这些包按照序列编号排队，但不存在任何形式的索引。
The packets are in sequence-number order but are not indexed in any way.
在pull交换中，应用程序通过给总线发送某种形式的请求并接收一个或多个在响应中排队等候的消息来启动传输。
In pull exchanges the application initiates the transfer by sending some form of request to the bus and receiving one or more queued-up messages in response.
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