Once pairing is complete, the link manager protocol sets up the links. Two
main kinds of links exist to carry user data. The first is the SCO (Synchronous
Connection Oriented) link. It is used for real-time data, such as telephone connections. This type of link is allocated a fixed slot in each direction. A slave may
have up to three SCO links with its master. Each SCO link can transmit one
64,000-bps PCM audio channel. Due to the time-critical nature of SCO links,
frames sent over them are never retransmitted. Instead, forward error correction
can be used to increase reliability.