Computer Network Unit 1 q what are the topologies in computer n/w ?

EA
Extended Address field signifies up to two additional bytes in the Frame Relay header, thus greatly expanding the number of possible addresses
FECN
Forward Explicit Congestion Notification (see ECN below).
BECN
Backward Explicit Congestion Notification (see ECN below).
DE
Discard Eligibility (see DE below).
Information
The Information field may include other protocols within it, such as an X.25, IP or SDLC (SNA) packet.
The protocol is similar to that of an X.25 network, except all circuits are permanently assigned. What is a circuit? A circuit is a link between user end points. In frame relay and X.25 networks, circuits are known as "permanent virtual circuits", or PVC's. The circuits are known as virtual because they are not electrical ciruits where there is a direct electrical connection from end to end. Rather, there is a "logical" connection, or virtual connection, where the user data moves from end to end, but without a direct electrical circuit.
X.25 circuits can be initiated and ended from the users terminals. Frame relay circuits are set up at the time of installation and are maintained 24 hours per day, 7 days per week. Frame relay circuits are not created and ended by user at their terminals or PC's. However, the user may have an application running over a frame relay circuit where computer to terminal sessions are initiated and ended by the user. These sessions are related to the application, not to the underlying frame relay network.
Frame relay relies on the customer equipment to perform end to end error correction. Each switch inside a frame relay network just relays the data (frame) to the next switch. X.25, in contrast, performs error correction from switch to switch. The networks of today are sufficiently error free to move the burden of error correction to the end points. Most modern protocols do error correction anyway, protocols such as SDLC, HDLC, TCP/IP, stat mux protocols, etc.