Tuesday, May 12, 2009

Why you'll never see 200Mbps from a 200Mbps 'Net connection

By Nate Anderson

Virgin announced the world's fastest cable modem system this week: a 200Mbps trial it is conducting in the UK. But DSL and fiber providers are angry that cable uses these numbers, saying that they don't reflect the shared reality of a cable network. Ars explains just what can go wrong on the way to 200Mbps.

The UK's luckiest Internet users live in Ashford, Kent. Virgin Media announced this week that it is trialing 200Mbps cable connections in Ashford, where 100 "lead adopters" will help Virgin test and evaluate the system. This is a real-world trial that has already escaped from the lab, but it won't be coming to a home near you any time soon; Virgin plans to run its trial for at least six months before looking at the results and considering further deployment.

Virgin used the opportunity to talk a bit of smack about cable operators in other countries, claiming that its 200Mbps service "is believed to be the fastest implementation of DOCSIS 3 technology in the world, running faster than services offered in Japan and the US, which currently reach 160Mbps and 101Mbps respectively."

This is true, except for the bit where Virgin appears to suggest that its 200Mbps service is, you know, actually available to more than 100 residents of Ashford, Kent.

In any event, these numbers (200Mbps, 160Mbps, and 101Mbps) make the DSL and fiber-to-the-home folks livid, because it's not an accurate representation of the speeds that any cable user can achieve in a real-world deployment. (See this post by Verizon's Eric Rabe, for instance, where he calls the cable numbers "a parlor trick.")

Who's right?

A shared architecture

Both sides are accurate, when considered from Obi-Wan's "a certain point of view." Cable can deliver these speeds, but it does so over a shared architecture that sees speed drop as more people use the Internet at the same time. Consider the throughput constraints on a cable system:

Total number of users

Each node on a cable system is set up in a loop, with every home on that node (up to several hundred) sharing the total bandwidth. Verizon's FiOS also shares, but it divvies up 2.4Gbps between 32 homes; DOCSIS 3.0 cable systems can share around 160Mbps with up to 400-500 homes. Even during peak periods, the line is filled with data only about 10 percent of the time, so the "oversubscription" model generally works well—but heavy use by many users will cause slowdowns, especially on the upstream link (see below).

Bottleneck speed

A connection to a distant server is only as fast as the slowest link. Even when the connection between home and the central office's CMTS (cable modem termination system) is humming along, upstream choke points in the backhaul network, the public Internet, or at the remote server can all lower perceived network speeds.

The gear in your house

As cable's DOCSIS data architecture continues to mature, the new high speed offerings may be too fast for some customers even to use. As Virgin notes when describing its 200Mbps trial, "there are no wireless routers able to deliver throughput of speeds as high as 200Mb, and computers require very high specification in order to be able handle data at such a high rate.

If a home still has a (not uncommon) 802.11b wireless router, for instance, the device can only move 11Mbps under normal circumstances. 802.11g routers can move around 54Mbps, while some 802.11n routers claim speeds in the 100Mbps+ range. None can hit 200Mbps, however.

200Mbps is also faster than fast ethernet (100Mbps), still common in plenty of home PCs tucked in dens, bedrooms, and basements across the country. These machines can never access 200Mbps speeds—and in fact won't even get 100Mbps speeds thanks to network overhead.

In addition, the cable modem needs to be fast enough to handle the new speeds, and customer wiring in the home needs to be able to support 100Mbps+ speeds reliably.

Your DOCSIS profile

When a cable modem is attached to the local loop, it contacts the CMTS and downloads the customers DOCSIS profile. The profile provides all sorts of variables to the modem, including the maximum upstream speed that it can send data (downstream speed to each modem is limited by the CMTS itself to avoid flooding the wire with data).

Cable operators can change this remotely to fix problems or allow more speeds; "enterprising" customers have also attempted to hack their devices to go faster than their pay tier. The CMTS can detect modems transmitting faster than their allowed rate and issue a hard bandwidth limit to such devices to prevent flooding the upload link with data.

Cable plant noise

Noise is the enemy of data, and cable lines can be full of it. Cable modems operate at different power levels in order to overcome any noise on the line that might interfere with transmission. The problem could be inside the house, where it's the customer's problem, or in the local node. Either way, too much noise degrades speeds and makes for a miserable experience.

Severe upstream congestion.

Even with the new DOCSIS 3.0, upstream bandwidth is quite limited in cable systems. When shared between an entire neighborhood, this can cause speed problems—and not just for uploads. (This is why Comcast's P2P blocking system only targeted the upload link.)

Users seeding .torrent files or uploading photo sets to Flickr will see a slowdown anytime the upstream link is congested, of course, but so will plenty of other users. TCP, the most common protocol on the Internet, relies on a stream of acknowledgements to make sure that data is arriving accurately at its destination. In the case of a severely overloaded uplink, such acknowledgements may be delayed, which can in turn affect a user's downloads. (This does not apply to "fire and forget" protocols like UDP.)

An overloaded CMTS

The CMTS in your cable company's local office terminates all the local loops and connects them to the cable company's backhaul network. Like a giant router, a CMTS must keep up with the tremendous packet flows that it sees; if for some reason it cannot, performance degrades.

A CMTS can be badly configured, thereby using too much processor time. It can also be loaded down running management functions and reports, or it might simply be handling a huge number of packets. Cisco, a CMTS maker, notes that in scenarios where CMTS processor usage runs above 80 percent during peak times, "end users may start to experience slower performance and increased latency."

DSL, fiber: we don't share (as much)

DSL and fiber share some of these issues, of course. With DSL, for instance, line noise can be a tremendous problem. But providers are publicly more concerned with that big number—101Mbps! 200Mbps!—even if customers will never see those speeds.

DSL is not a shared architecture in the last mile; each DSL link runs over dedicated copper from a DSLAM port in the local office to a customer's home. The whole office must be connected to a backhaul network, though, which is generally fiber, and problems can crop up here if a central office is underprovisioned. Speeds are typically less variable than with cable, but they don't "sound" as fast, either.

Fiber also shares, but with far fewer homes than cable. It also offers tremendous bandwidth now, with the possibility of more in the future as new wavelengths are added to the fiber.

But no one shares like cable, and that's why some of these speed numbers need to be considered carefully. When Cablevision in the US offers 101Mbps to customers but a DOCSIS 3.0 connection tops out at under 160Mbps—well, it's clear that customers won't get anything like 101Mbps during peak times. Similarly, Virgin's talk of 200Mbps connections is likewise about the total shared bandwidth on each local loop.

Still, traffic provisioning has gotten better in the last years, and cable can address local issues by splitting nodes. While cable's shared architecture means that users simply can't get anything like the peak 160Mbps or 200Mbps, speeds can remain surprisingly high. Most people, even when "using" their Internet connection, aren't actually using their Internet connection—especially when browsing the web, a notoriously "bursty" activity.

Oversubscription is the same technique used by airlines to sell seats on planes, since they know that some percentage of passengers won't show up. The difference is that, if too many people do arrive, the airline hands out vouchers; cable just hands out lower speeds.


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