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
参考答案：C
解析：
帧中继是(51)的简化形式，其原理类似(52)，在该网络上信息进行同步传输，根据消息头信息数据北路由到不同的目的地。包被路由通过一个或者更多的入我们所知道的(53)虚拟电路。大多数虚拟电路是(54)，这意味着网络运营商要在约定的时间建立所有的DLCI连接。(55)这同时也是讯框传送网络规范的一部分。他们提供一个链接，该连接只能维持大约一次会议的时间。