============================================================================
Date: 11-01-91 (13:51)
From: WM DUEMLER
  To: ALL
Subj: BAUD VS. BPS                   Conf: (51) V32-Modem
----------------------------------------------------------------------------
        The following text comes from an article I have written to try and
explain flow control to the uninitiated.  It also reiterates an excellent
piece written by Ken Sukimoto.
============================================================================

FASTER THAN 2400 BAUD?  IMPOSSIBLE!

        It is a misnomer to refer to high speed, V.32 transmissions as 9600
baud.  The word baud refers to the number of signal changes per second, and
a voice line can accomodate 2400 signal changes per second.  When we talk
about the line speed of a modem, we are speaking in terms of bits per
second:  not necesarily the same as baud rates.  The trick is for higher
speed modems to pack more into those 2400 changes.

        2400 baud encodes one data bit in each signal change.  That gives a
data speed of 2400 bps.  By transmitting four data bits in each signal, a
modem can support 9600 bps.  9600 bits per second is still 2400 baud.  When
you add data compression techniques to the modem, such as V.42bis, you can
increase the bps rate even more.  V.42bis is theoretically capable of a
4 to 1 compression, making it possible to get data speeds of 38,400 bits
per second.

        The connection is still 2400 baud, the line speed is still 9600 bps,
but the throughput is as high as 38,400 bps.  Bits per second, while often
used interchangably in conversation, is not the same as baud.

WHAT DOES FLOW CONTROL DO?

        Imagine a salesperson on the phone to their best customer. The
customer wants to place an order for 50 seperate items, and proceeds to name
them, one after the other, without so much as a pause for breath.  At the
beginning, the salesperson is able to make note of every item the customer
is mentioning, but eventually, the salesperson starts to fall a little
behind.  The salesperson is one item behind, then two.  Eventually the point
is reached where the salesperson is unable to keep up and must ask the
customer to stop talking long enough to get the items recorded.  Once the
salesperson is caught back up, the customer gets the go-ahead to continue.
Without the frequent pauses, sooner or later the salesperson would miss a
line or two of the order, and get confused.

        This is a more simplistic view of flow control than the reality, but
it serves to illustrate the difficulties in high speed modem transmissions.
Under normal circumstances, your computer talks to your modem faster than
the modems talk to each other.  This allows the sending modem's buffer to
fill to the point that it has enough data to begin compressing.  When the
buffer is nearly full, the modem must signal the PC or terminal to
momentarily stop sending data.  There are two methods of signaling:
XON/XOFF, or software flow control; and RTS/CTS or hardware flow control.

HARDWARE VS. SOFTWARE FLOW CONTROL

        Software flow control uses two characters, XON and XOFF, to start
and stop the flow of data.  The modem sends the computer an XOFF character
when its buffer is nearly full, causing the computer to stop sending data.
When the modem's buffer starts to empty, it sends an XON to the computer,
telling it to send some more data.  The biggest problem with this method is
that if the data being transmitted contains any XON or XOFF characters, the
modem will act on them as if they were the actual signals.  This has the
potential of slowing data transfers drastically.  To get around this
potential problem, the XON/XOFF characters are prefaced by control
characters, but this also slows the transfer.

        Hardware flow control uses two of the signals between the modem and
the computer.  The computer raises the signal on Ready To Send (RTS) to let
the modem know it is ready to send data, then checks Clear To Send (CTS) to
see if the modem is ready to accept the data.  The modem raises CTS when it
is ready to receive data, and checks RTS to be sure the computer is ready to
accept data. Hardware flow controls is much more reliable than software,
since there is no possibility of data interference, and since the signals
can be manipulated at the speed of electricity.

DATA COMPRESSION

        The two forms of data compression used in Ven-Tel's modems are MNP
level 5 and V.42bis.  MNP level 5 is a routine that uses an adaptive
mathematical shorthand to constantly analyze the data and adjust the
compression parameters to maximize data throughput.  MNP 5 is capable of a
two to one compression that can boost throughput speeds as high as
19,200 bps.  V.42bis is the CCITT standard for data compression and
incorporates a technique called BTLZ which can compress the data to a four
to one ratio and result in throughput speeds as high as 38,400 bps.
V.42bis includes LAPM, or Link Access Protocol for Modems error correction
for faster and 100 % accurate data transmissions.

        When a modem uses a data compression routine like V.42bis, the
actual data speed can be much higher than the connected line speed.  That's
why flow control is so important.  It is possible for two V.32 modems to be
connected to each other at 9600 bps, and have their DTE speed set to 38,400.
The modems send data to each other at 9600 bps, where it is decompressed and
sent to the computer or terminal at the highest speed possible.

        A voice grade line may be able to handle only 2400 baud, but using
those 2400 signals per second to maximize the information carried in them
makes it possible to communicate worldwide at a speed once thought to be a
fantasy.
============================================================================
             Application Support - SysOp - Ven-Tel Modems
                TIPS BBS - San Jose - CA - 408-922-0988
