MODEM/XMODEM Protocol Explained
                by Kelly Smith, CP/M-Net "SYSOP"
                         January 8,1980
                   Edited April 28, 1981 - FJW

     I  thought  that it may be of some interest to those of  you 
who use the MODEM/XMODEM file transfer capability to get a little 
insight  as  to the communications  protocol  (i.e.  "handshaking 
method") used by the system. 

     Herein lie the details of a very good (but not perfect) data 
communications  protocol that has become the "de facto"  standard 
for  various  remote CP/M systems (RCPM's) which  are  accessible 
across  the country.   (Refer to RCPMLSTx.DOC on all  RCPM's  for 
access  numbers and note that the "x" in that list changes as new 
system  are  listed).   I  also  wish  to  give  credit  to  Ward 
Christensen  (the "original" CBBS) for writing  MODEM.ASM  (CPMUG 
Volume 25.11) and Keith Petersen,  Bruce Ratoff,  Dave Hardy, Rod 
Hart,  Tom  "C"  (we  know who you are  Tom!),  and  others,  for 
enhancements to Ward's original program which we now call  XMODEM 
(external modem).

     Data is sent in 128-byte sequentially numbered blocks,  with 
a single checksum byte appended to the end of each block.  As the 
receiving  computer acquires the incoming data,  it performs  its 
own  checksum  and upon each completion of a block,  it  compares 
its  checksum result with that of the sending computers.   If the 
receiving computer matches the checksum of the sending  computer, 
it   transmits   an  ACK  (ASCII  code  protocol  character   for 
ACKNOWLEDGE  (04 Hex,  Control-F)) back to the sending  computer.  
The  ACK  therefore  means "all's well on  this  end,  send  some 
more...".

Notice in the following example,  that the sending computer  will 
transmit  an "initial NAK" (ASCII protocol character for NEGATIVE 
ACKNOWLEDGE (15 Hex,  Control-U))...or, "that wasn't quite right, 
please send again".

Due  to the asynchronous nature of the initial "hook-up"  between 
the two computers, the receiving computer will "time-out" looking 
for data,  and send the NAK as the "cue" for the sending computer 
to  begin  transmission.   The sending computer  knows  that  the 
receiving computer will "time-out", and uses this fact to "get in 
sync"...  The sending computer responds to the "initial NAK" with 
a  SOH  (ASCII code protocol character for START OF  HEADING  (01 
Hex,  Control-A)),  sends  the first block number,  sends the 1's 
complement  of the block number (VERY important,  I will  discuss 
this later...),  sends 128 bytes of 8 bit data (that's why we can 
transfer  ".COM"  files),  and  finally  a  checksum,  where  the 
checksum is calculated by summing the SOH,  the block number, the 
block number 1's complement, and the 128 bytes of data.

Receiving Computer:

----/NAK/------------------------/ACK/----------------------
    15H                           06H 

Sending Computer:

--------/SOH/BLK#/BLK#/DATA/CSUM/---/SOH/BLK#/BLK#/DATA/etc.
         01H 001H 0FEH 8bit 8bit     01H 002H 0FDH 8bit .... 

     This  process continues,  with the next 128  bytes,  IF  the 
block  was  ACK'ed by the receiving computer,  and then the  next 
sequential block number and its 1's complement, etc.

     But  what  happens if the block  is  NAK'ed?...   Easy,  the 
sending computer just re-sends the previous block.

     Now the hard part...  What if the sending computer transmits 
a block, the receiving computer gets it and sends an ACK, but the 
sender does not see it?...   The sending computer thinks that  it 
has  failed  and  after  10  seconds  re-transmits  the  block...  
ARGH!...  The receiving computer has "stashed" the data in memory 
or  on disk (data is written to disk after receiving 16  blocks), 
the  receiving  computer is now 1 block AHEAD of the  transmiting 
computer!   Here comes the operation of the block numbers...  The 
receiver  detects that this is the last block (all  over  again), 
and   transmits  back  an  ACK,   throws  away  the  block,   and  
(effectively)  "catches  up"...    clever!    What's  more,   the 
integrity  of  the  block number is  verified  by  the  receiving 
computer,  because  it  "sums"  the SOH (01 Hex) with  the  block 
number  plus  the 1's complement of the block  number),  and  the 
result  MUST BE zero for a proper transfer (e.g.  01+01+FE hex  = 
00, on the first block).  The sequence of events then, looks like 
this: 

Receiving Computer:

----/ACK/-----------------------/NAK/-----------------------
    06H                          15H 

Sending Computer:

CSUM/---/SOH/BLK#/BLK#/DATA/CSUM/---/SOH/BLK#/BLK#/DATA/etc.
8bit     01H 003H 0FCH 8bit 8bit     01H 003H 0FCH 8bit ....

     Normal completion of data transfers will then conclude  with 
an EOT (ASCII code protocol END OF TRANSMISSION, 04 Hex, Control-
D) from the sending computer,  and a final ACK from the receiving 
computer.   Unfortunately,  if  the receiving computer misses the 
EOT,  it will continue to wait for the next block (sending a  NAK 
every  10  seconds,  up to 10 times) and  eventually  "time-out".  
This is rarely the case however, and although not "bullet-proof", 
it is a very workable protocol.

Receiving Computer:

----/ACK/---/ACK/"Transfer Complete"/A>(or B>)
     06H     06H .............................

Sending Computer:

CSUM/---/EOT/---/A>(or B>)
8bit     04H .............

     In   some  cases,   where  the  telephone  transmission   is 
repeatedly "trashed" (weak signals, multiple noise "hits", etc.), 
the receiving computer (and operator) will be provided the option 
to  quit.   Here,  the operator enters "R" or "Q" in response  to 
"Retry or Quit?" (after 10 retries).

Receiving Computer:

----/NAK/...NAK's ten times.../"Retry or Quit?"(Q)/A>...
     15H

Sending Computer:

CSUM/---/...Garbled Data....../-----------------------/A>...
8bit

     A  final  consideration when using the MODEM program,  is  a 
timing  related  problems when transfer  status  messages  and/or 
textual  data  is directed to the screen of a slow (4800 Baud  or 
less)  terminal  or  to a hard copy  printer.   This  problem  is 
readily apparent (multiple NAK's) when using MODEM for the  first 
time, and can usually be "cured" by NOT SPECIFYING the "V" (VIEW) 
sub-option  when sending or receiving files.   Users of  Lifeboat 
Associates  BSTAM encounter the same problem,  but this is easily 
fixed  with  the  files  TQPATCH.ASM  and  RQPATCH.ASM  (transfer 
quiet/receive quiet) that Keith Petersen (Royal Oak CP/M,  "call-
back" remote system,  (313)-588-7054) wrote to solve the  problem 
of low speed terminal I/O.

     For  users of CBBS's that do not have MODEM.ASM (but DO HAVE 
a CP/M disk system...ESSENTIAL!),  let me suggest that you  "data 
capture" the file MBOOTx.ASM from one of the RCPM's (it's a small 
8  kilo-byte file that "fits" in most systems' memory) to get the 
larger  MODEM.ASM (40 kilo-bytes).   Check it very carefully  for 
errors  using the "data capture" (read ERROR PRONE method  here). 
Then edit and assemble for your modem configuration.

     If  you are tired of buying software where the  advertisment 
is written better than the program, then the RCPM's are just what 
you have been looking for...and FREE!
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