LAPDm stands for Link Access Procedure on D channel (modified). This is a modified version of LAPD and is optimized for the GSM Air interface.
This is done in order to judiciously use the scarce radio resource. Some parts of LAPD frame are removed to save the resources. Some differences are:
1. In LAPDm, checksum is not used, as channel coding on layer 1 takes care of it(for error identification and correction).
2. In LAPDm, some of layer 2 control frames, viz. SABM and UA frames carry layer 3 information.
3. LAPDm does not support extended header formats.
4. LAPDm frames (one LAPDm frame carry a maximum of 23 octets) are shorter than LAPD frames (one LAPD frame carry a maximum of 260 octets).
This is done in order to judiciously use the scarce radio resource. Some parts of LAPD frame are removed to save the resources. Some differences are:
1. In LAPDm, checksum is not used, as channel coding on layer 1 takes care of it(for error identification and correction).
2. In LAPDm, some of layer 2 control frames, viz. SABM and UA frames carry layer 3 information.
3. LAPDm does not support extended header formats.
4. LAPDm frames (one LAPDm frame carry a maximum of 23 octets) are shorter than LAPD frames (one LAPD frame carry a maximum of 260 octets).
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