Android: Sending data >20 bytes by BLE
There are a lot of misleads here.
BLE is capable of sending much more than 20 bytes, and it can be done easily in android.
What you need to change is the link MTU that is set to 23 by default(only 20 of them can be used to set a value).
Android provides fragmentation mechanism if the given packet to send is larger than the current link MTU(this is the purpose of the offset parameter in the onCharacteristicRead(...) API).
So you can make the MTU bigger, as a request from the central using: requestMtu(...) API. The latter will cause a callback call onMtuChanged at the peripheral side which will inform him of the new MTU.
After this action is done, you can send bigger packets without issuing the Android fragmentation mechanism.
The alternatives are to build yourself your own fragmetation mechanism and not to send packets that are bigger then the MTU. Or rely on the Android mechanism and work with it using the 'offset' parameter.
BLE allows you transfer maximum of 20 bytes.
If you want to send more than 20 bytes, you should define array byte[] to contain how many packets you want.
Following example worked fine if you want to send less than 160 characters (160 bytes).
p/s : Let edit following as you want. Do not follow me exactly.
Actually, when we are using BLE, mobile side and firmware side need to set up the Key (ex. 0x03 ...) to define the connection gate among both sides.
The idea is :
When we continue to transfer packets, not is the last one. The gate is
byte[1] = 0x01.If we send the last one, The gate is
byte[1] = 0x00.
The data contruction (20 bytes):
1 - Byte 1 - Define the Gate ID : ex. Message gate ID byte[0] = 0x03.
2 - Byte 2 - Define the recognization : Is the last packet 0x00 or continue sending packets 0x01.
3 - Byte 3 (Should be 18 bytes after eliminating Byte 1 & Byte 2) - Attach the message content in here.
please understand my logic before reading the code below.
Below is an example of sending a Message with many packets, each packet is an array of size 20 bytes.
private void sendMessage(BluetoothGattCharacteristic characteristic, String CHARACTERS){
byte[] initial_packet = new byte[3];
/**
* Indicate byte
*/
initial_packet[0] = BLE.INITIAL_MESSAGE_PACKET;
if (Long.valueOf(
String.valueOf(CHARACTERS.length() + initial_packet.length))
> BLE.DEFAULT_BYTES_VIA_BLE) {
sendingContinuePacket(characteristic, initial_packet, CHARACTERS);
} else {
sendingLastPacket(characteristic, initial_packet, CHARACTERS);
}
}
private void sendingContinuePacket(BluetoothGattCharacteristic characteristic,
byte[] initial_packet, String CHARACTERS){
/**
* TODO If data length > Default data can sent via BLE : 20 bytes
*/
// Check the data length is large how many times with Default Data (BLE)
int times = Byte.valueOf(String.valueOf(
CHARACTERS.length() / BLE.DEFAULT_BYTES_IN_CONTINUE_PACKET));
Log.i(TAG, "CHARACTERS.length() " + CHARACTERS.length());
Log.i(TAG, "times " + times);
// TODO
// 100 : Success
// 101 : Error
byte[] sending_continue_hex = new byte[BLE.DEFAULT_BYTES_IN_CONTINUE_PACKET];
for (int time = 0; time <= times; time++) {
/**
* Wait second before sending continue packet
*/
try {
Thread.sleep(200);
} catch (InterruptedException e) {
e.printStackTrace();
}
if (time == times) {
Log.i(TAG, "LAST PACKET ");
/**
* If you do not have enough characters to send continue packet,
* This is the last packet that will be sent to the band
*/
/**
* Packet length byte :
*/
/**
* Length of last packet
*/
int character_length = CHARACTERS.length()
- BLE.DEFAULT_BYTES_IN_CONTINUE_PACKET*times;
initial_packet[1] = Byte.valueOf(String.valueOf(character_length
+ BLE.INITIAL_MESSAGE_PACKET_LENGTH));
initial_packet[2] = BLE.SENDING_LAST_PACKET;
Log.i(TAG, "character_length " + character_length);
/**
* Message
*/
// Hex file
byte[] sending_last_hex = new byte[character_length];
// Hex file : Get next bytes
for (int i = 0; i < sending_last_hex.length; i++) {
sending_last_hex[i] =
CHARACTERS.getBytes()[sending_continue_hex.length*time + i];
}
// Merge byte[]
byte[] last_packet =
new byte[character_length + BLE.INITIAL_MESSAGE_PACKET_LENGTH];
System.arraycopy(initial_packet, 0, last_packet,
0, initial_packet.length);
System.arraycopy(sending_last_hex, 0, last_packet,
initial_packet.length, sending_last_hex.length);
// Set value for characteristic
characteristic.setValue(last_packet);
} else {
Log.i(TAG, "CONTINUE PACKET ");
/**
* If you have enough characters to send continue packet,
* This is the continue packet that will be sent to the band
*/
/**
* Packet length byte
*/
int character_length = sending_continue_hex.length;
/**
* TODO Default Length : 20 Bytes
*/
initial_packet[1] = Byte.valueOf(String.valueOf(
character_length + BLE.INITIAL_MESSAGE_PACKET_LENGTH));
/**
* If sent data length > 20 bytes (Default : BLE allow send 20 bytes one time)
* -> set 01 : continue sending next packet
* else or if after sent until data length < 20 bytes
* -> set 00 : last packet
*/
initial_packet[2] = BLE.SENDING_CONTINUE_PACKET;
/**
* Message
*/
// Hex file : Get first 17 bytes
for (int i = 0; i < sending_continue_hex.length; i++) {
Log.i(TAG, "Send stt : "
+ (sending_continue_hex.length*time + i));
// Get next bytes
sending_continue_hex[i] =
CHARACTERS.getBytes()[sending_continue_hex.length*time + i];
}
// Merge byte[]
byte[] sending_continue_packet =
new byte[character_length + BLE.INITIAL_MESSAGE_PACKET_LENGTH];
System.arraycopy(initial_packet, 0, sending_continue_packet,
0, initial_packet.length);
System.arraycopy(sending_continue_hex, 0, sending_continue_packet,
initial_packet.length, sending_continue_hex.length);
// Set value for characteristic
characteristic.setValue(sending_continue_packet);
}
// Write characteristic via BLE
mBluetoothGatt.writeCharacteristic(characteristic);
}
}
public boolean writeCharacteristic(BluetoothGattCharacteristic characteristic,
String data) {
if (mBluetoothAdapter == null || mBluetoothGatt == null) {
Log.w(TAG, "BluetoothAdapter not initialized");
return false;
}
if (ActivityBLEController.IS_FIRST_TIME) {
/**
* In the first time,
* should send the Title
*/
byte[] merge_title = sendTitle(data);
// Set value for characteristic
characteristic.setValue(merge_title);
// Write characteristic via BLE
mBluetoothGatt.writeCharacteristic(characteristic);
// Reset
ActivityBLEController.IS_FIRST_TIME = false;
return true;
} else {
/**
* In the second time,
* should send the Message
*/
if (data.length() <= BLE.LIMIT_CHARACTERS) {
sendMessage(characteristic, data);
// Reset
ActivityBLEController.IS_FIRST_TIME = true;
return true;
} else {
// Typed character
typed_character = data.length();
return false;
}
}
}
On Lollipop you can send up to 512 bytes. You need to use BluetoothGatt.requestMtu() with a value of 512. Also, as @Devunwired mentioned you need to wait until any previous operation is complete before calling this.