On 06/26/09 05:31, Warp wrote:
>    Btw, I have never understood why "kilobit" is so popular of a term.
> Many things are measured in kilobits for no apparent reason, even though
> other similar things are measured in kilobytes.

	Like Andrew, I've seen it only in communications (and encoding is also 
considered part of communications...).

	Think modems (the phone ones). In the old days, it was all baud. 1 baud 
is 1bit/s. It probably has stuck since.

	Incidentally, I think with old modems, sending 1 byte didn't entail 
sending 8 bits. I think they had two error correcting bits, making 
sending 1 byte the same as 10 bits. If there's truth to that, I can see 
why it'd make sense to specify bits.


-- 
Q: What do you call a half-dozen Indians with Asian flu?
A: Six sick Sikhs (sic).


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Mueen Nawaz wrote:
>     Anyway,  to the best of my knowledge, kilobyte is the only one 

And if you look back at the instruction manuals for old 8-bit computers, 
like the TRS-80 model 1 or the Apple ][, you'll see them described as having 
65K of memory. I suspect nobody wanted to advertise 64K and have someone buy 
the competitor's machine because it had 65K. :-)

-- 
   Darren New, San Diego CA, USA (PST)
   Insanity is a small city on the western
   border of the State of Mind.

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Warp wrote:
>   Btw, I have never understood why "kilobit" is so popular of a term.

It's a PSTN term. I.e., it's used in communication mediums like the public 
switched telephone network where data isn't always byte-aligned. For 
example, an American T1 circuit delivers 193 bits per frame.

Nowadays, it's more common to pad things out to the byte, since line speeds 
are much higher. Nobody really tries to speed things up by using 5-bit 
characters any more, so the need for kilobits is lessened.

Of course, audio is still in kilobytes per second, where a kilobyte is 1000 
bytes and not 1024.

> in order to comprehend the meaning. (Of course an easy approximation would be
> to divide by 10... assuming the 25% error isn't significant in the context.
> In many contexts that's a huge error.)

The 25% is probably insignificant if the kilobits is communication speed, 
because you have TCP overhead, framing overhead, etc.

>   The only rational reason for using kilobits rather than kilobytes would
> be if you need to express sizes which are not multiples of 8 bits. However,
> in practice that's *never* the case.

Not any more.
http://en.wikipedia.org/wiki/Baudot_code

Of course, on serial lines, you have start bits and stop bits, so you're 
really sending anywhere between 9 and 12 bits per character, sometimes 
including even half-bits.

> All practical sizes in computing are
> multiples of 8 bits. There's absolutely no reason to use kilobits. There's
> unnecessary accuracy in the unit.

Again, yes, that's true now. Once people started mass-producing machines, 
and the software got to where it was reasonable to run software from one 
machine on another machine, 8-bit seemed to become most popular.

The intel 4004 was four bits, the PDP-8 was 12 bits, etc.  Nowadays, 
machines are powerful enough that you have protected memory mapped virtual 
memory capable machines sitting in boxes with no user interfaces on them.

-- 
   Darren New, San Diego CA, USA (PST)
   Insanity is a small city on the western
   border of the State of Mind.

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Mueen Nawaz wrote:
>     Think modems (the phone ones). In the old days, it was all baud. 1 
> baud is 1bit/s. It probably has stuck since.

Technically, one baud is one symbol per second. A 9600bps modem is a 2400 
baud modem with 2 symbols per baud.

>     Incidentally, I think with old modems, sending 1 byte didn't entail 
> sending 8 bits. I think they had two error correcting bits, making 
> sending 1 byte the same as 10 bits. If there's truth to that, I can see 
> why it'd make sense to specify bits.

One or two start bits to synchronize, one stop bit (or sometimes 1.5 stop 
bits) to process, and perhaps a parity bit. No error correcting bits, but 
parity might give you error detection if you're lucky.

-- 
   Darren New, San Diego CA, USA (PST)
   Insanity is a small city on the western
   border of the State of Mind.

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Darren New <dne### [at] sanrrcom> wrote:
> Mueen Nawaz wrote:
> >     Think modems (the phone ones). In the old days, it was all baud. 1
> > baud is 1bit/s. It probably has stuck since.
>
> Technically, one baud is one symbol per second. A 9600bps modem is a 2400
> baud modem with 2 symbols per baud.

What? That's nonsense, the units don't match here. You probably mean 2 bits per
symbol.

But even after *prehistoric* times, when transmission was almost always binary
(i.e. 1 bit per symbol), the misconception of "baud = bps" stuck for quite a
while.


> >     Incidentally, I think with old modems, sending 1 byte didn't entail
> > sending 8 bits. I think they had two error correcting bits, making
> > sending 1 byte the same as 10 bits. If there's truth to that, I can see
> > why it'd make sense to specify bits.
>
> One or two start bits to synchronize, one stop bit (or sometimes 1.5 stop
> bits) to process, and perhaps a parity bit. No error correcting bits, but
> parity might give you error detection if you're lucky.

The parity would typically be used with only 7 data bits.


But as a matter of fact, Kilo-, Mega- and GigaBITS are actually a much more
natural unit of measuring information storage or transmission capacity.

