SATA硬盘驱动器连接比旧的PATA硬盘驱动器连接更快，对于外部布线标准也可以这样说，但这是违反直觉的：为什么并行传输不会更快？

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。

问题

超级用户对并行和串行连接的数据传输速率很好奇：

Intuitively, you would think that parallel data tran**ission should be faster than serial data tran**ission; in parallel you are transferring many bits at the same time, whereas in serial you are doing one bit at a time.

So what makes SATA interfaces faster than PATA, PCI-e devices faster than PCI, and serial ports faster than parallel?

虽然人们很容易就认为SATA比PATA更新，但肯定有一个比年龄更具体的机制在起作用。

答案

SuperUser contributor Mpy提供了对传输类型性质的一些见解：

You cannot formulate it this way.

Serial tran**ission is slower than parallel tran**ission given the same signal frequency. With a parallel tran**ission you can transfer one word per cycle (e.g. 1 byte = 8 bits) but with a serial tran**ission only a fraction of it (e.g. 1 bit).

The reason modern devices use serial tran**ission is the following:

• You cannot increase the signal frequency for a parallel tran**ission without limit, because, by design, all signals from the tran**itter need to arrive at the receiver at the same time. This cannot be guaranteed for high frequencies, as you cannot guarantee that the signal transit time is equal for all signal lines (think of different paths on the mainboard). The higher the frequency, the more tiny differences matter. Hence the receiver has to wait until all signal lines are settled — obviously, waiting lowers the transfer rate.
• Another good point (from this post) is that one needs to c***ider crosstalk with parallel signal lines. The higher the frequency, the more pronounced crosstalk gets and with it the higher the probability of a corrupted word and the need to retran**it it. [1]

So, even if you transfer less data per cycle with a serial tran**ission, you can go to much higher frequencies which results in a higher net transfer rate.

[1] This also explains why UDMA-Cables (Parallel ATA with increased transfer speed) had twice as many wires as pins. Every second wire was grounded to reduce crosstalk.

Scott Chamberlain回应了Myp的回答，并扩展了设计经济学：

The problem is synchronization.

When you send in parallel you must measure all of the lines at the exact same moment, as you go faster the size of the window for that moment gets **aller and **aller, eventually it can get so **all that some of the wires may still be stabilizing while others are finished before you ran out of time.

By sending in serial you no longer need to worry about all of the lines stabilizing, just one line. And it is more cost efficient to make one line stabilize 10 times faster than to add 10 lines at the same speed.

Some things like PCI Express do the best of both worlds, they do a parallel set of serial connecti*** (the 16x port on your motherboard has 16 serial connecti***). By doing that each line does not need to be in perfect sync with the other lines, just as long as the controller at the other end can reorder the “packets” of data as they come in using the correct order.

The How Stuff Works page for PCI-Express does a very good explination in depth on how PCI Express in serial can be faster than PCI or PCI-X in parallel.

TL;DR Version: It is easier to make a single connection go 16 times faster than 8 connecti*** go 2 times faster once you get to very high frequencies.

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