[mythtv-users] 1TB USB $225 US

[Dan Ritter]

On Mon, Feb 11, 2008 at 11:24:20AM -0700, Brian Wood wrote:
> True I guess, on a lightly loaded network.
> 
> It's just getting pretty stupid. "Gigabit" network can't actually 
> approach that speed, USB can't come close to the advertised speed, 
> "54Mhz." wireless can't come close to that...

A Gigabit ethernet system can certainly come very close to
delivering a gigabit/second of data. 

(See, for example, http://www.dssnetworks.com/v3/FAQs.asp)

But. A cheap gigabit switch may only support a total of 1 Gb/s
across all the ports on the backplane. That's why it's cheap. Or
it may lose lots of speed trying to match a gigabit source with
a fast-ethernet sink. The technology is up to it, but the
implementation isn't necessarily there. And that's all right,
because you can also buy switches where the backplane can move a
bazillion frames per second. They'll cost you more, of course.

USB can come pretty close to advertised speeds, but disks can't,
and USB doesn't handle contention by several high-bandwidth
devices well. It's great for random input devices and small
storage, but expecting 480Mb/s from a disk that can only push
150MB/s from cache and about 90MB/s from spinning iron is silly.
The fastest Ultra320 SCSI disk, spinning at 15K RPM, has a
maximum transfer rate of 135MB/s. 

Wireless is its own ball of wax. For some reason the
manufacturers have been using the base signalling rate, before
error correction (absolutely necessary), as the advertised
speed. Real data rates there have always been (a) half of that
at best and (b) directly influenced by electromagnetic
interference, contention from other devices, and (c) antenna
positioning and distance. 25Mb/s (say, 3.3MB/s) hasn't been too
unusual for 802.11g systems, and 802.11n systems are clocking in at
80Mb/s. Transcoding your MPEG2 HD streams downwards is going to
be desirable for some time to come.

> Why can't manufacturers tell the truth about hardware capabilities?

Because we (generic we; all consumers) don't reward them for
that. We see shiny new numbers and "All New And Improved" and we
hope that this time, for sure, we can pull a rabbit out of this
hat.  But that trick never works. Research pays off.

-dsr-


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