WiFi noise issues, help me pick new antennas

[Frex]

Signal Strength goes down when xmitting [was WiFi noise issues]

Hello, I have a simple WiFi network: 1 wireless station (HTPC/NAS) with a TP-Link WN951N card, and a TP-Link WR1043ND router: different rooms, same floor, about 6m apart. WiFi is horrible behind the HTPC. Tinkering with channels, antennas and cardboard reflectors brought the signal up to 57/70, yet noise is high enough to produce bad packet loss and lag spikes. Both the router and the card have detachable antennas: 3x 3dBi on the router and 3x 2dBi card side.

My original idea was to get some cables to move the HTPC antennas out of the well, until I realized antennas are not significantly more expensive than cables (!!)

I am on a tight budget and looking to replace the minimum amount of antennas that gives me full Samba speed with no lag. These are the models that look like the best bang for the buck (there are cheaper ones indeed but shipping costs can get in the way)

• TP-Link ANT 2408C is a dipole antenna rated at 8 dBi, 1.3 m cable. 10 €/each.
• TP-Link ANT 2409N is a patch antenna, rated at 9 dBi, 1m cable, 22 €.
• Netgear Prosafe 32405, is a 3x3 5 dBi antenna, 1m cable, 55 €.

The last one is just for completeness, 3x 2408C are basically half the price. So, I was thinking about these configurations:

Router: unchanged. HTPC: 2 or 3x 2408C
Router: 1x 2408C. HTPC: 2x 2408C
Router: unchanged. HTPC: 1x 2408C + 1x 2409N

would any of them work well enough?

TIA

 

[subinsu]

It depends.
If the signal is blocked by the building material or items in the environment, using a high gain antenna may not help much. So, this will be trial and error.

Also, keep in mind that the high gain antenna just reshape the radio coverage volume, it does not increase the radio’s power. This means that you may gain greater coverage horizontally, but losing coverage vertically. Other words, you may have better signal at some locations, but poor signal at others.

 

[stevech]

RE "yet noise is high enough to produce bad packet loss and lag spikes"
I think you've mis-diagnosed the noise issue. Tell us more.

Generally, higher gain antennas just cause the noise + desired signal to both increase. But I doubt you have a noise issue.

 

[Frex]

Thank you both for the replies.

I think you've mis-diagnosed the noise issue. Tell us more.

I think you're right, noise seems fine. The stats from /proc/net/wireless are:

TV on - HTPC idling: Link Quality 45/70; Signal Level -65 dBm; Noise -256
TV off - HTPC idling: Link Quality 52/70; Signal Level -57 dBm; Noise -256
(Heavy activity on the network makes LQ take a dive. Also LQ tends to be better in the evening/at night, usually 57-60, I guess there's just less interference)

The TV is an old, large-ish CRT model and is placed directly above the HTPC. I'm fairly confident that moving the antennas on top of it would be beneficial: before the upgrade, the HTPC had a USB dongle, WiFi was next to unusable when it was plugged in the back ports. However, add an extension cord and move it above the TV, and everything was OK.

(IIRC LQ 60/70 and SS -48 dBm, but if you think actual numbers might be useful, I can plug the dongle back in and run some tests)

This means that you may gain greater coverage horizontally, but losing coverage vertically. Other words, you may have better signal at some locations, but poor signal at others.

I'm not overly concerned about that, there's only one station I really care about. Other wireless clients are the occasional guests, it's no biggie even if they don't get perfect coverage/performance.

 

[thiggins]

Get a wireless bridge and you can put it wherever you need to since it will be connected via Ethernet cable. Examples:
Linksys WET610N Wireless-N Ethernet Bridge with Dual-Band Reviewed
D-Link DAP-1522 Review: Dual-band Draft 11n for the Masses?

 

[Frex]

Thanks for the suggestion, Tim. Both are far beyond my budget, but luckily I found out that the most annoying part of the problem was actually not a WiFi issue per se. The lag spikes was caused by a driver problem, after reverting to an older version, my ping stats have improved a lot and the "Tx invalid misc" discarded packets are now 0 (previously several thousands would pile up in an hour). Finally I can stream videos with no hiccups!

