What is LAPDm (Link Access Protocol on the Dm channel)?

Link Access Protocol on the Dm channel is being derived from Link Access Protocol Balanced (LAPB) protocol. LAPDm does not uphold expanded header designs. LAPDm upholds just the SABM, Disk, DM, UI and UA U-Frames. It is utilized in GSM to assist the transport of data between the portable and the network. LAPDm is derived from a much older link layer protocol called HDLC and determined in 3GPP specifications TS 04.05 and TS 04.06.