X.25 Model
Physical layer: X.21, RS-232C, V.35; roughly equivalent to OSI Physical layer
Link Access layer: defines transmission and framing sequence; protocol used with X.25 is Link Access Procedure Balanced (LAPB).
LAPB is derived from HDLC; roughly equivalent to the OSI Data Link layer
Packet layer: virtual circuit transmission through switched networks; describes point-to-point connection as opposed to point-to-multipoint; roughly equivalent to OSI Network layer