Re: Does more bits-per-character but less characters-per-second make better use of bandwidth?
- From: "Paul E. Schoen" <pstech@xxxxxxxxx>
- Date: Sat, 15 Sep 2007 13:06:07 -0400
"Green Xenon [Radium]" <glucegen1@xxxxxxxxxx> wrote in message
news:46ea9a2d$0$32480$4c368faf@xxxxxxxxxxxxxxxxx
On Sep 14, 7:18 am, _ <jtayNOSPAM...@xxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote
in
http://groups.google.com/group/sci.electronics.design/msg/5400f61a68bf9e84?hl=en&; :
What do you mean by "better use"?
As explained below, using more bits-per-symbol with fewer
symbols-per-second makes better use of bandwidth than using more
symbols-per-second with fewer bits-per-symbol.
http://en.wikipedia.org/wiki/Baud
?Conveying more than one bit per symbol has advantages. It reduces the
time required to send a given quantity of data, and allows modern modems,
FDDI and 100/1000 Mbit/s Ethernet LANs, and others, to achieve high data
rates. An optimal symbol set design takes into account channel bandwidth,
desired information rate, noise characteristics of the channel and the
receiver, and receiver and decoder complexity. A typical 2400-bit/s modem
transmits at 600 baud (600 symbol/s), where each quadrature amplitude
modulation symbol carries four bits of information. 1000 Mbit/s Ethernet
LAN cables use many wire pairs and many bits per symbol to encode their
data payloads. 1000BASE-T uses four wire pairs and two data bits per
symbol to get a symbol rate of 125MBd.?
It seems that the baud rate differs from bit rate on devices like modems,
which use combinations of frequencies and amplitudes to pack more bits into
each symbol. But at the level of a USART, or RS-232, there is only one bit
per symbol, consisting of a logic high/low, or mark/space. Asynchronous
communication also requires at least a start bit, and one or two stop bits,
with the actual data contained in a 5 to 8 bit code. So, for a given value
of bits/sec, you can transfer more data per second with an 8 bit code than
a 5 bit code. The sequence of bits is called a frame, which might be
considered a symbol. 8 bits are needed to transmit 6 bits of information,
and 10 bits for 8 bits of information. So, at 4800 bps, you can transmit
600 or 480 frames per second, corresponding to 600*6=3600 or 480*8=3840
bits/second of data.
There may also be more overhead if a parity bit is used, and even more with
CRC error correction. But without error correction, there is a chance of
bad data being received, which could be disastrous for critical binary
data, but more tolerable for text, which has some built-in redundancy which
makes it readable even if some characters are missing or incorrect.
I found a fairly simple explanation of the RS232 spec at:
<http://www.lammertbies.nl/comm/info/RS-232_specs.html>
I'm not a communications expert, but I have had to deal with transmitting
data on a serial port. For my Ortmaster product, I need to transmit 10 or
12 bit A/D readings at 2400 per second. I use two 8 bit frames, with 1
start and 1 stop bit each, so I have two extra bits per frame. I use these
for error detection, by incrementing a two bit counter for each reading. If
the counters do not match, I know that a frame has been missed, so I
resynchronize and set the erroneous sample to zero. I use 57600 bps for
communication. The absolute minimum would be 48k.
Paul
.
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