Digitizing That Rambunctious Return Path
by Leslie Ellis // October 22 2001
Some inventions happen by mistake: The guy who set out to make a compound to replace the use of ivory in billiard balls instead came up with the curious substance we now know as Silly Putty.
Other inventions come more painfully: Nikola Tesla struggled for years, enduring scientific scorn from powerful contemporaries (Thomas Edison, for example) before his zealous pursuit of alternating current (AC) power became a still-used reality.
Other inventions make us smack palm to forehead, muttering laments about not conceiving them ourselves: Retractable dog leashes. Picnic backpacks. Luggage with wheels. A 5-42 MHz upstream path, digitized remotely from the headend, with most of its gunk extracted algorithmically.
Ok, ok. The eureka of the latter invention wouldn’t naturally occur to most people, like you and me, who delight in simple, useful, easy to conjure inventions – like something to thump the excess snow off the encrusted windshield wiper blade during a big storm.
But to cable technologists, who fret about the scrawny, noisy state of cable’s upstream signal path, such an invention is garnering considerable attention. If digitization could do to cable’s snarly, 5-42 MHz return path what it’s done for the quality of sound and video, it can’t be all bad.
In fact, it could be quite good. Extracting upstream signals digitally, and not going through the many analog processes that define today’s gear, doesn’t just mean a reduction in noise. It also leads to more usable bandwidth, which is important as more and more two-way services require safe, swift upstream passage.
True, cable’s suppliers of plant-related equipment have already introduced digitization to the upstream signal path. But so far, their digital stretch reaches only as far as the optical node and not all the way to the home. An estimated 6 million cable homes are passed by this sort of equipment – not much, in plant terms.
This column is not, and never will be used for purposes of endorsement. But every so often, an invention comes along that seems plausible enough for wider translation. Such is the case with a recently-uncloaked Silicon Valley startup, Pacific Broadband Corp., and a method its 100 engineers devised to make the upstream path a safer, faster place, through remote digitization.
It starts with an Application Specific Integrated Circuit, or “ASIC” (pronounced “a-sick,” with a hard “a.”) ASICs are chips built for a single, customized purpose. In this case, ASICs are programmed to sit in the headend portion of cable modem systems, called “CMTS,” for “Cable Modem Termination Systems.” There, the chips “listen” to the entire, 5-42 MHz spectral chunk, then digitize it.
This “listening” happens across the time and spectral domains – or in real time, and including any noise that temporarily or permanently messes with the intended signals. Algorithms (secret number codes) inside the ASIC subtract the noise (called “noise cancellation”) and compensate for distortions (called “equalization”).
If you had this chip stuck into your forehead (and assuming it worked that way) you could stand at a crowded cocktail party, focus on a person at the other side of the room, and hear precisely what she was saying – minus the clinking of glasses, the clunking of forks, and the ridiculous babble of the large man standing next to you. Actually, you could tune up to 16 conversations you wanted to hear, and discard the rest.
In technology terms, noise cancellation and equalization improve the upstream carrier-to-noise (CNR) ratio – tech-speak for a baseline measurement that compares signals sent, against the noises they encounter. The improvement is quantified by as much as 7 decibels, or 7 dB. If true, that’s pretty huge. It’s the difference between, say, 2.5 Mbps and 10 Mbps, in the same 3.2 MHz upstream chunk.
Translation: Way more throughput, for the same bandwidth.
Plus, because everything happens digitally, this particular ASIC is equipped to automatically handle more, and simultaneous, upstream “channels” – sixteen as opposed to one, in contemporary CMTS gear.
The ability to dynamically assign more upstream channels could, in turn, be helpful with respect to node re-combining. In today’s analog cable modem gear, CMTS ports are often shared among four or so 500-home nodes, at least until subscriber penetrations get high enough to warrant a decoupling.
Node decoupling, as it exists right now, can cause operational angst, not to mention service interruptions. Thus, the ability to handle penetration-releated throughput issues by automatic node-realignment is of particular interest to data technologists. Every little bit helps.
Of course, a useful invention – whether accidental, tortured, or obvious – is one thing. Making it work is quite another, particularly in a setting as predictably hostile as is cable’s upstream signal path.
But, if the old adage about necessity being the mother of invention is true, then cable’s upstream path has never been more acutely needful of some new mothering. And maybe it all starts with a digital cleansing.
