数据库的数据体系结构分为三个级别，(44)最接近用户，是用户能看到的数据特性。(45)涉及到所有用户的数据定义。最接近物理存储设备的是(46)。这些模型用数据库的(47)描述。

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Network　managers　have　long　awaited　practical　voice-over-IP(VOIP)solutions.　VOIP　promises(71)network　management　and　decreases　costs　by(72)a　company's　telephony　and　data　infrastructures　into　one　network.　And　a　VOIP　solution　implemented　at　a　company'　s　head　-quarters　with　far　-reaching　branch　offices　can(73)tremendous　amounts　of(74)in　long　distance　phone　bills,　provided　that　solution　delivers　POTS　-like　voice(75)over　the　Internet.

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(66)is a protocol that a host uses to inform. a router when it joins or leaves an Internet multicast group.

(67)is an error detection code that most data conlmunication networks use.

(68)is an interior gateway protocol that uses a distance vector algorithm to propagaterouting information.

(69)is a transfer mode in which all types of information are organized into fixed form. cells on all asynchronous or nonperiodic basis over a range of media.

(70)is an identifier of a web page.

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