Cable Nodes Becoming a Choke Point

Cable companies have been talking about a network architecture called node-plus-zero for several years, and finally some have begun deploying it, with Comcast and Cox leading the way. Node+0 is finally being deployed now because only recently did it become technologically and economically feasible, thanks to the introduction of a new generation of power amplifiers based on gallium nitride (GaN). (See Comcast Goes N+0 in Gigabit Markets and Cox Reveals Next Steps for D3.1, CCAP.)

Cable operators are also considering increasing the amount of spectrum in their networks, expanding from 1GHz to 1.2GHz. Plus, they're evaluating an architectural change referred to as remote PHY. Although all three are distinct technologies, they all place new demands on cable nodes, and for now there's no way to accede to all those demands simultaneously.

Qorvo Inc. is one of the very small handful of suppliers of new GaN power doubler amps that help make all three upgrades possible. Macom has a gallium arsenide (GaAs) amp that also supports 1.2GHz of spectrum. Shenzhen Sanland Technology appears to have a similar part. A barely known startup called One Tree Microdevices claims to have a device that might serve the purpose in its portfolio.

Cable operators, in their hybrid fiber coaxial (HFC) networks, run fiber from their central headends to nodes placed in every neighborhood, and then run coaxial cable from nodes to customers' premises.

The coaxial run starts with amps in the nodes, supplemented by as many as four or five more amplifiers spaced along the line to maintain signal strength on the way to customers. A network with four or five amps per line would be denoted as node+4 or node+5, respectively. Node+0, then, is an architecture in which all amplifiers are removed from the line, leaving only those in the nodes -- commonly four per node.

Because the terminology isn't obscure enough already, Comcast Corp. (Nasdaq: CMCSA, CMCSK) and some other companies also refer to node+0 as "fiber deep." Not only are some companies ditching the cascade of amps on their coax lines, they're moving their nodes closer to their customers, extending the fiber portion of the HFC network "deeper." That naturally makes the coaxial runs shorter.

Migrating to node+0 eliminates the cost of periodically replacing all those amps, and also the expense of maintaining them all, including the cost of powering them. A cable operator might end up adding 20% more nodes, but eliminating 80% of the active devices in its network compensates enough that an operator might be able to cut its overall power bill by as much as 50%.

Amps tend to have 15- to 18-year lives, and the last major wave of amp upgrades occurred between 12 and 15 years ago. At the time, node+0 was not an option because the approach requires amps in the nodes to compensate for removal of the subsequent daisy chain of amps on the line. Not only did they have to be more powerful, they would have had to do it without blowing the power budget for the nodes. Amps that could do all that simply did not exist back then.

During the last wave of amp upgrades, MSOs had another factor to consider, though. The spectrum available in a cable network is dependent on the frequency range that amplifiers installed in the network can support. The latest amps at the time wouldn't support node+0, but they would support higher frequencies up to 1GHz.

So the choice operators had was that they could buy amps that would get them to 870MHz or amps that went to 1GHz. If they bought the 870MHz amps, it would be a problem if they needed to go to 1GHz shortly after, because then they'd be buying and installing yet another round of amps, a waste of money and effort. The alternative was to buy amps that supported 1GHz from the get-go. If the operator needed the additional 130MHz, it was there. If not, the company had to replace its amps anyway. There was no single correct answer; different MSOs had different needs.

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Today, cable companies are in a similar situation. Installed amps are starting to age out, and MSOs are going to have to do something about it. This time around, though, the latest amps -- the new GaN models -- can not only support yet another expansion of the frequency range, up to 1.2GHz, but are also capable of compensating for the removal of the daisy chain of amps on the line, enabling node+0.

It is absolutely no coincidence at all that the latest iteration of cable's DOCSIS standard, DOCSIS 3.1, was engineered to support a spectrum expansion to 1.2GHz. (It also anticipates a subsequent step probably to 1.7GHz, and allows for additional expansions beyond that, should they ever become useful.)

GaN amps represent a legitimate technological leap, but they aren't magic. If operators want to do node+0 and add spectrum, that's difficult. If operators want to do those two things and also prepare for remote PHY, they're asking too much of what's available now.

