USB speeds vs device speed ratings

[bxf]

The answer to this must be something rather fundamental, but it escapes
me. Perhaps someone is able to explain.

There are several broadband USB modems on the market these days. Mine,
for example, is rated at 2.4mbps. How can a USB device have a speed
that by far exceeds the speed of which USB is capable? Not only that,
but my modem appears to be a USB1 device, which should mean a limit of
12mbps. Something doesn't add up. In reality, I only manage speeds that
occasionally reach a maximum of about 20% of the 2.4mbps rating, but I
suspect that is for reasons other than those posed by my question, but
in any event certainly exceeds USB1 speeds.

To further complicate the question, I can mention that the modem is
shown to be connected to a COM port, and these things have a speed
limit of 115,200 bps. This throws another variable into what I see as
inconsistency in all these numbers.

What am I missing?

Thanks for any input.

 

[R. McCarty]

Part of the problem is the "Notation" used. the other is that there are
actually 3 USB speed ratings, Low, Full & High ( USB 1, 1.1 & 2.0)

1.) mbps denotes Mega'Bits' per Second
2.) Mbps denotes Mega'Bytes' per Second

Unfortunately, most USB devices/peripherals describe their
performance is the 'Bits' notation not 'Bytes' like memory and
disk space. You have to do some math to convert the bits to
bytes. A conversion chart can be found here:
http://en.wikipedia.org/wiki/Mbit/s
*Near the bottom of the page

 

[bxf]

Thanks for the reply, but I don't think that's quite it.

For one thing, I'm used to seeing a "B" to represent Bytes, as opposed
to a small "b" to represent bits. And, as far as I'm able to ascertain,
the speeds of 12 for USB1.1 and 480 for USB2 represent megabits, not
megabytes. USB2 has a limit of 480 Megabits per second, or 60 MegaBytes
per second.

No, there must be another explanation, but thanks anyway.

 

[Eric]

The answer to this must be something rather fundamental, but it escapes
me. Perhaps someone is able to explain.

There are several broadband USB modems on the market these days. Mine,
for example, is rated at 2.4mbps. How can a USB device have a speed
that by far exceeds the speed of which USB is capable? Not only that,
but my modem appears to be a USB1 device, which should mean a limit of
12mbps. Something doesn't add up. In reality, I only manage speeds that
occasionally reach a maximum of about 20% of the 2.4mbps rating, but I
suspect that is for reasons other than those posed by my question, but
in any event certainly exceeds USB1 speeds.

To further complicate the question, I can mention that the modem is
shown to be connected to a COM port, and these things have a speed
limit of 115,200 bps. This throws another variable into what I see as
inconsistency in all these numbers.

What am I missing?

Thanks for any input.

Hi,

It looks like it fits to me.

If all the planets were lined up perfectly:

USB1.1 is capable of 12 megabits per second.
USB2.0 is capable of 480 megabits per second.
Your modem can bring in 2.4 megabits per second.

The USB speeds are burst speeds, of course, but even the sustained speed of
USB1.1 should be a wide enough pipe.

As for the COM port, where are you getting the number of 115.2 kilobits per
second?

Are you seeing this number somewhere in device properties?

Remember, that this COM port being "used" by the USB cable modem is a
virtual COM port.

I'm not familiar using USB for network things that require a virtual COM
port, but does it emulate a serial, parallel, or it's own funky
interpretation of a port? Remember that COM ports have evolved, at the
hardware level, over the years from their original vanilla "standard".
I.e., in the early to mid 90's, when devices other-than-printers starting
appearing that required a "high" (relative) transfer rate -- such as
scanners and external Zip drives that did funky SCSI emulation --, parallel
ports enhanced at the hardware level started showing up on the scene. (Such
as ECP parallel, which transfer up to 3 megabits per second.)

