Bluetooth link layers define two types of data links, Synchronous Connection Oriented (SCO) link, being one of them. SCO is a symmetric, point-to-point link between the master device and the slave device connected via Bluetooth.
Features of Bluetooth SCO links
In SCO, a dedicated, point-to-point link is established between the master device and the slave device before communication starts.
SCO is a symmetric link, i.e. fixed slots are allocated for each direction.
Since fixed slots are reserved, SCO provides a circuit switched connection.
SCO radio links are used for time critical data transfer, particularly for voice data.
Both the master and the slave device transmit encoded voice data in their reserved timeslots.
A master device can support three SCO links with the same or different slaves.
A slave device can have a maximum of three SCO links with its master device.
A SCO link can transmit a 64,000 bps (bits per second) PCM audio channel.
SCO frames are not retransmitted. This is because the transmission is time critical and it is not feasible to retransmit real-time voice data.
Since retransmission is not possible, forward error correction is the only error correcting option for transmission of reliable data.
An improvement over SCO links is enhanced SCO (eSCO) links. The eSCO links permit more flexibility in setting up links, support limited retransmissions, and allow more types of data packets than SCO.