Measuring Digital and DOCSIS Performance

Measuring Digital and DOCSIS Performance
- Poll the audience on what they do – line tech, service, maintnenance Randy Francis JDSU

Today’s Agenda Definitions / Background
Digital Signals / QAM DOCSIS 1, 1.1, (Data Over Cable Service Interface Spec.) VoIP Digital Testing/Troubleshooting MER, BER, QAM Ingress, Equalizer Stress, C/N vs. BER vs. MER Constellations / Symbols and Digital Bits, Phase Noise DOCSIS & IP Testing Packet Loss, Throughput VoIP Testing Voice Quality Measurements, E-Model, Delay, Packet Loss, Jitter VoIPCheck Testing, PacketCable Testing Return Path Why monitor it? RF Trending Results Ingress & Leakage Q and A Quiz Time Will be discussing: What are digital signals and how do they work. How do they contrast vs. Analog Signals. QAM signals. - What is DOCSIS? What is the history of it and what flavors are available today? - What test should be conducted when looking to test and troubleshoot digital signals. MER, BER, etc… DOCSIS testing has correlation to digital testing but not totally the same. - Why is the return path important? Especially as it applies to DOCSIS. You need 2-way communication. - Will end up with Questions and Answers .. About 1.5 hour presentation.

Acronyms & Terms QAM - Quadrature Amplitude Modulation
Symbols - Collection of Bits Symbol Rate - Transmission Speed I & Q - Components of QAM data Constellation - Graph of QAM Data MER - Modulation Error Ratio BER - Bit Error Rate FEC - Forward Error Correction These are some of the key terms that you’ll need to keep in mind during and after the presentation Intent is to more level set the audience. Don’t get hung up on these words. Qadrature Amplitude Modulation (modulates up to RF signal) (slang for Dig. Channel)-- allows you to have 8-11 digital channels in 6MHz spacing (Bit Error Ratio) In telecommunication, an error ratio is the ratio of the number of bits, elements, characters, or blocks incorrectly received to the total number of bits, elements, characters, or blocks sent during a specified time interval E-9 is good for Cable; 10 E-7 is becoming marginal. Pre – BER are errors b/4 FEC was able to correct. Post-BER are errors seen after FEC did it’s correction. Forward Error Correction - forward error correction (FEC) is a system of error control for data transmission, whereby the sender adds redundant data to its messages, which allows the receiver to detect and correct errors (within some bound) without the need to ask the sender for additional data. The advantage of forward error correction is that retransmission of data can often be avoided, at the cost of higher bandwidth requirements on average, and is therefore applied in situations where retransmissions are relatively costly or impossible

A Typical Home Installation
TAP House TV Drop Cable High Pass Filter GROUND TV BLOCK 3-Way 2-Way Splitter This chart is also for level setting. This is typically the type of home wiring that we think you’ll see out in the field. You’ll have Digital/Analog Signals, Voip and Cable Modem type signals to deal with. Splitter TV CABLE MODEM

Why Go Digital? Efficiency Quality Flexibility
Source signals are digital Standard and High Definition TV (SDTV, HDTV) High Speed Data and Digital Video is more efficient than analog Transmit equivalent of 6 to 10 analog channels (VCR quality) over one 6 MHz bandwidth Quality Better Picture and Sound Quality Less Susceptible to noise Error detection and correction is possible Flexibility Data easily multiplexed into digital signal Higher Data Security Why do we go digital? Why switch from analog? It’s a lot more efficient. Now you get more channels through. Generally limited to 870MHz of space. Eventually you will run out of space using 6MHz. Your STB today goes to several hundred channels. HD of course takes more b/w. Digital Picuture quaility is also better. Unlike Analog – you have Cliff Effect which you’ll see a bit later. You need to test because you won’t know. You can take more impairments before the channel breaks.

What is Digital? Source and Destination is digital data
Assign unique patterns of 1’s and 0’s Transmission path is via an analog carrier Choice of modulation is the one that optimizes bandwidth (data versus frequency ‘space’) and resiliency to noise 00 01 10 11 So what is a digital signal? Has anyone seen a spectrum analyzer? You’ll see a large spike for the Analog Carrier and a small spike for audio. For digital you’ll see a “haystack” on the analyzer. The content on the dig channel is in 1’s and 0s. 00 01 10 11 Generate Digital Receive Digital Transmit Analog

The Carrier, by the way, is ANALOG Modulation
Analog Content – Analog Carrier So take a look and look at analog vs digital. Digital Content – Analog Carrier

Some good news…and some bad news
Digital TV, DOCSIS, Digital Voice all use SAME type of RF Modulation – QAM Same measurements apply Signal level, Modulation Error Ratio, Bit Error Rate, in-band frequency response, are all similar, if not identical. Bad Each service has different threshold of impairments What is noticeable in Digital voice may not be perceptible in DOCSIS, what is bothersome in DOCSIS is different than digital video. There is some good and bad news. With digital services using the same type of QAM you can use the same type of tests. It doesn’t matter if its VoIP, BER, MER, DOCSIS Bad thing is that the measurments won’t always be the same. Different types of services have different issues. Sometimes I’ve even seen where there are different tech’s who do specific type testing. Let’s take a poll here and see: How many of you do A) Video B) VoIP C) Data testing?

QAM and CATV 16 QAM is part of the DOCSIS® 1.0/1.1 upstream specifications 64 QAM and 256 QAM is used for both digital video and DOCSIS downstream, allowing more digital data transmission using the same 6 MHz bandwidth Transmit equivalent of 6 to 10 analog channels (VCR quality) over one 6 MHz bandwidth DOCSIS is the Standard for data over Cable Cable systems provide higher signal to noise ratios than over-the-air transmission. A well designed and maintained cable plant meets these QAM signal to noise requirements. DOCSIS w/QAM you can use 16QAM or QPSK upstream and 64 or 256QAM in the downstream. DOCSIS The higher the number means that you can transmit more information. DOCISIS has standardized… Most of you are doing 256QAM in the downstream – correct vs. 64QAM. This means you will need more maintenance. QPSK is more forginving than QAM

QAM Data Capacity (Annex B)
(Upstream) 64 QAM (Downstream) 256 QAM Symbol Rate (Msps) 2.560 MHz) 5.0569 6 MHz) 5.3605 Bits per symbol 4 6 8 Channel Data Rate (Mbps) 10.24 Actual Information bit rate (Mbps) 9.0 Overhead 12.11% 11.11% 9.5% QAM 256 allows ~ 2 more channels vs. QAM 64 No Real Cost Advantages Why not go all QAM 256? Here are some numbers to look for. What do you get out of the higher level QAMS? In real life the tranlates to ~ 9MBps The Info bit rate is what you are likely to actually see.

