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DOCSIS 3.0 Tutorial – Introduction

October 12, 2009 by Brady

Fast Train

DOCSIS 3.0 Gets Fast

This is the first of a new series of Tutorials focused on the Data Over Cable Service Interface Specifications (DOCSIS) version 3.0.  I will make the assumption that you are familiar with the DOCSIS 1.x / 2.0 standards or have already reviewed my DOCSIS Basics Tutorial as I will be using many terms without explanation since they were previously covered.

The DOCSIS 3.0 specification is an extension of the DOCSIS 1.x and 2.0 specification which dramatically increases the data throughput by adding a technology known as channel bonding to the DOCSIS downstream and upstream, adding increased security, adding support for IPv6, and substantially improving the back-office management support (MIBs, SNMP, IPDR, etc.) for DOCSIS.  Each of these topics will covered in much greater detail in this DOCSIS 3.0 tutorial in multiple posts yet to come.

First and foremost, DOCSIS 3.o is most recognized for its dramatic downstream and upstream IP data throughput capabilities.  Typically these are four times those that DOCSIS 2.0 can support in the downstream / upstream and 12 times what DOCSIS 1.x can support in the upstream.  Without getting into the details yet, this is accomplished by using a rather simple concept called “channel bonding”.  In DOCSIS 1.x and 2.0, the CMTS transmits data to cable modems using one downstream QAM RF upconverted channel (“DS channels” for short).  The DOCSIS 3.0 specification has developed some unique methods to allow new CMTS architectures to communicate with DOCSIS 3.o modems using four DS channels.  So if a 256-QAM channel can transport 38 Mbps, then four 256-QAM bonded channels can transport 152 Mbps.  Similarly, DOCSIS 1.x/2.0 cable modems transmit data to the CMTS using one upstream RF channel using a number of different digital modulation schemes in a TDMA (or S-CDMA) format.  In DOCSIS 3.0, the specification allows for up to four upstream channels to be bonded together.

The following table shows the speed increases that can be expected for the downstream and upstream in both DOCSIS and Euro-DOCSIS systems when channel bonding is used in a DOCSIS 3.0 system.

DOCSIS 3.0 Speed Compariso

DOCSIS 3.0 Speed Comparison

Note that this chart also shows a DOCSIS 3.o downstream with “(8 channels)” bonded, yielding a data throughput of 343.04 Mbps.  When I stated channel bonding allowed four DS channels to be bonded, that indicated this was just a minimum.  Broadcom, one of the chipset vendors that makes cable modem chipsets, currently has a chipset solution which supports eight bonded downstreams.  Many cable operators are looking at using more than four bonded downstream channels.  Either to have the ability to provide more data bandwidth to the end user or to provide both data and MPEG/IPTV set top box capabilities in the same chipset solution.  It is important to understand this capability of the DOCSIS standard, the CMTS, the home (premise) equipment and potentially the test equipment used for installation, all of which will be covered later.

Summary

DOCSIS 3.0 offers tremendous capabilities over previous revisions to the specification.  The drivers for the new specification have been part do to new data intensive applications, but much more as a response to competitive threats such as Verizon’s FIoS and AT&T’s U-Verse.  DOCSIS 3.0 does offer much more than just speed, such as a distributable architecture, enhanced security, support for IPv6, enhanced back office management support and much more.  DOCSIS 3.0 does require a fork-lift upgrade if existing headend equipment is very old, however some vendors have migration paths for their carrier class platforms such as Cisco’s ubr10k, Motorola’s BSR 64k and Arris’ C4 CMTSs.  In either case, the future (at least for the next five years) is DOCSIS 3.0 and there are significant business cases to justify the investment in migrating to the new technology.

Posted in DOCSIS, DOCSIS 101, DOCSIS 3.0 | Tagged , , , , , , , | 7 Comments »

Speeding Upstream – Part II

October 7, 2009 by Brady

DOCSIS 3.0 Tips

(Extended version from that published in Communications Technology, June 1, 2009)
By John Downey and Brady Volpe

In Part I of this extended edition (original abbreviated version appeared in CT’s March 2009 issue) John and I discussed many general DOCSIS upstream issues that should be understood prior to deploying DOCSIS 3.0.  In this post we will focus more specifically on DOCSIS 3.0 issues that will occur as you are deploying DOCSIS 3.0 or post -deployment.

