Re: USB v1 and v2 compatibility -- OHCI v. EHCI ...

["Bryan J. Smith" <b.j.smith@ieee.org>]

On Thu, 2004-10-28 at 20:36, Tom Dison wrote:
> I've run an external USB 2.0 CR-RW off a 1.1 port on
> my laptop many times, it works fine (under windoze,
> linux and BSD). However, you can only burn at 4X.
> Better to get a USB 2 PCMCIA card. I only have one
> PCMCIA slot (cardbus), and I usually have a wireless
> NIC in it.

First off, remember that USB 2.0 does _not_ necessarily mean 480Mbps
(60MBps).  This is due to Intel's "renaming" of USB 2.0 to include _all_
ports on a USB 2.0 card.

You _must_ plug a device into an EHCI host controller to get 480Mbps. 
There are typically on one (1) EHCI host controller to only 1-2 ports on
any USB 2.0 card.  Use "usbview" to find out if you are connected to a
EHCI host controller.

Secondly, understand that 480Mbps is _not_ achievable.  Intel has
admitted that it can't even get half that speed theoretical.  Long story
short, USB was _never_ designed for high speed block devices, but only
lower speed character devices.

Many things like memory mapped I/O were retrofitted to USB _after_ Intel
decided to not license IEEE1394 Firewire.  And such features are done in
_software_, not hardware (unlike Firewire).

EHCI USB is fine for lower speed block devices like 22-54Mbps
(2.5-6.8MBps) WLAN or 48x (6.1MBps) CD-RW or even 4x (5.4MBps)
DVD-RW/+RW drives, possibly even 3.6MBps DV camcorders.  But once you
start talking external hard drives pushing over 20MBps, EHCI can't
compare to FireWire.

And if you are talking device-to-device, forget it, because USB cannot
do it.  FireWire is like on-board intelligence Ultra80 SCSI, USB 2.0 is
like old, pre-DMA Enhanced IDE (EIDE).

-- 
Bryan J. Smith                                  b.j.smith@ieee.org 
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