SATA Drive Transfer Rate Confusion

In studing for the A+ I am confused about SATA drive transfer rates. For instance SATA300 upped transfer rates to 300MB/s. What then is the reference to 3Gb/s I see for the same revision? 300MB/s does not translate to 3Gb/s. What gives?

tx
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  1. If I am not mistaken, 300 megaBYTES/second is about the same as 3 gigaBITS/sec.
  2. 300MB/s is the same as 3Gb/s because they use an 8b/10b ECC scheme in which each 8 data bits are supplemented by an extra 2 error checking bits to ensure reliable transfer. 3 gigabits per second is the peak raw transfer rate, but since each byte (8 bits) of data is transmitted as 10 bits with the error checking bits, only 300 megabytes per second of actual data is transferred.
  3. Great answer cjl - thanks! How would someone know when an extra 2 bits is included for parity checking? So it applies to SATA but what about other transfer rates for storage devices? How would you know?

    tx again
  4. The transfer rates you are talking about are not drive transfer rates, but cache to host transfer rates. No current conventional drive transfers data from the platter to the interface at 300mb/sec currently. Also, the ECC extra bits are in the block of data that is transferred, not necessarily in each byte. It is somewhat of an academic point because the drive transfer rate is variable, and depends on where the data is coming from.
  5. Any online resources I could visit to get a better understanding of what cache to host is and what is TRULY meant by transfer rates such as SATA300, USB 2.0 480 Mbps? etc
  6. dmurray9133 said:
    Any online resources I could visit to get a better understanding of what cache to host is and what is TRULY meant by transfer rates such as SATA300, USB 2.0 480 Mbps? etc


    Start with wickipedia.
  7. dmurray9133 said:
    Great answer cjl - thanks! How would someone know when an extra 2 bits is included for parity checking? So it applies to SATA but what about other transfer rates for storage devices? How would you know?

    tx again

    It depends on the specific connection used. 8b/10b encoding is quite common though - USB, FireWire, gigabit ethernet, PCI-E, and HyperTransport all use it as well for example (and that's not even close to a complete list).
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