DOCSIS 3.0 US Considerations

When DOCSIS 2.0 US speed is exhausted, then DOCSIS 3.0 US can be implemented.  Some considerations include:

  1. Frequency Stacking Levels – What is the maximum output with multiple channels stacked, is it p0wer/Hz, could it cause laser clipping?
  2. Diplex Filter Expansion to 85 MHz – If amplifier upgrades are planned for 1 GHz, then pluggable diplex filters may be warranted to expand to 85 MHz in the US.  We still must address existing CPE equipment in the field and potential overload.
  3. Monitoring, Testing, & Troubleshooting – Just like DOCSIS 2.0, now test equipment needs to have D3.0 capabilities.

DOCSIS 3.0 US Issues

As with DS issues, there are also US issues that need to be addressed. US bonding has not been pursued at this point because most people haven’t even exploited D2.0 US capabilities.  This does not mean we should avoid the potential issues that will arise.  Eventually, we will want to offer US speeds greater than what a single channel modem can offer of ~ 25 Mbps.  This will require more US spectrum, D3.0 CMs, and CMTS linecards with US bonding capability.  Some of the potential issues are:

  1. Levels – Activating multiple frequencies per US connector on a 3.0 CM has different maximum power per channel vs. a D2.0 CM.  Maximum transmit for a D2.0 CM using 64-QAM for one channel is 54 dBmV.  D3.0 US channel max power is 57 dBmV when using 32 & 64-QAM, 58 dBmV when using 8 & 16-QAM, & 61 dBmV when using QPSK. D3.0 S-CDMA US max power is 56 dBmV for all modulations.  As explained above, it can be seen that the max power for one channel on the connector has been raised by 3 dB over a D2.0 CM, but max transmit per channel for four frequencies stacked using 64-QAM ATDMA is only 51 dBmV & 53 for S-CDMA.
  2. Upstream Passband – The US upper edge has changed in the D3.0 specification to 85 MHz, whereas it was previously 42, 55, & 65 MHz for Euro-DOCSIS.  The option of going higher is good for future spectrum re-allocation and avoiding known bad frequencies on the US.  Some things need to be considered though and that includes; diplex filters, line EQs, step attenuators, and CPE overload.  If incorporating any of these devices in the plant, they may need to be replaced for the new frequency split.  Also, can current customer premise equipment (CPE) like settops and TVs handle a potentially high level of “noise” from a modem at 50 MHz or higher?
  3. Channel Placement – Since each US channel used for bonding is an individual channel, frequencies can be anywhere in the US passband and do not need to be contiguous.  Although, it may be wise to keep relatively close so plant problems like attenuation and tilt don’t cause issues.  The CM should have some dynamic range to allow specific channels to be a few dB different vs. other channels.  Since the transmitters (channels) are separate, they don’t have to be contiguous and can have different physical layer attributes like; modulation, channel width, TDMA or S-CDMA, etc.
  4. Total Power – Significant consideration must be given to the total RF power loading that will now be realized with US channel bonding in DOCSIS 3.0 CMs.  In previous DOCSIS standards, only one US channel was present.  For DOCSIS 3.0, up to four (4) US channels will be transmitting at the same time, possibly with a 6.4 MHz bandwidth each, resulting in nearly 26 MHz of US channel loading.  This is a lot of power hitting the return path fiber optic transmitter.  The probability of laser clipping is increased, especially if one has legacy Fabry-Perot (FP) lasers in the fiber nodes.  It is a good idea to upgrade to Distributed Feedback (DFB) lasers, which have significantly more dynamic range.  It’s wise to have a comprehensive plan to monitor laser clipping in the return path.  Using a return path monitoring system capable of looking above 42 MHz, like PathTrak, will enable you to see second and third order harmonics.  Remember, any burst noise above the diplex filter (i.e. 42 MHz) coming out of the return path receiver in the HE is usually indicative of return path laser clipping.

