XMODEM is a simple
file transfer
File transfer is the transmission of a computer file through a communication channel from one computer system to another. Typically, file transfer is mediated by a communications protocol. In the history of computing, numerous file transfer protoc ...
protocol developed as a quick
hack
Hack may refer to:
Arts, entertainment, and media Games
* Hack (Unix video game), ''Hack'' (Unix video game), a 1984 roguelike video game
* .hack (video game series), ''.hack'' (video game series), a series of video games by the multimedia fran ...
by
Ward Christensen for use in his 1977 MODEM.ASM
terminal program. It allowed users to transmit files between their computers when both sides used MODEM. Keith Petersen made a minor update to always turn on "quiet mode", and called the result XMODEM.
XMODEM, like most file transfer protocols, breaks up the original data into a series of "
packets" that are sent to the receiver, along with additional information allowing the receiver to determine whether that packet was correctly received. If an error is detected, the receiver requests that the packet be re-sent. A string of bad packets causes the transfer to abort.
XMODEM became extremely popular in the early
bulletin board system
A bulletin board system (BBS), also called a computer bulletin board service (CBBS), is a computer server running list of BBS software, software that allows users to connect to the system using a terminal program. Once logged in, the user perfor ...
(BBS) market, largely because it was simple to implement. It was also fairly inefficient, and as modem speeds increased, this problem led to the development of a number of modified versions of XMODEM to improve performance or address other problems with the protocol. Christensen believed his original XMODEM to be "the single most modified program in computing history".
Chuck Forsberg collected a number of common modifications into his
YMODEM protocol, but poor implementation led to a further fracturing before they were re-unified by his later
ZMODEM protocol. ZMODEM became very popular, but never completely replaced XMODEM in the BBS market.
Packet structure
The original XMODEM used a 128-byte data packet, the
block size used on
CP/M
CP/M, originally standing for Control Program/Monitor and later Control Program for Microcomputers, is a mass-market operating system created in 1974 for Intel 8080/Intel 8085, 85-based microcomputers by Gary Kildall of Digital Research, Dig ...
floppy disk
A floppy disk or floppy diskette (casually referred to as a floppy, a diskette, or a disk) is a type of disk storage composed of a thin and flexible disk of a magnetic storage medium in a square or nearly square plastic enclosure lined with a ...
s. The packet was prefixed by a simple 3-byte header containing a
< SOH> character, a "block number" from 1-255, and the "inverse" block number—255 minus the block number. Block numbering starts with 1 for the first block sent, not 0. The header was followed by the 128 bytes of data, and then a single-byte
checksum
A checksum is a small-sized block of data derived from another block of digital data for the purpose of detecting errors that may have been introduced during its transmission or storage. By themselves, checksums are often used to verify dat ...
. The checksum was the sum of all 128 data bytes in the packet
modulo
In computing and mathematics, the modulo operation returns the remainder or signed remainder of a division, after one number is divided by another, the latter being called the '' modulus'' of the operation.
Given two positive numbers and , mo ...
256. The complete packet was thus 132 bytes long, containing 128 bytes of
payload data, for a total
channel efficiency of about 97%.
The file was marked "complete" with a
< EOT> character sent after the last block. This character was not in a packet, but sent alone as a single byte. Since the file length was not sent as part of the protocol, the last packet was padded out with a "known character" that could be dropped. In the original specification, this defaulted to
or 26 decimal, which CP/M used as the end-of-file marker inside its own disk format. The standard suggested any character could be used for padding, but there was no way for it to be changed ''within the protocol'' itself – if an implementation changed the padding character, only clients using the same implementation would correctly interpret the new padding character.
Transfer details
Files were transferred one packet at a time. When received, the packet's checksum was calculated by the receiver and compared to the one received from the sender at the end of the packet. If the two matched, the receiver sent an
< ACK> message back to the sender, which then sent the next packet in sequence. If there was a problem with the checksum, the receiver instead sent a
< NAK>. If a
was received, the sender would re-send the packet, and continued to try several times, normally ten, before aborting the transfer.
A
was also sent if the receiver did not receive a valid packet within ten seconds while still expecting data due to the lack of a
character. A seven-second timeout was also used ''within'' a packet, guarding against dropped connections in mid-packet.
