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.

25（）

A．Datagram Switching

B．Datagram Routing

C．Packet Switching

D．packet Routing

主视图和俯视图的（）相等.

A、高度

B、宽度

C、长度

D、都相等