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PACKET SWITCHING NETWORKS
Revised 4-Mar-85
Contains former file GERMAN.TXT (modified)
If you are interested in CompuServe access and file transfer through packet
switching networks like those found in some 50 countries all around the world
- read on. I have solved the problem of downloading from CompuServe through
DATEX-P (Germany) and jumped 3 meters high when it finally worked. The
solution applies to other countries also.
Contents:
-------------- [1] QUICK INTRODUCTION -----------------------------
-------------- [2] DETAILED EXPLANATION ---------------------------
-------------- [3] INTERNATIONAL PAD PARAMETERS -------------------
-------------- [4] NATIONAL PAD PARAMETERS (DATEX-P AND OTHERS) ---
-------------- [5] XMODEM FILE TRANSFER ---------------------------
-------------- [6] PLEASE WRITE -----------------------------------
-------------- [1] QUICK INTRODUCTION ----------------------------------------
If you are not interested in the details - the next time you enter CompuServe
via a packet switching network do this:
- Logon until you see the "User ID:" prompt. (Any time later than this will
also work.)
- Enter a Ctrl-P (hold down the Ctrl key and press P once). You are now
talking to your network instead of CompuServe.
- Enter "set 3:126,4:0,5:1,9:0,12:0,118:8,119:21,120:22,125:10" without the
quotes and press Return.
- Press Return a second time.
- Enter your user ID and continue as usual.
This gives you
- prompt response to Ctrl command characters like Ctrl-C and Ctrl-O,
- uninterrupted text uploads
- no disturbing fill characters,
- cheaper local PAD line editing with Backspace, Ctrl-U and Ctrl-V and mostly
clean lines when in an online conference.
IMPORTANT NOTES:
(1) Check if your network's command prefix is really Ctrl-P by entering
Ctrl-P, then a nonsense command, then one Return. If you get a network error
message Ctrl-P is allright. If you get a CompuServe message like ?XXXXXX -
INVALID USER ID - TRY AGAIN / User ID: then Ctrl-P is wrong for you and you
have to ask the network operator for the correct network command prefix or
attention character.
(2) Your computer may freeze especially after finishing a connection. Type a
Ctrl-Q (hold down Ctrl and press Q) and continue normally.
(3) See chapter --- XMODEM FILE TRANSFERS --- if necessary.
-------------- [2] DETAILED EXPLANATION --------------------------------------
THE SYSTEM: Connection to CompuServe is normally done in the following way.
Async Terminal or Microcomputer - PAD - Packet Switching Network - Gateway -
CompuServe Network
THE PROBLEM: Using a microcomputer I can hardly afford a direct X.25 channel
to the network. So I use the public PAD (Packet Assembly and Disassembly)
unit provided by the "Post" which accepts the standard async signals, 300 or
1200 bps full duplex. The PAD's behaviour is the source of all potential
trouble.
Fortunately the PAD can be controlled by the user to such an extent that even
XMODEM and similar file transfers can be facilitated. The standard settings,
however, are completely inadequate, especially for binary file transfers.
Uploading (from you to CompuServe) is more difficult than downloading because
the PAD normally interprets some special characters send by your asynchronous
terminal or microcomputer which do not occur in the other direction, e.g.
X-ON, X-OFF and PAD command prefixes.
THE SOLUTION: Let us look at the problems in detail. In the following the PAD
parameter numbers and settings apply directly to the German DATEX-P network.
The international parameters as well as the basic problems and processes,
however, apply to all packet switching networks that are accessed by
asynchronous terminals through PADs.
PAD COMMANDS: "SET parameter_no : value , parameter_no : value , ..." sets PAD
parameters. "PAR?" lists the current parameter settings. "PROF profile_no"
resets all parameters to predefined values and "PROF? profile_no" lists the
predefined values of profile_no without applying them. Do not key in the
quotes. In most cases you will only need the SET command.
-------------- [3] INTERNATIONAL PAD PARAMETERS ------------------------------
PAD COMMAND PREFIX: When connected with CompuServe you can still issue
commands to the PAD. A special character (DLE=^P, Ctrl-P, check for your
particular network) switches the PAD into command mode. The following lines
are not sent to CompuServe any longer but are taken as commands by the PAD.
Two consecutive Returns get you back into the connection. For example to set
parameter 3 to the value of 126 and parameter 4 to zero you have to do this:
Key in ^P to switch the PAD into command mode, then key in "set 3:126,4:0".
Do not key the quotes. Finally press Return twice to get out of command mode
again.
This enables you to change PAD parameters while you are alredy connected. We
need this facility to adjust the PAD to our needs, especially because setting
the PAD parameters before establishing the connection does not always work.
In DATEX-P the parameters change when the connection is established and these
changes are not always favourable.
You can tell the PAD to let DLE (^P) characters pass unnoticed with "set 1:0".
However you will be able to get into command mode never again during the
course of that connection. For binary uploads "set 1:0".
PAD ECHO: The PAD will usually echo everything you send back to you. This
enables you to use full duplex transmission so you can see all transmission
errors. For file transfers switch the echo off with "set 2:0". Afterwards
switch the echo back on with "set 2:1".
FORWARD DATA CHARACTER: The PAD can be told to form a packet and forward it
before the packet is filled completely. This is necessary because often you
will not fill up a packet. Imagine you want to enter a menu selection. You
key just one digit and a Return. Without a Forward Data Character setting the
PAD would now wait for you to fill the remaining 126 bytes of this packet
before it is sent on its way to CompuServe. You will also want the PAD to
forward control characters like ^C and Escape immediately.
For file transfers, especially binary uploads, it is not desired to forward
packets that are not completely filled for economic reasons. After all you
pay for the packet, not the characters in it (more exactly for the segment).
