Stepping Up the Upstream Path
by Leslie Ellis // September 30 2002
In a market reeling with jailed executives, growth-suppressing accounting changes, and a depressing increase in severance package discussions, any bit of good news is refreshing.
Which is why it was vaguely comforting to learn of a zero-cost way to double the upstream capacity for cable modems, as part of a longer-term move to a 12-fold increase.
As quirky as it is, the upstream signal path matters. It matters because every significant new service slated for launch on cable plant requires it. On-demand video needs it to pass along the customer-generated fast forward, rewind, play and pause commands inherent to the service. Telephony, whether IP or circuit switched, needs it to haul everything that happens after a person picks up the phone to make a call. Broadband Internet service needs it to carry requests, like for a Web page. And so on.
Yet the upstream path – also known as the reverse path, and the return plant – is skinny, especially compared to the forward, or “downstream,” signal path (from headends to homes.)
How skinny? Picture a bookshelf. On it are six paperbacks, and 115 hardbacks. The ratio of paperbacks to hardbacks is roughly the same as upstream to downstream bandwidth, in a 750 MHz, two-way cable system.
Currently, engineers employ two architectural methods to better the upstream bandwidth experience for cable modem partakers. One is to “un-combine nodes,” and another is to “split nodes.”
Most operators, when they installed the gear to enable broadband Internet service, four or five years ago, combined as many as four, 500-home nodes to one port of the Cable Modem Termination System, or CMTS – the headend part of broadband Internet service. As penetration levels increase, and more people share the same slice of bandwidth, those nodes can be “un-combined,” by putting only two nodes, and then one node, per port. That’s node recombining.
Node splitting happens when that 500-home node is halved, so that 250 homes share the upstream signal path.
And now, methods are emerging to double upstream throughput by stretching the width of the upstream channel, from 1.6 MHz to 3.2 MHz. The extra room doubles the carrying capacity of the upstream channel, from 2.5 Megabits per second (Mbps) to 5 Mbps.
On an operational level, it means that when a system calls in for the resources needed to split a node, they may be asked to first set the CMTS at 3.2 MHz channel spacing.
Engineers familiar with the doubling process call it a no-cost no brainer: First, make sure nothing else is using the spectral chunk earmarked for the widening. Some set-tops, like those with impulse pay-per-view (IPPV) capabilities, use the upstream path, as does any telephony equipment. Once the coast is clear, type in a command that tells the CMTS to get wider. No cost; double the bandwidth.
The ability to stretch upstream channel widths is, in part, another plum salvaged from the remains of Excite@Home. Some MSOs didn’t have control over how CMTS units were configured or upgraded when they were @Home constituents; only now can they adjust CMTS parameters at will.
Splitting nodes and going to 3.2 MHz channel width means that the 5 Mbps of upstream throughput is available to half as many customers: Fewer customers get more bandwidth. Example: Splitting a 500-home node means that 50 homes, not 100 homes, in a 20% penetrated broadband Internet market, share the 5 Mbps.
And there are two other upstream throughput boosts on the horizon. One involves upshifting to 16-QAM (quadrature amplitude modulation), which, in a 3.2 MHz channel width, lifts the upstream carrying capacity of that channel to 10 Mbps.
The big kicker comes with DOCSIS 2.0, which somehow managed to harmonize two competing types of advanced modulation. Both use impressively nerdy descriptors: Synchronous Code Division Multiple Access (S-CDMA), and Frequency Agile Time Division Multiple Access (FA-TDMA).
With DOCSIS 2.0, the capacity of an upstream channel bolts to 30 Mbps, with noise safeguards. Noise management matters, a lot, because moving data that quickly tends to make it more susceptible to noise hits — and noise hits are legendary in the 5-40 MHz spectral range.
Put it all together, and the carrying capacity of the upstream signal path progresses like this: 2.5 Mbps upshifts to 5 Mbps, then 10 Mbps, then 30 Mbps, as a function of upstream channel width and modulation improvements.
Suddenly, cable’s 5-40 MHz upstream path isn’t just a curiosity because of the weird types of noise and interferences that lurk within it. Instead, today’s conversations about the upstream path center on efficiency: Which services need how much speed and space to thrive.
In other words, persistence is outweighing obstacles. It’s not a return to EBITDA accounting, nor an end to layoff anxieties. But it’s something.
This column originally appeared in the Broadband Week section of Multichannel News.