UBT(4) | Kernel Interfaces Manual | UBT(4) |
USB Bluetooth dongles provide two interfaces, both of which the ubt driver claims. The second interface is used for Isochronous data and will have several alternate configurations regarding bandwidth consumption, which can be set using the hw.ubtN.config sysctl(8) variable. The number of alternate configurations is indicated by the value in the hw.ubtN.alt_config variable, and the isoc frame size for the current configuration is shown in the hw.ubtN.sco_rxsize and hw.ubtN.sco_txsize variables.
By default, configuration 0 is selected, which means that no bandwidth is used on the Isochronous interface and no SCO data can be sent. Consult the Bluetooth USB specification at https://www.bluetooth.org/ for complete instructions on setting bandwidth consumption. The following extract may be useful as a general guidance though details may differ between manufacturers.
For example: I want one voice channel (which is all that is supported, for now) so am using configuration #2, with a frame length of 17 bytes. This gives possible values of:
(17 * 1) - 3 = 14
(17 * 2) - 3 = 31
(17 * 3) - 3 = 48
(17 * 4) - 3 = 65
(17 * 5) - 3 = 82
etc.
btconfig(8) shows the maximum SCO payload as 64 bytes, so I am using the next smaller size of 48, to minimize the overhead of the 3 header bytes.
The SCO packet size can be changed using the ‘scomtu' option to btconfig(8).
The failure mode is that the USB Bluetooth dongle locks up though generally removal/reinsertion will clear the problem.
August 27, 2006 | NetBSD 6.1 |