What is it? What differs among versions?
DOCSIS® So now, what is DOCSIS. What is it? What differs among versions?

DOCSIS Defined? DOCSIS Definition Data over a digital QAM signal
CableLabs® Certified Cable Modem Project Data Over Cable Service Interface Specification Not required but highly adhered to standard – allows interoperability Data over a digital QAM signal Digital signal Downstream usually 64 or 256 QAM Upstream usually QPSK or 16 QAM Uses Data Packets Information broken into chunks Asymmetrical communication More Info Downstream Less Info Upstream Analog signal components are visibly discernable using a spectrum analyzer Digitally modulated signals only show a “haystack” on a spectrum analyzer regardless of modulation or content – (more tools needed) What is DOSIS – 64, 256 QAM. All this data is just broken up into packets. Aysmmetrical – one side that is a lot bigger than the other… Downstream pumps out a lot more than the upstream. You probably have different tiers (gold, sivler, et…). Usually like 6Mbps down and maybe 1.5 upstream. Depending on what you do (video games, music, etc.. ) determines your performance Digital “haystack”

DOCSIS® Versions at a Glance
DOCSIS 1.0 (High Speed Internet Access) 23 million products shipped worldwide as of YE2002 228 CM Certified, 29 CMTS Qualified DOCSIS 1.1 (Voice, Gaming, Streaming) Interoperable and backwards-compatible with DOCSIS 1.0 “Quality of Service” and dynamic services, a MUST for PacketCable™ In the field NOW - 64 CM Certified, 22 CMTS Qualified DOCSIS 2.0 (Capacity for Symmetric Services) Interoperable and backwards compatible with DOCSIS 1.x More upstream capacity than DOCSIS 1.0 (x6) & DOCSIS 1.1 (x3) Improved robustness against interference (A-TDMA and S-CDMA) Available NOW – Number of CM & CMTS Qualified growing DOCSIS 3.0 (Channel Bonding) Interoperable and backwards compatible with DOCSIS 1.x & 2.0 Specification released earlier this month – Devices not yet available Good Resource Site: So here are the vairous flavors of Docsis? What are you using? My guess is 1.1… There hasn’t been a lot of DOCSIS 2.0 rolled out yet. DOCISIS 1.0 – I can pass the information and let you serve the web. Didn’t afford much QoS. L

DOCSIS 1.1 Overview Interoperable with DOCSIS 1.0, plus more…
Enhanced “Quality of Service” (QoS) Guarantees and/or limits for data rates (ex. Gold, Silver service levels) Guarantees for latency Improved security - designed to reduce possibility of “theft of service, provide secure software downloading.” – BPI+ (Baseline Privacy Plus) Interoperability - DOCSIS 1.0 and DOCSIS 1.1 cable modems and CMTSs on the same plant. Better operation and OSS features Transmit Equalization - more robust transmission Max Modulation 256 QAM Downstream (~40Mbps) 16 QAM 3.2MHz (~10Mbps) DOCSIS 1.1 established QoS. Ex. You pay for 6Mbps downstream and 1.5Mbps upstream. How do you gurantee this for your customers? DOCSIS1.1 did this. It also improved security – hackers were getting in to services but DOCSIS 1.1 introduced BPI+ which disallowed CM spoofing and added other security Before we wer limited to 9Mbps downstream So next Cablelabs improved it again – 2.0 spec. Around for ~ 2 years. More popular in UK.

DOCSIS 2.0 Overview – Key Upstream Advances
Symmetrical services are enabled by DOCSIS 2.0 1.5x greater efficiency operates at 64 QAM 2x wider channels new 6.4 MHz wide channel 3x better upstream performance than V1.1 6x better upstream performance than V1.0 DOCSIS 2.0 widens the pipe for IP traffic, allowing cable providers to create more and better services for voice, video, and data It does this by using enhanced modulation and improved error correction Superior ingress and impulse noise performance 100% backward compatible with DOCSIS 1.0/1.1 2.0 makes the service more symetrical… Easier to xmit data in the upstream. Now you can do 64QAM in the upstream. The gap is closing with v2.0. Another thing about v1.1 was that the b/w of the signal was limited to 3.2MHz wide. Now the b/w has been moved up to 6.4MHz wide. Another thing… we have been able to reclaim some of our dirty spectrum – ie. 0 – 15MHz spectrum. If the end of the upstream is 45MHz then we are really limited but now we can reclaim some of the lower space using SCDMA, Etc.. You don’t have to go replace all your installed CMs Next the new version is v3.0

DOCSIS 3.0 Overview Specification late last year
DOCSIS 3.0 Interface Specifications (Released December 2006) CPE equipment in development stages Downstream data rates of 160 Mbps or higher Channel Bonding 4 or more channels Upstream data rates of 120 Mbps or higher Internet Protocol version 6 (IPv6) Current System (IPv4) is limited to 4.3B numbers IPv6 greatly expands the number of IP addresses Expands IP address size from 32 bits to 128 bits IPv6 supports 3.4×1038 addresses; Colon-Hexadecimal Format 100% backward compatible with DOCSIS 1.0/1.1/2.0 256QAM => ~40Mbps 4 x 256QAM => ~160 Mbps 64QAM => ~30Mbps 4 x 64QAM => ~120 Mbps Came out last year…. Verizon and ATT are losing money to the cablecos… Now they are diving in to video and high speed data using fiber to the home, node, etc… as close as possible to the home.. This gives them huge data rates … have done up to 100Mbps.. Lots more b/w. Cablelabs are trying to fight this by doing Channel Bondign. They take the info on 4 channels and pump it through giving 160Mbps throughput, assuming you can get 40Mbps for a 256QAM modulation. This will helpo the cablecos be able to compete better. Works for both the downstream and the upstream. Who’s ever heard of an IP Address? Today we are using IP v 4 (ex xx.xx.xx). This numbering scheme is limited to 5 or 6 billion numbers. This means we can pump out ~ 1IP address per person in the world. Today people have Xboxes, laptops, etc… so more than 1 IP address per person. IP6 has been around in the 70s but no one ever imagined that it would need to be used. This is what Bill Gates said about 64K memory too… Imagine that. IPv6 fixes a number of problems in IPv4, such as the limited number of available IPv4 addresses. It also adds many improvements to IPv4 in areas such as routing and network auto configuration. IPv6 is expected to gradually replace IPv4, with the two coexisting for a number of years during a transition period. V3.0 - It will also be backwards compatible to v1.0/1.1/2.0. 4923:2A1C:0DB8:04F3:AEB5:96F0:E08C:FFEC

DOCSIS 3.0 – Channel Bonding
Individual QAM 256 DOCSIS channel Versions 1.0/1.1/2.0 used only one channel for upstream and one channel for downstream communications 256QAM => ~40Mbps 4 Bonded QAM 256 DOCSIS channels DOCSIS v3.0 Spec requires devices to be able to bond a minimum of 4 upstream channels into one and 4 downstream channels into one for 4 times increased throughput in both directions The MSO does not have to use all 4 channels, but the devices which are 3.0 compliant must have the ability to bond 4 or more channels in both directions Will you take your 4 6MHz channels and have a 24MHz wide channel? No is the answer. You are just putting the data together in that same space. Today’s v3.0 CMs are about the size of a STB. It will be like having four CMs in one box. It acts like a big funnel. It’s up and coming and can’t gaurantee when it will be mass produced. It may be a while… Just a matter of time. 4 x 256QAM: 4 x 40Mbps = 160 Mbps

Digital Testing and Troubleshooting
Now let’s look at some digital testing. How? What does it mean?

