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Was wondering about the transfer rates.

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March 6, 2011 11:14:57 AM

Hispeed should be around 57MB
Fullspeed around 1.5MB
Lowspeed Around ??

Anyways my point is in theory they are supposed to be hitting that point but in practice they never are even close to achieving that transfer rate.
I would like to know why ?
Is it a bottleneck ?


Incase you didnt get what i ment to say.
I mean in theory Hispeed is 480 Mbits which is estimated at 60mbytes which is more like around 57 Mbytes but in reality if you transfer a file it usually is way way slower.
a b G Storage
March 6, 2011 12:32:00 PM

What interface are you talking about - USB? SATA?

Many interfaces have more available bandwidth than the devices run at, so future devices can improve over time.
March 6, 2011 12:47:47 PM

Sorry i forgot to mention that i was talking about USB 2.0
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March 6, 2011 1:11:18 PM


I was done calculating the speed of transfer rate in theory of Low speed USB 2.0 it is supposed to be 183KB per second.
But none of the speeds actually transfer at the rate they are supposed too...



Basically i am asking what are the factors that are affecting the speed.
a c 415 G Storage
March 6, 2011 3:56:07 PM

USB 2.0's theoretical transfer rate is 480Mbit/sec which works out to 60MByte/sec when you take the protocol overhead into consideration. But real-life transfer rates seem to be limited to the high 30's MByte/sec. I'm not 100% sure, but I think this is a consequence of the tick/tock nature of the USB protocol (time slices are reserved for host->device and device->host transmissions).
March 6, 2011 5:12:35 PM

sminlal said:
USB 2.0's theoretical transfer rate is 480Mbit/sec which works out to 60MByte/sec when you take the protocol overhead into consideration. But real-life transfer rates seem to be limited to the high 30's MByte/sec. I'm not 100% sure, but I think this is a consequence of the tick/tock nature of the USB protocol (time slices are reserved for host->device and device->host transmissions).


Thank you for taking your time and posting.
This is my problem i would like to understand why, for example i can understand if you got a slow HD that does not read and write as fast that would be a bottleneck no ? Or am i wrong...

I just need to understand the reasons behind the slower transfer rate, and what are the factors that cause it to be slower.
a c 415 G Storage
March 6, 2011 6:15:52 PM

Well if you have a hard drive connected via USB 2.0 and if the maximum sustained transfer rate of the drive itself is, say, 30MByte/sec or less then yes - the drive would be the bottleneck.

But if the drive can sustain a transfer rate of, say, 100MByte/sec then it will be the USB 2.0 connection that's the bottleneck.

If you're wondering why USB 2.0 can only transfer less than 40MB/sec when the data rate is theoretically 60MByte/sec, I believe it's largely due to protocol overhead. Although bits are transmitted at 480 million per second, not all of the bits are allocated to carrying data. There are headers, checksums, and acknowledgements involved as well. In addition to this the USB protocol "ticks" a message packet every millisecond, and some ticks are reserved for things like discovery of new devices.
March 6, 2011 6:18:43 PM

sminlal said:
Well if you have a hard drive connected via USB 2.0 and if the maximum sustained transfer rate of the drive itself is, say, 30MByte/sec or less then yes - the drive would be the bottleneck.

But if the drive can sustain a transfer rate of, say, 100MByte/sec then it will be the USB 2.0 connection that's the bottleneck.

If you're wondering why USB 2.0 can only transfer less than 40MB/sec when the data rate is theoretically 60MByte/sec, I believe it's largely due to protocol overhead. Although bits are transmitted at 480 million per second, not all of the bits are allocated to carrying data. There are headers, checksums, and acknowledgements involved as well. In addition to this the USB protocol "ticks" a message packet every millisecond, and some ticks are reserved for things like discovery of new devices.


Do you think that is the only reason or can the hardware on the pc affect it in any way? i mean such as i dono single core cpu vs dual vs quad ? would that also make a difference? Ram and same goes for the rest of the hardware...
a c 415 G Storage
March 6, 2011 6:36:16 PM

The CPU, RAM or other components in the system won't affect USB speed, but it does appear that some USB chipsets are slower than others. Most motherboard chipsets are as good as you can expect, it's mostly the ones in external devices such as cheap memory card readers that seem to be the issue.
March 6, 2011 9:37:03 PM