The blue trace in Figure 1 below shows the case of strong laser clipping, while the green line represents a flat noise floor from the return path laser with no clipping.  Note that this return has four bonded channels in the US.

Bonded Upstream Laser Clipping

Bonded Upstream Laser Clipping

Figure 1

Using Return Path monitoring tools like PathTrak or Cisco Broadband Troubleshooter (CBT) to view 5-65 MHz for apparent laser clipping as in the above figure is critical for ongoing preventative maintenance.  Also needed is an analyzer that can read < 5 MHz for AM radio or ham radio ingress which can quickly leak into the network and contribute additional power to the return laser causing clipping, as well as cause problems at the input of the CMTS.  PathTrak (RPM-3000 card only) also has the ability to demodulate and display live cable modem US constellations as seen in Figure 2 below.

64-QAM Upstream Constellation

64-QAM Upstream Constellation

Figure 2

This monitoring system also provides average, maximum, and minimum MER of DOCSIS 3.0 and lesser CMs on each node.  In monitor mode it provides the ability to determine relative node quality.  It can determine which node has better CM MER than another and during which time of the day MER is the worst, since historical analysis is built-in.

D3.0 CM Transmit Levels

To address the potential issue where a CM today transmits near max power of 54 dBmV for 64-QAM, the specification has changed the CMTS US port level setting to allow it to be 6 dB lower as shown in figure 3 below.

CM Upstream Transmit Level

CM Upstream Transmit Level

Figure 3

This means the CMTS can be set lower so modems can be placed on those high value taps without changing HE or plant losses.  This is at the expense of lower MER/SNR readings.  The lowest setting on the CMTS today is -1 dBmV for 6.4 MHz wide channel. The range allowed on the CMTS is dictated by DOCSIS 2.0 and lower and says -1 to + 29 dBmV for 6.4 MHz and related to channel width, also known as symbol rate or baud.  D3.0 identified this potential issue and forced D3.0 CM vendors to support a transmit level of 3 dB higher than the 2.0 spec.  Therefore, 64-QAM has to transmit at least a maximum output of 57 dBmV with a single channel.

To keep the CM inexpensive and act as a constant power device, the max output will be dictated by how many US frequencies are active on the port (known as TCS for transmit channel set) and the highest modulation used.  Four channels stacked will be 57-6 = 51 dBmV per ch.  So, the overall effect from 2.0 to 3.0 is really a 3 dB difference.  The spec lowered the nominal setting allowed on the CMTS so cable operators didn’t need to lower tap faceplates or drop padding on every US port on the CMTS to keep D3.0 CMs online if they maxed out.  Most systems will leave the default of 0 dBmV configured and adjust padding appropriately.  Cisco has a command to keep CMs online that are maxed out, power-adjust continue 4 (default).  Many customers set it to 6 dB to give more room to work with.

New Architectures

When fiber is run deeper into the network as in the case of RF over Glass (RFoG) or DOCSIS Passive Optical Networks (DPON), a new conundrum is raised.  Many of these architectures will incorporate 32-way optical splitter/combiners.  Having a laser Tx in your house combined with 32 other houses feeding 1 Rx in the HE is addressed with lasers timed with the actual traffic from the house; unlike how it is done today where the US laser is on all the time.  Having US bonding and/or load balancing presents a potential issue where an US laser could be transmitting the same time as another US laser.  This may be address with the fact that even though multiple lasers are transmitting the same instant in time, if they are carrying different frequencies, then it may be acceptable.  The next question, will S-CDMA pose the same problems?  This multiplexing scheme allows multiple modems to transmit the same instant in time.

Isolation Issues

Figure 4 below depicts possible US isolation issues that can occur when an US frequency is narrowcast and another US frequency is introduced across multiple nodes.  Creative placement of pads and filters and/or isolation amplifiers can be used to prevent the signal from back-feeding when architectures like these are implemented.

