By Richi Jennings. January 16, 2011.
Yesterday, Michael Horowitz described an author's disappointing upgrade to Comcast's 50 Mbit/s cable broadband service. Scott Mueller (pictured) only got half the throughput he expected. Was that because his router didn't support Gigabit Ethernet? Or are there other reasons? Let's see, in The Long View...
Mueller is sort-of on the right track, but it's nothing to do with the network equipment supporting Gigabit Ethernet (GbE). Here's what Michael had to say yesterday:
Scott Mueller ... signed up for an even faster Comcast service, one offering a whopping 50 Mbps downstream and 10 Mbps upstream. But, after the upgrade, speed tests showed he was only getting 27 Mbps downstream.
His problem? A Linksys WRT54G router. ... To get download speeds faster than 27 Mbps you need the faster Gigabit Ethernet. ... The cable modem and the router ... need Gigabit Ethernet ports.
What's that smell? My urban legend detector is chiming; the needle is off the scale.
After allowing for protocol overheads, the real-world TCP/IP throughput of a 100Mbit/s Ethernet connection should be at least 70Mbit/s, so there's plenty of headroom to spare. That's 2.5 times the throughput that Mueller's seeing.
How come I can transfer files between two Windows 7 PCs via my 100Mbit/s router at these sorts of speeds?
The reality, like many things in life, is more complex than, "it needs GbE ports." I think there's several aspects to this story:
1. The Linksys WRT54G is a positively ancient design, first launched in 2002. Many of the later hardware revisions likely can't cope with the required throughput of a 50/10 connection.
2. Linksys Ethernet hardware is notorious for aging. I used to recommend Linksys routers to family and friends. But no longer: I've lost count of the number of routers where the Ethernet ports have gone bad -- this usually manifests as only being able to negotiate a 10Mbit/s connection, then becoming intermittent, before giving up the ghost entirely.
3. Some Comcast service levels include a temporary speed burst. Depending on which service Mueller bought, Comcast's carefully-worded "up to 50Mbit/s" may include the effects of PowerBoost, in which Comcast throttles file downloads without slowing down Web traffic. The speed it drops down to depends on congestion on the local cable, headend cabinet, and the various parts of Comcast's network (not to mention the vagaries of the Internet beyond).
4. Auto-negotiation may have failed. With 100BASE-T, it's worthwhile taking the time to explicitly configure each Ethernet port to use 100Mbit/s, full-duplex. Sometimes, the auto-negotiation doesn't give the best speeds. On Windows, run ncpa.cpl. (Note that this isn't a good idea with a GbE connection: auto negotiation is generally required here.)
5. Try enabling bi-directional flow control. Network adapter configurations often disable these by default. Again, ncpa.cpl is your friend -- you want RX and TX flow control enabled.
One or both of the first two points are most likely causing Mueller's problem, not the lack of GbE.
Any other suggestions for Scott and Michael? Leave a comment below...
|Richi Jennings is an independent analyst/consultant, specializing in blogging, email, and security. A cross-functional IT geek since 1985, you can follow him as @richi on Twitter, pretend to be richij's friend on Facebook, or just use good old email: TLV@richij.com.|