单项选择题

Frame Relay is simplified form of (66) , similar in principle to (67) , 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 (68) . Most Virtual Circuits are (69) , which means that the network provider sets up all DLCI connections at subscription time. (70) are also part of the Frame Relay specification. They privide a link that only lasts only as long as the session.

67（）

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