baffling speed problem

We've got a weird speed problem at work. Here's the setup:

wireless link to our main building ^ | | | | v wireless hardware on our building ^ | | | v router ^ | | | v switch ^ | | v hub

There are a bunch of computers connected to the switch, and a bunch connected to the hub. Here's what we see on *most* of our computers:

  1. Connections between computers in our building are fast.

  1. Uploading data from our building to the main building is fast. (Around

1.2 megabytes/second, which is the expected speed of the wireless link).

  1. Downloading data from the main building to ours is slow (30 kbytes/second is common, with frequent fluctuations up to around 80-100 kbytes/second, and occasional bursts up to 1 megabyte/second).

I said "most of our computers", because some get fast downloads. I have a test lab, which has more computers than there are network jacks in the room, so I brought in an unused DLink home router/switch from home, and am using the switch portion of it. So, one of the network jacks is connected to one of the 4 "LAN" ports on the router, and then 3 test computers are hooked to the others. These all get fast downloads.

Upon experimenting around the building, what we've basically found is this:

  1. If a computer is plugged into a network jack, it is slow to download (except for a single exception, which I'll cover later). It doesn't matter where that jack connects to on the other end--we've tried moving the other end from the hub to the switch, or the switch to the hub. It's slow no matter what.

  1. If we take one of the slow computers, and instead of plugging it directly into the wall jack, we plug it into a cheap switch, and plug the switch into the wall jack, that computer gets fast downloads.

  2. It doesn't seem to have anything to do with the OS. We see this on Windows, Linux (SuSE and Gentoo), and my PowerBook. It doesn't seem to be a specific brand of network card in the PCs, either...we've got a variety of onboard and add-in cards.

The one exception is there is one Windows box that is fast even when plugged straight into the wall jack. It's specific to that computer--if we unplug the cable from the back of it and move it over to my PowerBook, for example, my PowerBook is slow (and then if we stick a switch between the PowerBook and the wall, the PowerBook is fast).

We also tried using a hub instead of a switch between the computers and the wall. They stayed slow. (And if we went "computer -> switch -> hub ->

wall", they were fast).

(Oh, and it isn't something freaky about the wiring between the jacks and the hub or switch, because it was slow when we took my PowerBook into the wiring closet and plugged it right into the hub with a short cable that is known to be good).

It would seem that the fact that it only affects downloads from the other building would indicate a problem with the wireless hardware, or the router. However, if the problem is on that side of the switch, then it would seem that it would not be affected by whether or not the computers go through an extra switch between them and the wall jacks. Conversely, it would seem that anything that would be affected by that would affect internal traffic as well as traffic from the other building.

Anyone have any ideas as to what the heck could be going on here?

Reply to
Tim Smith
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This would have been my first guess. Asymmetric speed on a symmetric link is often due to a homemade cable with a split pair.

You talk of "downloading files" but it would be good to eliminate the protocol as a possible cause. Try some tests with ttcp and ping.

-- Robert

Reply to
Robert Redelmeier

I've done speed tests with a pair of programs that consists of a server that listens for a connections, and a client that connects, and then just loops send data using TCP. The server reports the speed it is getting.

When I say it gets slow, I mean that program gets around 30-60 kbytes/second. When I say fast, I mean 800-1200 kbytes/second.

I also test with doing file downloads (e.g., "curl -O http://192.168.0.50/big.file" were 192.168.0.50 is a machine on our LAN in the other building, big.file is a megabyte of data from /dev/urandom, and the machine is a very lightly loaded box). Same result as with my speed test program.

If it were a 5 or 10% slowdown, I'd worry about file transfer protocols, but we're talking a factor of 30 slowdown here.

Reply to
Tim Smith

In article , Tim Smith wrote: :We've got a weird speed problem at work. Here's the setup:

:2. Uploading data from our building to the main building is fast. (Around :1.2 megabytes/second, which is the expected speed of the wireless link).

:3. Downloading data from the main building to ours is slow (30 kbytes/second :is common, with frequent fluctuations up to around 80-100 kbytes/second, and :occasional bursts up to 1 megabyte/second).

Assymetric speeds often indicate a duplex problem.

Reply to
Walter Roberson

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