Tämänpäiväinen kysymys- ja vastaus-istunto tulee meille Hyvinkään, Super-käyttäjän, Stack Exchange-jaosta, joka on yhteisöllinen Q & A-sivustojen ryhmittely.
Kuva courtesy of GiocoVisione (Flickr).
Kysymys
SuperUser-lukija Space Ghost haluaa tietää, mitä SATA 3: n todellinen nopeus on:
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 ?
Mikä on SATA 3: n todellinen nopeus?
Vastaus
SuperUser-avustaja MariusMatutiaelle on meille vastaus:
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 telecommunications, 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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