2010年軟考網(wǎng)絡(luò)工程師模擬試題及答案解析二(上)

　　●Frame Relay is simplified form of (71) ,similar in principle to (72) , 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 (73) .Most Virtual Circuits are (74) ,which means that the network provider sets up all DLCI connections at subscription time. (75) are also part of the Frame Relay specification .They privide a link that only lasts only as long as the session.

　　(71) A. Datagram Switching

　　B.Datagram Routing

　　C. Packet Switching

　　D.packet Routing

　　(72) A. X .21

　　B.X .25

　　C.X .28

　　D.X .29

　　(73) A. DLCIs

　　B.HDLCs

　　C.SDLCs

　　D.DLs

　　(74) A.Permanent Circuits

　　B.Permanent Virtual Circuits

　　C.Switched Virtual Circuits

　　D.Switching Circuits

　　(75) A.Permanent Circuits

　　B.Permanent Virtual Circuits

　　C.Switched Virtual Circuits

　　D.Switched Circuits

　　【解析】幀中繼是報(bào)文交換的一種簡(jiǎn)化形式，原則上類似于X.25，每個(gè)數(shù)據(jù)幀通過(guò)頭信息被路由到不同的目的端。報(bào)文通過(guò)標(biāo)有DLCI值的一條或多條虛電路傳輸。大部分的虛電路是永久虛電路，這就是說(shuō)網(wǎng)絡(luò)提供者在簽署的時(shí)候就建立了DLCI連接。交換虛電路幀中繼規(guī)范的一部分，它們提供的鏈路只是維持在本次會(huì)話時(shí)間內(nèi)。