Basically, if you want to answer your COM port question, you'd have to find
out what kind of port is being emulated. Again, I'm not familiar with USB
for networking, so as far as I know USB probably has it's own funky type of
COM port it emulates. I wouldn't be surprised if that was the case. Either
way, the virtual COM port isn't a bottleneck for your cable modem.

In my opinion, using USB for networking is really "gimmicky", anyway. The
whole virtual COM port thing is a holdover from yesteryear design. If you
don't think you are getting good performance out of this USB modem, why not
get a standard cable modem that has a standard ethernet port? Down the road
when you add new hardware, especially networking hardware, you'll be much
less limited.

Cheers,
Eric

 

[bxf]

It looks like it fits to me.

If all the planets were lined up perfectly:

USB1.1 is capable of 12 megabits per second.
USB2.0 is capable of 480 megabits per second.
Your modem can bring in 2.4 megabits per second.

Duh, I must be going senile. Although the numbers clearly speak for
themselves, my mind somehow "saw" the 480 megabits as .480 megabits. To
put it another way, I saw the 2.4 as millions, and the 480 as
thousands. Please don't ask me why. Not only that, but if I'm not
mistaken, I already "solved" this "puzzle" myself some time back, and
have since forgotten. I'm not sure. I've been sitting on my rear end
for almost 10 months doing not much more than playing with my laptop,
and it must be time to take a long break.

The USB speeds are burst speeds, of course, but even the sustained speed of
USB1.1 should be a wide enough pipe.
Understood.

As for the COM port, where are you getting the number of 115.2 kilobits per
second?

Are you seeing this number somewhere in device properties?

Device Manager > Modems > my modem > Properties > Modem tab shows Port:
COMx, Maximum Port Speed 115200. I understand this to be the maximum
speed of any serial port.

Remember, that this COM port being "used" by the USB cable modem is a
virtual COM port.

I'm not familiar using USB for network things that require a virtual COM
port, but does it emulate a serial, parallel, or it's own funky
interpretation of a port? Remember that COM ports have evolved, at the
hardware level, over the years from their original vanilla "standard".
I.e., in the early to mid 90's, when devices other-than-printers starting
appearing that required a "high" (relative) transfer rate -- such as
scanners and external Zip drives that did funky SCSI emulation --, parallel
ports enhanced at the hardware level started showing up on the scene. (Such
as ECP parallel, which transfer up to 3 megabits per second.)

Basically, if you want to answer your COM port question, you'd have to find
out what kind of port is being emulated. Again, I'm not familiar with USB
for networking, so as far as I know USB probably has it's own funky type of
COM port it emulates. I wouldn't be surprised if that was the case. Either
way, the virtual COM port isn't a bottleneck for your cable modem.

OK, I had no awareness of the concept of an emulated port. From what
I've seen, I'd say it emulates a Serial Port. But, if it's emulated
then I can accept that normal hardware constraints would not
necessarily apply, so that makes the 115200 bps limit of Serial ports a
non-issue, I suppose.

In my opinion, using USB for networking is really "gimmicky", anyway. The
whole virtual COM port thing is a holdover from yesteryear design. If you
don't think you are getting good performance out of this USB modem, why not
get a standard cable modem that has a standard ethernet port? Down the road
when you add new hardware, especially networking hardware, you'll be much
less limited.

I work as a freelancer, moving around countries and continents, and I
just happen to be at home for an extended period at present,
unexpectedly, but there would be no point in getting a cable modem and
committing myself to at least a year's payments if I'm not likely to
stick around for very long, The USB modem is completely portable, and
in theory, if I can find an appropriate ISP that supports the device, I
can use it anywhere.

Cheers,
Eric

Thanks for the wake-up (slapping self in face).

 

[DandyDon]

Not to beat a dead horse, but the simple answer is that the USB ports are
speed rated as explained previously. The upload / download speed of the
devices that plug into them, however, can be whatever the manufacturer
chooses to set them at; depending on the device. Hope this doesn't muddy the
waters.