一旦系统有空闲信道，且定时器未超时，则切换呼叫可以占用这些空闲信道，接受服务。

When there is free channel in the system and time does not run out, the handoff call can take the free channel and receive service.

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因此，在网关的实现过程中，会涉及到多路呼叫的资源分配与维护、多线程的处理与同步、呼叫状态的迁移与超时处理等问题。

So there are some related problems to be solved, such as the allocation and management of multi calls, the synchronization of multi threads, the state transference and time-out processing.

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