可选信号标识后续的一些帧用来标识呼叫位置。

Optional signal indicates that the following frame is an identification of the calling station.

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连接到信号灯的路边标识能在距离路口数百米处帮忙提供信息，但是这样的标识除了价格高昂之外，也未被广泛部署。

Roadside signs wired to traffic lights can help get the message across a couple hundred metres from a junction, but such signs are expensive, and have not been widely deployed.

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这个函数返回一个信号量集标识符，它与一个参数的键值关联在一起。

This function returns the semaphore set identifier associated with the argument key.

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