100Mb/s ethernet on usb 1.1

Greetings:

(sorry for the cross post if not appropriate: similar queries were cross
posted to these two groups)

I would appreciate info on the behavior of USB <-> ethernet adapters
which support 100 Mb/s ethernet when used on USB 1.1 hosts; manufacturers
descriptions are lacking in specifics.

Some essential questions:

1.  Will these adapters negotiate 100 Mb/s when used on USB 1.1?
2.  If yes to (1) will there be a discernable speed improvement over
    10 Mb/s (depending on available onboard buffers, etc?)
3.  Some mfgrs advertise full duplex only on 100baseT; are there
    USB adapters that will do full duplex on 10baseT?

All replies much appreciated.

Michael Grigoni
Cybertheque Museum

Michael Grigoni wrote:
> 1.  Will these adapters negotiate 100 Mb/s when
> used on USB 1.1?

Why not?  The process negotiates Ethernet capabilities for the devices,
irrespective of abilities behind those interfaces.  Of course, the
developer could chose to have the product behave otherwise, but there's
no benefit to doing so.


> 2.  If yes to (1) will there be a discernable speed
> improvement over 10 Mb/s (depending on available
> onboard buffers, etc?)

USB1.1 runs at 1.5MBytes/sec (12Mbps), so it's possible to see better
performance with 100Mbps Ethernet.  In practice, half-duplex 10Mbps
Ethernet caps out just above 5Mbps (aggregate for both directions).


> 3.  Some mfgrs advertise full duplex only on
> 100baseT; are there USB adapters that will do full
> duplex on 10baseT?

Full-duplex and auto-sensing are not part of the 10Mbps Ethernet spec. 
Some Ethernet chipsets support a configurable full-duplex mode for 10Mb,
but it's prone to create tech support issues and only a fraction of the
market would use / appreciate it.  Far simpler to implement 10/100 into
the product and let it auto-negotiate.