Finally, this section describes the start-up and clear-down procedures of a handshake session. In G.994.1, there are two types of start-up procedures: duplex and half-duplex start-up modes. Typically, the duplex start-up mode is used by ADSL transceivers, and the half-duplex start-up is used by SHDSL transceivers. Either the CO unit or the CP unit may initiate either type of start-up. Manufacturers of DSL modems are generally encouraged to support both modes of operation to provide interoperable handshake communication with other types of DSL equipment. The following subsections describe the duplex and half-duplex start-up procedures and the clear-down procedure as defined in G.994.1.