Frame Relay is simplified form of (25) , similar in principle to (26) , in which synchronous, frames of data are routed to different destinations depending on header information . Packets are routed throught one or more Virtual Circuits known as (27) . Most Virtual Circuits are (28) ,which means that the network provider sets up all DLCI connections at subscription time. (29) are also part of the Frame Relay specification . They privide a link that only lasts only as long as the session.

26（）

A．X.21

B．X.25

C．X.28

D．X.29