The block numbers were also examined in a simple way to check for errors. After receiving a packet successfully, the next packet should have a one-higher number. If it instead received the same block number this was not considered serious, it was implied that the
had not been received by the sender, which had then re-sent the packet. Any other packet number signalled that packets had been lost.
Transfers were receiver-driven; the transmitter would not send any data until an initial
was sent by the receiver. This was a logical outcome of the way the user interacted with the sending machine, which would be remotely located. The user would navigate to the requested file on the sending machine, and then ask that machine to transfer it. Once this command was issued, the user would then execute a command in their local software to start receiving. Since the delay between asking the remote system for the file and issuing a local command to receive was unknown, XMODEM allowed up to 90 seconds for the receiver to begin issuing requests for data packets.
Problems
Although XMODEM was robust enough for a journalist in 1982 to transmit stories from Pakistan to the United States with an
Osborne 1
The Osborne 1 is the first commercially successful portable computer, released on April 3, 1981 by Osborne Computer Corporation. It weighs , cost US$1,795, and runs the CP/M 2.2 operating system. It is powered from a wall socket, as it has no ...
and
acoustic coupler over poor-quality telephone lines,
the protocol had several flaws.
Minor problems
XMODEM was written for
CP/M
CP/M, originally standing for Control Program/Monitor and later Control Program for Microcomputers, is a mass-market operating system created in 1974 for Intel 8080/Intel 8085, 85-based microcomputers by Gary Kildall of Digital Research, Dig ...
machines, and bears several marks of that
operating system
An operating system (OS) is system software that manages computer hardware and software resources, and provides common daemon (computing), services for computer programs.
Time-sharing operating systems scheduler (computing), schedule tasks for ...
. Notably, files on CP/M were always multiples of 128 bytes, and their end was marked within a block with the
character. These characteristics were transplanted directly into XMODEM. However, other operating systems did not feature either of these peculiarities, and the widespread introduction of
MS-DOS
MS-DOS ( ; acronym for Microsoft Disk Operating System, also known as Microsoft DOS) is an operating system for x86-based personal computers mostly developed by Microsoft. Collectively, MS-DOS, its rebranding as IBM PC DOS, and a few op ...
in the early 1980s led to XMODEM having to be updated to notice either a
''or''
as the end-of-file marker.
For some time it was suggested that sending a
character instead of an
or
should be supported in order to easily abort the transfer from the receiving end. Likewise, a
received in place of the
indicated the sender wished to cancel the transfer. However, this character could be easily "created" via simple noise-related errors of what was meant to be an
or
. A double-
was proposed to avoid this problem, but it is not clear if this was widely implemented.
Major problems
XMODEM was designed for simplicity, without much knowledge of other file transfer protocols – which were fairly rare anyway. Due to its simplicity, there were a number of very basic errors that could cause a transfer to fail, or worse, result in an incorrect file which went unnoticed by the protocol. Most of this was due to the use of a simple checksum for error correction, which is susceptible to missing errors in the data if ''two'' bits are reversed, which can happen with a suitably short burst of noise. Additionally, similar damage to the header or checksum could lead to a failed transfer in cases where the data itself was undamaged.
Many authors introduced extensions to XMODEM to address these and other problems. Many asked for these extensions to be included as part of a new XMODEM standard. However, Ward Christensen refused to do this, as it was precisely the ''lack'' of these features, and the associated coding needed to support them, which led to XMODEM's widespread use. As he explained:
:It was a quick hack I threw together, very unplanned (like everything I do), to satisfy a personal need to communicate with some other people. ONLY the fact that it was done in 8/77, and that I put it in the public domain immediately, made it become the standard that it is...
:...People who suggest I make SIGNIFICANT changes to the protocol, such as 'full duplex', 'multiple outstanding blocks', 'multiple destinations', etc etc don't understand that the incredible simplicity of the protocol is one of the reasons it survived.
Batch transfers
Another problem with XMODEM was that it required the transfer to be user-driven rather than automated. Typically this meant the user would navigate on the sender's system to select the file they wanted, and then use a command to put that system into the "ready to send" mode. They would then trigger the transfer from their end using a command in their terminal emulator. If the user wanted to transfer another file, they would have to repeat this process again.
For automated transfers between two sites, a number of add-ons to the XMODEM protocol were implemented over time. These generally assumed the sender would continue sending file after file, with the receiver attempting to trigger the next file by sending a
NAK
as normal at the start of a transfer. When the
NAK
s timed out, it could be assumed that either there were no more files, or the link was broken anyway.
