When I enable notification on a characteristic on my client app, why does the app only see 20 bytes?

Hi,
in my application I have defined aCustom Profile, which has aCustom Serviceand this service has twoCustom Characteristics (1, 2). The second characteristic is used to send an array of bytes to the clientwith notification. I've followed the sample128 example (BTW such a good document) to write my application. I have configured my timer for5 secondsand every time it expires in the timer interrupt handler I'm calling theke_msg_send()to send an array of 20 bytes. Meanwhile, on my client app (BLE Scanner mobile app) when I enable the notifications for Characteristic 2, I'm able to see20 bytesconstantly arriving at 5 seconds interval.

Next, instead of 20 bytes, I'm sending64 bytes每次计时到期。但我注意到my client application (BLE Scanner mobile app) that, when I enable the notification for Characteristic 2, I see that only 20 bytes are displayed on the app. However, if I explicitly do a read of Characteristic 2 I see all the 64 bytes.

1. Is it a limitation of the BTLE. Are the notifications only able to send 20 bytes?
2. Or Is it a limitation of the client app I'm using?
3. The changes I've made to my application are as below, but I don't know what else needs to be done.

What I've tried so far to achieve this are as follows:

Step 1: Increase the ATT_MTU size to 67
In the app_template_proj.c file, inside theapp_configuration_func(...)function, I've increased the:
...
...
// Maximum transmit unit size
cmd->max_mtu = 67; // 23
...
...


Step 2: Increase thetotal_sizeparameter ofattmdb_add_service()function.
In the template_task.c, insidetemplate_create_db_req_handler()function.
...
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nb_att_16 = 4; // 4 UUID16 Attribute declaration types
nb_att_32 = 0; // 0 UUID32 Attribute declaration types
nb_att_128 = 2; // 2 UUID128 Attribute declaration types
status = attmdb_add_service( &(REDS_env.REDS_shdl),
TASK_XXXX,
nb_att_16,
nb_att_32,
nb_att_128,
121 // See calculation below);
// Total Data portion of GATT database = 58 data bytes:
// 16 Primary service declaration
// + 19 Declaration of characteristic 1
// + 1 Value declaration of characteristic 1
// + 19 Declaration of characteristic 2
// + 64 Value declaration of characteristic 2
// + 2 Client configuration declaration of characteristic 2
// = 121 Data bytes total
...
...