Describe a window of frames. 20. X.25 deals primarily with which OSI layer? LAPB, its access protocol, resides in layer 2 ? LAPB is almost identical to and derives from which data-link layer protocol? . Differentiate permanent virtual circuits (PVCs) and virtual calls.? X.25 was designed as the basic protocol for a packet network. However, experience has shown that X.25 generally uses permanent virtual circuits. Discuss this with respect to full packet operation.