MODEM7
MODEM7, also known as MODEM7 batch or Batch XMODEM, was the first known extension of the XMODEM protocol. A normal XMODEM file transfer starts with the receiver sending a single
NAK
character to the sender, which then starts sending a single
SOH
to indicate the start of the data, and then packets of data.
MODEM7 changed this behavior only slightly, by sending the filename, in
8.3 filename format, before the
. Each character was sent individually and had to be echoed by the receiver as a form of error correction. For a non-aware XMODEM implementation, this data would simply be ignored while it waited for the
SOH
to arrive, so the characters would not be echoed and the implementation could fall back to conventional XMODEM. With "aware" software, the file name could be used to save the file locally. Transfers could continue with another
, each file is saved under the name being sent to the receiver.
Jerry Pournelle
Jerry Eugene Pournelle (; August 7, 1933 – September 8, 2017) was an American scientist in the area of operations research and ergonomics, human factors research, a science fiction writer, essayist, journalist, and one of the first bloggers. ...
in 1983 described MODEM7 as "probably the most popular microcomputer communications program in existence".
TeLink
MODEM7 sent the filename as normal text, which meant it could be corrupted by the same problems that XMODEM was attempting to avoid. This led to the introduction of TeLink by
Tom Jennings, author of the original
FidoNet
__
/ \
/, oo \
(_, /_)
_`@/_ \ _
, , \ \\
, (*) , \ ))
______ , __U__, / \//
/ FI ...
mailers.
TeLink avoided MODEM7's problems by standardizing a new "zero packet" containing information about the original file. This included the file's name, size, and
timestamp
A timestamp is a sequence of characters or encoded information identifying when a certain event occurred, usually giving date and time of day, sometimes accurate to a small fraction of a second. Timestamps do not have to be based on some absolu ...
, which were placed in a regular 128 byte XMODEM block. Whereas a normal XMODEM transfer would start with the sender sending "block 1" with a
header, the TeLink header packet was labeled "block 0" and began with a
. The packet contained the file creation date and time, filename up to 16 characters, the file size as a 4-byte value, and the name of the program sending the file.
A normal XMODEM implementation would simply discard the packet, the assumption being that the packet number had been corrupted. But this led to a potential time delay if the packet were discarded, as the sender could not tell whether the receiver had responded with a
because it did not understand the zero packet or because there was a transmission error. As TeLink was normally used only by
FidoNet
__
/ \
/, oo \
(_, /_)
_`@/_ \ _
, , \ \\
, (*) , \ ))
______ , __U__, / \//
/ FI ...
software, which demanded it as part of the FidoNet standards, this did not present a real-world problem as both ends would always support this standard.
The basic "block 0" system became a standard in the FidoNet community, and was re-used by a number of future protocols like
SEAlink and
YMODEM.
XMODEM-CRC
The checksum used in the original protocol was extremely simple, and errors within the packet could go unnoticed. This led to the introduction of XMODEM-CRC by John Byrns, which used a 16-bit
CRC in place of the 8-bit checksum. CRCs encode not only the data in the packet, but its location as well, allowing it to notice the bit-replacement errors that a checksum would miss. Statistically, this made the chance of detecting an error less than 16 bits long 99.9969%, and even higher for longer error bit strings.
XMODEM-CRC was designed to be backwardly compatible with XMODEM. To do this, the receiver sent a
C (capital C) character instead of a
to start the transfer. If the sender responded by sending a packet, it was assumed the sender "knew" XMODEM-CRC, and the receiver continued sending
C's. If no packet was forthcoming, the receiver assumed the sender did not know the protocol, and sent an
to start a "traditional" XMODEM transfer.
Unfortunately, this attempt at backward compatibility had a downside. Since it was possible that the initial
C character would be lost or corrupted, it could not be assumed that the receiver did not support XMODEM-CRC if the first attempt to trigger the transfer failed. The receiver thus tried to start the transfer three times with
C, waiting three seconds between each attempt. This meant that if the user selected XMODEM-CRC while attempting to talk to ''any'' XMODEM, as it was intended, there was a potential 10 second delay before the transfer started.
To avoid the delay, the sender and receiver would generally list XMODEM-CRC separately from XMODEM, allowing the user to select "basic" XMODEM if the sender didn't explicitly list it. To the average user, XMODEM-CRC was essentially a "second protocol", and treated as such. This was not true of FidoNet mailers, however, where CRC was defined as the standard for all TeLink transfers.