"set 3:126" to forward data after all control characters and DEL. "set 3:2"
to forward data after Return characters only. "set 3:0" (no Forward Data
Character) for file transfers.
FORWARD DATA TIME LIMIT: If you have no Forward Data Character you have to
tell the PAD to forward data anyway after a certain time because the transfer
protocol (e.g. XMODEM) will not always fill the packet completely. "set 4:8"
for file transfers. This yields a .32 s limit (8 * 40 ms). "set 4:0" for
normal operation with a Forward Data Character (no time limit). The maximum
value for this parameter is 255.
X-ON/X-OFF FROM PAD TO DTE: This parameter enables the PAD to stop and restart
your transmission by sending X-OFF and X-ON bytes to your computer. "set 5:0"
if this is not desired. "set 5:1" for uploads.
There is a minor problem when you allow the PAD to send X-OFFs to you.
Sometimes, especially after finishing or breaking a connection, the PAD sends
an X-OFF and your computer seems to freeze. Simply key a Ctrl-Q (X-ON) and
everything is allright again.
PAD MESSAGES: You may forbid the PAD to send its own messages to you with
"set 6:0" during a straight text download if you are afraid of "Parity Error"
or similar messages in the middle of a received file. "set 6:1" normally.
BREAK: There are different PAD reactions to a break signal sent by you.
Parameters 7 and 8 deal with these. Leave them alone, we do not normally use
break signals.
NUMBER OF FILL CHARACTERS AFTER RETURN: Always "set 9:0" unless you have a
real Teletype that cannot return the carriage in time.
LINE LENGTH: Always "set 10:0" unless you want the PAD to break long lines
with additional Returns and Linefeeds. (Max value 255)
X-ON/X-OFF FROM DTE TO PAD: "set 12:0" for binary uploads to make the PAD
ignore X-ON and X-OFF characters. "set 12:1" otherwise.
-------------- [4] NATIONAL PAD PARAMETERS (DATEX-P AND OTHERS) --------------
DELETE CHARACTER, DELETE LINE, REPEAT LINE, ADDITIONAL FORWARD DATA
CHARACTERS: These parameters allow local line editing performed by the PAD.
With "set 118:8,119:21,120:22" the PAD can be instructed to perform the duties
of the Backspace, Ctrl-U and Ctrl-V commands locally which saves you money
whenever you use these commands. If in doubt
"set 118:0,119:0,120:0,121:0,122:0".
PARITY: If you use 7 bit with parity you may "set 123:1" to make the PAD check
your parity bit. "set 123:0" for 8 bit character length and for all binary
file transfers.
DELAY OUTPUT DURING INPUT: Parameter 125 can make the PAD hold incoming data
until you have finished typing a line, a very handy feature for online
conferences. "set 125:10" for a maximum hold time of 10 seconds. Don't
worry, the PAD will not hold all incoming data for 10 seconds when you type.
As soon as you press Return all upheld data will start flowing again.
"set 125:0" for file transfers under protocol.
INSERT LINEFEED: Try whether "set 126:0" works with your equipment. If the
Return key does not advance to the next line let the PAD echo a linefeed after
each Return sent by you with "set 126:4" which is the standard setting.
(Other settings are: 1 = insert linefeeds after Returns sent by host through
PAD to DTE, 5 = both 1 and 4.)
-------------- [5] XMODEM FILE TRANSFER --------------------------------------
CompuServe's XMODEM has a special problem. When you initiate an XMODEM file
transfer CompuServe automatically sets a Transparent Profile (which is nice)
but does this just an instant too late. Thus the initial handshaking is
spoiled and the file transfer always gets stuck.
Fortunately we now know enough to take things into our own hands and control
the PAD ourselves until the CompuServe programmers get this fixed.
Being too lazy to SET all those parameters individually we can make use of the
Transparent Profile our networks offer. In Germany and Canada and probably
many or all other countries the Transparent Profile is called "PROF 3".
To start an XMODEM file transfer do this:
1. Go to the point in CompuServe approximately one command before starting
the download or upload.
2. Enter Ctrl-P (^P, DLE, the network's command prefix, check for your
particular network).
3. Type "prof 3" without the quotes and press Return.
4. Press Return a second time. You are now back in connection with
CompuServe.
5. Issue the last command(s) to start the download or upload process. There
is no echo any more, i.e. you don't see what you are typing. Don't worry,
just carry on.
6. After the transfer, if you don't like the standard parameter setting, SET
the parameters again by using a ^Pset command.
-------------- [6] PLEASE WRITE ----------------------------------------------
Please drop me a line if you have used the information in this file
unsuccessfully or successfully! I will update this file whenever new
knowledge becomes available. Do not send me SIG messages since I do not come
here often, use electronic mail. I would especially like to know:
Does Ctrl-P work in your network?
Is PROF 3 the transparent profile in your network?
Could you upload straight text?
Could you download with XMODEM?
Could you upload with XMODEM?
Could you locate any error or missing information in this text?
Do you have any other information you think should be included here?
Do you have any other information that might be of interest to me?
Thank you very much for your interest and co-operation.
NOTE: CompuServe can be reached directly with "0 3132" and also through Tymnet
and Telenet. The numbers are "0 3106,CPS01", "0 3106,CIS02", "0 3106,CIS03",
"0 3106,CIS04", "0 3106 001133", "0 3106 001134", "0 3106 00337300" for Tymnet
and "0 3110 20200202" and "0 3110 61400227" for Telenet. Do not key the
quotes. You may have to substitute the leading 0 by your network's
international prefix like 1 or C or P 1. Often you may omit spaces. You may
substitute "0 3107" for "0 3106" which presumably enforces the use of an ITT
gateway. There is a surcharge for all connections except 0 3132.
Hans G. Michna 74776,2361
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