Maybe uts just a faulty drive, try it on another PC. Maybe your USB interface chip got stuck(i.e. faulty) on USB 1.1 thus low data rates(12Mb/s = 1.5MB/s) and cannot switch to 480Mb/s speeds or its just a USB ver 1.1 compatible only!. Number of connected devices to the USB HUB also can cause the same( bandwidth is shared!)
I'm unsure about this but its possible that certain USB controller chips set the MAX speed on the bus to the slowest of the devices attached(could be wrong do)
March 6, 2011 9:45:17 PM

sylmarils said:
Maybe uts just a faulty drive, try it on another PC. Maybe your USB interface chip got stuck(i.e. faulty) on USB 1.1 thus low data rates(12Mb/s = 1.5MB/s) and cannot switch to 480Mb/s speeds or its just a USB ver 1.1 compatible only!. Number of connected devices to the USB HUB also can cause the same( bandwidth is shared!)
I'm unsure about this but its possible that certain USB controller chips set the MAX speed on the bus to the slowest of the devices attached(could be wrong do)


I am not having problems its more like i need the knowledge, to help me understand better my course :)  my studies do not explain the reasons of why it is not performing as it should.
I tried to research on the internet about it, but i could not find anything about it.

So i was hoping to understand the roots of this problem and why it exists :D 
March 6, 2011 9:47:54 PM

sminlal said:
The CPU, RAM or other components in the system won't affect USB speed, but it does appear that some USB chipsets are slower than others. Most motherboard chipsets are as good as you can expect, it's mostly the ones in external devices such as cheap memory card readers that seem to be the issue.


Interesting...

I am wondering about something could it be that it was falsely advertised to run at that speed ? Companies have been known to false advertise so that they can sell more.

Or is the 20mbyte slower speed because of the factors you said before ?

Tick tock and such...
March 6, 2011 9:56:08 PM

Ok if you read about USB in general you will find a few speeds that this BUS can operate. and the minimum speed is 1.5mb/s for the first version of USB 1.0 the rest are posted up above except USB3 and that is different physically.

See here http://en.wikipedia.org/wiki/Universal_Serial_Bus

Why it doesn't work at maximum ? Your answer is up above! plus there are overheads to be taken in consideration i.e. error correction coding and so on but you need to see more on the USB protocol(certain bit streams mean certain things and those take up BW i.e. signaling TX, RX,speed negociation,port ID and so on).Probably this answers your question i hope that helps
March 7, 2011 9:39:06 AM

I was wondering if i by any chance made a mistake calculating the theoretical speed of a HiSpeed USB 2.0
480 Mbits / 8 / 1024 / 1024 = 57.220458984375 Mega Bytes
57 not 60 :/  or am i wrong or is it an estimation "60"
March 7, 2011 10:05:29 AM

sylmarils said:
Ok if you read about USB in general you will find a few speeds that this BUS can operate. and the minimum speed is 1.5mb/s for the first version of USB 1.0 the rest are posted up above except USB3 and that is different physically.

See here http://en.wikipedia.org/wiki/Universal_Serial_Bus

Why it doesn't work at maximum ? Your answer is up above! plus there are overheads to be taken in consideration i.e. error correction coding and so on but you need to see more on the USB protocol(certain bit streams mean certain things and those take up BW i.e. signaling TX, RX,speed negociation,port ID and so on).Probably this answers your question i hope that helps


According to Wikipedia its only 10-15 % needed for the communications protocol, so what happens to the rest of the missing speed ?
a c 415 G Storage
March 7, 2011 3:22:45 PM

serenatous said:
I was wondering if i by any chance made a mistake calculating the theoretical speed of a HiSpeed USB 2.0
480 Mbits / 8 / 1024 / 1024 = 57.220458984375 Mega Bytes
57 not 60 :/  or am i wrong or is it an estimation "60"
USB 2.0 really, truly does run at 480MBits/sec. And that's 480,000,000 bits/sec (transfer rates never use "binary" prefixes).

The real issue is that not all of the bits are used to carry data all of the time. The protocol is not 100% efficient - if it was then there'd be no error checking and no way to do some of the other things that USB does like dynamic device discovery. Those things are important and are an essential part of USB.

What you're struggling with is the difference between what the engineers who have to design the circuits deal with (the real, physical bits being transferred) and the practical world of "how long does it take me to transfer my file?".
March 11, 2011 8:56:27 AM

pretty much, but i got the point now, so thank you very much for your time and patience with me :bounce:  I think i got the point now :D 
March 14, 2011 10:51:19 AM

This thread is resolved. You could close the thread.
Thank you for your time and all your help.
!