题目内容：
根据下列材料，回答题：Frame Relay is simplified form of（ 1 ），similar in principle to（ 2 ），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（ 3 ）.Most Virtual Circuits are（ 4 ），which means that the network provider sets up all DLCI connections at subscription time.（ 5 ）are also part of the Frame Relay specification.They privide a link that only lasts only as long as the session.
请回答第（ 1 ）题。
参考答案：
答案解析：