Say what?
Say what?
Posted Jun 11, 2009 21:11 UTC (Thu) by sbergman27 (guest, #10767)Parent article: Linux first to offer USB 3.0 driver (Linux Devices)
The spec is said to offer physical-layer throughput speeds of up to 5Gbps (gigabits per second), compared to USB 2.0's 480Mbps (megabits per second). By comparison, Firewire (IEEE 1394) currently tops out at 3.2Gbps.
It's said real-world transfer rates for USB 3.0 may be as much as 500Mbps, compared to 25Mbps to 35Mbps for typical USB 2.0 mass storage drivers, offering the potential for 20x faster speeds.
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Translating all of this to MBps (megabytes per second), for easier understanding...
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The spec is said to offer physical-layer throughput speeds of up to 625MBps compared to USB 2.0's 60MBps. By comparison, Firewire (IEEE 1394) currently tops out at 400MBps.
It's said real-world transfer rates for USB 3.0 may be as much as 62.5MB per second, compared to 3MBps to 4.5MBps, offering the potential for 20x faster speeds.
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So what's with the 10:1 ratio between wire speed and real world speed for USB 3.0? And I could have sworn my current USB 2.0 drives have managed better than 4.5MB/s.
And why are we still quoting speeds in terms of bits per second in the year 2009? I thought we agreed upon a standard size for a byte a while back, and the proponents of '7' lost?
Posted Jun 11, 2009 21:16 UTC (Thu)
by tzafrir (subscriber, #11501)
[Link] (3 responses)
Posted Jun 11, 2009 21:39 UTC (Thu)
by sbergman27 (guest, #10767)
[Link] (2 responses)
Posted Jun 12, 2009 6:40 UTC (Fri)
by shalem (subscriber, #4062)
[Link] (1 responses)
Posted Jun 12, 2009 6:48 UTC (Fri)
by yootis (subscriber, #4762)
[Link]
Completely wrong. Gigabit ethernet is 1 gigabit in each direction simultaneously. Fiber or copper doesn't matter, they get the same speed.
I routinely get 960 megabits per second in each direction at the same time.
Posted Jun 11, 2009 21:20 UTC (Thu)
by martinfick (subscriber, #4455)
[Link] (12 responses)
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).
Posted Jun 11, 2009 22:51 UTC (Thu)
by drag (guest, #31333)
[Link] (11 responses)
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.
Posted Jun 11, 2009 23:10 UTC (Thu)
by sbergman27 (guest, #10767)
[Link] (10 responses)
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.
Posted Jun 12, 2009 7:36 UTC (Fri)
by ncm (guest, #165)
[Link] (2 responses)
Posted Jun 12, 2009 13:45 UTC (Fri)
by sbergman27 (guest, #10767)
[Link] (1 responses)
Posted Jun 20, 2009 20:07 UTC (Sat)
by giraffedata (guest, #1954)
[Link]
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.
Posted Jun 12, 2009 11:06 UTC (Fri)
by khim (subscriber, #9252)
[Link] (3 responses)
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...
Posted Jun 12, 2009 13:51 UTC (Fri)
by sbergman27 (guest, #10767)
[Link] (2 responses)
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?
Posted Jun 12, 2009 19:12 UTC (Fri)
by admcd (subscriber, #5415)
[Link]
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.
Posted Jun 12, 2009 20:37 UTC (Fri)
by gilb (subscriber, #11728)
[Link]
Posted Jun 12, 2009 14:15 UTC (Fri)
by da4089 (subscriber, #1195)
[Link] (2 responses)
Posted Jun 12, 2009 20:39 UTC (Fri)
by gilb (subscriber, #11728)
[Link] (1 responses)
That would make 802.11a/g 250 kbaud for all of its data rates (from 6 to 54 Mb/s).
Posted Jun 20, 2009 20:17 UTC (Sat)
by giraffedata (guest, #1954)
[Link]
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.
Posted Jun 11, 2009 21:54 UTC (Thu)
by sf_alpha (guest, #40328)
[Link] (2 responses)
At time USB 2.0 released, most early generation controller does not have enough processing power and buffer to reach 100Mbps throughput. But now I can copy files from my USB 2.0 drives at 20MBPS++ (200Mbps++).
