MODEM PROTOCOL OVERVIEW

1/1/82 by Ward Christensen.  I will maintain a master copy of
this.  Please pass on changes or suggestions via CBBS/Chicago
at (312) 545-8086, CBBS/CPMUG (312) 849-1132 or by voice at
(312) 849-6279.

Rev: 08/09/82 Ward C. Change ACK to 06H (was error as 05H)

1/13/85 by John Byrns -- CRC option.
Please pass on any reports of errors in this document or suggestions
for improvement to me via Ward's/CBBS at (312) 849-1132, or by voice
at (312) 885-1105.

4/17/85 by Keith Petersen -- Merge of the two above.


	    Table of Contents

 1. DEFINITIONS
 2. TRANSMISSION MEDIUM LEVEL PROTOCOL
 3. MESSAGE BLOCK LEVEL PROTOCOL
 4. FILE LEVEL PROTOCOL
 5. DATA FLOW EXAMPLE INCLUDING ERROR RECOVERY
 6. PROGRAMMING TIPS.
 7. OVERVIEW OF CRC OPTION
 8. MESSAGE BLOCK LEVEL PROTOCOL, CRC MODE
 9. CRC CALCULATION
10. FILE LEVEL PROTOCOL, CHANGES FOR COMPATIBILITY
11. DATA FLOW EXAMPLES WITH CRC OPTION

-------- 1. DEFINITIONS.

  <soh> 01H
  <eot> 04H
| <ack> 06H
  <nak> 15H
  <can> 18H
  <C>	43H

-------- 2. TRANSMISSION MEDIUM LEVEL PROTOCOL

Asynchronous, 8 data bits, no parity, one stop bit.

    The protocol imposes no restrictions on the contents of the
data being transmitted.  No control characters are looked for
in the 128-byte data messages.  Absolutely any kind of data may
be sent - binary, ASCII, etc.  The protocol has not formally
been adopted to a 7-bit environment for the transmission of
ASCII-only (or unpacked-hex) data , although it could be simply
by having both ends agree to AND the protocol-dependent data
with 7F hex before validating it.  I specifically am referring
to the checksum, and the block numbers and their ones-
complement.
    Those wishing to maintain compatibility of the CP/M file
structure, i.e. to allow modemming ASCII files to or from CP/M
systems should follow this data format:
  * ASCII tabs used (09H); tabs set every 8.
  * Lines terminated by CR/LF (0DH 0AH)
  * End-of-file indicated by ^Z, 1AH.  (one or more)
  * Data is variable length, i.e. should be considered a
    continuous stream of data bytes, broken into 128-byte
    chunks purely for the purpose of transmission.
  * A CP/M "peculiarity": If the data ends exactly on a
    128-byte boundary, i.e. CR in 127, and LF in 128, a
    subsequent sector containing the ^Z EOF character(s)
    is optional, but is preferred.  Some utilities or
    user programs still do not handle EOF without ^Zs.
  * The last block sent is no different from others, i.e.
    there is no "short block". 

-------- 3. MESSAGE BLOCK LEVEL PROTOCOL

 Each block of the transfer looks like:
<SOH><blk #><255-blk #><--128 data bytes--><cksum>
    in which:
<SOH>	    = 01 hex
<blk #>     = binary number, starts at 01 increments by 1, and
	      wraps 0FFH to 00H (not to 01)
<255-blk #> = blk # after going thru 8080 "CMA" instr, i.e.
	      each bit complemented in the 8-bit block number.
	      Formally, this is the "ones complement".
<cksum>     = the sum of the data bytes only.  Toss any carry.

-------- 4. FILE LEVEL PROTOCOL

---- 4A. COMMON TO BOTH SENDER AND RECEIVER:

    All errors are retried 10 times.  For versions running with
an operator (i.e. NOT with XMODEM), a message is typed after 10
errors asking the operator whether to "retry or quit".
    Some versions of the protocol use <can>, ASCII ^X, to
cancel transmission.  This was never adopted as a standard, as
having a single "abort" character makes the transmission
susceptible to false termination due to an <ack> <nak> or <soh>
being corrupted into a <can> and cancelling transmission.
    The protocol may be considered "receiver driven", that is,
the sender need not automatically re-transmit, although it does
in the current implementations.

