Sector performances

qwerty00 · June 8, 2017, 9:16am

Hello,

I have a theoretical question to ask you:
in a situation where I have different clients at maximum modulation, if I add one client at a lower modulation, the entire sector decreases its performances.
Is it right?
If it is so, why it happens?

Thank you

ste · June 8, 2017, 12:18pm

In an ideal world sector should only decrease performance when this lower modulation client communicates. This is due to the fact that this client needs more airtime for the same amount of datatransfer. In reality we see performance decrease with higher number of cpes connected idle. This is due to weakness of the TDMA-Protocol handling the scheduling of packets.

qwerty00 · June 9, 2017, 3:55pm

Hello,

thank you for the answer.

By setting the TDMA period size of the NV2, are the time slots allocated for each clients the same, regardless the modulation?

juniores · June 16, 2017, 1:31pm

Any news ? Can I set 1 ms? Will I have a problem? Or do I leave it automatic?

honzam · June 16, 2017, 4:43pm

Yes, you can.

tdma-period-size - specifies size in ms of time periods that Nv2 AP uses for media access scheduling. Smaller period can potentially decrease latency (because AP can assign time for client sooner), but will increase protocol overhead and therefore decrease throughput. On the other hand - increasing period will increase throughput but also increase latency. It may be required to increase this value for especially long links to get acceptable throughput. This necessity can be caused by the fact that there is “propagation gap” between downlink (from AP to clients) and uplink (from clients to AP) data during which no data transfer is happening. This gap is necessary because client must receive last frame from AP - this happens after propagation delay after AP’s transmission, and only then client can transmit - as a result frame from client arrives at AP after propagation delay after client’s transmission (so the gap is propagation delay times two). The longer the distance, the bigger is necessary propagation gap in every period. If propagation gap takes significant portion of period, actual throughput may become unacceptable and period size should get increased at the expense of increased latency. Basically value of this setting must be carefully chosen to maximize throughput but also to keep latency at acceptable levels.
https://wiki.mikrotik.com/wiki/Manual:Nv2

juniores · June 16, 2017, 5:15pm

When automatic set does it alone? I think it’s the best option. Right ?

qwerty00 · June 21, 2017, 5:24pm

honzam:

Yes, you can.

tdma-period-size - specifies size in ms of time periods that Nv2 AP uses for media access scheduling. Smaller period can potentially decrease latency (because AP can assign time for client sooner), but will increase protocol overhead and therefore decrease throughput. On the other hand - increasing period will increase throughput but also increase latency. It may be required to increase this value for especially long links to get acceptable throughput. This necessity can be caused by the fact that there is “propagation gap” between downlink (from AP to clients) and uplink (from clients to AP) data during which no data transfer is happening. This gap is necessary because client must receive last frame from AP - this happens after propagation delay after AP’s transmission, and only then client can transmit - as a result frame from client arrives at AP after propagation delay after client’s transmission (so the gap is propagation delay times two). The longer the distance, the bigger is necessary propagation gap in every period. If propagation gap takes significant portion of period, actual throughput may become unacceptable and period size should get increased at the expense of increased latency. Basically value of this setting must be carefully chosen to maximize throughput but also to keep latency at acceptable levels.
https://wiki.mikrotik.com/wiki/Manual:Nv2

So, if I set manually the period size, should I have anyway the decrease of performances due to the low modulation of some clients?