SATA硬碟驅動器連線比舊的PATA硬碟驅動器連線更快，對於外部佈線標準也可以這樣說，但這是違反直覺的：為什麼並行傳輸不會更快？

今天的問答環節是由SuperUser提供的，SuperUser是Stack Exchange的一個分支，是一個由社群驅動的問答網站分組。

問題

超級使用者對並行和序列連線的資料傳輸速率很好奇：

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?

雖然人們很容易就認為SATA比PATA更新，但肯定有一個比年齡更具體的機制在起作用。

答案

SuperUser contributor Mpy提供了對傳輸型別性質的一些見解：

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迴應了Myp的回答，並擴充套件了設計經濟學：

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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