BASEBAND LINKS
The Bluetooth baseband provides transmission channels for both data and voice, and is capable of supporting one asynchronous data link and up to three synchronous voice links (or one link supporting both). Synchronous connection-oriented (SCO) links are typically used for voice transmission. These are point-to-point symmetric connections that reserve time slots in order to guarantee timely transmission. The slave device is always allowed to respond during the time slot immediately following an SCO transmission from the master. A master can support up to three SCO links to a single or multiple slaves, but a single slave can support only two SCO links to different masters. SCO packets are never retransmitted.
Asynchronous connectionless (ACL) links are typically used for data transmission. Transmissions on these links are established on a per-slot basis (in slots not reserved for SCO links). ACL links support point-to-multipoint transfers of either asynchronous or isochronous data. After an ACL transmission from the master, only the addressed slave device may respond during the next time slot, or if no device is addressed, the packet is considered a broadcast message. Most ACL links include packet retransmission.