LoRa nodes and simultaneously communication with LoRa Gateway

[sairfan1]

I'm learning LoRa did not get answer to a question

For example if I have a Gateway that supports 8 nodes in parallel (node could be SX1276/78 or any other node, or you can recommend some for such task)
I have 50 nodes to send data to Gateway
What will happen if more than 8 nodes sends data to Gateway at the same time,
Is there some mechanism that can handle such situation in LoRa devices/protocol

My question is not regarding any project, its only about concept, that's why I'm not mentioning any specific gateway or node devices, if its something necessary you can assume some most suitable for the task

Once i understand it enough then i will go further to buy devices and develop proof of concept
thanks,

 

[kripacharya]

Simultaneous transmission by multiple nodes is a common situation in communication networks.

There are some standard ways to resolve this. I do not know which method LoRa has implented.

1) do not allow contention. Instead each node is polled by a master and either responds then or returns a Nack.
2) nodes transmit whenever they want, but also listen to ensure the transmission did not clash. If it did, then wait for a random time and try again. (CSMA Method)
3) repeatedly resend (with some delay) until receive an Ack. (Aloha)
4) check if there is ongoing transmission. Send only when there is none, along with random delay (CSMA/CD)
5) maybe others I am not listed, like pre-defined transmit slots.(TDM/TDMA)

These are all DataLink layer protocols. You can check which one LoRa implements
It could be a combination, or also make use of SpreadSpectrum methods, which are similar to Frequency Division methods.

 

[andre_luis]

What will happen if more than 8 nodes sends data to Gateway at the same time,
Is there some mechanism that can handle such situation in LoRa devices/protocol

Collisions are not intrinsically handled with LoRa protocol, which means that proper packet management have to be externaly done by end application at each node in order to ensure that all command issued were acquired at the target. If some paket loss is acceptable, there are studies regarding the probability of their occurrence in function of the signal strenght and the number of nodes, which obviously does not cover all possibilities. So an approach that offers a reasonable reliability is the allocation in time at which each module wakes up and goes into the receiving mode; nedless to say that time accuracy is essential and a setup procedure is needed to be accomplished every time a new device is detected on the mesh. I did not have the opportunity to work with LoRa in the mesh topology, but if I'm not wrong, the Xbee seems a little more robust in this sense since it has an option to detect another channel, avoiding collision.

 

[kripacharya]

Yes that's correct. The LoRa per se itself only specifies the Physical layer. It requires some form of Datalink Layer to manage contentions/ collisions.

LoRa allows much larger distances than Xbee. Or so I am told.