ABR则是每个帧都有一个可变的比特率，但是其平均比特率是恒定的。
ABR has a variable bitrate for each frame, but its average bitrate is a constant.
Verizon的V CASV就是一个很好的例子；在平均400kbps的比特率，一个小时的电视需要180MB大。
Verizon's V CAST is a good example of this; at an average bitrate of around 400kbps, an hours worth of television would require 180MB of data.
针对双总线形、星形、环形和双环形四种光纤网络拓扑，比较它们的光纤需求长度、平均传播延迟、传输衰减、最大比特率及容错。
The comparison is in term of fiber required, average propagation delay, transmission loss, max bit rate, fault tolerance among four type topologies included bus, star, ring and dual ring.
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