LAPM (Link Access Procedure for Modems) is an error-control protocol for modems defined in
CCITT (Comité Consultatif International Téléphonique et Télégraphique) spec
V.42. This is similar to MNP-4. A V.42 modem, if it cannot connect with V.42 will
fall back to MNP-4. Since only the basic level 1 link layer is shared by MNP-4 and
LAP-M, this renegotiation can take a fair bit of time.