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I have fiber internet into my house. When I plug directly into the fiber to Ethernet converter, I easily can pull down at 30 Mbps, as verified from trying speedtest.net. The ethernet jack on the fiber converter is 10/100 Base-T.

My problem is when I plug in an intermediate Gigabit switch or router between my laptop and the fiber converter. Once I do that ( fiber converter > gigabit switch > laptop ), the same speed test gives me results at around 9 Mbps.

All of the cables involved are known to be good. I've tried both a Netgear gigabit switch, as well as an Apple Airport Extreme router as the intermediate nodes, and they have both show similar speed drops.

Where did all the speed go? What can I do to get the full connection speed with an intermediate switch in the mix?

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  • 9Mbps is consistent with the converter and/or switch port negotiating at 10Mbps, does your switch have an led indicator on the line to say what speed the port is running at?
    – Paul
    Commented Jul 8, 2012 at 5:01
  • @Paul That's what I was thinking, but he said that he gets 30Mbps when he plugs in directly.
    – imtheman
    Commented Jul 8, 2012 at 5:07
  • @cravemusic How long are your cables? Are any other computers connected to the intermediate router? What category are your cables?
    – imtheman
    Commented Jul 8, 2012 at 5:08
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    @Paul - Using Network Utility on OS X Lion, the wired connection reports Gigbit network speed when connected to the router. The same tool reports 100 Mbps when plugged directly in to the fiber converter.
    – cravemusic
    Commented Jul 8, 2012 at 5:16
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    I changed the network topology slightly. Now, I'm going fiber converter > 10/100 switch > Gigabit router or switch > laptop. Speed results are 12.5 Mbps down and 14.5 Mbps up with the gig router, and 4.5 Mpbs down and 11.5 Mbps up with the gig switch. If I drop the gig devices from the mix, but keep the 10/100 switch, I still can get the 30 Mbps down speed.
    – cravemusic
    Commented Jul 8, 2012 at 6:37

3 Answers 3

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It sounds like you may be having flow control problems. Ethernet flow control is generally more trouble than it's worth. It's better to just drop packets when the link gets overloaded than to let them pile up due to flow control; TCP watches for dropped packets to know when to throttle back. If you use link-layer flow control to decrease drop by increasing latency, you end up with a laggy link and TCP is kept in the dark about the problems, so it can't do the right thing. Try turning off Ethernet flow control on all devices on your network.

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Could be a duplex mismatch. If your switch doesn't support the IEEE standard 802.3u, then try forcing half-duplex on all clients/ports. See if that helps the speed issues.

I have seen speed issues similar to what you are describing on Cisco switches; auto-negotiation is often the culprate.

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When a fast switch with fast hosts are sending data to a slower port, the hosts will send more data than the switch can offload over the slow port, causing dropped frames, retransmits, and so-forth.

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