Frame Relay is simplified form of (51) , similar in principle to (52) , 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 (53) . Most Virtual Circuits are (54) , which means that the network provider sets up all DLCI connections at subscription time. (55) are also part of the Frame Relay specification. They privide a link that only lasts only as long as the session.

A．X.21

B．X.25

C．X.28

D．X.29

卡斯特认为，领导有两个基本含义：①是一种职能；②是( )

A．一种责任
B．一个具体的个体
C．一个地位集团
D．一个组织