QAM Digital Measurements
Spectrum & Digital Average Power Level MER QAM Pre/Post FEC BER Constellation Display Advanced Tests QAM Ingress Under The Carrier Group Delay In-Channel Frequency Response Equalizer Stress (Adaptive Equalization) We will focus on the ones at the top. Let’s cover the basics first.

QAM Digital Measurements
Measuring the quality of QAM digital carriers is significantly different than measuring analog carriers. Measurement of digital carriers is important to determine how close the carrier is to failing (how much margin) since there may be no quality degradation. When you do a measurement for a QAM channel it is very different from Analog (Level, C/N, Delta). Doesn’t look snowy then you are ok…turn on the TV and. For digital… things could look good and levels could look good but this doesn’t mean that things are ok on the digital channel. Something as simple as adding a splitter can add more loss and put you over the into the broken zone.

Digital Average Power Level Measurements
Digital Average Power Measurements and Measurement Bandwidth The spectrum analyzer view is an excellent tool to see discreet RF-carriers. Caution is needed when viewing digital modulated signals (haystack). The signal’s level measurement is derived from the selected measurement bandwidth (resolution bandwidth). At an RBW of 300 kHz, a 64QAM - 6 MHz wide digital signal reads in the spectrum analyzer trace 3 dB too low. The Average Power principle takes small slices from the integrated RF-energy, summing them together to one total power reading in the LEVEL-mode. Digtal analog level differs from analog levels. You might check the video over some KHz. Digital carriers space the power over the full 6MHz. When measuring a digital signal for average power it is best to use an SLM that measures the actual average power or to use the automated functions of a spectrum analyzer to calculate the average power. The spectrum analyzer can often lead the user to misadjust power levels by erroneously interpreting the actual power level. Analog and digital (broadcast) QAM signal. The recommended delta in level should be 6 to 10 dB.

Measuring the Digital “Haystack”
Non measured area based on 280 kHz step size within 6 MHz total Bandwidth Digital carrier under test (6 MHz BW) IF Measurement Bandwidth = 280 kHz The most practical way of measuring a digital Qam carrier is the average-power method. An average-power method takes small slices of the integrated RF-energy, summing them together to one total power reading. This method of measuring the total integrated power under the “haystack” is very accurate, especially for “haystacks” which aren’t perfectly square. Using an instrument that uses one slice of the “haystack”, or peak hold in spectrum mode, may be less accurate. Digital signal level measurements should be made with this integrated power technique for QAM, QPR, QPSK (Non-bursted) digital signals. User selectable bandwidth is needed for accurate power measurements. Sometimes a quick mode, which measures signals based on bandwidth estimation / normalization technique is warranted for quick verification. Again, this is different from the analog measurement. Summing slices of the total integrated energy Frequency -2.5 MHz +2.5 MHz +/- 140 kHz

Measuring the Digital “Haystack”
Measuring the Peak Level of the Digital Haystack Measuring the Average Level of the Digital Haystack The most practical way of measuring a digital Qam carrier is the average-power method. An average-power method takes small slices of the integrated RF-energy, summing them together to one total power reading. This method of measuring the total integrated power under the “haystack” is very accurate, especially for “haystacks” which aren’t perfectly square. Using an instrument that uses one slice of the “haystack”, or peak hold in spectrum mode, may be less accurate. Digital signal level measurements should be made with this integrated power technique for QAM, QPR, QPSK (Non-bursted) digital signals. User selectable bandwidth is needed for accurate power measurements. Sometimes a quick mode, which measures signals based on bandwidth estimation / normalization technique is warranted for quick verification. This shows how different the measurements are. But it’s more than just dB or Pwr measurements. 280 kHz Bandwidth power 6 MHz Bandwidth power

Digital – more than just dB’s
MER and Pre and Post BER measurements are key to insuring Digital Quality MER is the digital version of the analog C/N reading. Taking the difference. So this is what you should see for 64 and 256QAM. (see chart) This gives you a little head-room and you don’t get the customer call the next day.

Modulation Error Ratio (MER)
Analogous to S/N A measure of how symbols (I vs. Q) are actually placed, compared to ideal placement MER(dB) = 20 x log RMS error magnitude average symbol magnitude Good MER 64 QAM: 28 dB MER 256 QAM: 32 dB MER Unlike the analog C/N, Average symbol magnitude RMS error magnitude

MER determines how much margin the system has before failure
More MER Modulation Error Ratio (MER) in digital systems is similar to S/N or C/N used in analog systems MER determines how much margin the system has before failure Analog systems that have a poor C/N show up as a “snowy” picture A poor MER is not noticeable on the picture right up to the point of system failure - “Cliff Effect” Can’t use the TV as a piece of test equipment anymore With MER you don’t notice a problem until it tiles and then you are gone. MER gives an indication on how close you are to the Cliff Effect.

Digital TV Waterfall Graph
As noise floor increases – the analog picture gradulally goes down but the digital signal degredation is totally difrrerent.

C/N vs. BER vs. MER No FEC On the top it’s an anallog signal. ON the bottom is a digital signal. For analog you can gradually see the decline… With the Digtial signal it’s different (see the chart). You see the change within ~ 3 db. This is an ex. Of a 64QAM channel Now the customers give you a call. The MER is done on 1 channel.

Lets have some MER review…
Some Guidelines for MER 64 QAM - 28dB or better 256 QAM - 32dB or better Try to add at least 3db to above figures to allow headroom. “MER” is to Digital, what signal to noise is for analog MER is affected by high noise, low signal Also ANY other continuous impairments MER readings are relatively immune to “brief bursty” interference MER is a predictor of BER Now a quick review of MER…. Bursty – quick, gone quickly… not really shown in the MER… MER is more of an average measurement. So MER won’t likely find this. But you do see noise floor issues. So how do we look for more itermittant issues like this… BER..