Upstream Isoloation

Upstream Isoloation

Figure 4

Because of different combining schemes for different nodes as shown in figure 4 above, two USs which are at the same frequency, could interfere with each other.  Upstream 2 is being fed from two nodes, while US0 is from one node and US1 is from the other separate node.  The 24 MHz signal for US0 will travel to the US2 combiner and back-feed to the fiber receiver number 2, then possibly back-feed to US1 of the CMTS.  This 24 MHz signal will be about 40 dB lower than the expected signal if the isolation in each splitter is at least 20 dB.  Some ways to increase the isolation are: use amplifiers, add filters, or selectively move attenuation to different points in the network.  This situation may also cause issues with load balancing since one port is shared across two other ports.  Can US2 load balance with US0 and a different load balance with US1?

Plant Variations

System Plant Design Variations

System Plant Design Variations

Figure 5

Figure 5 above illustrates system plant design variations; not the in-home wiring variations!  This is the level that a home user device would need to transmit (if attached directly to the tap) to arrive at the associated amplifier at its recommended level.  If step attenuators, EQ taps, or line equalizers with reverse padding are used to make end of line CMs transmit higher, it addresses two concerns.  Since more attenuation is added where there was once less loss for ingress, it allows “ALL” CMs to have a better CNR and maybe a better MER/SNR.  The loss from step attenuators and these other options attenuates the ingress from those low value taps and houses attached to those taps.  These options also solve the potential issue of modems “ramping up” to max power when a power outage occurs.  A typical unit with 55 dBmV output would overdrive the system if located at the end of the cable span.

Frequency Expansion & Tap Change-outs

If systems are looking to do 1 GHz amplifier upgrades, they should look into removable diplex filters in case we change to 5-85 MHz, which D3.0 supports.  Also, the truck roll for this is expensive, so doing some tap change outs now would be optimum.  The first tap off of the active used to be a 29 dB tap, then got dropped to 26, then dropped to 23 all because of CM US transmit levels, very close to 55 dBmV or so.  Modems farther away on low value taps transmit much less, sometimes as low as 35 dBmV.  To get this delta much closer (tighter bell curve of CM transmit levels) we need to either add loss (attenuation) to low value taps or maybe we could just change the first few taps off the actives to cable simulator taps.  So the tap would look like a 32 dB tap at 1 GHz, but maybe 17 dB at 5 MHz.  This solves two issues:

  1. DS level and tilt hitting the house
  2. US max transmit levels

Since these modems would go from say 55 dBmV to 49 dBmV, we have room for 3.0 CMs with US bonding and/or we could add padding at the node to force all the CMs higher again and get better SNR.  If CM transmit levels are still a concern, using a D3.0 CM in 2.0 mode would allow higher US Tx level because the use of a single frequency and a D3.0 CM offers 3 dB higher power.  Running D3.0 in low modulation schemes allows higher power as well.  Using S-CDMA with more codes may also allow higher transmit power, but will depend on implementation.

Summary

It is important to recognize that customers and competition are driving the need for increasingly faster speeds in our DOCSIS networks. DOCSIS 3.0 provides an effective migration path for customer satisfaction, exceeding competitive pressures and enabling the delivery of new services and penetration of new markets. With more than four times the data throughput of its predecessors, DOCSIS 3.0 is apt to be nearly an order of magnitude more complex to deploy in such a manner as to take full advantage of its capabilities. Careful planning, proper plant maintenance and a well-developed continued preventive maintenance program from the onset will help pave the road to a speedy and fiscally fruitful deployment.

Posted in DOCSIS, DOCSIS 101, DOCSIS 3.0, Troubleshooting DOCSIS | Tagged , , , , , , , , , , , , , , | Leave a Comment »

Speeding Upstream – Part I

October 2, 2009 by Brady

DOCSIS 3.0 Tips

(Extended version from that published in Communications Technology, June 1, 2009)
By John Downey and Brady Volpe

The first part of this article (in CT’s March 2009 issue) discussed downstream potential issues, while this one focuses on the potential issues associated with upstream deployments. In particular, this article covers the critical upstream areas that one should be aware of when getting ready to deploy or already deploying DOCSIS 3.0.

