Dec 292017
 
USRobotics Sportster 14,400 modem manual.
File SP144MAN.ZIP from The Programmer’s Corner in
Category Communications
USRobotics Sportster 14,400 modem manual.
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Contents of the APA.TXT file


APPENDIX A. ERROR/FLOW CONTROL CONCEPTS


This appendix includes detailed
information on how Sportster 14,400
modems use error control and,
especially useful, some statistics and
guidelines on using the modem for the
best throughput.

ERROR CONTROL AND THROUGHPUT OVERVIEW

Error control protects the integrity of
data transferred over phone channels and
is available for calls at 1200 bps and
above. It can be disabled, although
high speed calls (above 2400 bps) should
always be under error control. The
operations defined in an error-control
protocol include the following.
Establishment of compatibility
Data formatting into blocks or
(MNP) frames
Error detection through Cyclic
Redundancy Checking (CRC)
Positive acknowledgment of error-
free blocks and negative
acknowledgment of corrupted data
blocks
Retransmission of corrupted data
blocks
The Sportster 14,400 is set at the
factory to &M4, causing it to try for
an error-control connection and, if
that isn't possible, to proceed with
the call in Normal mode. The modem
first tries for a V.42 connection, then
an MNP connection. The following infor
mation is based on the Sportster
14,400's setting of &M4.

CCITT V.42 HANDSHAKING

The exchange of signals between two
devices in order to establish a
communications link is called
handshaking. CCITT V.42 includes a two-
stage handshaking process.
A Detection phase that is based on
an exchange of predefined
characters.
LAPM (Link Access Procedures for
Modems) Negotiation. In this
phase, the modems identify their
capabilities concerning maximum
data block size and the number of
outstanding data blocks allowed
before an acknowledgment is
required.

MNP HANDSHAKING

This protocol is supported by the CCITT
V.42 Recommendation. It was originally
developed by Microcom, Inc., and is now
in the public domain.
MNP handshaking begins with an MNP Link
Request sent by the calling modem. If
the remote modem doesn't recognize the
request, error control isn't possible.

DATA COMPRESSION

If the modems successfully establish a
V.42 connection, they also negotiate for
V.42 bis data compression. If they
successfully establish an MNP
connection, they negotiate for MNP5 data
compression.
Modems using V.42 bis compression
negotiate the following options.
Dictionary size, that is, the
amount of memory available for
compression table entries.
(Entries are codes devised for
redundant data. The data is packed
into shorter data units, called
code words, and unpacked by the
receiving modem.)
Possible sizes are as follows.
USRobotics modems use 11-bit, or
2048-entry dictionaries, but drop
down if the remote modem uses a 512-
or 1024-entry dictionary.
BitsEntries
9 512
10 1024
11 2048
Maximum string length of each
entry. As the dictionary fills,
the modem deletes the oldest unused
strings.
V.42 bis compression is more efficient
than MNP5 compression in part because it
dynamically deletes unusable strings.
In addition, it works better with files
that are already compressed. These
include already compressed files, such
as the .ZIP files downloaded from many
Bulletin Boards, and 8-bit binary files,
which appear to the modem to be
compressed.
MNP5 compression should not be used
with such files because it adds data to
them, which lessens throughput. (The
additional data is stripped when the
file is decompressed by the remote
modem.) When transferring such files,
it's best to set the modem to &K3:
this allows V.42 bis compression to
work dynamically with the compressed
data, but disables MNP5.

FLOW CONTROL

Flow control of data from the computer
or terminal is required under error
control for two reasons.
1.The transmitting modem buffers a copy
of each frame it transmits to the
remote end until it is acknowledged
by the receiving modem.
2.If errors are encountered, the
transmitting modem must resend the
corrupted data. This retransmission
activity, combined with the steady
stream of data from the computer or
terminal, can overflow the buffer.

