Say what? [LWN.net]

Say what?

Posted Jun 11, 2009 21:20 UTC (Thu) by martinfick (subscriber, #4455)
In reply to: Say what? by sbergman27
Parent article: Linux first to offer USB 3.0 driver (Linux Devices)

Serial buses tend to be quoted in bits per second. Disk throughput (not traditionally serial) mostly uses Bytes.

I suspect that the first statement was meant to be bits, and the "real world" statement was meant to be bytes (because dividing by 10 is hard).

to post comments

Say what?

Posted Jun 11, 2009 22:51 UTC (Thu) by drag (guest, #31333) [Link] (11 responses)

* This. *

This is the answer.

The guy f*d up on the math, did a typo, or simply lacked understanding. I regularly see speeds of 10-20MBps on fast flash devices over USB 2.0.

Sure there is overhead to maintaining USB devices, but your looking at 10-20% overhead, not 90%. That's worse then Wifi which your lucky to get data transfer at 50% of the speed your connected at.

Say what?

Posted Jun 11, 2009 23:10 UTC (Thu) by sbergman27 (guest, #10767) [Link] (10 responses)

FWIW, if I don't get at least 30MB/s from a mechanical USB 2.0 drive, I start looking around for what the problem is.

The purpose of my original post was to point out how quoting bits/s instead of bytes/s causes great confusion for essentially no benefit. Mbps? MBps? Divide by 10? Divide by 8? 7 bit? 8 bit? Start bit? Stop bit? Parity bit? Frame bits?

I've dealt with this crap for 21 years or more, and it's time for the madness to end.

Computone? Digiboard? Okidata? TI? Do you cross 2 and 3? 4 and 5? Tie together 4, 8 and 20? Does 7 go to 7... or to 5? DB25? DB9? God save us if it's a conversion cable. Or a conversion crossover cable.

We've dumped most of the insanity. But let's get rid of the rest. Stop my nightmares.

Just tell me how many Megabytes Per Second the damned interface is nominally supposed to transfer and I'll be as happy as a clam at high tide.

DE-9

Posted Jun 12, 2009 7:36 UTC (Fri) by ncm (guest, #165) [Link] (2 responses)

That's "DB-25" and "DE-9". "D" is the shape, "B" or "E" the size, and "25" or "9" the number of pins. A "DA shell" has room for lots of pins, a "DF" not many.

DE-9

Posted Jun 12, 2009 13:45 UTC (Fri) by sbergman27 (guest, #10767) [Link] (1 responses)

Except that in practice, virtually nobody ever says DE-9.

DE-9

Posted Jun 20, 2009 20:07 UTC (Sat) by giraffedata (guest, #1954) [Link]

Oh, lots of people get DE-9 right. And there's nothing to lose by saying it right even if most people don't.

If you want to see a connector name that is almost universally misused, look at RJ-45. Virtually everybody calls the 8P8C modular telephone connector RJ-45, even though the real meaning of RJ-45 (an ancient hookup standard for modems) is formally documented.

Let's stop at bytes per second

Posted Jun 12, 2009 11:06 UTC (Fri) by khim (subscriber, #9252) [Link] (3 responses)

Just tell me how many Megabytes Per Second the damned interface is nominally supposed to transfer and I'll be as happy as a clam at high tide.

But how many bytes per megabyte? Some still think 1048576 (or may be 1024000 like on floppy disk?) is the right asnwer... The only sane unambigous measurement is byte per second...

Let's stop at bytes per second

Posted Jun 12, 2009 13:51 UTC (Fri) by sbergman27 (guest, #10767) [Link] (2 responses)

Bytes per second is fine with me.

FWIW, to me the only place where it makes any sense at all to use kibibytes (an how I despise that term!) is for RAM. Durable storage like hard drives, CDs, DVDs, and even flash memory, really, should just use simple powers of 10. RAM is a special case.

By the by... does anyone happen to know what style of "megabits per second" are referenced by USB 2.0's 480 mbit/s spec?

Let's stop at bytes per second

Posted Jun 12, 2009 19:12 UTC (Fri) by admcd (subscriber, #5415) [Link]

Ah, but how many bits in a byte.

I'm not sure whether there are any current systems where a byte isn't 8 bits. However, some standards bodies use "octet" to avoid any possible ambiguity.

Let's stop at bytes per second

Posted Jun 12, 2009 20:37 UTC (Fri) by gilb (subscriber, #11728) [Link]

USB 2.0 is 480x10^6 bits/second. PHY people tend to think in powers of 10.

Say what?

Posted Jun 12, 2009 14:15 UTC (Fri) by da4089 (subscriber, #1195) [Link] (2 responses)

i can't help but mention bps vs baud as well :-)

Say what?

Posted Jun 12, 2009 20:39 UTC (Fri) by gilb (subscriber, #11728) [Link] (1 responses)

In baud, I expect that USB 3.0 would be listed as 500 Mbaud as the symbols are made up of 10 coded bits.

That would make 802.11a/g 250 kbaud for all of its data rates (from 6 to 54 Mb/s).

Say what?

Posted Jun 20, 2009 20:17 UTC (Sat) by giraffedata (guest, #1954) [Link]

In baud, I expect that USB 3.0 would be listed as 500 Mbaud as the symbols are made up of 10 coded bits.

The fact that you can separate those bits in time means it's 5000 Mbaud.

In fact, it's directly analogous to a classic baud-measured protocol, Bell 103A: A 300 Baud modem transmits 300 bits per second, with every 10 representing one byte analogous to a byte in the USB stream.

On a transmission line, it is posible to represent multiple bits in a single indivisible unit of time -- a symbol --, and that's where baud and bps differ. For example if you have 4 defined voltages, a single symbol carries two bits.