The business objective for many operators today is to provide faster speeds to compete with Verizon’s FiOS. These speeds are for downstream and upstream. DOCSIS 2.0 can provide approximately 37 Mbps on the downstream and 27 Mbps on the upstream, aggregate speed. Some per-cable modem speeds are approaching these peak rates and exceeding them. The only way to offer higher rates than what DOCSIS 2.0 can offer is to upgrade to DOCSIS 3.0. Instead of reducing node sizes, which can exceed $10,000 per node split, the business case can quickly be made to migrate to DOCSIS 3.0.

Potential Pitfalls

As explained in the first part of this article, any new technology will have some trade-offs and potential pitfalls that we must understand and plan for in advance.  The following will discuss some of these issues:

  1. Why it’s Needed – This can range from competitive pressure, to higher tiers of service, to more customers signing up.
  2. Frequency Stacking Levels & Placement – What is the modem maximum US output with four channels stacked and do the channels have to be contiguous?
  3. Isolation Concerns – Whenever applications have different service groups, we have overlaid networks.  Signals destined for one node could “bleed” over to another.
  4. US Frequency Expansion to 85 MHz – Amplifier upgrades are occurring now.  It’s best to make the truck roll once.  Think about diplex filters, line EQs, step attenuators, taps, etc.

Note: DOCSIS 3.0 CMs support a minimum of 4 US and 4 DS channels even though it could be more.  Keep in mind that more USs in a mac domain requires more maps and “eats” into the DS throughput.  Approximately every US port uses 0.25 Mbps of DS capacity or more for maps.

DOCSIS 2.0 – ATDMA

It would be good to understand how to exploit the upstream (US) capabilities we have with DOCSIS 2.0 and its corresponding issues before moving into DOCSIS 3.0 US issues.  A quick way to increase upstream data rates is to deploy DOCSIS 2.0 in a DOCSIS 1.x system.  A 6.4 MHz wide channel using 64-QAM will provide up to 27 Mbps vs. the 9 Mbps available with a DOCSIS 1.x system.

DOCSIS 2.0 US Considerations

There are a few considerations that need to be addressed when utilizing ATDMA:

  1. 64-QAM at 6.4 MHz – What levels are supported, how do modulation profiles affect levels and modulation error ratio (MER), what about frequency allocation?

Note – MER is the same as signal-to-noise ratio (SNR) as reported from a Cable Modem Termination System (CMTS).

  1. Linear Impairments – How do group delay & micro-reflections affect per-CM MER and US port average MER, how to use pre-equalization to your advantage?
  2. Laser Clipping – More channels means more power and potential laser clipping. Do you have FP or DFB lasers deployed in your return path?  If you don’t know, you should do some research.
  3. Monitoring, Testing, & Troubleshooting – Proactive vs. reactive and testing with a signal that is realistic.

Linear Impairment Effect

Many times doubling the US channel width will indicate more issues with the plant than actually increasing the modulation to 64-QAM.  Linear impairments like group delay and micro- reflections will not be apparent with a spectrum analyzer, but will severely degrade US MER.

After increasing the channel width to 6.4 MHz, it’s imperative to measure and document unequalized US MER at multiple test points in the plant.  Unequalized means per-CM US MER without pre-equalization activated.  JDSU PathTrak Return Path Monitoring System with an RPM-3000 linecard can demodulate live CM signals along or a continuous 16-QAM to 64-QAM signal generated from the JDSU DSAM meter.  This provides invaluable insight into the monitoring and troubleshooting of non-linear impairments.

The recommended unequalized MER is 25 dB or higher.  Less than 25 dB reduces operating margin.  Be sure to check US MER as well as per-CM MER.  The Cisco CMTS command, “show cable modem phy”, can be used to display per-CM MER (SNR).  If diplex filter group delay is suspect in addition to long amplifier cascades, it may be necessary to pick a frequency below 30 MHz, away from diplex filter bandedge.  If group delay is causing per-CM low MER issues and a lower frequency is not an option, it may be possible to activate pre-equalization.  Be sure the latest IOS version is running on the CMTS with proper modulation profiles.