And ... USB 3.0 now support FULL-DUPLEX, real-world throughput is much improved and theorically may reach maximum throughput at 4Gbps+ if controller is good enough.
Bit rate is mostly describe rate of serial of bits that transfer on the wire. Byte per second is usually use to measure throughput of real data or payload.
Posted Jun 12, 2009 13:22 UTC (Fri)
by clugstj (subscriber, #4020)
[Link] (1 responses)
Posted Jun 12, 2009 14:17 UTC (Fri)
by khim (subscriber, #9252)
[Link]
Connector
A, Connector
B and Connector
Micro B - two new wires for SuperSpeed Up and two new wires for SupeSpeed
Down. Like Gigabit Ethernet, just shorter and few times faster...
Posted Jun 12, 2009 12:39 UTC (Fri)
by robert_s (subscriber, #42402)
[Link] (2 responses)
Nope. That's not how it works. Take 8b10b coding for example. It takes 10 bits to transmit a(n 8 bit) byte, so the raw bitrate is indeed ten times that of the datarate.
Posted Jun 12, 2009 15:10 UTC (Fri)
by k8to (guest, #15413)
[Link] (1 responses)
Posted Jun 20, 2009 20:33 UTC (Sat)
by giraffedata (guest, #1954)
[Link]
But bytes aren't a unit of measure to be quoted. They're units of data.
8 bits don't make a byte like 12 inches make a foot. It's more like 24 beers make a case. You could point to a keg of beer and say it contains 6.9 cases of beer, but in fact there are no cases of beer in it.
Computer memory and disk sectors, on the other hand actually have bytes in them.
The only unambiguous measure you can give is the raw bit signalling rate. Try to get more real-world than that, and you have to get specific about which real world you mean.
Posted Jun 12, 2009 20:31 UTC (Fri)
by gilb (subscriber, #11728)
[Link]
If you rated 802.11a/g the same way it would be 72 Mb/s. The 54 Mb/s data rate uses a 3/4 coding, i.e., for every 3 bits input, you get 4 coded bits on the air. (4 us symbol time, 288 coded bits per symbol, 216 data bits per symbol.)
This sort of exaggeration seems to be becoming more common. I sometimes hear that HDMI 1.0 is 4.5 Gb/s, when it is really 3.6 Gb/s (TMDS encodes 8 bits for every 10 symbols as well).
The good news is that the 4 Gb/s is bi-directional, there are separate up and down link wires, as opposed to USB 1.0 and 2.0 in which the same wire and hence the available bandwidth is shared for up and down link.
The protocol for USB 2.0 is not particularly efficient either, which is part of what gives the much lower throughput for a single device. The aggregate throughput (i.e., with more than one device) can be higher. OTOH, the simplicity of the protocol makes endpoints cheap.
USB 3.0 looks to be a much more efficient protocol, so it is possible that the throughput improvement will be significant, aside from the improvement in the PHY rate.
Say what?
Say what?
Say what?
multiplied by 2 as its fullduplex, with that reasoning 100Mbit nics are 200Mbit). Yip, another score for the marketing department.
Say what?
Say what?
Say what?
Say what?
DE-9
DE-9
Oh, lots of people get DE-9 right. And there's nothing to lose by saying it right even if most people don't.
DE-9
Let's stop at bytes per second
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.
Let's stop at bytes per second
Let's stop at bytes per second
Let's stop at bytes per second
Say what?
Say what?
Say what?
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.
Say what?
Say what?
Four new wires
4Gbps full-duplex on a single wire? How do they do that, or did
they add more wires for USB 3.0.
Say what?
Say what?
Say what?
That wouldn't stop us from quoting in bytes.
It complicates the picture a bit, but there are plenty of complications.
Co-opting the word "byte" as a unit of measure does complicate the picture, and it doesn't make anything less confusing, because there are so many levels on which to count the bits. Most people would say it's fraudulent to say 5E9/8 bytes travel down this line every second, since less than 80% of that number end up as actual bytes in computer memory.
Say what?
