DevAddr Prefix / Duplicated DevAddrs

Question

Hello,

i have a question regarding to DevAddr Prefix in de Chirpstack Configuration.

Regarding to this old post https://forum.chirpstack.io/t/is-the-device-address-automatically-generated-when-a-node-joins/10924/12 it is normal, to have dublicated devAddrs if you have many devices.

myself, i have about 10'000 Devices (with the plan to have arround 20'000) - and never had duplicated devAddrs. A few days ago i started with DevAddr Prefix /17 - which should give me arround 32'000 DevAddrs. Now i started rejoining the devices, and i have now a few duplicated devAddrs. Most used devAddr is used 11 times from different devices.

now my question:

why should i use a devAddr Prefix with 32'000 Addresses, if they duplicate anyway?
does it have any performance issues, when MIC is needed to find the correct Device?
can i just use /24 (for example), so i have only 256 devAddrs. I will have a lot of duplicated devAddrs, but can filter a smaler range on the Gateways.
whats the best practice for devAddr Prefix? should it be as high as planed devices, or not? what do you guys use there?

thanks for your feedback,
Sil

Orne Brocaar · Answer

Generally, the DevAddr prefix is based on the configured NetID. Unless there is an use-case for it, you should use the full DevAddr space. However, there are use-cases where you want to limit the DevAddr space.

E.g. in case of integrating with Helium, it is possible to configure the Helium NetID, but only use the DevAddr space that you have acquired from Helium (you need to buy a block of DevAddrs). In that case you would only use a sub-set of the available DevAddrs from the (Helium) NetID.

Note that a LoRa Alliance assigned NetID has a NetID Type, and each Type has a certain prefix length / DevAddr space.

When there are duplicates, it means ChirpStack needs to do more cryptographic work to correlate the DevAddr with the correct device in the database (it will fetch more records from the database / it needs to calculate the MIC multiple times).

DevAddr Prefix / Duplicated DevAddrs

1 Answers