How to increase credit in L2CAP

RE: How to increase credit in L2CAP

Kenneth — Tue, 06 Apr 2021 13:49:42 GMT

Maybe this can be useful: https://devzone.nordicsemi.com/f/nordic-q-a/65490/le-credit-based-for-l2cap-connetion-oriented-channels---initial-credit/268270#268270

RE: How to increase credit in L2CAP

Rickon — Fri, 26 Mar 2021 11:34:17 GMT

Hello,

How can I set the credit? I set below, but credit is still 1;

How can I use *p_credits?

sd_ble_l2cap_ch_flow_control(uint16_t conn_handle, uint16_t local_cid, uint16_t credits, uint16_t *p_credits))

RE: How to increase credit in L2CAP

Kenneth — Thu, 18 Mar 2021 10:23:06 GMT

I think the entire L2CAP packet may be 251bytes*5+2bytes = 1257bytes? However since a BLE packet can only contain 251bytes, the packet is fragmented on the link, and the last 2bytes are split into a separate packet. The last packet is slightly inefficient, it would be more efficient if you reduce the L2CAP to 1255bytes.

Kenneth

RE: How to increase credit in L2CAP

Rickon — Thu, 18 Mar 2021 02:50:01 GMT

Thank you for your reply. Another problem, why only 2 bytes is in the last pack, other are 251 bytes?

RE: How to increase credit in L2CAP

Kenneth — Wed, 17 Mar 2021 14:01:10 GMT

Unfortunately the nRF5 SDK OTS example is not optimized for throughput.

To increase throughput I suspect one of these two must be changed, either increase queue size:
ble_cfg.conn_cfg.params.l2cap_conn_cfg.rx_queue_size = 1; // Need to be more than 1

or increase credits to 2, 3, or 4:
sd_ble_l2cap_ch_flow_control(uint16_t conn_handle, uint16_t local_cid, uint16_t credits, uint16_t *p_credits)) // with credits set to 2, 3, or 4 might be possible

There is a message sequence chart here showing the above:
https://infocenter.nordicsemi.com/topic/com.nordic.infocenter.s140.api.v7.2.0/group___b_l_e___l2_c_a_p___c_h___f_l_o_w___c_o_n_t_r_o_l___m_s_c.html

Looking at the logs I can see that iphone 8 and 11 behave a bit differently:

It looks like the second to last packet don't have the MD bit set on iphone 11, in other words it is as if the last 2bytes is not ready in the link layer of the host on iphone 11 yet, while it is ready on iphone 8. This cause the transfer to be slower on iphone 11, since it wait until next connection interval to send the last 2bytes, but I do not know why there is a difference.

Hope it helps,
Kenneth