Note: US interleaving has been added in DOCSIS 2.0 and can be applied to the D2.0 A-long burst in the mod profiles for added protection to impulse noise events.

DOCSIS 2.0 has many benefits and one of those comes in the DOCSIS 2.0 CMTS linecards and is called pre-equalization.  The CMTS first analyzes the signal coming from the CM and sends correction information back to it.  The CM uses this information to pre-distort, or pre-equalize, its signal before transmission.  Now the signal travels through the HFC network and is impaired by group delay and frequency response.  The pre-distorted signal is distorted back and the CMTS actually receives a nearly ideal signal.  Pre-equalization is very useful for supporting 6.4 MHz wide channels whether using 16-QAM or 64-QAM.

The divide and conquer troubleshooting method recommended is to exclude the MAC address of a field meter (i.e. DSAM) from the CM pre-equalization process.  This will cause the DSAM to report un-equalized MER in the field, while the customer CMs are operating at a higher, pre-equalized MER.  The objective is to then use the non pre-equalized DSAM to troubleshoot the HFC network until impairments are identified and resolved, such as bad connectors and taps.  The goal is to have the non pre-equalized MER of the DSAM nearly as good as the pre-equalized MER of CMs on the same US leg.

Note: Increasing the channel width from 3.2 to 6.4 MHz keeps the same average power for a single carrier, for the Cisco implementation.  This means the MER will drop by 3 dB, and possibly more because wider channels incur more group delay.  If the CMTS kept the same power/Hz, it could cause maximum transmit levels from CMs and/or laser clipping or overload.

Understanding equalized vs. unequalized MER readings is paramount to quantifying plant issues.  Regardless if the CMs have pre-eq activated; the CMTS linecard will also have adaptive equalization.  The end user must know if the CMTS US MER is reported before or after this adaptive EQ and preamble lengths in the modulation profile could affect this.

Modulation profile choices include QPSK for maintenance, 64-QAM for Data, and possibly 16-QAM for VoIP.  Many options are possible and the station maintenance(SM) burst modulation could affect level reporting and subsequently MER.  The level reported by the CM could be based on the long burst while the CMTS level is based on the SM burst.  If Pre-EQ is activated in 1.1 & > CMs, it can greatly enhance the US per-CM MER readings, but could mask plant issues.  DOCSIS 1.1 CMs have an 8-tap EQ and DOCSIS 2.0 CMs have a 24-tap EQ.  In a simplified explanation, this can be thought of as 8 or 24 sampling points to get a “good” digital representation of the “haystack”.

Monitoring the CMTS for Plant Health

By monitoring US MER and FEC counters, a generalization can be made for the US health of the plant, but further investigation is needed.  It is recommended to also monitor per-CM FEC and MER numbers.  Another data point can be formulated with the Flap-list.  This Cisco-patent pending feature has been included in DOCSIS 3.0 as the Modem Diagnostics Log and can indicate US issues.

Table 1 below lists some recommended thresholds for alarms of different parameters.

FEC Counters vs. MER

Table 1

After deploying ATDMA, it will be necessary to monitor MER on a per US basis with the ability to drill-down for per-CM MER. Uncorrectable / Correctable FEC per US with ability to drill-down for per-CM counters will also be used.  Use Return Path monitoring tools like Cisco Broadband Troubleshooter (CBT) or PathTrak to view 5-65 MHz for apparent laser clipping.  It’s also needed to have an analyzer that can read < 5 MHz for AM radio or ham radio ingress. PathTrak can look at the 0.5 MHz – 85 MHz spectrum and also look at “real” modem US constellations.  Since the CBT tool is in the CMTS and understands minislot time assignments, it can be used to see the US with no modem bursts and also display specific modem bursts.

