Prowid course material: https://kurslar.prowid.com/en/index.php?id=lesson&m=3025
ISDN: BRI and PRI, Q.921 and Q.931 · Layer 2: Q.921 (LAPD)
LAPD frame types: I, S (RR, RNR, REJ), U (SABME, UA, DISC, DM, FRMR, UI)
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