Comments on: Network speed varies from PC to PC--why?

Question: Network speed varies from PC to PC--why?

teddyb109 — Tue, 14 Mar 2006 16:30:47 -0800

I work in an office with several not-for-profits share a high speed internet connection. My internet connection on my Mac Book seemed slow, so I tried the online test at Speakeasy. (I disabled the Airport connection so that I could test Ethernet) I ran the test several times and always found I had around a 400 K connection. Wireless, same test, circa 150 K. So I asked my collegue to run the same test from his Dell desktop---he got almost 1200K across several tests. I thought maybe it was the jack in my office, so I tried yanking out the plug and trying it in my collegue's office with another Dell desktop--she got 1200k--I got 400. I thought it might have been the MBP, so I dug out a Dell laptop--guess what--the most it would get in any office location was 400K. Same for a Mac mini that's on the same network. Now here's the thing--if I take that same MBP laptop home I get 3000K over my cable modem. Why would some computers get higher throughputs than others? They have an outside vendor administer the network and they seem to have disabled ping and traceroute. I started to wonder if maybe they limit bandwidth to the IP address, but all I really do during the day is work in Basecamp and reply to emails with an occassional look at Bloglines. Any ideas for the MeFi community about why this might happen and how I can intelligently describe the problem to the network admin so it doesn't sound like I'm nuts?

By: RichardP

RichardP — Tue, 14 Mar 2006 17:28:23 -0800

Unfortunately, I can think of several other mechanisms that would explain your symptoms. However, you mentioned that the administrator of your network has disabled ping and traceroute (which both usually use the ICMP protocol). Is the shared connection a DSL connection shared with a router? If so, I'd first try to rule out that the problem isn't PPoE in combination with broken path-MTU discovery due to erroneous ICMP filtering (also here).

As an experiment, on your Mac Book Pro (and assuming the Mac is connected via Ethernet), try the following:

From the Apple menu, choose System Preferences.
Select the Network pane.
From the Show pop-up menu, choose Built-in Ethernet.
Click the Ethernet tab.
From the Configure pop-up menu, choose Manually (Advanced).
For Maximum Packet Size (MTU) select the radio button labeled Custom.
In the field to the right of Custom, enter a new value lower than 1500 - I'd try 1492, 1484, or perhaps 1480.

If this fixes the problem, you can either talk to your provider or network administrator to get the ICMP filtering fixed (ask them to let ICMP can't fragment (type 3, code 4) messages through the filter) or leave the MTU lower on your machine. If you leave the MTU lower it will slightly degrade your performance when communicating with other machines in your local network.

By: qwip

qwip — Wed, 15 Mar 2006 00:40:00 -0800

I was gonna suggest what RichardP suggested, so there's that...

By: teddyb109

teddyb109 — Wed, 15 Mar 2006 03:38:38 -0800

Our tech person told me that it was a shared cable connection. I'll give this a whirl. Thanks!

By: teddyb109

teddyb109 — Wed, 15 Mar 2006 11:11:50 -0800

OK - Ran these tests today and here were the results--no significant change than the default of 1500.

== bandwidth data ==

1500 (default)

Download Speed: 390 kbps (48.8 KB/sec transfer rate)
Upload Speed: 49 kbps (6.1 KB/sec transfer rate)

Download Speed: 355 kbps (44.4 KB/sec transfer rate)
Upload Speed: 102 kbps (12.8 KB/sec transfer rate)

Download Speed: 74 kbps (9.3 KB/sec transfer rate)
Upload Speed: 66 kbps (8.3 KB/sec transfer rate)

1480
Download Speed: 437 kbps (54.6 KB/sec transfer rate)
Upload Speed: 367 kbps (45.9 KB/sec transfer rate)

1484

Download Speed: 424 kbps (53 KB/sec transfer rate)
Upload Speed: 290 kbps (36.3 KB/sec transfer rate)

1000

Download Speed: 410 kbps (51.3 KB/sec transfer rate)
Upload Speed: 351 kbps (43.9 KB/sec transfer rate)

1472

Download Speed: 415 kbps (51.9 KB/sec transfer rate)
Upload Speed: 80 kbps (10 KB/sec transfer rate

1468

Download Speed: 260 kbps (32.5 KB/sec transfer rate)
Upload Speed: 269 kbps (33.6 KB/sec transfer rate)

500

Download Speed: 424 kbps (53 KB/sec transfer rate)
Upload Speed: 373 kbps (46.6 KB/sec transfer rate)

400

Download Speed: 377 kbps (47.1 KB/sec transfer rate)
Upload Speed: 179 kbps (22.4 KB/sec transfer rate)