The packet division by setting of ATT_MTU and DLE？

[Yoko]

Hello,

We try to use "ATT_MTU" and "Data length extension (DLE)" to expand a packet in S132 SDK 14.0.0. When we set "ATT_MTU" > "DLE", please tell me what happens?

We thought data was sent by 1 packet when I set "ATT_MTU"=158 and "DLE"= 137. But, I confirmed the packet was divided when I set "ATT_MTU"=158 and "DLE"= 137. Additionally, division of a packet doesn't occur anytime.

We checked consumption current from an oscilloscope. The data was sent by 2 packets. But the packet interval of this data is very short. And the data was sent by 1 packet at receiving side.

1.Why does division of a packet occur?

2.The 2nd packets is empty?

[Ulrich Myhre]

The maximum length of a logical ATT PDU is ATT_MTU long (so for HVN, the actual data payload is ATT_MTU-3 long due to handle + opcode).

The maximum length of an on-air PDU is DLE_LEN long. Since 4 bytes are used for the L2CAP header (length + channel identificator), you should set DLE to be ATT_MTU+4.

In your setup, MTU is 158 and DLE is 137. For an HVN or write command, that will be divided into two PDUs as follows:

PDU #1: L2CAP Header (4) | ATT Header (3) | ATT payload (130)
PDU #2: ATT payload (28)

So you get some saving with high ATT_MTU by not sending multiple headers, but by increasing the DLE length it should be sent as one PDU.

[Ulrich Myhre]

Stupid question maybe, but are you also initiating an ATT MTU exchange before this? Are you giving a proper length to the hvx call? (Should be agreed MTU size minus 3). It sounds like your hvx call is still just requesting to notify about 20 bytes, and the other side is doing it correctly, but the ATT MTU exchange prodecure has not yet completed.