Sometimes high latency in wireless network

jekakm · Wed Mar 26, 2014 11:41 am

Hi to all! I have Mikrotik RB2011UAS-2HnD. With 30 wireless clients, traffic is approximately 20 Mbps for all. Channel to Internet is around 80 Mbps. The most time it works good, but sometimes latency goes to 8-10 seconds(usual it's not more than 40 ms)! I tried to change wireless channel, tune wireless option in mikrotik without any luck.
Now I install old dlink 320 in this network and it's works satisfactorily.

My current configuration:

2 X  name="wlan1" mtu=1500 mac-address=D4:CA:6D:70:F1:B3 arp=enabled disable-running-check=no interface-type=Atheros AR9300 radio-name="D4CA6D70F1B3" mode=ap-bridge ssid="1111" area="" frequency-mode=superchannel 
      country=no_country_set antenna-gain=0 frequency=2437 band=2ghz-b/g channel-width=20mhz scan-list=default wireless-protocol=802.11 rate-set=default supported-rates-b=1Mbps,2Mbps,5.5Mbps,11Mbps 
      supported-rates-a/g=6Mbps,9Mbps,12Mbps,18Mbps,24Mbps,36Mbps,48Mbps,54Mbps basic-rates-b=1Mbps basic-rates-a/g=6Mbps max-station-count=2007 distance=indoors tx-power=17 tx-power-mode=all-rates-fixed noise-floor-threshold=default 
      nv2-noise-floor-offset=default periodic-calibration=enabled periodic-calibration-interval=60 dfs-mode=none wds-mode=disabled wds-default-bridge=none wds-default-cost=100 wds-cost-range=50-150 wds-ignore-ssid=no 
      update-stats-interval=disabled bridge-mode=enabled default-authentication=yes default-forwarding=yes default-ap-tx-limit=0 default-client-tx-limit=0 proprietary-extensions=post-2.9.25 wmm-support=disabled hide-ssid=no 
      security-profile=default disconnect-timeout=15s on-fail-retry-time=100ms preamble-mode=both compression=no allow-sharedkey=no station-bridge-clone-mac=00:00:00:00:00:00 ht-ampdu-priorities=0 ht-guard-interval=any 
      ht-supported-mcs=mcs-0,mcs-1,mcs-2,mcs-3,mcs-4,mcs-5,mcs-6,mcs-7,mcs-8,mcs-9,mcs-10,mcs-11,mcs-12,mcs-13,mcs-14,mcs-15,mcs-16,mcs-17,mcs-18,mcs-19,mcs-20,mcs-21,mcs-22,mcs-23 
      ht-basic-mcs=mcs-0,mcs-1,mcs-2,mcs-3,mcs-4,mcs-5,mcs-6,mcs-7 ht-txchains=0,1 ht-rxchains=0,1 ht-amsdu-limit=8192 ht-amsdu-threshold=8192 tdma-period-size=2 nv2-queue-count=2 nv2-qos=default nv2-cell-radius=30 
      nv2-security=disabled nv2-preshared-key="" hw-retries=7 frame-lifetime=0 adaptive-noise-immunity=ap-and-client-mode hw-fragmentation-threshold=disabled hw-protection-mode=rts-cts hw-protection-threshold=0 frequency-offset=0 
      rate-selection=advanced multicast-helper=disabled multicast-buffering=enabled

Could someone help me with it, please?

jekakm · Wed Mar 26, 2014 11:50 am

And TX/RX CCQ > 90% for all client's.

CyberTod · Wed Mar 26, 2014 1:09 pm

Check the bitrates of clients when this happens, especially rx bitrate. Sometimes a client's rx rate will go to 1mbit or something very low and this will bring down the entire ap if he is uploading (torrents).

rmichael · Wed Mar 26, 2014 3:21 pm

Change disconnect-timeout to 1s, hardware retries to 2 and watch for stations that disconnect - those are the ones causing latency (by making AP retransmit for a long time). It could also be ROS bug - which version are you running?

jekakm · Tue Apr 01, 2014 1:18 pm

Change disconnect-timeout to 1s, hardware retries to 2 and watch for stations that disconnect - those are the ones causing latency (by making AP retransmit for a long time). It could also be ROS bug - which version are you running?