The more bits that are incorrect, the more the signal will be affected
Bit Error Rate (BER) Bit errors result when the receiver interprets the wrong symbol and hence the wrong bits The number of bit errors versus the number of bits transmitted is Bit Error Rate (BER) The more bits that are incorrect, the more the signal will be affected Good signal: BER (1.0 E-9) Threshold for visible degradation: BER 1E-6 One error in every 1,000,000 bits BER will sample this information. You are expecting 1 and 0s. BER looks at the bits coming in not such much doing a measurement. The more bits are incorrect then you will see it represented as “1 error” in a Billion (1x10E-9) transmitted. 1E-6 means 1 error in every million bits sent.

More on BER “BER” is HOW many errors per HOW many bits of data
BER is affected by bursty interference Also, any other impairments will adversely affect BER Some amount of errors can be corrected by digital receivers. (PRE errors) POST errors are uncorrected errors – always unacceptable!!! Measurement reads in scientific notation 1.0 E-9 is the best on a handheld test meter BER is good as catching the quick impairments. Post errors – things that the customer will notice. It is unacceptavel to have post BER errors. 1E-9 is best for 1E-9. Rhode and Swartz can go to

Pre and Post FEC BER Number of bad bits for every good bit.
Forward Error Correction when working will output >10-11 1 error in 100 billion bits 1 error every 42 minutes MPEG-2 likes good BER FEC will work to about 10-4 1 error in 10,000 bits 1 error every 276 uses QAM256  42Mb/s FEC can correct BER from 1-E-4 to 1E-9 FEC causes Cliff Effect What’s Pre and what’s Post? QAM channel can pump out 42Mb/s  42Mins == FEC – helps correct errors that are sent down the line. They can be fixed in the STB and CM.

BER Example A 256QAM channel transmits at a symbol rate of 5M symbols per second Bit rate = 8 bits per symbol X 5M symbol per second =40M bits per second Error Incident = Bit rate X BER = Errors Per Second So just a bit more on scientific notation. What does it mean? (see chart)

What Causes MER and BER to degrade? “ Noise”
The most common problem with digital-TV and cable modem services is interference under the digital carriers and Noise(stated by most larger operators with experience in digital transmission services). Most common sources are: Ingress due to off-air UHF TV channels Intermittent ingress due to pager transmitters or two-way radio base stations. Cracked or broken cables CSO/CTB-intermodulation – due to unterminated TAPs or impedance mismatch Laser Clipping What causes the MER or BER to degrade? Many times its noise. Off air ingress via broken, cracked cables also. CSO/CTB doesn’t go away. The haystack somewhat hides it sometime.

QAM Constellations Now let’s discuss QAM Constellation.
Don’t want to go too deep into it, though. Poll: How many of you are familiar with the diagrams? How many of you use them?

Constellations, Symbols and Digital Bits
Each “dot” on constellation represents a unique symbol Each unique symbol represents unique digital bits Digital data is parsed into data lengths that encode the symbol waveform Each box is called a boundary. Think of a QAM Channel as being someone given directions on where info should go and the QAM diagram as a map. Each box is an address. 16 QAM

Constellation: headend or field problem ?
Constellation is an ideal tool to find QAM modulator problems. Modulator issues in the headend or noise and interference (ingress, CTB, CSO, etc.) in the field? It’s possible to see interference in the constellation diagram if interference is very severe, however, one can’t distinguish noise from micro-reflection problems. Much better alternatives to find ingress, noise and micro-reflection problems is an in-service spectrum analyzer view and equalization stress characteristic. Constellation diagram showing severe ingress Constellation is an ideal tool to find QAM modulator problems. The very distinguish shapes of the constellation diagram reveals in one view if the problem is caused by the modulator issues in the headend or noise and interference (ingress, CTB, CSO, etc.) in the field. In theory, it’s possible to see in the constellation diagram if the field problem is caused by noise or interference, but practically this only works if the interference is very severe. Subtle problems can not be seen easy. Also the constellation diagram can’t distinguish noise from micro-reflection problems. Much better alternatives to find ingress, noise and micro-reflection problems is an in-service spectrum analyzer view and equalization stress characteristic. Typical which errors originates from the headend. Phase Noise The constellation appears to be rotating at the extremes while the middle ones remain centered in the decision boundaries. Phase Noise is caused by headend converters. Gain Compression The outer dots on the constellation are pulled into the center while the middle ones remain centered in the decision boundaries. Gain Compression is caused by filters, IF equalizers, converters, and amplifiers. I Q Imbalance The constellation is taller than it is wide. This is a difference between the gain of the I and Q channels. I Q Imbalance is caused by baseband amplifiers, filters, or the digital modulator. Carrier Leakage

Constellation Display
This is a visual representation of the I and Q constellation plots on a 16 QAM carrier

64 QAM and 256 QAM 64 QAM has 8 levels of I and 8 levels of Q making 64 possible locations for the carrier. 256 QAM has 16 levels of I and 16 levels of Q making 256 possible locations for the carrier. QAM 256 is somewhat less robust vs. QAM 64 8 Levels of I Channel 16 Levels of I Channel For each different QAM there are different number of boxes, 16, 128, 64, 256 etc… As modulation goes up things are less robost because there is a lot less room to play around with. 8 Levels of Q Channel 16 Levels of Q Channel

Effects of Noise and Interference
Noise and Interference moves the carrier away from its ideal location causing a spreading of the cluster of dots. Ideal Locations When the

Data Received Correctly
Decision Boundaries Data that falls outside the decision boundaries will be assumed to be the adjacent data. Data Received Correctly Q I When the dots don’t go where they should then you get a Bit Error. Error Decision Boundary

Constellation Typical errors which originate from the headend:
Phase Noise The constellation appears to be rotating at the extremes while the middle ones remain centered in the decision boundaries. Phase Noise is caused by headend converters. Gain Compression The outer dots on the constellation are pulled into the center while the middle ones remain centered in the decision boundaries. Gain Compression is caused by filters, IF equalizers, converters, and amplifiers. I Q Imbalance The constellation is taller than it is wide. This is a difference between the gain of the I and Q channels. I Q Imbalance is caused by baseband amplifiers, filters, or the digital modulator. Carrier Leakage Gain Compression: Slight Laser clipping also causes GC Constellation can help show where the problem is: field, home, et…

System Noise and/or Leakage Issues
A constellation displaying significant noise Dots are spread out indicating high noise and most likely significant errors An error occurs when a dot is plotted across a boundary and is placed in the wrong location Meter will not lock if too much noise present Dots are spread out showing error Leakage or Noise coming in on the line.