However, my signal strength is still low and my Samba transfer rate suffers. So, I still need help to pick new antennas

 

[stevech]

However, my signal strength is still low and my Samba transfer rate suffers. So, I still need help to pick new antennas

In most situations, I recommend adding an access point to cure the weak signal issue. This can be an older unused wifi router reconfigured.

 

[Frex]

Steve, your post got me thinking. I went around with the router in hand to see how much can be gained by focusing only on getting the best possible signal. Of course the router ends up in places where it is unsightly and/or unpractical to use), but the results are interesting nonetheless.

The answer is from 5 to 10 dB, with most of the gain seemingly coming from increased height (although in some position going higher than some makes no difference). However, a 5 dB SS increase is only worth a few hundred kB/s in Samba performance.

I think the problem is, as soon as the HTPC starts transmitting, signal strength and link quality both go down about ten units, e.g. LQ from 60 to 50 and SS from -50 to -60 dB.

Maybe the antennas are too close to each other and to the HTPC case, and this is causing reflections and interference?

 

[stevech]

if SS means signal strength reported in units of dBm, then -50 to -60 is an excellent/ideal signal.

Beware your jargon: kB is bytes by convention; kb or Kb is bits. 8 bits in a byte. So KBps (per second) = Kbps divided by 8. And Kbps = KBps times 8

Yes, if you look at an 802.11 device's specs, you should find the raw bit rate over the air. There are a small number of standard rates, in kbps, per the IEEE standard. Absent interference, your IP layer yield will be about 60% of the raw bit rate, e.g., at best, 54Mbps times 0.6.

Antennas of two client devices can be very near one another without affect.
If a client device is very near an access point or WiFi router, there is a chance of receiver overload. Close, meaning 3 ft. or less. Most WiFi products will tolerate a signal as strong as -30dBm or so.

 

[Frex]

Something is not right. I have borrowed a brand new laptop, Atheros AR9285 card (the HTPC has an AR5416).

Wired PC -> laptop 11.76 MiB/s
Laptop -> Wired PC 11.90 MiB/s
HTPC -> Wired PC 2.8 - 4.2 MiB/s (dropping to 300-600 KiB/s on select occasions, can't figure out what's the problem..)

I placed the laptop on the floor near to the HTPC, and have timed the transfers myself using a stopwatch.

 

[Frex]

I finally found the culprit, and that is the Linux ath9k driver. I was misled by people saying online that the card works: it would seem for most that just means the card associates and doesn't drop out, regardless of performance.

After countless hours of googling I realized issues with AR5008 are sort of known of. One prominent Atheros employee advised to avoid AR5008 and AR9001 under Linux, and choose AR9002 based cards instead.

I still don't know what to make of that, since the router apparently has an integrated AR9001 and runs Linux, yet it works pretty well?

So now I'm back to square one. Finding a Wireless-N card that works well under Linux seems a daunting task. Many cards use the AR5008, for some I couldn't figure out the chipset they're using. Broadcom apparently is the worst choice and there's some Ralink but I don't know how good the driver is.

My options are: switching to wired Ethernet (royal pain, and not sure it can actually be done). Switching to Windows. Or change the network so that the HTPC is no longer a NAS.

Thanks everybody for all the help so far (and sorry for the long post)

 

[stevech]

For Linux, you could buy a low cost WiFi bridge. Connect its LAN port to the PC's ethernet port (or to a switch). Config the bridge to become a client of the WiFi network. So Linux isn't aware of the WiFi. Not so convenient for a laptop though. But there are small bridges to be had.

 

[Frex]

The bridge's an idea I may have dismissed too hastily. TP-Link has a couple APs that can be turned into bridges, the WA801 can be had for about 40 € s&h included; there's also a 3 antenna model, WA901 for about 50 €. Also, at that price point I could get a second WR1043 router and turn it into a bridge using OpenWrt/Gargoyle (WR1043 is basically a WA901 + a GbE switch, bigger Flash and an USB port)

Think this will be my plan B if it turns out switching to wires is unfeasible.