about setting "NRF_SDH_BLE_GAP_DATA_LENGTH" value

Question

I've set "NRF_SDH_BLE_GAP_DATA_LENGTH" value to 27 on peripheral side, and it've changed to 251 after connection.
On the central side, the value is fixed as 27.
I believe the connection params affect each other, when it changes on peripheral side, the value also changes on the central side. Is it wrong?
Should I change the value on the central side?

Einar Thorsrud · Accepted Answer

Hi, 
 I think there is a misunderstanding here DLE is not related to advertising at all, just the packet length when in a connection. (You can use longer advertising packets with extended advertising but that is a different and completely unrelated topic, so you can ignore that). 
 As you write that you want longer packets in a connection, but not longer advertising packets, set NRF_SDH_BLE_GAP_DATA_LENGTH to 251 in both the central and peripheral. If you refer to the NUS examples (central and peripheral) from a recent SDK, this will give you long packets in a connection out of the box. 
 Bi_ said: But I want to send it in 251 size after connection. If so, is there a need for a routine to change the data length after the connection event even in the case of (2) central? 
 If you use the NUS examples, no, as it is supported and used out of the box. This is becasue the GATT module which is used in those examples will do the data length update procedure for you. 
 Bi_ said: If this value is set to 251, it is understood that if you want to transmit 247 data, you only need to send one chunk. 
 Yes. 
 Bi_ said: If this value is 27, we know that 247 is divided into 27 and sent. right? 
 Correct. 
 Bi_ said: I wonder if it is correct that the higher this value, the faster the speed. 
 Yes, you can say that. Because there is less overhead. For maximum throughput you may also want to increase the event length as mentioned above, by setting NRF_SDH_BLE_GAP_EVENT_LENGTH to be longer in some cases (up to the same length as the connection interval). You also want to set NRF_SDH_BLE_GATT_MAX_MTU_SIZE set to 247, as is already done in the NUS examples.

about setting "NRF_SDH_BLE_GAP_DATA_LENGTH" value

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