[wellylug] FYI: Firewire vs USB2

[Andrew]

Hi all,

Sorry the info wasn't 100% right, more like general info. So - FireWire 
clarifications:

Both FW and USB have fixed bandwidth available. As devices are added 
(and use that bandwidth), the amount available drops. The difference is 
HOW the bandwidth is allocated (centralized at the CPU for USB, 
distributed among the devices for FW). USB can do video just fine, just 
not as well. The real problem is that USB devices can't talk directly 
to each other. A USB camcorder copy would have the video stream be 
packets sent first from one camcorder to the CPU and then from the CPU 
to the other camcorder. FW camcorders talk directly to each other.

FW400's peak speed is 50MByte/sec, USB2 can do 60MByte/sec. Actual 
measured throughput for a disk drive is about 42MByte/sec for FW, 
38MByte/sec for USB2. The USB2 number can be increased as Intel 
improves their chipset ... But it will never be much more than about 
50Mbyte/sec ... FW400 is doing about as good as it can right now, but 
we've seen FW800 disks running at up to 60MByte/sec  and we know we can 
get that to about 85-90Mbyte/sec as we tune the SW and HW.

FW power is specified as 8-30V, up to 1.5A (or a maximum of 45W) per 
port.

USB can only provide 500ma at 5V, or 2.5W.

Both FW and USB are shared busses ... And FW400 traffic cuts into the 
amount available for FW800 ... This is *important* since FW devices 
need to talk directly with each other without the CPU getting in the 
way.

In fact, Mac USB implementations are better than normal because the USB 
1.1 traffic is not shared. You can run a full 12Mbit/sec out of every 
Mac USB port at the same time. (This is because USB devices only talk 
with the CPU, not each other.) Note that a USB hub can't do this trick 
since it must direct all the data back to the CPU ... A hi-speed USB2 
hub can, however, combine several USB 1.1 streams together since it 
talks to the CPU at the higher 480Mbit/sec rate.


On Wednesday, September 17, 2003, at 11:03  AM, Brenda Wallace wrote:

> On Wed, 2003-09-10 at 22:13, Andrew wrote:
>> 3. FireWire supplies power to devices. The FireWire bus can power
>> devices needing up to 15 volts of power, so that means that compact
>> "pocketable" hard drives can be powered by the FireWire bus, whereas
>> USB 2 portable drives need an external power brick.  This is why the
>> iPod can be charged by the computer it's connected to, as it uses
>> FireWire.  If the iPod used USB 2, it would require a separate AC
>> adapter to recharge the battery.
>
> is this true?? usb1 devices are powered from the bus.. i've seen plenty
> mp3 players that charge their battery from usb.
> can you really no longer do that under usb2? or have i misunderstood
> this?
>
>
> -- 
> Brenda Wallace <brenda at wallace.net.nz>
>
>
> -- 
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