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USB 2.0 speed

By indi
Mar 8, 2006
  1. USB 2.0 seem to have 480Mbps speed of transfer. Can anyone explain it in detail.

    USB 2.0 tranfers data is microframe (125us) compared to a frame (1ms) in USB 1.1. In a single microframe u can send 3 packets i..e 3 x 1024 = 3072 bytes.

    Hence 3072 x 8 bytes in 1ms
    3072 x 8 x 8 bits in 1 ms
    3072 x 8 x 8 x 1000 in 1 second
    ...
    ...
    it comes to 196Mbps

    Then how is 480 Mbps achieved
     
  2. A_DOG73

    A_DOG73 TS Rookie Posts: 170

    Who cares how!

    I just love USB. It is so convenient. I don't know what I would do without it. :haha:
     
  3. Nodsu

    Nodsu TS Rookie Posts: 5,837   +6

    Look up bulk transfers.. You can fit 13 512-byte bulk packets into a microframe and you get 13*512*8*8*1000 bits per second for useful data (426Mbps)
     
  4. indi

    indi TS Rookie Topic Starter

    Thanks a lot, but still we are not reaching 480Mbps
    Then i checked the USB 2.0 spec where I came across bulk ttransfer as 7500 bytes per micro frame which on calculation results into 480Mbps (60MBps)....but no details about data payload details or Max transfers



     
  5. Nodsu

    Nodsu TS Rookie Posts: 5,837   +6

    That 480 is not "useful" bandwidth (who has ever heard of anyone advertising anything with its real speeds and not some arbitrary numbers? :p ). It is just the amount of bits that go through the wire when using the hi-speed clock rate.
     
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