交换机的主要作用是同时开通数个通信通道，使网络总吞吐量远大于单个端口的速率。它根据(34)进行帧转发。

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In　low　-　speed　network　,it　is　usually　adequate　to　wait　for　congestion　to　occur　and　then　react　to　it　by　telling　the　source　of　packets　to　slow　down.　In　high　-　speed　networks,　this　approach　often　works　poorly,　because　in　the(71)between　sending　the　notification　and　notification　arriving　at　the　source,　thousands　of　additional(72)may　arrive.　In　ATM　network　,　a　major　tool　for　preventing(73)is(74)control.　　When　a　host　wants　a　new　virtual(75),it　must　describe　the　traffic　to　be　offered　and　the　service　expected.

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The　Simple　Network　Management　Protocol(SNMP)is　an(66)protocol　that　facilitates　the　exchange　of　management　information　between(67).　It　is　part　of　the　Transmission　Control　Protocol/Internet　Protocol(TCP/IP)protocol　suite.　SNMP　enables　network(68)to　manage　network　performance,　find　and　sole　network　problems,　and　plan　for(69)An　　SNMP　-managed　network　consists　of　three　key　components:　managed　devices　,(70),and　network　-management　systems　.