With a normal Characteristics Read only the MTU Size (20bytes) of data will be read.
My customer will offer a characteristics with a larger size (about 100bytes).
I saw that BLE offers a "Long Read" feature which reads until the size of the characteristics is reached.
(https://bluegiga.zendesk.com/entries/25053373--REFERENCE-BLE-master-slave-GATT-client-server-and-data-RX-TX-basics)
attclient_read_long command - Starts a procedure where the client first sends normal read request to the server, and if the server returns an attribute value with a length equal to the BLE MTU (22 bytes), then the client continues to send "read long" requests until rest of the attribute is read. This only applies if you are reading attributes that are longer than 22 bytes. It is often simpler to construct your GATT server such that there are no long attributes, for simplicity. Note that the BLE protocol still requires that data is packetized into max. 22-byte chunks, so using "read long" does not save transmission time.
But how can I use this feature in Android?
The BluetoothGatt class only offers a simple "Read()" - same for iOS.
Increasing the MTU is not possible since we need to support devices with AP Level < 21 (increaseMTU was introduced at API 21)
I can confirm for iOS that a read operation as per standard will occur first. Then if the server returns a completely filled PDU, the iOS device will then continue to perform the blob read operation. Tested with iPhone 7 running iOS 11.2.x
You do NOT need to call the peripheral.readValue(characteristic) multiple times for long attributes. CoreBluetooth does all of this under the covers.
Refer to the Bluetooth Spec Core v5.0, specifically Vol 3, Part F. "Long Attribute Values".
Experiment to prove above.
I have an Android Thing acting as the server that I'm making return the maximum length with my iPhone during a read operation. iOS and my RPI3 exchange a MTU of 185. So the read response is (MTU - 1) 184 bytes long. The server (RPI) then receives a new read request with an offset of 184, which you can then return more data. This is continued until the offset is > 512, or the last read response returns less than the MTU - 1 length.
Based upon the fact that the BluetoothGattServer supports long attributes, I'd assume the BluetoothGatt object does as well. Since there is no way via the API to set the offset to be read, I'd assume you can invoke read just once.
Related
I understand that in Android, the MTU for writing data to a Characterisitc is around 23 bytes (3 bytes used so you have around 20 bytes free) and that you can request a higher MTU (up to 512) to allow you to write/send larger data packets to a bluetooth device.
However, do you also need to do this when reading data from a characterisitic?
Both Reading and Notification updates?
If say, I have a characteristic that sends data in 123 byte chunks, but I only ever have to send at most 2 bytes to it, do I need to negotiate a large MTU?
I can recommend you to read the ATT and GATT chapters in the Bluetooth Core standard. Those explain the protocol.
By default Android does not negotiate a larger MTU than the default (23 bytes). You can do that yourself though by calling the requestMtu function.
Android automatically under the hood uses "Write Long Characteristic Values" and "Read Long Characteristic Values" when the MTU is not big enough when reading/writing values in order to transfer the whole value. However these procedures are very inefficient since they require multiple roundtrips. The read operation also is not atomic.
Notifications and Indications don't have any "Long" variant with multiple roundtrips, so these will be truncated to fit the MTU.
We are facing one issue when reading characteristics from remote BLE device.
This issue happen in Android OS 5.0 and above.
Points are below to generate issue :
Make one peripheral device with one service and one characteristics.
Characteristics will have only read permission. Now set the value of this characteristics with more than 20 characters i.e. 20 bytes.
Now let peripheral device broadcast itself with one service and one characteristics.
Now launch any BLE scanner app from market and connect with this peripheral device.
Once successfully connected with peripheral device just try to read characteristics.
In this case it will not show any data and when debugging the app it show that it returns null data.
The above same case not working in the Android OS 5.0 and above.
Same case working in android 4.4.
So there is something change in Android OS 5.0 and above that internally disable readblob() request that can read data having more that 20 characters.
This can be simply achievable by splitting your data into 20 byte packets and implementing a short delay (i.e. using sleep()) between sending each packet.
You can use BluetoothGatt.requestMtu(). See the Official document of BluetoothGatt.requestMtu
Request an MTU size used for a given connection.
When performing a write request operation (write without response), the data
sent is truncated to the MTU size. This function may be used to request a larger MTU size to be able to send more data at once.
A onMtuChanged(BluetoothGatt, int, int) callback will indicate whether this operation was successful.
Requires BLUETOOTH permission.
If you want send more 20 bytes, you should define array byte[] include how many packet you want.
