[OpenWrt-Devel] Alternatives do TDMA

Fernando Frediani fhfrediani at gmail.com
Mon Feb 16 18:07:48 EST 2015

Hello bkil,

Many thanks for your detailed response.
I would gladly post it to openwrt-users if that worked, which doesn't 
seem to be the case as far as I know.

But also taking the opportunity in this devel list to ask if anyone 
worked of ever saw any work to develop a open TDMA implementation that 
could be merged to OpenWRT. I personally have read a while ago about 
some material that was developed for FreeBSD, but there wasn't much 
information really and no much other than that I could find unfortunately.

Regarding your response I was particularly interested in the RTS/CTS 
configuration and hear about optimal RTS Threshold values.
Also does that AP and Clients have to have exactly same RTS/CTS 
configuration and RTS Threshold values or only the AP is enough ?
This is more common in WISP providers, but would that be also for 
example in internal areas with many clients (e.g: a conference) where 
the clients aren't aware about having to enable the RTS/CTS protection 
and eventually the threshold ?


On 16/02/2015 19:24, bkil wrote:
> 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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