Abis is a BSS internal interface linking the BSC and BTS. It allows control of the radio equipment and radio frequency allocation in the BTS. It allows control of the radio equipment and allocation of the radio frequency in the BTS. This interface is generally carried by a DS-1, ES-1, or E1 TDM circuit. It uses TDM subchannels for traffic. For BTS supervision and telecom signaling LAPD (Link Access Protocol for D channel) is used. It also carries synchronization.