Azimout's Linux weblog

802.11n speed

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Transferring large files (350MB) from my desktop to my wife’s netbook reaches a maximum speed of 3.5 – 4.0 MB/s, which is far less than what I was hoping for (802.11n claims a maximum of 600 Mbps, i.e. 75 MB/s), so I decided to investigate.

  • The netbook (Asus EeePC 1005HA) has an Atheros AR9285 wireless chip, which only seems to do a single spatial stream. Even in the optimal case (40 MHz channel, 400ns guard interval), this lowers my ceiling from 600 Mbps (MCS=31) to 150 Mbps (MCS=7), i.e. 18.75 MB/s.
  • Then I discovered my desktop was connected to the router at 100Mbps, not 1Gbps as I expected. This is because my motherboard is crappier than I thought: the Asus M2N-MX SE Plus has the NVIDIA MCP61P chipset, which only does fast ethernet. So my ceiling goes down to 100Mbps, or 12.5 MB/s
  • I haven’t been able to find numbers on WPA2 bandwidth overhead, but I used nttcp to benchmark the network speed between the two machines, and got 65-70 Mbps, i.e. 8-9 MB/s. By default nttcp transfers 8MB, but transferring more data had no effect on the result.

This is still more than double the actual transfer rates I get, but at least it shed some light on my problem.

(For reference, if I tell my router to do mixed 802.11b-802.11g, nttcp gives me 23 Mbps, so with 802.11n it’s still 3 times faster)


Written by azimout

18/10/2010 at 15:38

Posted in Problems

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