---- 4B. RECEIVE PROGRAM CONSIDERATIONS:

    The receiver has a 10-second timeout.  It sends a <nak>
every time it times out.  The receiver's first timeout, which
sends a <nak>, signals the transmitter to start.  Optionally,
the receiver could send a <nak> immediately, in case the sender
was ready.  This would save the initial 10 second timeout.
However, the receiver MUST continue to timeout every 10 seconds
in case the sender wasn't ready.
    Once into a receiving a block, the receiver goes into a
one-second timeout for each character and the checksum.  If the
receiver wishes to <nak> a block for any reason (invalid
header, timeout receiving data), it must wait for the line to
clear.  See "programming tips" for ideas
    Synchronizing:  If a valid block number is received, it
will be: 1) the expected one, in which case everything is fine;
or 2) a repeat of the previously received block.  This should
be considered OK, and only indicates that the receivers <ack>
got glitched, and the sender re-transmitted; 3) any other block
number indicates a fatal loss of synchronization, such as the
rare case of the sender getting a line-glitch that looked like
an <ack>.  Abort the transmission, sending a <can>

---- 4C. SENDING PROGRAM CONSIDERATIONS.

    While waiting for transmission to begin, the sender has
only a single very long timeout, say one minute.  In the
current protocol, the sender has a 10 second timeout before
retrying.  I suggest NOT doing this, and letting the protocol
be completely receiver-driven.  This will be compatible with
existing programs.
    When the sender has no more data, it sends an <eot>, and
awaits an <ack>, resending the <eot> if it doesn't get one.
Again, the protocol could be receiver-driven, with the sender
only having the high-level 1-minute timeout to abort.

-------- 5. DATA FLOW EXAMPLE INCLUDING ERROR RECOVERY

Here is a sample of the data flow, sending a 3-block message.
It includes the two most common line hits - a garbaged block,
and an <ack> reply getting garbaged.  <xx> represents the
checksum byte.

SENDER					RECEIVER
				times out after 10 seconds,
			<---		<nak>
<soh> 01 FE -data- <xx> --->
			<---		<ack>
<soh> 02 FD -data- xx	--->	(data gets line hit)
			<---		<nak>
<soh> 02 FD -data- xx	--->
			<---		<ack>
<soh> 03 FC -data- xx	--->
   (ack gets garbaged)  <---		<ack>
<soh> 03 FC -data- xx	--->		<ack>
<eot>			--->
			<---		<ack>

-------- 6. PROGRAMMING TIPS.

* The character-receive subroutine should be called with a
parameter specifying the number of seconds to wait.  The
receiver should first call it with a time of 10, then <nak> and
try again, 10 times.
  After receiving the <soh>, the receiver should call the
character receive subroutine with a 1-second timeout, for the
remainder of the message and the <cksum>.  Since they are sent
as a continuous stream, timing out of this implies a serious
like glitch that caused, say, 127 characters to be seen instead
of 128.

* When the receiver wishes to <nak>, it should call a "PURGE"
subroutine, to wait for the line to clear.  Recall the sender
tosses any characters in its UART buffer immediately upon
completing sending a block, to ensure no glitches were mis-
interpreted.
  The most common technique is for "PURGE" to call the
character receive subroutine, specifying a 1-second timeout,
and looping back to PURGE until a timeout occurs.  The <nak> is
then sent, ensuring the other end will see it.

* You may wish to add code recommended by John Mahr to your
character receive routine - to set an error flag if the UART
shows framing error, or overrun.  This will help catch a few
more glitches - the most common of which is a hit in the high
bits of the byte in two consecutive bytes.  The <cksum> comes
out OK since counting in 1-byte produces the same result of
adding 80H + 80H as with adding 00H + 00H.

