DSL

Broadband Then and Now

Thirteen years ago today (Sept. 10), this column debuted, with a long look at cable modem vs. telco DSL (Digital Subscriber Line) speeds. At the time, 2.3 million people in the U.S. subscribed to high-speed data, and 750,000 to DSL.

Today, and depending on whose numbers you like, cable counts 46.8 million (18x growth), and telco DSL 34.6 million (46x growth.)

Back then, telcos like Pacific Bell aired (very funny!) advertisements, featuring the once-neighborly people of Laurel Lane, at war because cable’s shared bandwidth was slowing down their Internet connections. One guy dips his weed whacker over the fence, beheading his neighbor’s zinnias. Another pops a little kid’s balloon with a gardening tool. The postman shows a can of mace: “I used to use this just for dogs.”

The tag line: “Don’t share a cable line. Get Pacific Bell DSL.”

(Here’s a link to the ad, for the online edition: http://creativity-online.com/work/pacific-bell-neighborhood/13693)

That first-ever column pointed out that DSL is also shared bandwidth, and that the technology would likely run into other problems, as penetration rose, like “crosstalk.” That’s what happens when the twisted-pair wires telcos use radiate RF (Radio Frequency) energy from one to the next, because they’re so physically close to one another in the sheath. Symptom: Sluggishness.

DSL technology progressed, of course. From “ADSL” (Asynchronous DSL) there came “VDSL,” for Very High Speed DSL. Then there was “pair bonding,” which aggregates the bandwidth in homes with two phone lines.

The latest new-new thing in DSL advancement goes variously by “vectoring” and “vectoring with bonding.” Here’s an example of how people talk about it, from a recent batch of notes: “They’re offering 50 to 80 Mbps down, with vectoring.”

Guess what vectoring does? Hello again, crosstalk! Vectoring cancels it, which boosts downstream DSL speeds to a theoretical max of 150 Mbps downstream, and 50 Mbps upstream. (See above quote for a more realistic speed assessment.) With vectoring and a bonded pair: 300 down, 100 up.

Cable’s broadband technology progression came mostly through the DOCSIS (Data Over Cable Service Interface Specification) chapters – 1.0, 1.1, 2.0, 3.0. Coming soon, the whopper, DOCSIS 3.1, which puts operators on a comfortable path toward offering multi-Gigabit per second speeds.

Both types of networks still “share” bandwidth, although that’s changed, too. For the longest time, the cable “node” served about 500 homes. These days, “node splitting” is a routine activity in cable plant. It takes about a half a day, and, if enough “dark fibers” are present at the node, it cuts the number of home sharing bandwidth to 125 or less.

So now, as it was back then, the supposed big bummer of cable’s shared bandwidth isn’t really that big of a bummer after all. And telcos will remain bandwidth-hobbled, but rallying hard, until they draw fiber right to the side of the house.

Meanwhile. Someone should find the creators of that PacBell ad, and engage them on some new broadband material. Everybody needs a little for-real funny from time to time, even when it’s directed right at you.

This column originally appeared in the Platforms section of Multichannel News.

 

Thank You, Data Fans, For Sharing

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’ve not seen it, go to http://bit.ly/SKTKeC.)

It’s a clever attack on what DSL proponents perceive as cable’s Achilles Heel: That hybrid-fiber coax (HFC) 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, 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, to cable’s 2.3 million cable modem subs. All I’m suggesting here is the old one about throwing stones in glass houses.

Fact number one: DSL is a shared network. Not from the home to the central office, true. After that.

A quickie on DSL topology: The DSL modem at a house connects to a companion modem at the central office, about 3 miles away. There, any voice conversation on the line heads over to the phone switch, and the data traffic enters a DSLAM —  a “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 DSL Access Multiplexer, or DSLAM.

Multiplex means smoosh. As in, cramming many inputs into one output. In this case, 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, that have to be re-sent. First come, first served. (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 is specific to twisted-pair wires, like 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% 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%. That’s 100 cable modem customers. Then, estimate how many of them are online at the exact same time. Say that’s 40%. 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% 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.

This column originally appeared in the Platforms section of Multichannel News.