Constellation with Phase Noise Zoomed Constellation with Phase Noise
Display appears to rotate at the extremes HE down/up converters can cause phase noise Random phase errors cause decreased transmission margin Caused by transmitter symbol clock jitter Rotation Rotation Show’s up more like a rotation. Can be caused from converters w/in the HE… Garbage coming in… escalate the ticket back to the HE… Constellation with Phase Noise Zoomed Constellation with Phase Noise Key Cause: Converter within the HE (garbage in produces garbage out)

Gain Compression If the outer dots are pulled into the center while the middle ones are not affected, the signal has gain compression Gain compression can be caused by IF and RF amplifiers and filters, up/down converters and IF equalizers May be caused by line amps or laser clipping Not a customer issue – maintenance techs issue Corner dots are pulled into the center. Can be caused by line amps or laser clipping. Also not a customer issue. Outer edges pulled in

Coherent Interference
If the accumulation looks like a “donut”, the problem is coherent interference CTB, CSO, Off-Air Carriers (ingress) Sometimes only a couple dots will be misplaced This is usually laser clipping or sweep interference Circular “donuts” Donuts – if there is CSO or CTB.

What is important? What can be done?
DOCSIS & IP Testing Let’s now dig into DOCSIS What is important? What can be done?

DOCSIS® Testing - Levels
Verify proper receive level at cable modem Should be seeing: 0dBmV Pass/Fail Indicator: Downstream Information Frequency, Modulation type, Channel MER shows that downstream is clean and clear with margin Upstream Frequency, Modulation type, Channel BW, DOCSIS version BER shows that downstream is clean and clear of impulse noise Of primary concern is that the DOCSIS® channels are able to operate properly. Verification of ranging provides the necessary information to determine two-way capability. By checking ranging you can verify the following Should be seeing -10 and 0dBmv– you specs might be tigher. Looking specifically at the CM type data not the TV service. Downsteam QAM signal is okay and within service margins of Level, MER, and BER. Upstream – the modem is able to range properly and obtain a working upstream channel. One important parameter to look at is the upstream transmit level. It is important to have the upstream transmit level be in the proper range. A suggested nominal range for transmit level is between 35dBmV and 45dBmV. If the modem settles out with a high transmit level, 56dBmV for example, you are at risk for the modem to drop offline as the system changes. Suppose the modem settles out at a low value, 22dBmV for example because it is on a low value tap. Then the modem is knocked off line and has to range again. Due to too much traffic it doesn’t get a response from the CMTS, the modem will continue to ratchet up its output level – potentially all the way up to 58dBmV in an effort to get a response. The result of this excess output level, 36dB higher than nominal, could cause reverse laser clipping. In this situation you can install a frequency selective attenuator that attenuates the reverse path but not the forward path. Shows that upstream is properly aligned and CMTS has “ideal” receive level with margin to spare. Recommend: (35dBmV – 45dBmV)

DOCSIS – Packet Loss Testing
Check Packet Loss and determine if upstream is good Overall loop information, Up+Downstreams You also want to do some IP testing…. Whole point is to test data transmission. Packet loss testing is good to do… to many packets lost cause very slow “surfing”. View Downstream performance Upstream Signal to Noise Ratio

DOCSIS – Throughput Testing
Check Throughput for proper speeds Config. File You also want to check the throughput… If the customer pays for 10Mbps then you want them to get it so you don’t get called back out. Ensure customer can get what they pay for

Correlate Data & Voice with RF
Compare Node Outages with your Return Path performance history to QUICKLY identify the cause of the problem – RF or Data layer! Monitoring the reverse path can take shape by more than just RF performance monitoring. Many systems are implementing modem status monitoring which will alarm on outages. By using the data tools in conjunction with the RF monitoring tools you can quickly determine the cause of the problem, RF or Data layer. To do this you will need to have history of the RF and Data to compare outages at particular times. Here you see 0 out of 15 can’t get online. If it’s a node issue then you can do more checking at the node vs. the home. HSD Node Outage Report Courtesy of Auspice Corporation

Why is it important? What can be done?
VoIP Testing Return path is ½ the communication. Why is it important? What can be done?

VoIP – Bullet Train Analogy
Ideal World: Packets like train Cars through a station – 1 at a time, evenly spaced, and Fast Bullet Train… Cold & Refreshing… Keep in mind what the service should look like… a bullet train is fast, sleek and well designed; cars going through one at a time with no hiccups.

VoIP – Train Analogy Real World
VoIP Packets don’t always do what you want… Multiple railcars taking different directions, uneven spacing Missing railcars… all = TRAIN WRECK!

Voice Quality - Measures
Models to Answer: “Does it ‘Sound Good’?” International Telecommunications Union (ITU) has multiple models Subjective (Opinion of users) Mean Opinion Score (MOS) Defacto standard ‘score’ “Judged” summary of call quality In-service test, can be used on live network Algorithms have replaced but mimic opinionated scores Objective (Not an opinion) Perceptual Analysis/Measurement System (PAMS) and Perceptual Evaluation of Speech Quality (PESQ) Not based on network issues: Packet loss, Jitter and Delay Do not factor in end-to-end delay Require HW probes (Not used for in-service testing) E-Model, R-Value Addresses all issues with PAMS and PESQ particularly as it places focus on Packet loss, Jitter, and Delay How you measure/summarize what actual VoIP is; cable industry piggybacking off of telephone world. True MOS is people with golden ear: scoring calls; In service test on live network, live telephone call; people rating from 1-5 what same phone call sounded like to come up with MOS score. MOS algorithm that matches with score that a person would give PAMS: device at each end , meant for copper testing, out of service test Emodel: takes call brings in what affects calls most pl jitter delay: call quality for VoIP R-Value: good for figuring MOS score with live network; convert into actual MOS score. MOS= mean opinion score, the standard scoring for how a call would sound on the network.

Voice Quality - E-Model
E-Model  R-Value Most complete, objective test R = Ro + A – Ls – Ld – Le Ro: Basic SNR or circuits – handsets, environment A: Advantage Factor – GSM poor service, example. Ls: Simultaneous impairment factor – loudness/volume, hissing, sidetone, quantization distortion Ld: Talker/listener echo, Overall Delay Le: Equipment impairment factor – Packet loss, Jitter R-Value: converts to MOS score, takes into consideration packet loss, jitter, and delay Ro: Real world – every component induces its own noise onto the network. Solution- more expensive equipment. A: what type of service is being used? How you are doing transmission. Ls: how much volume? Hissing? Can’t change. Equipment impairment factor: ingress, noise on system: router overcapacity shitty equipment Biggest effect delay, jitter, packet loss Chart: 3.6 MOS score (about 73 on R-Value) is the cross over point from always satisfied to marginally satisfied. Things you will take action on!

