LAPDm

LAPDm is a data link layer protocol used in GSM cellular networks. LAPDm forms Layer 2 of the Um interface between the Base Transceiver Station and Mobile station, which is to say that it is used in the radio link between the cellular network and the subscriber handset.

LAPDm is derived from a much older link layer protocol called HDLC and specified in 3GPP specifications TS 04.05 and 04.06. LAPDm is similar to the ISDN Layer 2, LAPD, but with these simplifications:

Another important difference between LAPDm and LAPD is the establishment contention resolution procedure of GSM 04.06 Section 5.4.1.4, wherein the MS sends an L3 message in the information field of the SABM frame which is then echoed back by the BTS in the corresponding UA frame. This procedure is required in LAPDm because of the possibility of a handset accidentally attempting to use the wrong channel. LAPD does not require contention resolution, since hard-wired ISDN devices cannot accidentally use a wrong channel.

The Associated Control Procedure used in Layer 2 of the iDEN RF interface is very similar to LAPDm.

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