XMODEM File Transfer Protocol
By Larry Jordan
When transferring files between computers using the telephone
system, there is always the chance that electrical noise will
result in data transmission errors. To ensure proper transfer of
files it is necessary to detect data transmission errors and to
retransmit data that contains errors. Most people think that
asynchronous parity error detection provides that capability. It
does not. Parity error detection does tell you when a data
transfer error has occurred, but it is up to you to retransmit
the data to correct errors. The problem is that parity error
detection is not actually performed by most IBM PC communication
packages. If a package does perform the error detection, it may
not inform you of errors in such a way that you know to
immediately retransmit the data. ASCOM, for example, places an
asterisk in a file where parity errors are detected, but you
may not realize the errors occurred until long after the file is
transferred. To ensure "error-free" data transfer you need a
protocol file transfer technique. Andrew Fluegelman has added
such a technique to PC-TALK.III called the XMODEM protocol.
A protocol is a set of rules and conventions that apply to a
specific area of communications that allow participants to
properly communicate regardless of the hardware brand or software
package being used. The protocol file transfer is a set of rules
for transferring files which specifies a set of ASCII handshaking
characters and the sequence of handshaking required to perform
certain file transfer functions. Protocol handshaking signals
allow communication software to transfer text, data and machine
code files, and to perform sophisticated error-checking. The
handicap in using protocol file transfer techniques is that the
computers on both ends of the communications link must be using
compatible software; there is no standard that controls these
protocols and almost all communication packages that have a
protocol file transfer option use a protocol unique to that
package. This means that a business or group of people must
standardize its microcomputer communications software to take
advantage of protocol transfers.
The Ward Christensen XMODEM protocol is one specific file
transfer protocol that may become a default standard in personal
communications because of its widespread use on bulletin
boards and because of its inclusion in low cost personal computer
communication packages such as PC-TALK. It has not gained
widespread acceptance in business communication packages partly
because the protocol is public domain; most business
communication package designers use unique protocols to force
businesses to use their software on both ends of communication
links. By providing you with this insight into protocol transfer
and explaining in detail the operation of the XMODEM protocol, I
hope to add momentum to the development of a "standard protocol"
whether it be the XMODEM model or some other model. Users of
communication software deserve a standard protocol that will
allow them to use the technique with any microcomputer regardles
of the software packages employed.
The XMODEM protocol is illustrated in Figure 1. As you can see
from that figure, XMODEM does not begin the transfer of data
