Office network bottleneck

Question

At work our IP is the devil Comcast. When I do a speedtest I get a jump of upwards of 20Mbs and then it slowly goes down to average out at around 10Mbs. The upload speed stays at stead 2-3Mbs. My ping is normal at 30ms.

Now, when I download something from the internet larger than your average webpage the speed bottlenecks. Today, for example, I go download an Azure update for Visual Studio (still downloading) and get 20-200KBs.

I read this article about bottlenecks. It was very vague and didn't really provide troubleshooting steps (what I am looking for) but rather just listed potential problems.

I realize first I must find the problem; I need to isolate it. I have access to practically everything as far as networking goes in my environment, so that's not really an issue. However, I can't just start unplugging and moving cables around, either.

I am very ignorant when it comes to the real technical side of networking (for instance, I can read a Wireshark capture but I cannot interpret what it means).

What I am asking for are some specific troubleshooting steps I can take to isolate the problem. It's a typical small office setup with about 10-15 computers, a hefty fileserver, a few tiny servers for testing, about 3 working switches that go out a Netgear Prosafe Firewall (I was going to provide a link SE won't let me). At least one of the switches is a gigabit switch. (Is my PC going through the gigabit switch? I don't know. Can I find out? I am not sure.) I know I could just start unplugging to figure out which is mine, but like I said, I can't do that. Is there any other way to tell?

Another obvious question, "Does anyone else have the same problem?" Answer: Sometimes. It is really hit or miss. (which makes me think it's not just an issue of who's on the gigabit switch and who's not).

To sum up, are there any troubleshooting steps recommended for my situation?

Answer 1

The problem with diagnosing speed problem is that it can be so many factors it's difficult to find the exact cause.

Your firewall should be giving a summary of overall network usage - that 20mb pipe (may be less) is shared amongst your entire network, so you have to take this into account. Your switch speed does not matter since even 100mbit is fast enough for your current Internet speed. For reference, you can tell by looking under Network Connections and viewing the status of the adapter - that shows your link speed.

My bet is that there's other machines consuming some bandwidth which is what's causing the slow download speeds for you. It also highly depends on the load of the remote connection. From my experience, Microsoft-esque downloads (Azure, Technet, etc) can be fairly slow depending on your location. Try the CacheFly speed tests. The static file download is helpful too. Try a download manager that can download the test file with multiple threads too.

If your testing still shows you're getting nowhere close to your advertised speed, contact Comcast.

Answer 2

The "20 drops to 10" pattern is a perfectly normal pattern when your service provider is selling you a 10, but gives you some credit for not being a hog and provides "burst" speeds for a little while (typically a matter of seconds) if you have not been eating 10Mbit before the "Burst" - this makes "normal user" web-browsing (sporadic) act a lot like it's got a 20 Mbit connection. Put a constant load on it and the burst times out. So unless you are paying for a full-time 20mbit (or more) connection, it's normal.

The gigabit switch, or lack thereof, is a red herring - if your other switches are 100mbit, they are still 10 times faster than your uplink (or 5x faster than the burst speed.) What can be a problem is an older router/firewall (I'm not sure how old what you have is) as a lot of early routers could only manage 20-30mbit, and (nearly) nobody knew that until 50 mbit connections became available and the weakness started to show. But if all you have is a 10/20mbit connection, that isn't the issue, yet.