I check bitrates and change disconnect-timeout and hwretries - it's not help. Now I'm using 6.11, I will try contact with support, maybe it's real some bug in ROS.

CyberTod · Tue Apr 01, 2014 1:52 pm

Don't always assume there is a bug in ros. 30 clients with 802.11 protocol is a lot and if some of them are not connected with good quality these things are undesirable, but normal.
So as I said check bitrates and ccq when the problem occurs.

jekakm · Tue Apr 01, 2014 2:12 pm

Don't always assume there is a bug in ros. 30 clients with 802.11 protocol is a lot and if some of them are not connected with good quality these things are undesirable, but normal.
So as I said check bitrates and ccq when the problem occurs.

Yeah, but I can't setup ethernet here.

I check this, minimum rx bitrate when problem present is 24 Mbps and TX/RX CCQ > 90%

CyberTod · Tue Apr 01, 2014 4:43 pm

I'm assuming here this is an outdoor installation. Maybe there are some hidden node issues.
Try setting rts/cts on clients and cts to self on the mikrotik ap.

rmichael · Tue Apr 01, 2014 5:14 pm

Change disconnect-timeout to 1s, hardware retries to 2 and watch for stations that disconnect - those are the ones causing latency (by making AP retransmit for a long time). It could also be ROS bug - which version are you running?
I check bitrates and change disconnect-timeout and hwretries - it's not help. Now I'm using 6.11, I will try contact with support, maybe it's real some bug in ROS.

It's not supposed to help only pinpoint where problem is. After you've made a change didn't you notice higher disconnect rate of one (or few) stations?

jekakm · Wed Apr 02, 2014 10:31 am

I'm assuming here this is an outdoor installation. Maybe there are some hidden node issues.
Try setting rts/cts on clients and cts to self on the mikrotik ap.

I tried enable this option(look at configuration on first message), it's also not helps. Also I enable adaptive-noise-immunity option.

jekakm · Wed Apr 02, 2014 10:34 am

It's not supposed to help only pinpoint where problem is. After you've made a change didn't you notice higher disconnect rate of one (or few) stations?

I got it, unfortunately, no one client was disconnected when latency goes high again.

CyberTod · Thu Apr 03, 2014 9:21 am

I've looked at the configuration, but you have set rts/cts on the ap. That is wrong. Basically is has to ask before sending data, so some other device will tell him to wait and then you will have this exact problem with the high latency. By using 'cts to self' the ap tells itself it can send data so you minimize the effect of someone else using the air. The rts/cts option should be used on all clients, because when they cannot hear each other they can try send at the same time, causing collisions and again causing high latency.
Also you need to set some threshold for the packet size. I'm using 512, but this is something you need to tune. Some use 256. If the packet is too big it will rarely send the frame, but if too small it will cause too much overhead, so it's a tradeoff.

jekakm · Thu Apr 03, 2014 12:57 pm

I've looked at the configuration, but you have set rts/cts on the ap. That is wrong. Basically is has to ask before sending data, so some other device will tell him to wait and then you will have this exact problem with the high latency. By using 'cts to self' the ap tells itself it can send data so you minimize the effect of someone else using the air. The rts/cts option should be used on all clients, because when they cannot hear each other they can try send at the same time, causing collisions and again causing high latency.
Also you need to set some threshold for the packet size. I'm using 512, but this is something you need to tune. Some use 256. If the packet is too big it will rarely send the frame, but if too small it will cause too much overhead, so it's a tradeoff.

Oh, got it about cts to self, thank you. One strange problem with hw-fragmentation-threshold - when I set it to 512 the part of clients can't send big packets. 64bytes icmp - ok, but if I try send 1500 for example - no reply. So this client can't load page via http for example. All this clients has RT3290 wireless chip, maybe it's some driver trouble.

CyberTod · Thu Apr 03, 2014 2:29 pm

No, no. I didn't meant that. Leave the fragmentation threshold to default. You don't want to fragment packets.
What I meant was this property : hw-protection-threshold

Sometimes high latency in wireless network

Sometimes high latency in wireless network

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