Why? Node+0 requires more powerful amps for the nodes to drive signals the full distance from node to customer premises, and they have to do it without drawing significantly more current. The only way to increase the power while minimizing the current draw is to switch to GaN-based amps, and even then, the switch to GaN amps is not a perfectly ideal solution. GaN cannot reduce the power required; but it will draw less additional power than all alternatives.

Previous amps operate at 11W or so. The new GaN amps draw about 18W, so with four per node that's 28 extra watts. So far that's been acceptable for those operators eager to do node+0.

There's a complication for cable operators that want to do node+0 and simultaneously increase spectrum to 1.2GHz, however.

A node+0 system at 1GHz typically operates at 58 dBm out, but attenuation increases at higher frequencies, so if an MSO wants to do node+0 and go to 1.2GHz, it needs more power.

Comcast is upgrading its amps now and it wants the option to go to 1.2GHz, so it's asking for 64 dBm out, tickling technological limits all up and down the supply chain.

What about adding remote PHY? Remote PHY doesn't put any stress on the amps, but what it does, however, as it moves resources from the headend to the node, is add another increase in power required by the node.

"You have fixed power consumption the node can carry. You're asking for more performance, but you're asking to cut the power down? Well, it's just not going to happen," observed Kellie Chong, director of CATV and Broadband Access products at Qorvo.

What does appear possible is amp manufacturers reducing GaN device power consumption just enough to keep the overall power budget for the node unchanged when remote PHY is factored in.

Qorvo said it is looking at a technique called digital predistortion (DPD), which is commonly used in the wireless industry but is less well known in the cable industry. DPD algorithms predict the non-linear behavior of amplifiers, Chong explained.

"If you can correct for that, then you might be able to save power from the amplifiers so you can have remote PHY and node+0," she said.

The cable industry can expect that in maybe in a year, a year and a half she said.

There are also rumors of some unspecified digital alternative still being worked on in laboratories that might bear fruit in a year or two.

Every MSO has different needs, and all of these considerations will factor into their decisions on when to embark on all of these upgrades.

It turns out that many MSOs who have networks capable of 1GHz still aren't using the extra 130MHz they bought themselves when they upgraded from 870MHz. Toss in the complications associated with also going node+0 or opting for remote PHY, and many operators are apt to wait a year or two for the next generation of improved amps before they embark on their upgrades.

And it's unlikely that those MSOs already doing node+0 will also do remote PHY any time before that.

— Brian Santo, Senior Editor, Components, T&M, Light Reading

brooks7 12/5/2016 | 6:19:22 PM
Re: my two-cents The problem is then getting fiber through walls...


jbtombes 12/5/2016 | 6:11:04 PM
Re: HFC vs. FTTH Altice in metro NYC (Cablevision) is indeed compact. The rest of what Altice has acquired in the US (Suddenlink) is scattered across Texas, Oklahoma, Louisiana, Arkansas, Kansas, Missouri, W. Virginia, North Carolina, Ohio. (Am I missing anyone?) Deployments there would likely be less effiient and more costly.
Duh! 12/5/2016 | 3:51:19 PM
Interesting... What am I missing? I understand that 1.2 GHz Remote (MAC/)PHY node would certainly require higher power/higher gain/higher bandwidth power amplifiers than a current generation 4X4 node. It would also need DAC, ADC (for the upstream), RF front-end, baseband and possibly MAC logic. Some of that would be made up in  the upstream lasers.  Still, more power for sure - but as much as four or five amplifiers? Really? Can you share your source?

Also, shedding amplifiers and replacing linear optics with baseband will boost SNR, perhaps enough to enable higher order modulation and thus better spectral efficiency. Wouln't that delay any need to go to 1.2GHz?
msilbey 12/5/2016 | 2:24:56 PM
Re: HFC vs. FTTH This is the first place I've seen it discussed that cable nodes will have a tough time handling N+0, 1.2GHz and Remote Phy all at once. I find that particularly interesting because it seems to me that once a cableco has moved forward with one of those strategies, it's more likely to want to move forward with the rest. For example, once an operator is down to N+0, there's a better case for moving to Remote Phy. And at the same time, if the use case that cable operator has sold is around high-capacity bandwidth, there's equally a desire to increase total spectrum availability, i.e. upgrading to 1.2GHz.

