[OpenWrt-Devel] Alternatives do TDMA

[bkil]

Dear Fernando,

You should have posted this question to OpenWrt-User, but I will answer it here.

I haven't personally deployed such a configuration, yet. I don't think
you can do much besides enabling RTS/CTS at every CPE (client). Much
fewer connected clients will be supported compared to a TDMA system.

However, here are some other non-default settings you could test:
* coverage class/distance optimization
* try narrow 5-10MHz channels in case of a crowded neighborhood -
sometimes less is more
* if link speed is already maxed out for the closest nodes, you may
try to reduce their tx power while maintaining the link speed and
error rate, though I wouldn't expect much effect
* after you've measured the link margin and its fading characteristic
at each of your clients, you could consider increasing the mandatory
basic_rate and mcast_rate to reduce airtime a bit more
* you could experiment with increasing the beacon interval, though
each station should already sync and avoid interference with those,
and this could reduce stability
* you may increase dtim_period a bit - again not much effect
* consider blocking most kinds of broadcast/multicast packets if your
network doesn't need it
* compared to AP mode, 802.11s mesh mode has various promising
techniques for precise node coordination and time slot reservation in
the standard which may or may not have been implemented, so you should
have a look
* RTS/CTS should be enough, but another option would be to reduce max
packet size (fragmentation threshold), which will also gravely reduce
your throughput
* you may reduce the number of retries as a last resort and hope for
the upper layers to limit rate (black magic)
...

Hardware considerations:
* use good directional or sector antennas and/or shielding at the base
to reduce noise from the surrounding buildings
* 5GHz is less crowded
* the best solution would be to insert a few intermediary nodes to
form a mesh instead of a star topology - unslotted and uncoordinated
medium access has its limits
* or at least offload the clients to multiple hotspots operated at the
same location, but using different frequencies or polarization

Note that not all options are displayed on Luci, but you could add
them to /etc/config/wireless manually (some could require capability
overrides):
http://wiki.openwrt.org/doc/uci/wireless

An interesting hack come to mind. What if we turned the situation
around? You could operate each CPE in AP mode with a very long beacon
interval. The portal (gateway) itself would operate in multi-STA.
After some u-APSD/PS-POLL tweaking, you could power save on all but
one AP similar to how it's done by default on multi-frequency
multi-STA. The portal would essentially unmute a single CPE at a time
in a round-robin fashion. It sounds a bit quirky and it would surprise
me if the solution scaled beyond a few dozen CPEs, but it would
enforce a kind of TDMA and it might theoretically eliminate collisions
without RTS/CTS if that is your thing. Bandwidth utilization and
latency would leave much to be desired, however.

I'm all into radio propagation, so please do share your views and
findings about this question.

Regards

> Hi guys,
>
>What is the best alternative to TDMA when using OpenWRT and Outdoor /
>PtMP access ? Any specific configuration to be done in OpenWRT in order
>to deal with multiple clients in different ranges ?
>
>Thanks
>Fernando
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