Thank You, Data Fans, for Sharing

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By now, you've probably seen or heard about PacBell's TV ads for DSL, where cable's shared bandwidth escalates into an absurdly amusing neighborhood war.

Even cable people laugh at the spots: Friendly neighbors become sworn enemies, skulking around with spray paint to single out "bandwidth hogs." (If you haven't seen it, go to www.adcritic.com.)

It's a clever attack on what DSL proponents perceive as cable's Achilles' heel: Hybrid-fiber coax architectures, like cable's, are configured to share bandwidth among 500 or more homes hanging off a node. The assumption: Sharing causes insufferable slowdowns for cable-modem users. Consumers should pick DSL and never have any slowdowns.

Sorry, it's just not that easy. As is usually the case, the truth about sharing lies somewhere in the middle.

Before we even get into who's sharing what with whom, here's an even simpler truth: Residential DSL penetration, while climbing quickly, is still very low. At the end of the second quarter, there were about 750,000 residential DSL customers versus cable's 2.3 million cable-modem subs. All I'm suggesting here is the old one about throwing stones in glass houses.

Fact No. 1: DSL is a shared network-not from the home to the central office, true, but after that.

A quickie on DSL topology: The DSL modem at a house connects to a companion modem at the central office, about three miles away. There, any voice conversation on the line heads over to the phone switch, and the data traffic enters a DSLAM, or "digital-subscriber-line access multiplexer."

Telcos use DSLAMs because the alternative-dedicating a router port to each individual DSL subscriber-would be outrageously expensive. Something was needed for router-port sharing. Enter the DSLAM.

Multiplex means smoosh, as in cramming many inputs into one output. In this case, it means combining multiple DSL flows into a composite data stream, out to routed Internet pipelines. Smoosh means share.

With low DSL penetration, it's easy to dismiss potential DSLAM-sharing bottlenecks. But two things can happen when penetrations rise.

First, more traffic hurtles through the DSLAM. The Internet's language, TCP/IP, juggles overloads by dropping packets, which have to be re-sent-first come, first served. (This is true for cable, too.) This all happens transparently to the person surfing away. Symptom: sluggishness.

The second thing that can happen when DSL penetrations rise is lesser known. It has nothing to do with sharing, but it's notable. It's called "crosstalk," and it is specific to twisted-pair wires, like what telcos use to give us phone and DSL service.

Translated: When phone wires are bundled into one sheath to the central office, they're physically close enough to one another that the DSL traffic, because of where it is in the RF spectrum, could radiate from one pair of wires to the next. That's crosstalk.

To the DSL equipment, it looks like noise. As DSL traffic increases, the noise floor rises. As the noise floor rises, data rates decrease. Symptom: sluggishness.

What's DSL's fix? Adding DSLAMs is one solution, and it's a pay-as-you-go capital expense. But fixing crosstalk means driving fiber deeper-an expensive operational expense that's time-consuming.

Cable is shared. But it's incorrect to characterize 500 homes all sharing the bandwidth dedicated to cable-modem traffic. That'd be 100 percent penetration for high-speed-data services-unlikely anytime soon.

Translating cable's sharing issues isn't hard. Start with node size. Say it's 500 homes. Then apply the penetration rate. Say it's 20 percent. That's 100 cable-modem customers. Then estimate how many of them are online at the exact same time. Say that's 40 percent. You're down to 40 people sharing 27 megabits per second. Evenly distributed, that's 675 kilobits per second each. That's pretty fast.

The hardest part is anticipating what those 40 customers are doing. Some activities like e-mail, chat and surfing are "bursty"-you don't need 675 kbps to send an e-mail. But streaming video chews up bandwidth in a shared environment.

Cable's fix is to split nodes so that fewer homes are sharing the same 27-mbps data channel. It takes about half a day to split a node. In today's systems, even penetrations in the high 30 percent range haven't yet needed to be split.

In reality, most of the well-publicized cable-modem slowdowns so far had more to do with improper router configuration-human error-than bandwidth sharing.

For both cable and DSL, sharing is an issue that will require careful attention. But it's not as bad as PacBell's clever ads make it out to be. Nor is DSL immune.

Stumped by jargon? Heard a term that makes no sense? Ask Ellis: She loves this stuff. E-mail your question to ellis299@aol.com, with "translation please" in the subject header.

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