Comcast, for example, is already N+0 in a number of markets. Comcast is also interested in Remote Phy. Will it try to move forward with both and forego an upgrade to 1.2 GHz? Interesting dilemma. 
comtech3 12/5/2016 | 1:36:15 PM
Re: my two-cents Inside wiring is not a probem with Comcast FTTH becaue ONU used is a bit similar to that used by Verizon,  except that it does not function as a MODEM as does Fios where a MoCa router is used.The Comcast ONU output is to a wireess gateway MODEM, which can be customer's owned, or rented. That flexibiltiy is not given to Fiios customers, although they can  have CAT5, of CAT6 UTP cables ran from their own switch, or router. However, for troubleshooting purposes, I guess, Verizon customers are forbidden to use their own equipment.

Comcast has embarked on FTTH in some markets already where they have deployed DOCSIS 3.1 gigabit Internet speeds.

The problem that Comcast may have some problems that has also beset Google, is the run fiber underground in pre-existing development where they already provide service.Residents are highly recluctant to having their yard dug up to have it done.However, there is a technigue that can be use to circument this by using a heat process to remove the center conductor from the hardline feeder cable that separates it from dielectric to insert the fiber through and use it as a conduit.

Bottom line though, I don't see any of the major MSOs going the route of full FTTH in my lifetime as the that old copper l clad hardline still has some resilience and adapatabiltiy left in it.

inkstainedwretch 12/5/2016 | 11:54:29 AM
HFC vs. FTTH Yes, an MSO retires these issues when it deploys fiber to the home, and this is one of the reasons why most MSOs install FTTH in greenfield deployments.

But ripping up coax is hard, especially for a large MSO whose network is actually a conglomeration of systems sprawling across a lot of geography.

The task of ripping up coax and going all-fiber is less onerous for a  company operating in a relatively small, relatively circumscribed area. We saw Videotron in Canada (essentially Montreal) do it a few year back, and Altice/Cablevision (New York City and adjacent suburbs) just announced it will do likewise. A lot of smaller MSOs have already done so too.

So we might see some companies wait it out as long as they can to see if  technology/economics progresses to the point where skipping ahead to FTTH becomes a practical option. 

--Brian Santo
brooks7 12/5/2016 | 10:57:17 AM
Re: my two-cents The PON specs actually are a lot more than 20Km, you are thinking about the maximum distance between ONTs on a PON.  I think the big issue for cable is where it does the O/E conversion in a FTTH scenario.  I am sure they would like it to be inside the home.  Problem is how to do this and not deal with all the possibilities in home wiring.  If they go outdoor ONTs (like the bulk of FiOS), then actually the number of OSP active electronic elements increases.


comtech3 12/5/2016 | 6:51:21 AM
my two-cents It is my understanding that a single mode fiber can carry a signal without regeneration for over 20km. Now,the distance between a node and a subsriber at an end of line tap is on average 500 feet. If the notion is to eliminate the coaxial output from the node, and replace with fiber, which would give rise to taps where subscribers drop would be connected.The issue now is whether the drops would still coax, or fiber. If the drops used are still coax, then some kind of powering would required to convert the light in the fiber to RF.That could be achieved by using a separate output from the node to power the taps.

But here lies the problem. The idea behind the HFC was to eliminate the issues of power comsumption,outages, and a host of RF problems that were presented with the old tree and branch system. The node+4, +5 didn't work out that way when suddenly "as built" cable map add to be modified to accomodate a new development, which meant that was once end of line tap may have to be opened up for another line extender, or a mini bridger amp that is either powered by removing the power block to the tap, or keeping block and add a power supply to the newly extended plant.

What is giving a lot of maintenance techs some sleepless nights at one particular MSO that all I alluded above is meaningless because the is great emphasis on FTTH, which would require less maintenance techs. Already the company has stopped hiring maintenance techs in one of it's subscriber areas.
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