Delay, Packet Loss and Jitter- What is it?
Time it takes a packet to ‘transverse’ the network Point A to Point B Time it takes is shown here as ‘X’ Too much delay affects the quality of a call See this on PSTN with international calls (Over-talk) Echo (More delay = more potential echo) Usually an architecture (traffic/capacity) issue Longer distances and more routers add delay Too many calls/downloads add delay (slows routers) Generally not a HFC issue with equipment such as amplifiers X-Time X-Time Network D D C C B B A A Point A Point B What causes delay: Traffic capacity, digital processing, under 100km is still within standard delay Over-talk: large delay between time the information left from person making call to person listening= unnatural pause in conversation, causes other person to start talking, then other persons voice starts talking, both voices colliding, causing “over-talk” Echo: person talking can hear themselves speak. Echo cancellers monitor the communication for a set amount of time to decipher if a piece of information passed through in one direction and came back in opposite direction; if delay is too great, the time window for echo canceller to work effectively is beaten and the echo will come back through the network and be presented on the other side. Echo then comes from opposite side and becomes greater as delay becomes greater. Architecture: too much visual IP or too much data communication; router overflowed and unable to distribute packets quickly; too much distance between the two locations increases delay. Not usually attributed to HFC issue/amplifiers.

Packet did not arrive (Point B) or out of sequence VoIP telephony is different than data If it was out of order, computer reorders for data If lost, just retransmit data Shown here as infinite time – did not show up Worse if it is ‘bursty’, many lost in a row – “lossy” Can be architecture or physical layer Ingress (especially upstream) Routers over capacity (too full to hold any more) ∞ -Time X-Time Network D D C C B B A A Point A Point B Major causes of packet loss: physical layer ingress, noise, return path Architecture: data layer Bursty: harder to trouble shoot, looses chunks, Continuous packet loss – examples: Unscrewing a cable; Transient Noise on a line (non-lingering issues) Cell: phone example, going into tunnel packets get lost, sound like underwater Packet Loss- Measurable statistic; whether or not a piece of information that was transmitted ever showed up on other side. VoIP- if a packet gets lost in the network, it is permanently gone/lost, a chunk of information is gone. “bursty”- usually a packet getting lost in upstream, 75%-80% Routers over capacity: amount of information going out is disproportionate to incoming information. 10% of issue; usually an HFC issue.

Packets not arriving at the same time (different from X-Time) – time between packets is different Different than data You never notice with Data, doesn’t matter how the information arrives, just care that it shows up VoIP is Real-Time Key Causes IP based equipment having packet routing issues X-Time Network D D C C B B A A C B D A Point A Point B Slower than X Slower than X Faster than X Most difficult to comprehend… The difference of delay of when it was expected- to when it actually showed up. Diagram: ABCD all go out same time; A shows up correct time, B shows up later than it should have, C is faster than it should have, D is creeping in. Keep jitter down to 0 if possible Data (VoIP is real time service) Freight train: always one after another Different rates of delay Causes: IP based equipment, routing different for each packet, packet scheduling priority

Packet Knowledge D C B A Z Point A Point B FULL
Right before packet B goes out to network, the chosen router is full and picks a different route that’s not being utilized; packet B is choosing top route instead of bottom route which is longer but more reliable at that time; before packet C goes out, the bottom router is becoming empty, and packet C chooses bottom router and shows up before packet B= conversation is out of order and different timing. Sounds like a bunch of garbage, not a natural conversation; or results in packet loss. Z

Jitter – A problem? Creates two problems D C B A Direct speech quality
Part is FF, part is normal, part is ‘Slow’ Could result in Packet Loss Walk on top of packets if some are slow and consecutive ones are fast. Could be capacity, physical or core architecture Routers spitting out packets at varying intervals Packets take different routes to destination D C B A X-Time Faster than X Overlap: B and C show up same time it dumps one Slower than X Slower than X

Jitter – Solution? Jitter Buffer D C B A D C B A
Add a place to store the data, and ‘spit’ it out at a constant rate If packets come in fast – slow them down Packets are slow – OK, we have added time to allow for this Very similar to Windows Media Player Jitter Buffer D C B A D C B A Point B Y-Delay X-Time Slower than X Slower than X Real player, Windows media player, downloading video off web page, you see the bar going up ahead of the play bar, buffering information coming off web example loading Too much buffer in a call, people wont stand for it Too small buffer, inadequate spacing/timing Does Jitter Buffer solve the problem? Yes- allows for packets to be re-arranged, slowed down, sped up but also incurs additional delay. Dynamic jitter buffer optimizes itself for that particular communication. Faster than X Buffer bar

Jitter Buffer – Problem Solved?
Yes and No We have taken care of jitter (that isn’t extreme), but have added delay: Y (buffer size) If packets come in fast – slow them down Packets are slow – OK, we have added time to allow for this X-Time X-Time Y-Delay D C B A D C B A Just Right Dynamic Jitter Buffer Self adjusting buffer, based on system conditions which optimizes the jitter buffer C A D B

Jitter Buffer – Tradeoffs
Too large Too much delay Too small Have jitter Have packet loss Just Right Dynamic Jitter Buffer Can set this option so the system will monitor and optimize the jitter buffer

VoIPCheck™ over DOCSIS®
HFC Performance Packet Statistics Packet loss Delay Jitter VoIP Quality MOS R-Value Test Result Totals Current Min Max Average DSAM- 2 different types of VoIP Communication Send out RTP packets, VoIP style packets, will be able to turn out those packets, receive packets, do calculation, get a packet loss, jitter, delay calculation and can also pull off MOS and R-Value for real life, the best and worst, and the average since test began. Look at average, current- typically sporadic. Gives you a good overall of how well a VoIP communication would be over a DOCSIS channel.

VoIPCheck™ over DOCSIS®
Segment HFC and IP impairments Quick quality (MOS) verification of VoIP over DOCSIS® channel Good to check VoIP packet statistics Can also dive into the segmented view which allows you to go beyond the CMTS. Need to be able to test the communications past the first router or past the CMTS. VoIP check sets up advanced IP server on a location around the gateway that you have control of… testing more of your IP backbone for any kind of potential issues. If call would be good over regular docsis channel then the call should be clear of impairments, unless provisioning or back office equipment problems Does not check proper MTA provisioning, signaling, communication to back office servers, or other call degradation such as echo.

