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I have been performing the same test with 3 different Wi-Fi routers (ASUS RT-N 16, Netgear WD3700 and Netgear WD4500):

  1. Turn on the router and plugin external HDD.
  2. Connect to the router from one of two different laptops (both using 802.11n), while placing the laptop inches from the router.
  3. Copy a big file from the laptop to the external HDD over Wi-Fi.

No matter what router, laptop or frequency (2.4 or 5.0) I used, my file transfer speed never went above 6.0MBps (with Netgear WD4500) or 3.5MBps (with any other router).

Why is it so much lower than theoretical maximum?

Few things that I have already ruled out:

  1. One of the laptops is brand new ASUS UX31a running Windows 7, another one is an older Toshiba with Windows XP. Transfer speed is almost identical on both of them, so it should not be an issue with the laptop.
  2. When HDD is connected directly to the laptop via USB, the transfer speed hovers around 20MBps, so HDD can't be limiting factor either.
  3. While doing the test, no other computers were connected to that Wi-Fi network.
  4. All routers were brand new, running stock firmware.
  5. InSSIDer shows that there is just one another router in vicinity, running only at 2.4 frequency and at different channel than my router.

Am I missing something obvious?

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migrated from stackoverflow.com Aug 14 '12 at 11:36

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offtopic for Stack Overflow, please delete the question and repost on Super User –  Mike Pennington Aug 14 '12 at 5:45

1 Answer 1

What are the exact capabilities of your wireless cards? What wireless mode did you have the routers in? What did you machine report as its 802.11 data rate for the connection? What speed can you get doing IPerf to a decent machine on wired Ethernet on the LAN port of the router? What speed can you get doing SMB file sharing with that same decent PC on the LAN port? What speed does that decent PC on the LAN port get when doing file transfers to/from the router?

The 6 MebiByte/sec speed you got is faster than what you could get over 802.11g (or A or B for that matter), and reasonable for some of the slower flavors of 802.11n. But if you could show that your client card is a fast 3x3 (3 spacial stream, 450 megabits/sec) card, then it would make sense to hope for better performance out of the WNDR4500, which is also a 450 megabit/sec N device. The other two routers you mentioned are middle-of-the-road 300 megabit/sec devices.

6 MebiBytes/sec is about 50 megabits/sec, which is the speed one might expect an app that uses TCP very efficiently to get out of a 100 megabit/sec link. But file sharing protocols like SMB tend to read and write in chunks and wait for one chunk to finish before starting a new one, so they don't keep the TCP pipe full, so maybe that's an okay speed to expect for SMB on a 130 to 150 megabit/sec link.

If you were using a 2x2 (2 spacial stream) 802.11n client but only on traditional 20MHz-wide channels, your max data rate would be 144.4, so this sorta fits. If you were running a 1x1 (single spacial stream) 802.11n client even on 40MHz-wide channels, your max data rate would be 150 megabits/sec, so after the overhead of SMB, TCP and especially Wi-Fi, a 50 megabit/sec = 6 MebiByte/sec data rate still seems within reason (not unreasonably slow).

Also, consumer home gateway products that happen to have a USB file sharing port thrown in as an afterthought don't always contain a fast enough embedded CPU to really handle the work of file sharing at the same time as everything else (especially handling the wireless interface, which tends to be a CPU-intensive interface compared to Ethernet on most designs). So it's hard to know what those three boxes are capable of, file sharing performance-wise. You did pick three decent quality routers, but it's still hard to know.

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