I'm doing a feasibility study on a proposed wireless/wired network topology for a coming project of ours. A rough description of its use is "like an industrial sensor system". It looks something like this:
+----+ +----------+ +------| AP | - - RF - - | Endpoint | | +----+ | +----------+ Ethernet | +--------+ +--------+ | +----+ | +----------+ | Server |-- Ethernet --| Router |---+------| AP | + - - | Endpoint | +--------+ +--------+ | +----+ | +----------+ | | | +----+ | +----------+ +------| AP | + - - | Endpoint | | +----+ +----------+ | | +----+ +------| AP | +----+
The Server and Access Points (of which there might be hundreds) talks IP over ethernet. The Acces Points talks proprietary RF with the Endpoints (of which there might be thousands per Access Point). The Access Points have infinite power, but the Endpoints runs on batteries, so we need to keep their draining scenarios to a minimum. Anyway, that's not the question.
In this study I've done a simulation of the system interaction on the Access-Point-to-Endpoint part to find radio congestion situations, optimal update intervals for the Endpoints and more to get as long battery life time as possible given the response time requirements of the RF endpoints.
The simulation is basically a time-stepping scheme where all states of this Endpoint cycle are stepped a time unit for each Endpoint in the simulation (hundreds to thousands): sleeping, starting the MCU, starting the radio, performing clear channel assessement, backing off, transmitting a payload of predefined size to the Access Point, waiting for the radio to turn over to receiving, going back to sleep.
We have a pretty solid understanding of all timings involved in the above situation where real RF is involved. Not so for the Ethernet part, however.
When an Access Point receive a payload from an Endpoint, it'll repack it and send it to the server, which will give back a reply to the Access Point with minimal payload, a payload that is repacked in the Access Point before transmitting it to the Endpoint.
Question: What order of magnitude can we expect for the time it'll take for the server query/reply, from the perspective of the Access Point? We can assume that the server is "close" to the router and Access Points, no Internet requests will be involved.
I realize that the question might be extremely under-specified, so please chime in with anything needed to give a rough guesstimate.