Flap-List

Cable Flap-List monitoring is used for CM issues caused by US noise impairments and timing issues.  The following configurations are recommended as a best practice:

cable flap-list miss-threshold 5

Modems are polled every 20 seconds (15 when linecard redundancy is configured) and correlates with a “hit” when the 3-way maintenance “handshake” is successful.  If a poll is missed, the CMTS will go into a fast mode and poll every second.  If there are five consecutive polls missed, the flap count increments by one and the miss count would increment by five.  “Misses” can be correlated with T3 timeouts from the CM log.

cable flap-list power-adjust threshold 2

If the CM has power adjustments of 2 dB or higher during one station maintenance interval, the flap count and power adjust count increment by one.

cable flap-list insertion-time 120

If the CM sends initial ranging two or more times within two minutes, the flap count increments by one.  This does not necessarily mean a modem going offline and online.  It could be a modem that goes through “init” states many times.

Some recommendations for flap-list monitoring include:

  1. Periodically poll the flap-list at an appropriate interval of every 30 minutes or so.
  2. Perform trend analysis to identify CMs that are consistently in the flap-list.
  3. Clear the flap-list periodically (daily?) to “re-calibrate”.
  4. Query the billing and administrative database for CM MAC address-to-street address translation and generate appropriate reports and work orders.  CMs in a specific area with lots of flaps can indicate a faulty amplifier or feeder lines.

Note: The bottom line is correctable & uncorrectable FEC.  If correctable FEC is incrementing, then eventually it will lead to uncorrectable FEC, which equals packet drops.  If uncorrectable is incrementing much faster than corr and/or SNR seems good, then it could be an impulse event like laser clipping, impulse noise, or sweep interference.

Summary

This concludes Part I of the extended version of our Speeding Upstream article.  Next week I will post Part II, at which time John and I will cover:

  • DOCSIS US issues such as
    • RF Levels
    • Passband
    • Channel Placement
    • Total Power
    • and Laser Clipping

We will also discuss the significant changes in upstream transmit power level of cable modems from DOCSIS 1.x and 2.0 to the new DOCSIS 3.0 specification.  Finally, we will cover isolation issues and future migration considerations in the upstream due to the new upstream expansion capabilities with DOCSIS 3.0.

John J. Downey is a broadband network engineer for Cisco Systems.
DOCSIS is a registered trademark of Cable Television Laboratories, Inc

Posted in DOCSIS, DOCSIS 101, DOCSIS 3.0, Troubleshooting DOCSIS | Tagged , , , , , , , , , , , , | 2 Comments »

Latest Article – DOCSIS 3.0 Tips

July 2, 2009 by Brady

CT_060109_CoverIf you have not had a chance to catch the June issue of Communications Technology, check out the article penned by John Downey of Cisco Systems and me on “the critical upstream areas that one should be aware of when getting ready to deploy or already deploying DOCSIS 3.0.”

The shortened version of this DOCSIS article can be found on CT’s website HERE.  The full version should be made available on this blog soon.

A permalink will also be added on the right column under “Articles and Publications“.

Posted in CATV, DOCSIS, DOCSIS 3.0 | Tagged , , | Leave a Comment »

DOCSIS and Cable Modems – How it works :: Tutorial Wrap Up

April 20, 2009 by Brady

If you have followed the “DOCSIS and Cable Modems – How it works” tutorials this far, congratulations!  You now have a basic foundation of how DOCSIS networks operate and the ability to pick up the DOCSIS specification and read and comprehend it – this is hard to do for the novice.  If you are just finding this blog for the first time, then I recommend that you go to the DOCSIS Tutorial Series and start at the beginning before proceeding.

For everyone else, there are a couple final items that I want to cover in this DOCSIS tutorial wrap-up blog before I move onto other cool topics.  They are Synchronous-Code Division Multiple Access (S-CDMA) and Adaptive Signal Cancellation Algorithms, both were introduced in the DOCSIS 2.0 specification and are carried over into DOCSIS 3.0.