-------- 7. OVERVIEW OF CRC OPTION

The CRC used in the Modem Protocol is an alternate form of block check
which provides more robust error detection than the original checksum.
Andrew S. Tanenbaum says in his book, Computer Networks, that the
CRC-CCITT used by the Modem Protocol will detect all single and double
bit errors, all errors with an odd number of bits, all burst errors of
length 16 or less, 99.997% of 17-bit error bursts, and 99.998% of
18-bit and longer bursts.

The changes to the Modem Protocol to replace the checksum with the CRC
are straight forward. If that were all that we did we would not be
able to communicate between a program using the old checksum protocol
and one using the new CRC protocol. An initial handshake was added to
solve this problem. The handshake allows a receiving program with CRC
capability to determine whether the sending program supports the CRC
option, and to switch it to CRC mode if it does. This handshake is
designed so that it will work properly with programs which implement
only the original protocol. A description of this handshake is
presented in section 10.

-------- 8. MESSAGE BLOCK LEVEL PROTOCOL, CRC MODE

 Each block of the transfer in CRC mode looks like:
<SOH><blk #><255-blk #><--128 data bytes--><CRC hi><CRC lo>
    in which:
<SOH>		  = 01 hex
<blk #>     = binary number, starts at 01 increments by 1, and
	      wraps 0FFH to 00H (not to 01)
<255-blk #> = ones complement of blk #.
<CRC hi>    = byte containing the 8 hi order coefficients of the CRC.
<CRC lo>    = byte containing the 8 lo order coefficients of the CRC.
	      See the next section for CRC calculation.

-------- 9. CRC CALCULATION

---- 9A. FORMAL DEFINITION OF THE CRC CALCULATION

To calculate the 16 bit CRC the message bits are considered to be the
coefficients of a polynomial. This message polynomial is first
multiplied by X^16 and then divided by the generator polynomial
(X^16 + X^12 + X^5 + 1) using modulo two arithemetic. The remainder
left after the division is the desired CRC. Since a message block in
the Modem Protocol is 128 bytes or 1024 bits, the message polynomial
will be of order X^1023. The hi order bit of the first byte of the
message block is the coefficient of X^1023 in the message polynomial.
The lo order bit of the last byte of the message block is the
coefficient of X^0 in the message polynomial.

---- 9B. EXAMPLE OF CRC CALCULATION WRITTEN IN C

/*
This function calculates the CRC used by the "Modem Protocol"
The first argument is a pointer to the message block. The second
argument is the number of bytes in the message block. The message
block used by the Modem Protocol contains 128 bytes.
The function return value is an integer which contains the CRC. The
lo order 16 bits of this integer are the coefficients of the CRC. The
The lo order bit is the lo order coefficient of the CRC.
*/

int calcrc(ptr, count) char *ptr; int count; {

    int crc, i;

    crc = 0;
    while(--count >= 0) {
	crc = crc ^ (int)*ptr++ << 8;
	for(i = 0; i < 8; ++i)
	    if(crc & 0x8000)
		crc = crc << 1 ^ 0x1021;
	    else
		crc = crc << 1;
	}
    return (crc & 0xFFFF);
    }

-------- 10. FILE LEVEL PROTOCOL, CHANGES FOR COMPATIBILITY

---- 10A. COMMON TO BOTH SENDER AND RECEIVER:

