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USB Throughput

I was thinking about high speed USB vs full speed and how hubs work. If I understand correctly, a high speed hub always uses high speed upstream with a high speed host port, but can use either high speed or full speed on the downstream port. So it would only make sense that once a full speed data stream reaches a high speed hub, it can be combined with other full speed streams into that same hub and utilize the higher speed of the high speed service? That is, more than one full speed stream can be merged with no apparent impact on performance?

I'm looking to verify that there is an advantage to using a high speed hub with multiple full speed devices.

Rick

Reply to
rickman
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Correct. The host will be doing 'split transactions'. Here's a good explanation of what happens on the bus:

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In between the start-split and complete-split transfers, the host and usb are free to service other ports. Of course, it depends on the actual implementation how efficiently this is done.

Reply to
Arlet Ottens

Look at it the other way round.

If you run only full speed (12Mbps) to host the max speed to host is

12Mbps to be SHARED between the devices on the hub(s). 4 port hub means 12Mbps / 4 if all active at the same time i.e 3Mbps THROUGHPUT theoretical max excluding overhead to the host.

Therefore if you have High speed (480Mbps) to host, this shared amongst the devices on the hub(s), so can take each full speed device (12Mbps) at their full 12Mbps to the host.

Now work out the throughput on high speed for a 4 port hub.

--
Paul Carpenter          | paul@pcserviceselectronics.co.uk
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Reply to
Paul

And here: usb_20.pdf, extracted from

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Section 8.4.2

-- Roberto Waltman

Reply to
Roberto Waltman

ed

can

b_20_081810.zip

Thanks to all, this will give me something to chew on.

Rick

Reply to
rickman

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