S-CDMA

Synchronous-Code Division Multiple Access indicates that multiple cable modems can transmit simultaneously on the same RF channel and during the same TDMA time slot, while being separated by different orthogonal codes (ref. DOCSIS 2.0 RFI pg. 40).  Whoa!  That is a mouthful, but it can be immensely powerful for a number of reasons.  First, one can have up to 128 cable modems transmitting simultaneously, since there are 128 spreading codes.  How is this possible?  Just by using some math and digital signal processing.  Before the data is transmitted it is randomized in a unique method such that each burst of data is “spread” out using one of the 128 code words.  After the data is spread, it looks more like noise than the usual TDMA signal.  The code words and spreading algorithm is created in such a way that when the data is received, the patterns have an orthogonal (or mathematically perpendicular) arrangement such that the receiver can isolate and demodulate one cable modem from another.  Simultaneous transmission means less time waiting for REQ-MAP cycles.

Much more importantly, S-CDMA signals are significantly more immune to certain types of ingress noise.  Specifically impulse noise, which is most often associated with the low frequency band of the RF spectrum, i.e. 5 to 20 MHz.  For this reason, S-CDMA is ideally suited for use in this region.  Why is S-CDMA more immune to impulse noise?  Because the data in an S-CDMA signal is spread out by the spreading code so in the event that a burst of impulse noise does impact a modem transmission, it will randomly catch bits which are not adjacent to each other in a single packet.  This makes it easier for the Reed-Solomon error correction to repair any damaged bits once the packets are re-assembled on the receiver side.

A couple of draw backs associated with S-CDMA are incompatibilities with earlier modems already in the field, significantly increased DOCSIS MAC overhead when mixing both S-CDMA and TDMA modems together on the same upstream channel, and potential return path laser clipping in the event that one actually allows 128 modems to talk at the same time – this can create a lot of total RF power, over-driving lasers.  The ideal use for S-CDMA is in the 5-20 MHz range, usually with a low modem count and ideally for a small to medium business deployment.

Adaptive Signal Cancellation Algorithms

When DOCSIS 2.0 chipsets arrived in the market place with S-CDMA, they also added features to TDMA to make it more resistant to ingress.  Specifically, adaptive signal cancellation algorithms in the DOCSIS 2.0 (and now D3.0) chipsets focused on quickly identifying and cancelling out any coherent interfering signals that could be present under a DOCSIS channel.  Depending upon the chipset and type of interferring signal, as many as 12 signals under a TDMA upstream DOCSIS channel can be identified by a chipset and eliminated.  The impact on a DOCSIS network is that signals which would normally cause an outage to many subscribers are suppressed in the CMTS, allowing cable modems to stay online, often error free.  Note that this is meant to be a temporary solution to keep subscribers online while plant technicians troubleshoot the root cause of the problem.

The following picture shows a DOCSIS channel just below 30 MHz with an upstream that has high level CPD only about 10 dB below the peak of the DOCSIS channel.  Notice that one of the CPD remnants falls directly under the DOCSIS channel.  You will have to take my word for it, but during the testing of this event there were no errors in the upstream as these cable modems were communicating with a DOCSIS 2.0 CMTS.  Had it been a DOCSIS 1.x CMTS substantial errors would have been present since the CNR was only 10 dB on a 3.2 MHz 16-QAM signal.

DOCSIS Channel with CPD
DOCSIS Channel with CPD

 Laboratory testing has demonstrated that adaptive signal cancellation is capable of removing coherent interferers sticking out of a DOCSIS channel as high as 10 dB.  This means one has a -10 dB CNR (for a signal carrier).  As more interferers are added the cancellation algorithm becomes less effective.  Additionally, as the interferers become wider in bandwidth, such as one would have from a modulated carrier as an FM or FSK signal, the less effective cancellation is, but nonetheless still much better than no cancellation at all.

Wrap-up

So I hope that you have found the DOCSIS 101 tutorials valuable or at least some parts of it relevant to your career.  As I often do, I urge you to visit www.cablelabs.comand take the time to read the DOCSIS 2.0 RFI as you should now have a solid basis to now understand much of this highly technical document.  This is my recommended reading before diving into the DOCSIS 3.0 specifications, which are substantially more heavy than D2.0.  I intend on putting together some D3.0 materials to follow, so please stop back.

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