In BLE Mesh network, about limitation of node number

[Vithan]

Hi, I have some issue about mesh network.

Using below developing environment.

- SEGGER Embedded studio for ARM V4.22

- nRF SDK 15.3.0

- nRF Mesh SDK 3.2.0

- chip : nRF52832

- s132_nrf52_6.1.1_softdevice

I developed light control sensor module using mesh example 'light_switch_server'.

Using one provisioner (Samsung GALAXY S9+ and nRF Mesh APP) and configure publish/subscribe one group '0xC000'.

When I did test about mesh networking with 40 module(node), mesh communication was fine.

But, the number of module(node) is above 40+, there some problem was occurred.

Some module(node) could not receive mesh message(or publishing message). Is there any reason? or I have to change some of SDK config?

[Vithan]

I found this post : https://devzone.nordicsemi.com/f/nordic-q-a/48966/about-number-of-nodes-that-can-participate-in-the-mesh-network/196467#196467

In "nrf_mesh_config_core.h", "REPLAY_CACHE_ENTRIES 40", this parameter is related to my issue.

So, I want to know how to calculate the parameter in my project, and limitation.

I plan to produce above 3000 modules.

[Hung Bui]

Hi Vithan, 
You are right, the REPLAY_CACHE_ENTRIES will limit the number of unique address you can receive data from. You can increase this number, the trade off is the bigger size of m_replay_cache table ( 8 bytes/entry) and the longer time it take for the node to check the list to avoid replay attack. 

However, would all of your nodes require high number of replay cache ? Meaning would all the nodes receive packets from all other nodes ? Or it will be some of them (for example the gateway, the client) will receive the packet from more nodes, and others will receive command/data from a handful of nodes ? 
Note that this configuration doesn't affect the relay capability. 

[Vithan]

Yes, all the nodes receive packets from all other nodes.

I tried some test, I think it would be only affected nrf52832 RAM usage.

If It was right, how much I can use RAM space? (I set 'REPLAY_CACHE_ENTRIES 2000', the free space is 16.5KB of 60KB.)

[Hung Bui]

As I mentioned, it will be 8 bytes / entry. So for 2000 node it requires 16000 bytes in RAM.