WISP Network bottlenecks – Read the text below or watch the 5-minute video to learn more.
Almost all of the Quality of Experience (QoE) issues that we see in fixed wireless setups are due to some kind of network bottlenecks. These network bottlenecks can be at the access points, backhaul, individual subscribers, routers or switches. Wherever you have some kind of network bottlenecks, that’ll either add latency or cause some kind of packet loss and thereby it impacts the subscriber’s QoE.
There are two key themes when it comes to network bottlenecks. The very simple way is you have a network bottleneck, any packet that doesn’t make it, just drop them off. This is kind of the least graceful way to do it but that’s often what you get for example if you have network congestion on a certain pipe but no shaper in front of it, that’s exactly the behavior you see. In this case, no latency is added but it results in a very high packet loss. The main issue with this is that just by dropping packets, it doesn’t give TCP a chance to slow down and start behaving the way it’s supposed to. The other issue is as TCP notices these dropped packets, you get all kinds of retransmits thus adding extra traffic on your network and making things more difficult.
Examples of this behavior we have seen are at networks where –
- Cambium 450s are used for plan enforcement
- Backhaul congestion exists
- Links/ports can’t keep up with the traffic requirements
The other mechanism is all around buffering. Basically, as you have the network bottleneck and all the traffic can’t make it through, the shaper will simply buffer some of the traffic thereby adding latency and causing TCP to back off. This can work very effectively if you have a shaper that isn’t too deep and just buffers it a little bit and smooth out bursty behavior. The thing to watch out is if this buffer gets too big, it’ll add way too much latency, thereby impacting QoE.
As a WISP, you should aim to fine tune this piece. Different kind of shapers behave differently. Examples that we have seen across wireless networks where network bottlenecks cause buffering are –
- Access point congestion – adding up to 100s of ms of latency
- Interference on your APs
- On the cambium side, if you have an EPMP for plan enforcement, it’ll add too much latency
- Software mismatch and more
The common theme in this discussion is about latency and packet loss. These are both very critical to your customer’s QoE. Higher latency will result in poor VoIP, real-time gaming, and even unresponsive/slow browsing. Putting it concretely, to have a good VoIP call, you need a round trip time of under 150 ms. That’s from phone to phone and back again – all the way across your network, the internet, and the other party’s network. As a WISP, you’re only controlling half of it so you want your network to stay under 75 ms to give rest of the network to have a chance to add latency and yet have a good experience for the subscriber.
On the packet loss side, lost packets get retransmitted. The overall effect beyond that is similar to latency but it also adds jitter and extra network traffic. We’ve seen that 2-4% range for TCP retransmit is pretty normal. If this goes over 5%, you know that your network needs some careful examination and fixes.
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