Frame Relay is simplified form of (51) , s
题型：单项选择题
问题：
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.
（55）是（）
A. Permanent Circuits
B. Permanent Virtual Circuits
C. Switched Virtual Circuits
D. Switched Circuits
