nd Layer The Bluetooth Baseband, sitting above the PHY layer in the Bluetooth protocol stack (Figure 10-2), manages the physical channels and links, including device discovery, link connection and management and power control. Time division multiplexing is used to divide access to the channel across devices in the piconet. Time is divided into slots of 650 us which are numbered according to the master device clock and allocated to links and devices by the master device. Two types of links are supported, synchronous connection-oriented (SCO) and asynchronous connection-less (ACL). SCO links mainly carry voice transmission data and are symmetric links between the master and a single slave. To maintain the link, the master reserves transmit/receive time slots at regular intervals and, since the link is synchronous, SCO packets are not retransmitted in the event of a packet error. ACL links connect the master to all the slave devices in the piconet (point to multi-point). The master device can establish an ACL link to any slave using time slots that are not reserved for any active SCO links. Only one ACL link can exist at a time, but the link can be to a slave that already has an SCO link to the master. For most ACL packets, packet retransmission is applied in the event of a packet error. The Bluetooth Baseband defines 13 packet types, including 4 specifically for high quality voice and voice + data transmission. Each packet consists of a 68-72 bit access code, a 54 bit header and a payload of up to 2745 bits. The access code is used during device discovery and to gain access to a specific piconet, and the header carries the slave address as well as information for acknowledgement, numbering and error checking of packets. The Baseband controls the process of device discovery through an inquiry procedure, which enables a device to discover other devices in range and determine their addresses and clock offsets, and a paging procedure which sets up the connection and synchronises the slave device clock to the master. On