What is the Diff between Xmodem/Ymodem

[Chuck Forsberg WA7KGX]

In article <2374 at psuvax1.UUCP> jte at psuvax1.UUCP (Jon Eckhardt) writes:
:Now, one question for all of you guys who know your communication 
:packages.  What is the difference between Xmodem/Ymodem/Umodem/Kermit.
:Which one do you guys think is the best of all of them and most versitile.

Ward Christensen wrote MODEM in the late 70's to transfer files between
the primitive Altair bus (S-100) micros available to hobbyists at the
time.  The press has since come to call this protocol XMODEM, and the
name has stuck.  XMODEM uses 128 byte blocks to transfer a single file
between two machines.  A path name must be given to both sending and
receiving programs.  The resultant file will have up to 127 bytes of
garbage appended to it.  Without proprietary enhancements, XMODEM
transfers are unreliable in the presence of line impairments. 

UMODEM is a popular Unix program that supports XMODEM protocol. 

XMODEM/CRC uses a 16 bit CRC instead of the original 8 bit checksum,
improving integrity but not reliability.  XMODEM-1k also allows 1024
byte blocks to be sent to improve throughput with computer networks and
timesharing systems. 

YMODEM sends one or more files per command by passing the pathname and
(optionally) file length, modification time, etc., in block 0.  This
allows the receiver to generate an exact copy of the file contents. 
CRC-16 is normally used with YMODEM.  1k blocks are a popular option,
but cannot be mandatory because some operating systems choke on the
longer blocks. 

The Unix rb and sb programs (now superceded by rz and sz) support
YMODEM file transfers as well as XMODEM, XMODEM/CRC and XMODEM-1k.

Kermit refers to a set of related file transfer protocols developed at
Columbia to allow file transfers in brain damaged communications
environments that do not allow all 256 8 bit codes to be sent and
received correctly.  As with YMODEM, Kermit sends the pathname to the
receiver, so multiple files can be sent with one set of commands.  To
transfer files, both Kermit programs must be set to use the same Kermit
sub-protocol (8th bit quoting .vs.  transparency).  An average Kermit
implementation is more reliable than an average XMODEM implementation. 
The design features that allow Kermit to work in brain damaged
environments make it less efficient than YMODEM or XMODEM-1k. 

In early 1986 Telenet commissioned the development of the  Public
Domain ZMODEM protocol to provide efficient and reliable file transfers
over PC Pursuit and similar environments.  More information on ZMODEM
is available in ZMODEM.ARC on various bulletin boards including TeleGodzilla.

Chuck Forsberg WA7KGX Author of Pro-YAM communications Tools for PCDOS and Unix
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