nd & 3rd order components)
CCN ratio degradation depends on CIN and CNR values

26. Distortions in an All-Digital Network
Composite noise
Thermal noise
Composite
intermodulation noise
For each 1 dB increase in system carrier levels:
CNR behaves as before with all-analog operation
CIN ratio degrades by 1 to 2 dB (mix of 2nd & 3rd order components)
CCN ratio degradation depends on CIN and CNR values

27. 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.

28. 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.

29. 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?

30. 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-the-air UHF channel slots and North American STD and IRC cable channel slots 14 15 16 17 18 65 66 67 68 69
468 MHz
470 MHz
474 MHz
480 MHz
486 MHz
492 MHz
498 MHz
476 MHz
482 MHz
488 MHz
494 MHz
500 MHz
UHF
Cable

31. 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-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, MHz for Ch. 7, MHz for Ch. 8, and so on) 7 8 9 10 11
174 MHz
180 MHz
186 MHz
192 MHz
198 MHz
204 MHz
OTA VHF
Cable VHF

32. 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.

33. 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

34. 1 dB optical attenuation increase
AM Optical Fiber Links
Did you know…
…assuming an optical fiber link is operating within its linear range, a 1 dB change in signal level at the RF input to the optical transmitter will result in a 1 dB change in signal level at the optical receiver’s RF output?
…and a 1 dB change in optical power at the input to the optical receiver will cause a 2 dB change in signal level at the optical receiver’s RF output?
Transmitter
Receiver
+15 dBmV
-10 dBm
+20 dBmV
+16 dBmV
Average optical power stays the same, as long as OMI remains in the 0% to 100% range
+21 dBmV
Transmitter
Receiver
1 dB optical attenuation increase
-10 dBm
+20 dBmV
+15 dBmV
-11 dBm
+18 dBmV

35. AM Optical Fiber Links
Did you know the 1 dB optical power-versus-2 dB RF power relationship in AM fiber links is not because “optical decibels are twice as big as RF decibels”?
From Modern Cable Television Technology, 2nd Ed.*:
In fact, optical transmitters and receivers are not linear devices but “square law” devices; that is, the instantaneous light output power of a transmitter is proportional to the input current and thus to the square root of the input signal power. At the other end of the circuit, the RF output power from the detector is proportional to the square of the optical power received, so the total link is nominally linear (predistortion is often used to overcome residual nonlinearities). As will be seen, however, the square law transfer function has an effect on noise and distortion addition. In particular, because of the square law detector transfer function, a change of 1 dB in optical loss will result in a 2-dB change in detected RF power, leading to the commonly stated, but incorrect, statement that “optical decibels are twice as big as RF decibels.”
* By Ciciora, Walter; J. Farmer, D. Large, M. Adams; Morgan Kaufmann Publishers; ©2004

36. Frequency Response
Did you know the broadband sweep gear we use to align and maintain our outside plants only shows half of the measured frequency response?
Well, maybe “half” isn’t the right word here, but a sweep display only shows one part of the complete frequency response
Graphic source: JDSU

37. Frequency Response
“Frequency response” is a complex quantity that has two components:
Magnitude (or amplitude)-versus-frequency
Phase-versus-frequency
The display of a sweep receiver shows us amplitude-versus-frequency, but not phase-versus-frequency.
Graphic source: Holtzman, Inc.

38. Frequency Response
Ideally, amplitude-versus-frequency should be flat, and phase should change in proportion to frequency. When amplitude-versus-frequency is not flat, we see amplitude ripple (“standing waves”), amplitude tilt or some combination of the two. When phase-versus-frequency is out of whack, we have group delay.

39. Ø Solar Transit Outages
Did you know that what are sometimes called “sunspot outages” have nothing to do with sunspots? Ø
Graphic source:

40. Geostationary satellite
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.
Geostationary satellite
Sun
Day 1
Sun
Day 2
Antenna beamwidth
Sun
Day 3
Sun
Day 4
Earth station antenna

41. 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 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!

42. Total Power
Did you know a quick way to estimate approximate total power is based on the rule-of-thumb that each time the number of channels doubles—assuming all channels have the same signal level—the total power increases 3 dB (3.01 dB)?
Number of Channels
Power per Channel
Total Power 1
0 dBmV 2
+3 dBmV 4
+6 dBmV 8
+9 dBmV 16
+12 dBmV 32
+15 dBmV 64
+18 dBmV
128
+21 dBmV

43. QAM Signal Amplitude Ripple
Did you know a quick way to determine the approximate in-channel flatness of a QAM signal is to use a properly adjusted spectrum analyzer to observe the top of the “haystack”?
Graphics sources: Agilent and Sunrise Telecom

44. 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 MHz, Channel Width MHz, QPSK Symbol Rate Msps
Spectrum Group is overridden
BroadCom SNR_estimate for good packets dB
Nominal Input Power Level 0 dBmV, Tx Timing Offset 2035

45. 128 RS symbols = 1 RS codeword
Codeword Errors
Did you know that in DOCSIS downstream Reed Solomon (RS) forward error correction (FEC), 7 bits = 1 RS symbol, and 128 RS symbols = 1 RS codeword? 1 1 1
7 bits = 1 RS symbol
128 RS symbols = 1 RS codeword
RS symbol #1
RS symbol #2
RS symbol #3
RS symbol #4
RS symbol #127
RS symbol #128
In each RS codeword: 122 RS symbols = data symbols, 6 RS symbols = parity symbols

46. Codeword Errors
Did you know that any number of bit errors in a RS symbol means the entire symbol is errored?
= good RS symbol
= errored RS symbol
= errored RS symbol
= errored RS symbol

47. Codeword Errors
Did you know that for a RS FEC configuration of “t = 3” the FEC decoder can fix up to any 3 errored symbols in a RS codeword?
128 RS symbols = 1 RS codeword
This is a correctable codeword error
Did you know that when there are more than 3 errored symbols in a codeword the entire codeword is errored?
This is an uncorrectable codeword error

48. Q and A