Maximum value for ATT_MTU

[Youssif]

Hi

When initialising BLE in the softdevice, the following value is used as part of enable_params:-

 * @brief BLE GATT initialization parameters.
 */
typedef struct
{
  uint16_t  att_mtu;          /**< Maximum size of ATT packet the SoftDevice can send or receive.
                                   If it is 0 then @ref GATT_MTU_SIZE_DEFAULT will be used.
                                   Otherwise @ref GATT_MTU_SIZE_DEFAULT is the minimum value.
                                   @mscs
                                   @mmsc{@ref BLE_GATTC_MTU_EXCHANGE}
                                   @mmsc{@ref BLE_GATTS_MTU_EXCHANGE}
                                   @endmscs
                              */
} ble_gatt_enable_params_t;

I know that the minimum (and default) size for ATT_MTU is 23 bytes as defined in ble_gatt.h:-

/** @brief Default MTU size, in bytes. */
#define GATT_MTU_SIZE_DEFAULT 23

However, there is no specification for the maximum size, so what is the maximum size that can be used?

I am using softdevice s132 v3.0.0 on nRF52.

[Ulrich Myhre]

Hi,

The maximum should only be limited by the amount of RAM you want to set aside for it. Performance-wise, the throughput gain per byte of RAM starts to fall off when ATT_MTU gets much larger than the ATT overhead of 4 bytes. Many peers will also not be able to utilize enormous payloads, and will limit you to what they can support. There is also a limit on how long a packet can be on the air, so some amount of fragmentation will always happen once you pass that threshold.

Currently, the full amount of RAM for the ATT_MTU is reserved for every possible link, but this is something that is likely to be changed in future releases.

[Nivetha]

If the connection interval is 10ms, whats the maximum value of ATT_MTU without DLE and with DLE?

[Ulrich Myhre]

Please try to raise new questions as separate topics. This increases visibility and increases the chance of you receiving the best response. It also makes it much easier for future visitors to find the answer to their own, similar question.

[Nivetha]

Thank you, Ulrich. I have posted my question here.