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简述GPRS参考模型。

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GPRS是要在一个发送实体和一个或多个接收实体之间,提供数据传递的能力。这些实体可以是一个移动台,也可以是一台终端设备。后者可以连接到一个GPRS网络,或连接到一个外部数据网络。基站为移动台提供无线信道,接入到GPRS网络。用户数据可以在四种类型的移动台之间传递。

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