VoIP Testing Based on CableLabs™ PacketCable™ Standard
Place and test the Quality of a PacketCable® Based VoIP Call to validate In-Service performance Measure Performance Statistics Packet Loss Total “End to End” Delay (ms) Jitter MOS & R-Value DSAM- 2 types of tests: VoIP check tests – sends out rtp packets, receive packets, do calculation, do a packet loss, jitter, delay statistics, and pull off MOS and R-Value at same time. Look at average. Full blown packet cable VoIP test= place a live telephone call with the DSAM. DSAM has an embedded mta built into it and establishes signal with network servers, is granted access, and allows technician to place phone call and qualify communication of coax between subscriber’s home and operator’s network. Enables them to get the packet loss, jitter, delay scores from their end but also from other side communication especially if on same network. Also to pull off the R-Value and the MOS score from both locations. Able to fully establish what’s going on its VoIP network. One mta to another mta Place a phone call on meter to eliminate potential CPE issues; Insure network connection/signaling

Segmented Error Budgets
Chart for goal direction, for example keep delay less than 100 ms, jitter less than 10 ms, packet loss less than 1, and keep R-Value above 70. Will have issues if delay starts getting above 150 ms one way, above 15 ms for jitter, above 2% packet loss, etc.

Wrap-Up

Troubleshooting is “Back to the Basics”
Majority of problems are basic physical layer issues Most of the tests remain the same Check power Check forward levels, analog and digital Check forward / reverse ingress Do a visual check of connectors / passives Replace questionable connectors / passives Tighten F-connectors per your company’s installation policy Be very careful not to over tighten connectors on CPE (TVs, VCRs, converters etc.) and crack or damage input RFI integrity Testing 256 QAM transmission of data over HFC By Marc Ryba, Senior Project Engineer; and Paul Matuszak, Senior Project Engineer, GI Communications Division, Eastern Operations, General Instrument Corp. December 1, 1996 Addressing industry demands for more efficient bandwidth utilization and building on its experience with 64 QAM transmission over cable, General Instrument has developed a 256 QAM transmission system that provides far more efficient use of cable system bandwidth and expands channel capacity. This expanded channel capacity results in a 44 percent increase in information rate and a 50 percent increase in video content as compared to 64 QAM. With it, broadband network operators will be able to carry two HDTV channels instead of just one in a 6-MHz space. The added capacity enables expanded video, modem, telephony and business data services. 256 QAM transmission also makes it possible to substantially increase the number of cable services on bandwidth-limited networks designed for analog video performance. This capability might allow deferral of costly upgrades/rebuilds. GI successfully conducted the first extensive field tests of the 256 QAM system in an actual cable environment with Rogers Cablesystems Limited, Canada's largest cable operator. The field testing discussed was performed at 21 locations served by three different Rogers headend sites servicing parts of Toronto, Newmarket, St. Thomas and Woodstock in Ontario, Canada. New and older cable plants were chosen to test the performance of 256 QAM transmission in systems typical of deployment scenarios. Background As mentioned above, the 256 QAM system's increased information rate enables a larger number of services to be compressed in a 6 MHz bandwidth. This increased information rate, resulting from 256 QAM's added spectral efficiency, provides the opportunity for carrying additional services such as increased quantities of digitized cable channels, video-on-demand, near-video-on-demand, Internet access and interactivity — without compromising existing features and services — which results in additional revenues for broadband network operators. On average, for equivalent picture quality, nine NTSC signals can be placed in the same bandwidth, as compared with only six signals for 64 QAM. Table 1 provides a comparison of 64 QAM and 256 QAM efficiencies. These values are based on an average bit stream for each video service. Assuming that film-based services are effectively digitized at a 3 Mbps (Megabits per second) rate, and live video at 4 Mbps, the 256 QAM transmission results in a 50 percent increase in both live video and movies per 6 MHz bandwidth. Also, with the HDTV bit rate specified by ATSC as 19.4 Mbps, 256 QAM is able to transport two HDTV signals in the same bandwidth, while 64 QAM can accommodate only one signal. The larger constellation size and concomitant reduced Euclidean distance associated with 256 QAM transmission does compromise some of the signal robustness seen with the 64 QAM signal. The recommended carrier-to-noise ratio for operating 256 QAM and 64 QAM through the cable system is 37 dB and 32 dB, respectively. The theoretical BER curve showing carrier level vs. additive white Gaussian noise (AWGN) is shown in Figure 1. The carrier-to-noise ratio for the theoretical coded 256 QAM signal has a 6 dB shift in noise performance as compared to 64 QAM and is therefore less tolerable to noise. The curve also shows the increase in performance obtained by the use of ITU J.83(B) FEC over the ITU J.83(A) with a 256 QAM constellation at MSps (Mega Symbols per second). Parameters such as CNR, CSO and CTB should be well controlled for 256 QAM transmission. It has been observed that peaking in the distortion components is a primary cause of bit errors. As Figure 2 illustrates, because of the denser 256 QAM constellation, it is less tolerant of these distortions. Therefore, for successful deployment of 256 QAM, cable plants should adhere to FCC technical standards as a minimum. Test setup All tests were bit error rate tests and were conducted using Broadcom transmission hardware, ITU J.83(B) Forward Error Correction (FEC) and prototype demodulators. A block diagram of a typical receive site test setup is shown in Figure 3. A pseudorandom data generator and FEC encoder were used to produce the input to the Broadcom 256 QAM modulator. Channel up-conversion was performed using a General Instrument C6M for the 256 QAM signal and was then combined with Rogers headend analog channels for transmission. The QAM signal transmission channels were varied from area to area, with the test channels usually operating at the upper edge of the cable spectrum. The 256 QAM average signal power level was adjusted at the headend for operation at 10 dB below the adjacent analog video's peak of sync power. The proof of concept receiving equipment which was used consisted of an 860 MHz bandwidth RF tuner and a 64/256 dual QAM demodulator incorporating an ITU J.83(B) FEC at an interleaver depth of 66us. Testing was performed in selected Rogers employee homes and at pedestal taps in residential neighborhoods through 100 feet of coax simulating the drop to other cable subscribers' homes. Extended duration testing was performed in the Rogers employees' homes to both assess longer term error performance as the cable system levels change with temperature and to determine the impact of in-home wiring on 256 QAM modulated signals. Performance tests at the pedestals consisted of BER measurements and input power level variations of the QAM and analog signals. Two PCs were used for each demodulator/BERT pair during the course of the tests: one for logging errored seconds from the HP3784 BER tester and the other for tuning and controlling the demodulator. Recording of BER data was accomplished via an RS232 link between the BER tester's printer port and a PC. Short-term tests were performed using 15-minute gating periods. Extended duration testing consisted of one-second gating periods for the duration of the test. Each test had an associated error log that recorded the error count and the time duration of the test period. The file was stored in ASCII format for later off-line analysis. Test results Initial testing consisted of a lab trial of the 256 QAM signal over an ALS DV6000 (8-bit) digital fiber link. The fiber link consisted of a 1550 nm laser and 20 km of Corning SMF28 fiber optic cable. No problems arose with transmission of the 256 QAM signal through the link. A BER vs. broadband noise response curve was verified for the QAM signal by introducing AWGN into the system after the modulator. Little degradation in BER vs. noise performance was seen on the QAM signal. The link was found to be transparent to the 256 QAM signal and ran error-free. This BER curve is shown in Figure 4. The IF-RF performance over cable vs. fiber link is virtually identical. System performance, shown in Figure 4, is degraded by approximately 0.6 dB for the following reasons: The 64/256 QAM dual-mode Broadcom demodulator chip, which interfaces directly to the ITU J.83(B) FEC, provides seven soft decision bits rather than the eight required by the FEC in 256 QAM mode. Since the LSB is not used, this results in 0.2 dB of performance loss; In order to transmit MSps in a 6-MHz channel, a filter roll-off (a) of 12 percent is required. A filter with an alpha of 12 percent is used in the transmitter, but the Broadcom demodulator chip implements a receive filter with a roll off of 20 percent. This mismatch between the transmitter and the receiver adds 0.4 dB degradation. The first set of system tests was conducted over a newly-upgraded HFC plant. Two fiber optic links were used and consisted of a 55 km fiber link using the ALS DV6000, and 10 km AM fiber links connecting the headend to several optical hubs, as shown in Figure 5. From the hubs, coaxial distribution was used with the longest runs tested being two equally long active runs. The first consisted of seven trunk amplifiers and two line extenders, and a second consisted of six trunk amplifiers and three line extenders. The 256 QAM signal was placed on EIA Channel 80. The lower adjacent channel supported cable modem traffic operating at 500 kbps QPSK. The upper adjacent channel was inactive. Five sites were tested under short-term conditions, and all ran error-free. One extended duration test was performed and resulted in percent error free seconds (EFS). The test duration was 37 hours, 3 minutes. Table 2 provides a summary of the extended duration tests performed. Threshold of visibility (TOV) also was performed at this location. TOV is defined by CableLabs as a BER less than or equal to 3E-6, obtained in three consecutive 20-second gating periods. If a BER greater than 3E-6 occurs in one of the three 20-second gating periods, another period is allowed to be tested. The limitation in TOV testing was found to be the signal level at the front end of the tuner. TOV levels were within amplitude variations that are expected to be seen on a typical cable drop over time because of temperature. Digital carrier-to-noise ratios for all sites were found to be between 31 dB and 33 dB. Analog carrier-to-noise ratios up to 45 dB were measured. Carrier-to-noise and distortions did not present a problem at this location. Subsequent system testing was performed at two different locations on older, non-rebuilt coaxial systems. The first system tested was specified as an "electronics drop-in upgrade" 450 MHz system. This location's longest active run that was tested consisted of a 30-trunk amplifier cascade. The 256-QAM signal was placed on EIA Channel 48. Both lower and upper adjacent channels were present and used sync-suppression for video scrambling. Five short-term tests were run at four locations. The tests ran error-free. One 256-QAM extended duration test was performed and resulted in percent EFS. The test duration was 5 hours, 36 minutes. Considerable in-band tilt, (approximately 3 dB), was observed on the 256 QAM signal at this site. The tilt was because of excessive system frequency/amplitude roll-off and exceeded the specification for the demodulator. The tilt is the cause of the degraded BER performance. TOV testing was performed on one site and found to be consistent with the previous measurement on the recently upgraded HFC system. Digital carrier-to-noise ratios for all sites were found to be between 30.5 and 36.2 dB. Analog carrier-to-noise ratios up to 46.6 dB were measured. The carrier-to-noise ratio did not present a problem at this location. Distortions did not appear to be problematic at this location and met FCC required specifications. The second fully coaxial system tested was also an older 450 MHz system. The longest active run tested consisted of a 31-trunk amplifier and one-line extender cascade. The 256 QAM signal was placed on EIA Channel 51. The lower adjacent channel was active, and the closest upper channel was EIA Channel 53. Three locations were tested under short-term conditions. All tests ran error-free. Two extended duration tests were performed simultaneously and resulted in percent and percent EFS. The time duration was 13 hours, 16 minutes. Digital carrier-to-noise ratios for all sites were found to be between 30.8 and 38.4 dB. Analog carrier-to-noise ratios up to 48.4 dB were measured. CNR, CTB, and CSO did not present a problem at this location. Conclusions Based on the test results obtained on the Rogers system, 256 QAM is a viable transmission format for properly maintained new and older cable plants and inside wiring. Short-term tests yielded error-free performance, and extended duration test results showed EFS performance of percent or better. Test results indicate minimal degradation in performance when operating over a digital fiber link, such as the ALS DV6000. On the headend systems tested, for the most part, RMS distortions measured were below the levels that would induce bit errors. Distortion levels (rms values) such as CTB and CSO were not the primary cause of errors, but the random peaking of these distortions was a cause for concern. In HFC plants, shorter runs and fewer active components minimize the potential for these effects. The FCC technical standard for cable is still a viable guideline for implementing both 64 and 256 digital transmission. At a minimum, operators should adhere to the FCC specification to ensure successful implementation of digital transmission.

