I'm implementing a bacnet library to read and write some object instances. But I don't understand, why with a PC and with an application like Yabe if I ask to a common controller to read more than 300 objects, the segmentation is supported, instead with a mobile phone and on the same request to the same controller the segmentation is not supported. Both the PC and the smartphone are in the same network on Wifi. So what is the problem/difference?
It is nothing to take with a PC or a mobile phone. Any IP compatible devices mobile phone, desktop, laptop, Arduino chips will work with BACnet. Your problem is segmentation. You will have to handle the segmented messages coming from the Controller. Every BACnet controller vendor specifies the MaxApduLength (i.e max length of data to be transferred within one UDP packet). The standard APDU length is 1476 bytes. So a BACnet controller cannot send the data more than the length of APDU specified within it.
Requesting 300 objects from a controller results definitely in a large APDU than the controller's limit. In this case Controller will send you segmented messages each with sequence number. You will have to handle this all segmented messages and combine them in one message then only you will get exact response. This is same with Yabe and other BACnet clients.
I have also written BACnet libraries for our mobile applications in Java and Swift 4.0 and both can read any number of objects from controller with segmentation support added. I have tested these with controllers having more than 400 objects.