Hi,
I have a wireless link that’s been running fine for about 7 days now.
This morning the link started disconnecting randomly and is very unstable at the moment.
The link consists of two SXT 5HPnD in bridge/station bridge mode, the link distance is about 1km with clear LOS.
Below are screenshots of the registration on both nodes.
I’m having some trouble interpreting the values on the registration page (mostly signal strength and signal-to-noise ratio)
Anyone willing to explain to me why the signal to noise ratio is about 50db apart on both nodes?
And why the TX value on both is about 50db apart as well, tx is transmit power right? so shouldn’t those values be nearly identical on two devices of the same model?
bridge.JPG
station bridge.JPG
Thanks!
Peter
Check Wireless > wlan1 > HT that both chains are ticked; looks like you have one end with just one chain enabled.
Thanks for the suggestion!
Both chains are ticked on both sides:
station bridge ht.JPG
Can you also explain to me how i need to read and explain the difference in strength and snr?
Thanks!
Peter
Receive signal strength is measured in dBm it’s signal you are receiving from other side.
Transmit signal strength is measured in dBm it’s signal other side is receiving from your device (reported by other device).
SNR - Signal-to-Noise ration is calculated = Received signal strength - Noise floor. The greater value the better signal. To achieve best rates at 802.11n you need at least 35dB, to achieve best rates at 802.11ac you need at least 45 dB.
Registration table depend on values reported by stations.
Sure? both SXT have all chains ticked?
The TX/RX signal stregth diference means that AP can “hear” (Rx) the station fine (in fact with a tad too powerful signal), but the station barely can hear (Rx) the AP.
You should do an spectral scan on the station, as maybe there’s a new radio transmitting in the same frequency near, “deafening” the station.
To do so you should be at the station end and issue:
/interface wireless spectral-history range=5ghz duration=30 wlan1
Bear in mind if you are at the AP end, issuing an spectral scan will drop the station link preventing you from seeing the results, so you should be connected to it via an alternative connection: either wired locally, or set a laptop connected through 3G with Teamviewer so that you can work on both AP and station even without them being wirelessly linked.
@Naurislv,
Thanks, that explains a lot!
Could you check my conclusion?
Bridge
tx -90dBm
rx -42dBm
snr 79db
The bridge is receiveing the station ok because of rx -42dBm
With a snr of 79dBm this side should have a perfect link.
The station is receiving the bridge poorly because the tx (which is actually the rx of the other side) is -90dBm
Station Bridge
tx -40dBm
rx -90dBm
snr 26dBm
The station is receiving the bridge poorly because of rx -90dBm (verified, 1 led signal strength on the back of unit)
A snr of 26 is not enough to supply a stable wifi-n link.
The bridge is receiving the station ok because the tx (which is actually the rx of the other side) is -40dBm
@pukkita
Yes double checked both ends, all two are enabled.
Thanks for the suggestion i will try this tonight!
I already snooped the wifi from winbox on the station, is this the same?
This gave no evidence of a busy channel.
I also tried changing channels but all of them present the same values give or take.
Very strange, i know for a fact that the snr leds on the station where all on when 5 days ago.
The link has clear LOS and is only 1 KM long and polarisation of both units is the same.
spectral-history is usually much more informative than a snooper.
It also could be interference from devices using the same band (have you a construction crane near?), this may (or not) show it:
/interface wireless spectral-scan show-interference=yes duration=30 range=5ghz wlan1
Post the results…
There is also the rx sensitivity of a wireless card, so at -90dBm this is completely unreliable link, don’t bother looking at SNR if your signal is that low. And if one direction has this low signal the entire link will be unstable.
Because your other direction is a very strong signal all I can think of is damaged transmitter at the ap side.
You could do a sanity check: reset wireless configuration on both SXTs and reconfigure wireless (Wireless > wlan, Reset configuration button).
So, nothing found on interference and spectral-history command.
Reset both wireless configs (ap and station) no change.
Replaced the station with a spare, no change. (So i changed back)
Replace the ap with the same spare, bam everything works again.
taylor-ap.jpg
paarden-st.JPG
Any idea what might have caused this?
Do i need to dial down the TX power?
Also, can you tell me how i can disable the lower-transmit speeds so the link becomes more stable and the link stops hopping speeds?
Thanks for your help!
CyberTod hit it! damaged Tx on the SXT5 AP side.
I think you got too powerful SXT for just 1km, I’m very keen on SXT5 Lites. I’d lower the power to get around -50dBm.
To stabilize the link what you have to do is actually the contrary, disable higher, not so stable datarates.
How do i decrease tx power?
Wireless → wlan → Wireless
Set country and after that increase antenna gain?
What gain do you recommend to start with?
Wireless → wlan click advanced mode button TX power tab. set to all rates fixed, 20-22dBm would be a good starting point.
Worked, link steady again.
Thanks for all your help!