The Abis interface connects the Base Station Controller (BSC) and the Base Transceiver Station (BTS) in a GSM network. It transfers synchronization information, signaling between the BSC and BTS, and traffic such as encoded speech. The Link Access Procedure on the D-channel (LAPD) protocol is used to provide acknowledged and unacknowledged signaling over the Abis interface using frames that contain flags, frame check sequences, commands, addresses, and information fields. Key information carried over Abis includes synchronization, signaling between the Transceiver Handler and Transceiver Controller, encoded speech using various coding schemes, and in-band signaling between the BSC and BTS components.