Have you ever had a network problem with a server connected at 100Mb/s, but don't have the ability to perform file transfers above about 1-2 Mb/s? There is an interesting, and surprisingly common, problem that can manifest in exactly this way.
The first step in determining if this problem is affecting you is to try running several simultaneous file transfers. If you run, for example, 5 file transfers, do you see 1-2 Mb/s for each indifividual transfer resulting in 5-10Mb/s of aggregate transfer overall? If you do, then you likely have a duplex mismatch problem in your network.
Duplex mismatch is where one end of the link tries to run in full-duplex mode while the other end tries to run in half-duplex mode. The full-duplex end will transmit whenever it has traffic to send over the link while the half-duplex end will only send when the link is idle. The result is that the full-duplex end will frequently clobber packets being sent by the half-duplex end. Since the full-duplex end has no concept of collisions, the clobbered packets are resent by the TCP layer of the connection which is where the "1-2Mb/s" limit on a per-connection basis comes from.
With a managed switch you will see late collisions and multiple collisions on the half-duplex end of the link and usually transmit and receive errors on the full-duplex end of the link. These error counters will increment quickly and it's an easy problem to spot when you know what you're looking for. There is an equally easy solution too -- just manually set BOTH ends of the link to operate in full-duplex mode. Once you make this one simple change you should see normal file transfer speeds restored to the affected server.
Once you've seen the unique behavior of the duplex mismatch problem it becomes easy to spot. Just remember to think about duplex mismatch as a possible problem when you see very slow transfer speeds across Ethernet links that should operate at much faster speeds. It's also worth noting that this problem only really applies to 10 and 100 Mb/s Ethernet links since gigabit links do not have a half-duplex operating mode.
Duplex mismatch is a problem that does rear it's head from time to time. It's a fast check that you can do to ensure it isn't the source of a problem, or won't become a future problem!
The first step in determining if this problem is affecting you is to try running several simultaneous file transfers. If you run, for example, 5 file transfers, do you see 1-2 Mb/s for each indifividual transfer resulting in 5-10Mb/s of aggregate transfer overall? If you do, then you likely have a duplex mismatch problem in your network.
Duplex mismatch is where one end of the link tries to run in full-duplex mode while the other end tries to run in half-duplex mode. The full-duplex end will transmit whenever it has traffic to send over the link while the half-duplex end will only send when the link is idle. The result is that the full-duplex end will frequently clobber packets being sent by the half-duplex end. Since the full-duplex end has no concept of collisions, the clobbered packets are resent by the TCP layer of the connection which is where the "1-2Mb/s" limit on a per-connection basis comes from.
With a managed switch you will see late collisions and multiple collisions on the half-duplex end of the link and usually transmit and receive errors on the full-duplex end of the link. These error counters will increment quickly and it's an easy problem to spot when you know what you're looking for. There is an equally easy solution too -- just manually set BOTH ends of the link to operate in full-duplex mode. Once you make this one simple change you should see normal file transfer speeds restored to the affected server.
Once you've seen the unique behavior of the duplex mismatch problem it becomes easy to spot. Just remember to think about duplex mismatch as a possible problem when you see very slow transfer speeds across Ethernet links that should operate at much faster speeds. It's also worth noting that this problem only really applies to 10 and 100 Mb/s Ethernet links since gigabit links do not have a half-duplex operating mode.
Duplex mismatch is a problem that does rear it's head from time to time. It's a fast check that you can do to ensure it isn't the source of a problem, or won't become a future problem!
Waveform Technology colocates your server and ensures you'll have service at all times and more importantly, when you need it! You can read more about Waveform Technology at http://www.waveform.net
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