Why can not set MAX_MTU_SIZE to 247?

Question

On 52832, SDK V14.0.0, SD132-5.0 
 I tried to set NRF_SDH_BLE_GATT_MAX_MTU_SIZE to 247, as from dev-doc i saw that SD132 supports BLE V4.2 DLE. 
 However it returns: 
 ble_gatt: sd_ble_gap_data_length_update() (request) on connection 0x0 returned unexpected value 0x13. 
 When set MAX_MTU_SIZE to 23 this issue disappear.

rolandash · Accepted Answer

Hi Petter, 
 I digged into nrf_ble_gatts.c deeper and checked the procedure, and i now get bit more clear picture of the problem.
I think this problem is exactly the same as in this post:
 devzone.nordicsemi.com/.../ 
 I think the root cause is still iPad Air 2 not supporting V4.2 DLE, even though after one year in latest ios 11. So the LL packet length is anyway to be only 27 bytes. 
 However, the ATT layer MTU size can be negotiated and 185 is the up limit on iPad side (as shown in the sniffer captured in above link). 
 sd_ble_gap_data_length_update() returning error is indeed because of the gap event length. I tried to check limits parameters and even with 4(5ms) it still 23bytes shorter so sd_ble_gap_data_length_update() returned 0x13. When set to 5(6ms) sd_ble_gap_data_length_update() succeeded locally and no error message printed, but then iPad refused the LL_LENGTH_REQ so BLE_GAP_EVT_DATA_LENGTH_UPDATE is returned with max_rx_octets: 27 that iPad can only support. 
 Seems we can only find larger LL PDU support on iPhone 8 and X .

Why can not set MAX_MTU_SIZE to 247?

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