通信的双方都可以发送信息，但是不能同时发送，也不能同时接收，通信时是一方发送一方接收，过一段时间再反过来，这种通信方式是(31)。

(82)

(81)

(80)

(79)

(77)

In　the　following　essay,　each　blank　has　four　choices.　Choose　the　most　suitable　one　from　the　four　choices　and　write　down　in　the　answer　sheet.　There　are　two　types　of　key　technology:　private　key　and　public　key.　Private-key　encryption　methods　are　called(71)ciphers,　information　is　encrypted　with(72)both　the　sender　and　receiver　hold　privately.　Public-key　encryption　methods　are　called(73)ciphers,(74)created　for　each　user,　encryption　key　and　decryption　key　are(75)

(76)

(75)

(74)

In　the　following　essay,　each　blank　has　four　choices.　Choose　the　most　suitable　one　from　the　four　choices　and　write　down　in　the　answer　sheet.　Heterogeneous　network　environments　consist　of　computer　systems　from(66)vendors　that　run(67)operating　systems　and　communication　protocols.　An　organization　that(68)its　computer　resources　is　usually(69)the　task　of　integrating　its　heterogeneous　systems.　Typically,　each　deparlment　or　division　has　defined　its　own　network　needs(70)OS,　LAN　topology,　communication　protocols,　applications,　and　other　components.