There is an example Android: Sending data >20 bytes by BLE
Also there is another example How to send more than 20 bytes data over ble in android?
If I want to transfer a lot of data (e.g. 1 MB file) over BLE, what's the best way to do it?
I control both sides of the connection, but the client side is iOS/Android so only has access to GATT. I can't do anything with L2CAP.
I also can't wait for Bluetooth 4.1, 6LoWPAN, Connection-Oriented-Channels or anything like that.
I would assume the answer is to have one "request" characteristic that you write a data request to ("Give me 3000 bytes starting at byte 0"), and a "data out" characteristic that sends lots of 20 byte notifications (the maximum characteristic size) containing the data.
Is there a better way?
Yes we are using the approach you have mentioned.
Request data with the last index number(First time the index is 0)
The server send you with data with index no.Store the index no for subsequent format
continue Step 1 and 2 till the time server sends end of data-probably with index -1 or something.
Make sure you transfer the data you required in the most space efficient format.See if you can zip the files and transfer it.
You can update connection interval to small value with smallest 6*1.25 ms in remote BLE device.
Actually, BLE is designed for Low energy, small packet, low data rate.
L2cap data will be transmitted in different data channel with frequency hop. Packets TX/RX happen within each connection interval and max number of packets TX/RX in an event is restricted by specification, finally implemented by manufacture. So we can change connection interval as small as possible to increase data rate.
Refer BT 4.0 spec Vol 2, 7.8.18 LE Connection Update Command.
Try to negotiate a larger MTU than the default.
Then each notification can be larger. Even though it will be fragmented by the L2CAP layer, you will get a slightly larger throughput since the packet header will be smaller.
Is there someone using blob request (long read) from an android device?
We work with a CC2540 from TI, connected to a android 4.4.
We try to read a long characteristic value (size more than 23 bytes). In the android API for BLE, we have not seen a readBlob or readLong method.
We expect that the Android BLE Stack do the job for us, by reading a characteristic presentation format (same way has notification), but it doesn't works.
We have no idea how to send Blob Request through Android.
Let me make this clear that Android has only one method to read the value of a characteristic, readCharacteristic(characteristic). You can use this method to get the value of a characteristic of any length. Android takes care of forming a ReadBlob request; it's all in the back end. You'd have to change the code of your CC2540 though, to make it work with ReadBlob request. Once you make all the required changes at your CC2540 end, on calling readCharacteristic() from Android, you'll get the entire value of the characteristic which you can access in the onCharacteristicRead() callback.
You can´t, BLE characteristic values are limited at 20 bytes. So if you want to send or receive more than 20 bytes, you have to split it into 20 byte chunks. See this topic on the issue.
I have an android client that functions as a central and have an app on my MAC (peripheral) that this central connects to and sends data.
At this point, I need to wait almost 100ms after I call writeCharacteristic(..) to receive the onCharacteristicWrite(..) callback. I am sending strings. If I send smaller strings, the throughput is great (understandably). When the string contains about 200 characters and I send 20 byte chunks, it takes almost a second before the entire string is seen at the peripheral. When I set the write type to NO_RESPONSE before writing the characteristic, I see no data on the peripheral.
After I connect, I have done the following to improve throughput:
Stopped discovery after services are discovered because it is an expensive operation
I set the write type to default first - When I do this, I see data on the peripheral. But, there is a significant delay. When I set the writeType to NO_RESPONSE, I see no data on the peripheral. I have no logic in onCharacteristicWrite(..) either. Sometimes, I see the data getting truncated on the peripheral.
I have set the desired connection latency to low on my mac app. Is there a way to set a value (as 7.5ms perhaps?).
When I set the write type to default and send a string of 200 characters - I split the string into 20 byte chunks. I now have 10 chunks to send. If I set characteristic value and call writeCharacteristic(..) in loop, I see no data. When I add a ~100ms delay after writeCharacteristic(..) before it executes the next iteration of the loop, I see data on the peripheral.
I see a huge increase in throughput between an iOS central - iOS peripheral. I don't see why Android central - iOS peripheral shouldn't work he same way. From my understanding, Android and iOS use the same chip.
Any reason the performance is so poor? Is there anything else I can do to improve throughput?
Please have a look at the MTU size. My experience:
Using a iOS central, the central automatically starts the MTU size negotiation with some large value. I think it is larger than 200 bytes.
On most Android devices I tested this does not start automatically but you have to start the MTU size negotiation by your app (central). If you do not do that, Android cuts your data into 20 byte pieces. This has big influence on your throughput.