Movement of network-layer data through the internetwork usually involves encapsulation
of datagrams inside media-specific frames. As each media frame
arrives at the router and is discarded, the router analyzes the
datagram and places it inside a new frame as it is forwarded.
The X.25 specification maps to Layers
1 through 3 of the OSI reference model. Layer 3 X.25 describes
packet formats and packet-exchange procedures between peer Layer 3
entities. Layer 2 X.25 is implemented by LAPB.
The same layer of the OSI reference
model occurs twice. As shown in the Figure, Layer 3 occurs twice:
once for the IP datagram and once for X.25. When the system forwards
a datagram to an X.25 interface, the maps are consulted to determine
the X.25 destination.
Similarly, in an X.25 environment,
the LAPB frame arrives at the router, which extracts the datagram
from the packet or packets. The router discards the encapsulating
frame and analyzes the datagram to identify the format and next hop.
Based on the route determination, the router re-encapsulates the
datagram in framing suitable for the outgoing media as it forwards
the traffic.