This column originally appeared in the Broadband Week section of Multichannel News.
You Must Remember This: How (RAM, DRAM, NV-RAM, & Flash) Memory Works
by Leslie Ellis // October 08 2001
Memory, both human and machine, is a tricky tapestry of pointers and storerooms, often yielding exasperation. The things we want desperately to forget – that sleepy, misguided turn in the hotel room that deposited you, blinking, in the hallway, not the bathroom – linger forever. The things we want desperately to remember – the faces and advice and essence of lost loved ones – hover only in the mind’s periphery.
As recent events in New York and Washington unfailingly submit, memory is frequently a trickster. As humans, we long to forget some things, and remember others, but our brains don’t always cooperate: Some things are just impossible to forget, and horribly simple to recall.
Machine memory, mercifully, is slightly more manageable than the woolly inner workings of the human brain. Machines can be ordered to always remember, never forget.
Take today’s digital set-top boxes. What we hear most vociferously about them is how much memory they don’t have. This awkward reality parallels various proverbs: A pint pot doesn’t hold a quart. The five-pound bag and the 10 pounds of … you get the idea.
But, as is usually the case in technological matters, there’s a lot more to be said about machine memory and how it works than the shrill refrain of “not enough.”
First, a few basics. Electronic memory is generally stored on chips. It is measured in kilobytes (abbreviated “kB”) and Megabytes (abbreviated “MB.”) Some types of memory are more expensive than others, but in general, memory prices are falling predictably.
The digital set-tops shipping in the highest volumes today – the “2000” series of both Motorola and Scientific-Atlanta – contain at least three different types of memory. In techno-speak, these three memory types go by “NV-RAM,” for “non-volatile, random access memory;” “flash;” and “DRAM” (pronounced “dee-ram.”)
It is flash and DRAM memory that gets the most conversational play among digital box aficionados. Maybe you’ve heard a technologist refer to set-top memory as “four by eight,” or “one by two.” The first number is the number of megabytes of flash memory. The second number is the number of DRAM megabytes. A four-by-eight configuration, then, is a box that contains 4 Megabytes of flash memory, and 8 Megabytes of DRAM.
In general, the difference between the three types of set-top memory involves what stays in the storage cells when the power goes out. Non-volatile, in this sense, means “must stay.” Both NV-RAM and flash memory chips are configured to keep stored information when the power fails. DRAM memory cells, by contrast, are volatile: They blank out when the power goes, and need to be refreshed.
NV-RAM is the tiniest of the three, capacity-wise. It is generally sized in kilobytes per unit. NV-RAM holds super-critical stuff: The identification number of the box. Customer-generated preferences, including any parental locks. Information about pay-per-view purchases, for inclusion in the next monthly bill.
NV-RAM is much like the first data you learned as a child, and the first data you teach your children to remember: Name, address, telephone number.
Flash is the most expensive of the three. A sort of re-writeable NV-RAM, flash memory used in contemporary digital set-tops is sized in the low Megabytes – 4 MB in the baseline case of the S-A Explorer 2000; 1 MB, in the baseline case of the Motorola DCT-2000.
Flash memory holds the software code that makes various applications work – the “applications code,” or “executables,” in technical parlance. This means the operating system, the program that evokes the electronic program guide, and any other “resident applications” present in the box.
Flash, in human terms, holds the things you tend to remember no matter what: Your birthday. The Pledge of Allegiance. Multiplication tables. Your grandmother’s rice pudding recipe. How to tie your shoe. Your greatest love. The words to your favorite songs. Things you hold dear, either by repetition or vigilance.
DRAM holds the information used by the applications held in flash – the “apps data,” in tech-speak. DRAM is volatile, meaning that if the power fails, all its storage cells are wiped clean. Guide data is a good example: When the power returns to the box, all of the titles and descriptions of TV shows must be re-loaded. In some cases, applications code, like the software files that make VOD work, are loaded into DRAM, and initiated when a digital customer evokes the application.
The constant in machine memory is this: No matter how cheap it gets, and no matter how much of it snaps into electronics devices like the digital set-top, there will never, ever be enough. Such is the nature of software: Like a gas, it tends to fill all available space.
While more malleable than quixotic human memory, machine memory will require intense supervision. That means making sure what’s available is used intelligently, and making sure interactive service suppliers color inside the lines of what will always be a precious resource.
This column originally appeared in the Broadband Week section of Multichannel News.