在研究SATA 3的速度时，“规定速度”和“实际结果”明显不同，那么到底是怎么回事呢？今天的超级用户问答帮助好奇的读者了解更多有关SATA 3的工作原理。

今天的问答环节是由SuperUser提供的，SuperUser是Stack Exchange的一个分支，是一个由社区驱动的问答网站分组。

图片由GiocoVisione（Flickr）提供。

问题

超级用户读卡器空间Ghost想知道SATA 3的实际速度是多少：

Google says it is running at 6 Gb/s. The throughput is at 600 MB/s. 600 MB/s equals 4.8 Gb/s. Does this mean that the bandwidth is 6 Gb/s, but the actual throughput is 4.8 Gb/s ?

SATA 3的实际速度是多少？

答案

超级用户贡献者MariusMatutiae为我们提供了答案：

• Does this mean that the bandwidth is 6Gb/s, but the actual throughput is 4.8 Gb/s ?

Yes it does. It is interesting to understand why.

While data is actually sent at 6 Gb/s, it is encoded to counteract two common defects in telecommunicati***, DC Bias and Clock Recovery. This is often accomplished using a specific coding algorithm called 8b/10b Encoding. It is not the only encoding algorithm which has been devised to this end (there is also a Manchester encoding), but it has become the de facto standard for SATA data transfer.

In 8b/10b encoding, eight bits of signal are replaced by 10 bits of (signal + code). This means that, out of the 6 Gb the channel sends in a second, only 8/10 (4/5) are signal. 4/5’s of 6 Gb is 4.8 Gb, which in turn equals 600 MB. This is what degrades the 6 Gb/s channel into a mere(?) 600 MB/s channel.

The advantages obtained by compensating for DC bias and allowing for Clock Recovery more than compensate for this slight degradation.

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