Getting to Know QoS, P-QoS and D-QoS of bandwidth

8/10/2003 8:00 PM Eastern

In last week's e-mail was a translation request from a reader named Arun, who wanted to know more about a way-techie term that tends to bump around conversations about cable modems and bandwidth guarantees.

The term in question: "P-QoS."

If you've never run into this one before, know that most people speak it as a word: "pea kwoss."

The "P" in "P-QoS" stands for "provisioned." The "QoS" stands for "quality of service," which is a more succinct, if clunky, way to say, "the ingredient that lets you assign minimum or maximum bandwidth levels on cable modems."

(By the way, "QoS," without a preceding letter, is usually pronounced as its constituent letters: Q-oh-S.)

An oldie but goodie

Technologists involved in the ongoing work of DOCSIS (Data Over Cable Service Interface Specification) say that P-QoS falls into the "oldie but goodie" category.

For the rest of us, it probably doesn't sound all that familiar — which is further reason why P-QoS is this week's translation.

Understanding the different "grades" or "qualities" of service in a broadband Internet system starts with understanding what happens when there's no QoS in place.

No QoS is the same as "best-effort" broadband Internet service. "Best effort" is what you get as one person with a cable modem, in a community where your neighbors are also using cable modems, and you're all riding the same plant. Short of paying more for guaranteed levels of bandwidth, your modem gives you its best effort at getting your stuff through, quickly.

The majority of cable's broadband Internet customers get "best effort" service right now. Today's bandwidth parameters are about upper limits: You get no more than X Megabits per second downstream, and Y Mbps upstream.

Bandwidth guarantees, where you always get at least so much bandwidth, are a thing of QoS.

Session candidates

The holy grail of QoS, for now, is "D-QoS," or "dynamic quality of service." It lets cable operators nail up bandwidth to a broadband customer on the fly — that means without having to download a new configuration file into the cable modem and then reboot the modem to activate the new file.

With D-QoS, bandwidth boosts occur in the background, without a download or reboot sequence.

Any session-based service, like a voice-over-IP (VoIP) telephone call, a stream of video to the PC, or a networked PC game, is a candidate for D-QoS. (See the Nov. 11, 2002 "Translation Please" for more on D-QoS.)

Business bits

P-QoS sits squarely in the middle, between "best effort" and D-QoS. It doesn't upshift customers to more bandwidth on the fly, but it does shift them to more bandwidth on a monthly billing basis.

In part, that's why it's called "provisioned" quality of service. Recall: To provision is to activate an account for service, as automatically as possible. So, when a customer wants a fixed level of upstream and downstream bandwidth, likely for a higher monthly rate, P-QoS makes sure the bandwidth is there.

The reason P-QoS is entering conversations now is the slow-but-steady rise of the commercial services, or "B2B," market.

When a small business calls, wanting to buy symmetrical T-1 service from a cable operator (that's 1.5 Mbps downstream and upstream, at all times), something needs to be in place to set up those parameters. That something is P-QoS.

P-QoS and D-QoS both fall under the wider general category of QoS. This isn't good-better-best, with QoS on the bottom, D-QoS on top and P-QoS in the middle. When people talk about QoS, they're usually talking generically about data service tiers.

If P-QoS and D-QoS were terms that bumped around conversations about video, instead of broadband Internet, the scenario would go something like this: P-QoS would be the thing that lets people upgrade themselves to Home Box Office or Showtime, for a higher monthly fee. D-QoS would be the thing that lets them magically watch that service in HD, for the duration of the show that's being transmitted.

(This latter analogy falls into the "don't try this at home" category – there is no such thing that can make an analog TV suddenly be HDTV.)

Spinach, sir

Operationally speaking, P-QoS and D-QoS are different enough that it probably doesn't make sense to hold off on one until the other is ready. Or, more specifically, holding off on P-QoS because D-QoS is better doesn't quite make sense. Besides, P-QoS is already built into cable modems based on the DOCSIS 1.1 specification; D-QoS comes with DOCSIS 2.0.

By now, you've probably come to the none-too-shocking conclusion that QoS isn't the sexiest topic in the world. But, as this column has noted before, sometimes you have to eat your spinach before you can have dessert.

Reader Arun, hope this helps.

Stumped by gibberish? Send translatables to

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