It's not only commonly used with transmission media, but also in
microelectronics, where (except in the "computer proper" business) you'll
virtually never find storage capacity specified in multiples of bytes *except*
when the storage device happens to be designed to access 8 bits in parallel. In
all other cases, chips are usually specified either at multiples of bits, or
multiples of a data word (the number of bits accessed in parallel), which would
typically be 1, 4, 8 (sometimes 9) or 16 bits.

A 2G DDR2 memory module should actually be designated as a 32Mx64 module. Would
be hard to explain to the customers, who are almost invariably locked with the
concept of a byte as the smallest unit of memory.

Happens to hardcore software developers, too. I once had some problems figuring
out why some intended-to-be-portable C software library I had developed didn't
run properly on a particular microcontroller - until finding out that a "char"
data type (the smallest data type in C, typically equalling a byte) was
actually 16 bits wide on that rascal. "A byte? What's that, Sir?" Doesn't make
life easier when you're developing a portable library for a communications
interface heavily based on the concept of the 8-bit thing.

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clipka wrote:
> Darren New <dne### [at] sanrrcom> wrote:
>> Mueen Nawaz wrote:
>>>     Think modems (the phone ones). In the old days, it was all baud. 1
>>> baud is 1bit/s. It probably has stuck since.
>> Technically, one baud is one symbol per second. A 9600bps modem is a 2400
>> baud modem with 2 symbols per baud.
> 
> What? That's nonsense, the units don't match here. You probably mean 2 bits per
> symbol.

Yes, sorry. Two bits per symbol aka two bits per baud.

> But even after *prehistoric* times, when transmission was almost always binary
> (i.e. 1 bit per symbol), the misconception of "baud = bps" stuck for quite a
> while.

Yes.  Except, of course, in the phone company, where it actually made a 
difference. :-)

> The parity would typically be used with only 7 data bits.

Usually, yes. Usually either 8-N-1 or 7-?-2, IME.

> all other cases, chips are usually specified either at multiples of bits, or
> multiples of a data word (the number of bits accessed in parallel), which would
> typically be 1, 4, 8 (sometimes 9) or 16 bits.

Yep. What about the serial type chips, like SD cards and stuff? How do they 
get rated technically, do you know?

> run properly on a particular microcontroller - until finding out that a "char"
> data type (the smallest data type in C, typically equalling a byte) was
> actually 16 bits wide on that rascal.

Yeah. This is why Ada has different types for "a memory unit" and "an I/O unit".

-- 
   Darren New, San Diego CA, USA (PST)
   Insanity is a small city on the western
   border of the State of Mind.

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Darren New wrote:
> Warp wrote:
>> Darren New <dne### [at] sanrrcom> wrote:
>>> I didn't mean to single you out personally in any way. I just thought
>>> you'd be amused that the two search engines couldn't agree. :-)
>> 
>>   The question remains: Which one is correct?
> 
> I find it interesting that google thinks 1024 meg is a gig, and wolfram
> thinks 1000 meg is a gig. :-) 

Maybe Wolfram thinks 1000 megabytes are a gigabyte, and 1024 mibibytes are a
gibibyte?

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clipka wrote:
> Happens to hardcore software developers, too. I once had some problems
> figuring out why some intended-to-be-portable C software library I had
> developed didn't run properly on a particular microcontroller - until
> finding out that a "char" data type (the smallest data type in C,
> typically equalling a byte) was actually 16 bits wide on that rascal. "A
> byte? What's that, Sir?" Doesn't make life easier when you're developing a
> portable library for a communications interface heavily based on the
> concept of the 8-bit thing.

A C 'char' type MUST be exactly 1 byte long (in particular, sizeof(char)
MUST be 1). However, a C implementation may define "byte" with more than 8
bits.

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Nicolas Alvarez <nic### [at] gmailcom> wrote:
> clipka wrote:
> > Happens to hardcore software developers, too. I once had some problems
> > figuring out why some intended-to-be-portable C software library I had
> > developed didn't run properly on a particular microcontroller - until
> > finding out that a "char" data type (the smallest data type in C,
> > typically equalling a byte) was actually 16 bits wide on that rascal. "A
> > byte? What's that, Sir?" Doesn't make life easier when you're developing a
> > portable library for a communications interface heavily based on the
> > concept of the 8-bit thing.

> A C 'char' type MUST be exactly 1 byte long (in particular, sizeof(char)
> MUST be 1). However, a C implementation may define "byte" with more than 8
> bits.

  Does the C standard use the word "byte", or does it simply say that
sizeof(char) must always be 1 (without specifying a name for the unit)?

  Of course if you need to take into account the amount of bits in your
integral types (including char) and you want your C program to be fully
portable even to exotic embedded systems, you have to use the CHAR_BIT
constant defined in limits.h rather than assume it's always 8.

-- 
                                                          - Warp

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On 06/26/09 11:13, Darren New wrote:
> Mueen Nawaz wrote:
>> Think modems (the phone ones). In the old days, it was all baud. 1
>> baud is 1bit/s. It probably has stuck since.
>
> Technically, one baud is one symbol per second. A 9600bps modem is a
> 2400 baud modem with 2 symbols per baud.

	Fine, be picky.<G>

	I was taught it meant 1 cps (character per second) - which could have 
been 1 bit/s, but not necessarily. I suppose character is the same as 
your symbol.

	It's just that in all the contexts that _I've_ used it, a baud was 1bit/s.


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                        >>>>>>mue### [at] nawazorg<<<<<<
                                    anl

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