Built in to DOCSIS is the ability to turn every modem in the field into a test probe
The best thing about DOCSIS 3.1 is that cable can ride the technology to multigigabit broadband speeds. The second best thing is far improved efficiency in spectrum use. The third best thing is that it will significantly decrease the cost per bit of transmitting data. The list of important benefits keeps going, and when you get down to, like, number 8 or 9 on the list, there is yet another truly nifty little advantage: DOCSIS 3.1 turns every single cable modem into a network probe that can be used to identify network impairments.
“The geek out factor is high, so if you are a techie you will love it,” according to an explanation of the technique written by Brady Volpe of The Volpe Firm, who has been contributing to the development of the technology.
“If you are more interested in the financial end of it you will really get excited about the cost savings,” Volpe continued. “For those of you that are into quality control and customer satisfaction you too are covered with this technology, and if you are the tech that has to find the impairment on the hottest or coldest day of the year you too will embrace this technology. Imagine knowing where to go to find the impairment and have it layout on a map for you."
The approach makes use of DOCSIS pre-equalization on the upstream. Different network impairments will have specific signatures that can be sifted out of the pre-equalization data. The technique not only identifies upstream problems, but it also locates them with gratifying precision.
Cable operators have been talking about cleaning up the upstream plant for years, but until recently it’s been mostly talk. Given the historical lack of traffic on the upstream, maintenance on that part of the network simply has not been prioritized.
But that has been changing. More and more customers are uploading increasing amounts of content. Expecting that to be the case, the industry has built ever more powerful capabilities into successive iterations of DOCSIS technology specifically to identify impairments on the upstream.
CableLabs, its members, and cable equipment vendors have been deliberately building monitoring and test capabilities into DOCSIS almost from the beginning. But with the 3.1 version of DOCSIS, this cable modem-based spectrum analysis technique is now becoming far more effective, far more accurate, and so much easier to use than it has been with earlier iterations of DOCSIS that there’s little justification to not use it.
Pre-equalization has been a means of compensating for distortion from a variety of impairments (frequency response, micro-reflections, group delay, etc.) in cable modem networks since DOCSIS 1.1. Operators’ ability to use that data has been inherent in DOCSIS since the 2.0 version, though in a somewhat rudimentary form, and not widely taken advantage of.
It became more powerful tool to use with improvements included in DOCSIS 3.0, and is now being exploited by a number of different MSOs, who have all developed home-grown approaches, each with a different name. Comcast, for example, calls its tools Scout Flux. Cox Communications, Charter Communications and Rogers Cable also have their own versions.
The technique is reaching a new level of sophistication with the introduction of DOCSIS 3.1 products. Broadcom and the Intel / MaxLinear team have been building 3.1 cable modem chips that significantly bolster these capabilities in cable modems.
CableLabs’ InGeNeOS (Intelligent Generation-Next Operational Systems) working group has been working on – and continues to work on – a variety of DOCSIS-based techniques to deal with impairments.
The first capability, now in use, was the upstream equalizer, explained Tom Williams a member of the InGeNeOS working group.
“Then we had downstream spectrum analysis at the cable modem. That’s really taking off because those chips are being deployed really aggressively. Then upstream spectrum analysis is being working on very aggressively.” That’s the upstream monitoring capability now being deployed.
That won’t be the end of it. “And then the new thing just beginning to get traction is distortion analysis,” Williams concluded.
CableLabs refers to the full set of impairment identification capabilities as DOCSIS Proactive Network Maintenance (PNM).
Right now, it’s the upstream monitoring technique that’s ready and moving into wider usage. The kicker with this technique is that the impairments identified and located on the upstream often enough are the same impairments that also affect the downstream channel.
“For instance, if you have a corrosion problem in the plant, it will affect both the upstream and the downstream,” said CableLabs' Alberto Campos, also a member of InGeNeOS.
Add to the list: animals chewing on wires, bad construction processes, cracks in the cable, the power company drilling through your lines, mailbox posts getting embedded in your lines.
“A lot of the problems are common to both,” Campos continued. “When you have a strong micro-reflection because of a crack in the cable, it impacts both. Many of the upstream issues are also downstream issues.”
What CableLabs has done is perform extensive analysis of pre-equalization coefficients. Each type of modem has individual characteristics, as do specific impairments.
