I have an application that requires to send 1000 bytes of data every second and I am trying to find the best way to do that.
I saw in the docs that the MTU size can be from 23 to 517. It is set by default to 23 and I cannot find a way to change its value.
Any feedback on this ?
I also want to send this data through a GATT_Notify but the length parameter of the notify value is uint8 limiting the notification to 256 (so cannot notify to 517)
Can the uint8 limit be changed to get to 517 ? The GATT_Notify is a library compiled routine so cannot really be accessed.
At last, I saw in the HCI doc (8.1.2) a way to send data differently than through a GATT Characteristics but there is not much information about it. When dealing with large amount of data, should it be the way ? Are there any functions that can be used to do that directly in the peripheral to send data from the peripheral to the USB Dongle ?
Thanks
Christian
Hi Christian,
I haven't messed with large data yet, but there are a few things which might not seem intuitive. The max data per connection event is limited to 80 bytes at app level. There is a 4X20 buffer right now, and when the connection event happensany data here will be sent out. The way around this may be to have shorter connection event intervals, and slave latency for power savings. There is a buffer limit of 4 buffers in link layer. If you are hitting this limit, then you should receive an error code 0x04 for "Buffer Not Available". I'm not aware of any fragmentation ability for this kind of data, so it may be up the app to handle this.
Br,
-Greg
Greg,
You can actually have more than 4 GATT buffers is you redefine the GATT profile but the big problem is that each of these buffers are limited to 20 bytes. This limit is set by the MTU Size (L2CAP_MTU_SIZE or 23 - 20+3 overhead) in the att.h - This cannot be change because when you change the L2CAP_MTU_SIZE, you will get all kind of compiler error and warning for functions that are encoded in the binaries libraries that TI is providing.
The limit of 23 when you are dealing for 1000 bytes of data is totally impractical. It adds too much overhead, requires data to be sent every 10ms and will consume too much power and this just does not work.
Per the BLE specification, I should be able to set the MTU_SIZE to up to 517. Is there an L2CAP function I can use to change that setting dynamically or any other function ?
After talking with developers here, we are going to be limited to around 80bytes of application data per connection event. The 512 size attribute will still be broken down and handled automatically, but the amount of data sent per connection event is still limited. With the small connection interval and some slave latency - you should only be on when you need to transmit data. There are some peak current power savings achieved with this implementation, although it does add overhead with large amounts of data which is the situation you are seeing.
Is there a plan to be compliant with the BLE spec on this and allow up to 512 bytes of data per connection ?
Hi Everyone,
I am a newbie to the BT LE world and I am trying to convert one of my existing IrDA devices. From the posts I have seen on this forum, there appears to be some need for large data transfers. I have up to 2MB that I would need to download to a host. Has anyone had any success and using GATT to transfer more than the single byte attributes I have seen in the two example programs? I am trying to determine whether this chip set or the upcoming BlueGiga BT LE module (suppose to be sampled next month) would work best for my application (or if BT LE is even the right interface to use!).
I apologize if this is covered in another thread.
Thanks,
Marty
The Bluetooth spec does not require that 512 bytes of data must be sent per connection event. The BT spec states that the length of each attribute may be a maximum of 512 bytes, but that does not mean that the entire value necessarily needs to be sent in a single event. GATT contains the following sub-procedures for reading or writing characteristics / descriptors who's values are greater than 20 bytes:
Read Long Characteristic Value
Write Long Characteristic Value
Read Long Characteristic Descriptor
Write Long Characteristic Descriptor
I must be on the wrong thread. That wasn't my question. I was trying to see if anyone has built something on top of GATT to do longer data transfers.
Is there any sample code that shows the usage of those sub procedures?