THROUGHPUT GUIDELINES

The following guidelines should help to
make the most of the modem's advanced
performance features. In many
instances, experimentation and
experience will indicate what works best
for your applications.
1.Optimal throughput is attained under
the following conditions.
The communications software allows
fixing the serial port rate higher
than the connection rate, for
example, setting the software to
38.4K bps and setting the modem to
&B1.
If the software automatically
switches bit rates to follow the
connection rate, the modem's serial
port rate also must be set to
follow the connection rate for each
call, &B0, and throughput will be
limited.
The call is under data compression.
The data is comprised of text files
rather than binary files such as
.EXE or .COM files. See the table
at the end of this appendix.
2.Disable MNP5 compression for files
that are already compressed, or 8-bit
binary files, which appear to the mo
dem to be already compressed. Set
the modem to &K3, which disables MNP5
and leaves V.42 bis compression
enabled.
3.Many non-text files require a file-
transfer protocol. The file transfer
may be slowed down by a file-transfer
protocol, but the results vary. For
example, certain public domain file-
transfer protocols have the following
effects.
KermitWith the basic Kermit,
throughput is severely reduced
due to short block lengths
(under 128 bytes) and
acknowledgment turnaround
time. Later enhancements to
Kermit permit larger data-
block lengths.
XmodemThroughput may be reduced if
your version uses short block
lengths, for example, 128
bytes. Some versions use
blocks of 1 Kbyte, which is
much better, although overhead
(error-control protocol
information) still affects
overall throughput.
YmodemThis protocol is similar to
Xmodem with 1-Kbyte block
lengths, and allows multiple
files to be sent in one
transfer.
These file-transfer protocols further
reduce throughput when an error-
control connection is established.
The accuracy of the data is checked
twice, by the file-transfer protocol
and the modem. To avoid redundancy,
use these protocols only for non-ARQ
connections.
For better throughput, we recommend
Ymodem-G, with the modem left at its
error-control default, &M4. Ymodem-G
assumes the modems are handling error
control: overhead is minimal, with
throughput almost equal to that
obtained with no file-transfer
protocol. However, keep in mind that
Ymodem-G is only useful if the modems
are using error control. In
addition, follow this recommendation
only if your machine and software
support hardware flow control.
On the other hand, the most current
version of Zmodem can yield even
greater efficiency. Leave the modem
at its error-control default (&M4)
and data compression default, &K1.
Zmodem performs the same kind of com
pression as V.42 bis; it turns off
its compression if files are already
compressed.
ONLINE FALLBACK/FALL FORWARD

Under error control, if a disturbance on
the phone line causes an error to a data
block, the receiving modem replies with
a negative acknowledgment. In response,
the transmitting modem retrieves a copy
of the original data block from its
Transmit buffer, and every block it sent
after that block, and retransmits them.
This keeps the data error-free and in
sequence.
However, there is a retransmit limit:
the modems hang up if line disturbances
are so severe that one of the modems has
retransmitted the same block of data
twelve times without a positive
acknowledgment.
High-speed calls are more vulnerable
than transmission at 2400 bps and
below. In order to keep V.32 bis
modems online above 2400 bps, instead
of reaching the retransmit limit and
hanging up, one of the modems requests
that they fall back, that is, that they
reduce their rate from 14.4K to 12K
bps, and then to 9600 bps or lower, if
necessary. When online conditions
improve, the modems fall forward to the
next higher speed, up to the link rate
of the call.

ACHIEVABLE THROUGHPUT STATISTICS

The table below indicates the maximum
throughput, in characters per second
(cps), that can be expected under the
following optimal conditions.
Serial port rate set at 57.6K bps;
modem set to &B1
(Note that both your software and
your computer must support 57.6K
bps in order for you to use that
rate)
Connection (link) rate of 14.4K bps
(assuming no protective fallback to
a lower speed is necessary)
V.42 bis compression negotiated for
the call, and the default size 11-
bit, 2048-entry dictionary
Straight data (that is, not already
compressed, no file-transfer
protocol)
Transmission from a fast (for
example, 286) computer

Throughput (cps) when
set to 14.4K bps
File Type MNP5 V.42 bis
Assembler or Compiler listing 2880
3840
Text file 2325-2625 3400-5760
Binary file: .EXE 2175-2400
2030-2600
Binary file: .COM 2100-2250
2050-2300
.ZIP files (common on BBS's)* 1500-1650
1700
Random binary 8-bit* 1460-1575
1700
* These files are already compressed
or appear to the modem to be com
pressed. Additional MNP5 compres
sion causes throughput lower than
what can be expected using MNP
without compression. We recommend
setting the modem to &K3 when
transferring these files, to allow
V.42 bis but disable MNP5.

The following table indicates the
maximum throughput, in characters
per second (cps), that can normally
be expected
in the same conditions as the
previous table , but with a serial
port rate of 38.4K bps.

Throughput (cps) when
set to 14.4K bps
File Type MNP5 V.42 bis
Assembler or Compiler listing 2880
3840
Text file 2325-2625 3400-3480
Binary file: .EXE 2175-2400
2030-2600
Binary file: .COM 2100-2250
2050-2300
.ZIP files (common on BBS's)* 1500-1650
1700
Random binary 8-bit* 1460-1575
1700

*We recommend setting the modem to &K3
when transferring these types of files.
See the note attached to the previous
table.




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