The only change to the File Level Protocol for the CRC option is the
initial handshake which is used to determine if both the sending and
the receiving programs support the CRC mode. All Modem Programs should
support the checksum mode for compatibility with older versions.
A receiving program that wishes to receive in CRC mode implements the
mode setting handshake by sending a <C> in place of the initial <nak>.
If the sending program supports CRC mode it will recognize the <C> and
will set itself into CRC mode, and respond by sending the first block
as if a <nak> had been received. If the sending program does not
support CRC mode it will not respond to the <C> at all. After the
receiver has sent the <C> it will wait up to 3 seconds for the <soh>
that starts the first block. If it receives a <soh> within 3 seconds
it will assume the sender supports CRC mode and will proceed with the
file exchange in CRC mode. If no <soh> is received within 3 seconds
the receiver will switch to checksum mode, send a <nak>, and proceed
in checksum mode. If the receiver wishes to use checksum mode it
should send an initial <nak> and the sending program should respond to
the <nak> as defined in the original Modem Protocol. After the mode
has been set by the initial <C> or <nak> the protocol follows the
original Modem Protocol and is identical whether the checksum or CRC
is being used.

---- 10B. RECEIVE PROGRAM CONSIDERATIONS:

There are at least 4 things that can go wrong with the mode setting
handshake.
  1. the initial <C> can be garbled or lost.
  2. the initial <soh> can be garbled.
  3. the initial <C> can be changed to a <nak>.
  4. the initial <nak> from a receiver which wants to receive in
     checksum can be changed to a <C>.

The first problem can be solved if the receiver sends a second <C>
after it times out the first time. This process can be repeated
several times. It must not be repeated a too many times before sending
a <nak> and switching to checksum mode or a sending program without
CRC support may time out and abort. Repeating the <C> will also fix
the second problem if the sending program cooperates by responding as
if a <nak> were received instead of ignoring the extra <C>.

It is possible to fix problems 3 and 4 but probably not worth the
trouble since they will occur very infrequently. They could be fixed
by switching modes in either the sending or the receiving program
after a large number of successive <nak>s. This solution would risk
other problems however.

---- 10C. SENDING PROGRAM CONSIDERATIONS.

The sending program should start in the checksum mode. This will
insure compatibility with checksum only receiving programs. Anytime a
<C> is received before the first <nak> or <ack> the sending program
should set itself into CRC mode and respond as if a <nak> were
received. The sender should respond to additional <C>s as if they were
<nak>s until the first <ack> is received. This will assist the
receiving program in determining the correct mode when the <soh> is
lost or garbled. After the first <ack> is received the sending program
should ignore <C>s.

-------- 11. DATA FLOW EXAMPLES WITH CRC OPTION

---- 11A. RECEIVER HAS CRC OPTION, SENDER DOESN'T

Here is a data flow example for the case where the receiver requests
transmission in the CRC mode but the sender does not support the CRC
option. This example also includes various transmission errors.
<xx> represents the checksum byte.

SENDER					RECEIVER
			<---		<C>
				times out after 3 seconds,
			<---		<nak>
<soh> 01 FE -data- <xx> --->
			<---		<ack>
<soh> 02 FD -data- <xx> --->	(data gets line hit)
			<---		<nak>
<soh> 02 FD -data- <xx> --->
			<---		<ack>
<soh> 03 FC -data- <xx> --->
   (ack gets garbaged)  <---		<ack>
				times out after 10 seconds,
			<---		<nak>
<soh> 03 FC -data- <xx> --->
			<---		<ack>
<eot>			--->
			<---		<ack>

---- 11B. RECEIVER AND SENDER BOTH HAVE CRC OPTION

Here is a data flow example for the case where the receiver requests
transmission in the CRC mode and the sender supports the CRC option.
This example also includes various transmission errors.
<xxxx> represents the 2 CRC bytes.

SENDER					  RECEIVER
			  <---		  <C>
<soh> 01 FE -data- <xxxx> --->
			  <---		  <ack>
<soh> 02 FD -data- <xxxx> --->	  (data gets line hit)
			  <---		  <nak>
<soh> 02 FD -data- <xxxx> --->
			  <---		  <ack>
<soh> 03 FC -data- <xxxx> --->
   (ack gets garbaged)	  <---		  <ack>
				  times out after 10 seconds,
			  <---		  <nak>
<soh> 03 FC -data- <xxxx> --->
			  <---		  <ack>
<eot>			  --->
			  <---		  <ack>

			-----END-----
