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What's killing my LAN speeds?

post #1 of 2
Thread Starter 
Hey OCN, got a bit of a quandry for you folks. I'm running my LAN mainly off a Sagemcom combo VDSL modem, gigabit router/switch, 802.11ac AP (proprietary stuff, no spec sheet, all I know is it's called F@st 5250, AKA Bell Home Hub 2000). I have a file server setup that I have confirmed hits 120MBps (gigabit) transfer speeds over plain ethernet, but when I'm transferring files over WiFi through the router to the server, the best speed I've seen is just 20MBps (tested via both SMB and SFTP).

Only 802.11ac is enabled on the router, and I'm using an Intel 802.11ac NIC that is showing link speeds well beyond 20MBps/160Mbps. Transfer speeds to my Pi server is also gimpy, closer to 20Mbps than to the theoretical 100Mbps max.

I've checked and fiddled with everything I could think of, the traffic only goes through 802.11ac and Cat 5e, the software says everything is OK, but speeds are inexplicably garbage. What else could be wrong? Is there a good way I could diagnose the traffic to pinpoint the bottleneck?
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post #2 of 2
Thread Starter 
Well I got no help here but made some progress so maybe some of what I learned can help anyone else with a similar problem.

First of all there's a non-issue that caused a bit of confusion. Win 7 just plain simply fails to report 802.11ac connection and reports it instead as 802.11n even though the link speed is reported accurately. I had to play with driver settings and AP settings a bit to ensure the link wasn't being downgraded.

Typically, 802.11ac works with 80MHz channel bandwidth but there are three variations: 1x1, 2x2 and 3x3. This basically means the amount of discrete antennas multiplied by the amount of individual data streams (one per antenna), and that alone has a huge effect on speed. My AP supports 3x3 but the NIC only does 2x2 so a theoretical max of 867Mbps (3x3 is up to 1300Mbps) and there's some interference so my link speed typically maxes at 650Mbps. After confirming all possible wireless-related issues were sorted out (I had already confirmed no issues on the NAS end previously) , I did an iperf test to the NAS and this is the result:
Code:
------------------------------------------------------------
Client connecting to 192.168.2.10, TCP port 5001
TCP window size: 1000 KByte
------------------------------------------------------------
[  3] local 192.168.2.41 port 49801 connected with 192.168.2.10 port 5001
[ ID] Interval       Transfer     Bandwidth
[  3]  0.0-10.0 sec   338 MBytes   284 Mbits/sec

284 Mbps is only about 35 MBps, which is roughly the new average file transfer speed I'm able to get to the NAS. I thought that was still kind of garbage for a link speed of 650 Mbps, which should translate to 80MBps+, so I did more digging and came across this post on superuser.com:
Quote:
If all goes well, and they're all able to sustain the 650Mbps signaling rate that your AP tops out at, I'd expect you to see a max throughput of 260Mbps, which is 31MiB/sec.

The calculation goes like this:

Take the signaling rate (PHY rate) the devices are able to sustain.
Divide it in half since both devices are sharing the wireless medium and the AP is having to do Intra-BSS Relay, which means every packets is sent across the channel twice; once from the source device to the AP, and then again from the AP to the destination device.
Take 80% of that (i.e. multiply it by 0.8) to account for Wi-Fi overhead, assuming frame aggregation (A-MPDU).
So (650/2)*0.8 = 260Mbps. 260/8.4 = 31MiB/sec (8.4 is to convert from 10^6 bits to 2^20 Bytes).

Wow. That is pretty close to the iperf test result (except I'm dividing by 8 for MB not MiB), so I guess it confirms I am already seeing the best speeds I can expect to see, short of seeing if I can stabilize the link at 867Mbps and maybe squeeze out a few extra MBps. Even still, according to the calculations above, the maximum nominal speed for a 3x3 802.11ac link would be only 65MBps, which is still roughly HALF the speed of gigabit Ethernet!!!

Long story short, 802.11ac is totally not worth the premium! Sigh, well, it sucks but looks like gigabit WiFi is still just a pipedream after all.
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