“Specific signatures are somewhat unique to the different cable modems. By analyzing these signatures, we’re able to determine if there are common problems, or not, that the cable modems have to deal with,” Campos said. “Power comes when you correlate information from multiple cable modems.”
These signatures also have related capabilities that are equivalent to a time domain reflectometer that will give an exact location of where the problem is located, Campos explained.
“Every cable modem becomes a probe,” Campos said. “The cable modem is going to be your network analyzer, your spectrum analyzer, your time domain reflectometer. This is very powerful, because it’s already deployed in your network. It’s in 2.0; it’s in 3.0 – we added it after the 2.0 spec was out. For 3.1, and even for EPOC, there is now the opportunity to design things from the start.”
(EPOC is a nested acronym, the EPON – Ethernet passive optical network – protocol over coaxial. It will enable MSOs to mix both HFC and EPON networks and manage them from the same management platform.)
Data from groups of cable modems can then be aggregated. If the cable modems in a group exhibit common problems, such as micro-reflections or group delay, the group can provide the estimated distance to the impedance mismatch causing the micro-reflection.
By mapping this distance, Volpe explained, pinpointing the exact location is possible.
“You get better spectral coverage,” Williams said. “You get wider analytics, meaning you can more accurately locate the source. Say you have a buried cable, someone bores into it, water gets in. When you have to go dig a trench, getting a location plus-or-minus 6 inches is so much better than plus-or-minus 50 feet.”
The pre-equalization data can be analyzed in real time, or it can be stored in databases for subsequent evaluation. Either way, it will enable an operator to watch impairments as they are generating. The MSO can assess what problems are most severe, and prioritize maintenance on that basis.
Some larger MSOs are already reporting amazing results. Williams noted that with the ability to precisely locate impairments, there is a great potential for reducing truck rolls. Volpe said various MSOs have reported a drastic reduction in maintenance costs and significantly improved operational efficiencies.
Enabling all of this also requires a PNM server, as well as a modification in CMTSs.
The change in the CMTS is almost trivial, Volpe explained. “By adding a simple, one-line command in the CMTS that enables DOCSIS pre-equalization, you can see an immediate 5 to 10 dB improvement in upstream MER (SNR).”
DOCSIS 3.1 cable modem chipsets will provide significantly more precision than chipsets for D3.0 or earlier DOCSIS versions, in part because they integrate full-band capture.
The new generation of DOCSIS 3.1 cable modem chipsets are so superior at taking advantage of this monitoring capability, it's giving MSOs even greater incentive to move 3.1 modems into the field as quickly as possible.
“The latest improvements in Proactive Network Maintenance (PNM) functions include analysis of upstream pre-equalizer coefficients, and downstream spectrum analysis by the cable modem,” said Broadcom technical director Bruce Currivan, in a written exchange.
The cable modems report their data only when it’s requested, he explained. “The CM reports its pre-equalizer coefficients and performs spectral analysis when queried by the PNM server.”
D3.1 cable modem chipsets have more than enough processing power for each cable modem to perform some of the PNM data analysis. “The existing processor in the cable modem is used as-is, performing PNM functions as a background task,” Currivan said.
The polling schedule, he said, “is determined by the tool implementation at the PNM server. Typically all CMs are polled several times a day, and those showing problems are interrogated more often for a ‘deep dive’,” he said.
“The PNM tool at the server compares metrics to thresholds,” Currivan continued. “For example, upstream in-band ripple (derived from FFT group delay of pre-equalizer coefficients) greater than, say, 3 dB peak to peak, is flagged as a potential problem.”
If the cable modem becomes a network probe, a network analyzer, and more, that is necessarily going to mean the requirements for traditional network test & measurement equipment is likely to change, if not diminish.
Currivan acknowledged this. “This is a challenge for test equipment manufacturers, as some functions previously performed by their test gear are now performed by PNM. However, test equipment manufacturers have responded by finding new creative ways to add value to PNM using specialized test equipment.”
At the most recent Cable-Tec Expo, ZCorum demonstrated its downstream channel monitoring app, called RF Inspector, which performed spectrum analysis, drawing spectrum data from cable modems via an SNMP-based DOCSIS 3.0 MIB.
In early February, ZCorum introduced its Pre-Equalization Analyzer, which uses technology from Nimble This (Volpe is involved with the company) to analyze the pre-equalization data from CMs to monitor the upstream.