Extremely low bandwidth between 2 radio modules - Page 1
Forum section: electronics / beginners

author: epigramx
author_rank: Regular Contributor
timestamp: March 21, 2019, 04:31:43 pm
content: I seem to be getting extremely low bandwidth (with uart at 115200 and rf at 19200) when trying to receive a stream of gnss data, distance not being the problem, with two e32-ttl-100 modules. Do I miss something basic like having to change default channels?

author: Benta
author_rank: Super Contributor
timestamp: March 21, 2019, 04:40:20 pm
content: What do you mean by low bandwidth?The signal from the satellite is only 50 baud, so the utilization of a UART port is of course low.

author: epigramx
author_rank: Regular Contributor
timestamp: March 22, 2019, 03:41:00 am
content: Quote from: Benta on March 21, 2019, 04:40:20 pmNo, that has nothing to do with gnss, the satellite data are plentiful locally. I'm talking about radio linking only (the data might as well be any arbitrary text).PS. "the satellite is only 50 baud". No, that's not how gnss modules work, they combine multiple satellite data etc.

author: epigramx
author_rank: Regular Contributor
timestamp: March 22, 2019, 06:07:35 am
content: Nevermind, it turns out my problem is of a different nature entirely. Apparently my confusion came from the fact that since the radio modules have an extremely low buffer size (512 bytes if I'm not mistaken), if the UART rate is not the same with the radio rate, it usually drops data without warning!

author: helius
author_rank: Super Contributor
timestamp: March 22, 2019, 06:37:47 am
content: All data links need to implement flow control, unless the source of data is strictly less than the worst-case channel throughput.This module appears to provide an AUX pin that can be used as a CTS (clear to send) signal.

author: cgroen
author_rank: Unknown
timestamp: March 22, 2019, 11:16:11 am
content: Quote from: epigramx on March 22, 2019, 03:41:00 amWell, there is actually 50 bps data from the satellites (although its not what you refer to in your original post)....https://en.wikipedia.org/wiki/GPS_signals#Navigation_message"In addition to the PRN ranging codes, a receiver needs to know the time and position of each active satellite. GPS encodes this information into the navigation message and modulates it ontoboth the C/A and P(Y) ranging codes at 50 bit/s. The navigation message format described in this section is called LNAV data (for legacy navigation)."I know, "apples" and "oranges"...

author: Buriedcode
author_rank: Super Contributor
timestamp: March 22, 2019, 03:16:05 pm
content: According to the datasheet for those modules, the over-the-air-rate can be configured up to 19.2k. However, given the overhead of radio packets, automatic resends, channel coding and the fact it's a LoRa (spread spectrum) means the actual channel through-put will be much lower. The datasheet doesn't provide a best-case throughput, so you will have to measure this yourself. I would have thought that 2.4k is probably the max.

author: tszaboo
author_rank: Super Contributor
timestamp: March 22, 2019, 04:36:55 pm
content: With LoRa, you can only transmit like 1% of the time, otherwise you violate spec.Some people think the name comes fromLongRange, while it comes from:low datarate, still bigger battery than you think, and network is built next year.

author: epigramx
author_rank: Regular Contributor
timestamp: March 23, 2019, 12:56:59 pm
content: Apparently my modules are just extremely low bandwidth by design. By the way they also have an error correction mode on by default which is an extreme hog so I guess one could disable it to achieve something more reasonable (e.g. 9600bps seems workable now for the data I need it for, though I want to lower it to 4800 if possible).PS. regarding their "long range", while the tech is that, I doubt this particular model is practically very long range since it's only the 100mW model.

author: Buriedcode
author_rank: Super Contributor
timestamp: March 23, 2019, 02:48:57 pm
content: I thought Lora modules with 100mW output could do a couple of km? I know the spread-spectrum adds a fair bit of coding gain at the cost of through-put.

author: epigramx
author_rank: Regular Contributor
timestamp: March 24, 2019, 07:58:07 am
content: Quote from: Buriedcode on March 23, 2019, 02:48:57 pmYeah I mean it's partly marketing though. e.g. for this model they advertise 3000 meters but they also mention in the small print, if the antenna is 5dbi (small ones people use are barely 2.5dbi) and at 2 meters height, and with no obstacles in view.I'll be happy if I get 500 meters with average equipment to be honest when some minor/medium obstacles are inbetween.[Ah, and they say if the bandwidth of air com is 2.4kb(bit!)s which is extremely low for many applications.]

author: epigramx
author_rank: Regular Contributor
timestamp: March 24, 2019, 08:30:14 am
content: Quote from: helius on March 22, 2019, 06:37:47 amThanks for that info, I was wondering what that AUX shenanigans was for and most people dismiss it with "just leave it floating". However, it's not entirely unreasonable to leave it unused if a specific application has determined in most realistic scenarios won't have congestion issues (though I understand it's best for something commercial to not leave it unused).[PS. In my particular personal project it won't have an important use beyond initial testing because the bandwidth required is relatively reproducible and with a max that is relatively constant, and in any case I would have to alter the sender software which is a third party creation and I'm not willing to delve into that.][Actually, it's pretty cool to use it for a led indicator, so there's that.]