Common problems typically identified in outside plant
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 lower value taps. Unused drop passive ports not terminated. Use of so-called self-terminating taps (4 dB two port; 8 dB four port and 10/11 dB eight port) at feeder ends-of-line. Such taps are splitters, and do not terminate the line unless all F ports are properly terminated.

Common problems typically identified in outside plant
Kinked or damaged cable (including 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.

Quiz Time

Quiz Time Define DOCSIS and QAM What is the visual difference in an analog channel vs. digital channel on a spectrum analyzer. How many channels can you get in a digital 6MHz b/w? When would someone use QAM256 modulation vs. QAM64? Why not use all QAM256 modulation? What’s the data rate expected for DOCSIS 3.0? Distinguish between Downstream and Upstream. What new technology enables this larger data rate? Please explain. How many IP addresses will IPv6 provide?

Quiz Time con’t. Why doesn’t Peak Level Power = Ave Digital Power?
Define MER and BER. Why do we use Constellation Diagrams? What are 2 good DOCSIS Tests? What are 3 things to check when testing VoIP? What are 2 accepted “scorings” used in the industry? What type testing should be done on the Reverse Path? Why is Leakage testing important? Name 2 ways that it can it be performed?

Questions Thank you for your time… Any Questions???

Measuring Digital and DOCSIS Performance

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