Sending over 20 bytes via BLE

Question

I am trying to send a json string over BLE, but, like json has more than 20 bytes, I have to send several packets of 20 bytes.

I read all the posts that I found. Some examples:

• Android: sending data> 20 bytes using BLE
• Android 4.4 BLE indicator data over 20 bytes
• https://devzone.nordicsemi.com/question/155/writing-more-than-20-bytes-at-a-time-to-a-characteristic/
• https://devzone.nordicsemi.com/question/12416/android-sending-data-20-bytes-by-ble/

All messages contain similar information. The first message implements some functions, but:

• What are "BLE.INITIAL_MESSAGE_PACKET" and BLE.DEFAULT_BYTES_IN_CONTINUE_PACKET "variables", where there is an declaration of each of them?
• Where is the function "sendLastPacket (characteristic, initial_packet, CHARACTERS)"?
• On the receptor side, how can I combine all the packets sent by the transmitter?

I did not find any information about this problem. Can anyone help me?