在第四部分中，重点介绍了多点单播子系统的设计与实现过程，并对实现中所使用的关键技术进行了探讨。

In the fourth part, the design, the developing process and the key techniques adopted in the development of this subsystem are introduced.

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IP组播技术实现了IP网络中点到多点的高效数据传输， 它能弥补单播通信和广播通信的不足。

IP multicast technique realizes efficient data transmission from one point to multiple points in IP network, and remedies the defects of unicast and broadcast communication.

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