Higher throughput
Since the XMODEM protocol required the sender to stop and wait for an
or
message from the receiver, it tended to be quite slow. In the era of 300 bit/s modems, the entire 132-byte packet required 4.4 seconds to send (132 bytes * (8 bits per byte + 1 start bit + 1 stop bit) / 300 bits per second). Assuming it takes 0.2 seconds for the receiver's
to make it back to the sender and the next packet to start hitting the receiver (0.1 seconds in both directions), the overall time for one packet would be 4.6 seconds, just over 92% channel efficiency.
The time for the
/
process was a fixed function of the underlying communications network, not of the performance of the modems. As modem speeds increased, the fixed delay grew in proportion to time needed to send the packet. For instance, at 2400 bit/s the packets took only 0.55 seconds to send, so if the
/
still took 0.2 seconds to make it back to the user's machine, the efficiency has fallen to 71%. At 9600 bit/s it is just under 40% – more time is spent waiting for the reply than is needed to send the packet.
A number of new versions of XMODEM were introduced in order to address these problems. Like earlier extensions, these versions tended to be backward-compatible with the original XMODEM, and like those extensions, this led to further fracturing of the XMODEM landscape in the user's terminal emulator. In the end, dozens of versions of XMODEM emerged.
WXModem
WXmodem, short for "Windowed Xmodem", is a variant of XMODEM developed by Peter Boswell in 1986 for use on high-latency lines, specifically public
X.25
X.25 is an ITU-T standard protocol suite for Packet switched network, packet-switched data communication in wide area network, wide area networks (WAN). It was originally defined by the CCITT, International Telegraph and Telephone Consultative Co ...
systems and
PC Pursuit. These have latencies that are far higher than the
plain-old telephone service, which leads to very poor efficiency in XMODEM. Additionally, these networks often use
control character
In computing and telecommunications, a control character or non-printing character (NPC) is a code point in a character encoding, character set that does not represent a written Character (computing), character or symbol. They are used as in-ba ...
s for
flow control and other tasks, notably
XON/XOFF will stop the data flow. Finally, in the case of an error that required a resend, it was sometimes difficult to know whether a
SOH
was a packet indicator or more noise. WXmodem adapted XMODEM-CRC to address these problems.
One change was to escape a small set of control characters:
DLE
,
XON
,
XOFF
and
SYN
. These were escaped by inserting a
DLE
in front of them, and then modifying the character by XORing it with 64. In theory, this meant the packet might be as long as 264 bytes if it originally consisted entirely of characters that required escaping. These inserted and modified characters are not part of the CRC calculation, they are removed and converted at the receiving end before calculating the CRC.
Additionally, all packets were prefixed with a
SYN
character, which meant the packet lead-in was
SYN
SOH
, reducing the chance that a stray
SOH
would be confused for a packet header in various error cases. An unescaped
SYN
found in the body of a packet was an error.
The major change in WXMODEM is the use of a
sliding window to improve throughput on high-latency links. To do so, the
ACK
messages were followed by the packet number they were
ACK
ing or
NAK
ing. The receiver does not have to
ACK
every packet; it is allowed to
ACK
any number between one and four packets. An
ACK
with the fourth packet sequence number is assumed to
ACK
all four packets. An error causes a
NAK
to be sent immediately, with all packets from that number and after being re-sent.
Requiring an
ACK
every four packets makes the system work like it has a packet size of 512 bytes, but in the case of an error, typically only requires 128 bytes to be re-sent. Moreover, it reduces the amount of data flowing in the reverse direction by four times. This is of little interest in the typical modem's
full duplex
A duplex communication system is a point-to-point system composed of two or more connected parties or devices that can communicate with one another in both directions. Duplex systems are employed in many communications networks, either to allow ...
operation, but is important in
half duplex
A duplex communication system is a Point-to-point (telecommunications), point-to-point system composed of two or more connected parties or devices that can communicate with one another in both directions. Duplex systems are employed in many commu ...
systems like
Telebit models which have 19 kB speed in one direction and 75 bits/s in the return channel.
SEAlink
One of the first third-party mailers for the
FidoNet
__
/ \
/, oo \
(_, /_)
_`@/_ \ _
, , \ \\
, (*) , \ ))
______ , __U__, / \//
/ FI ...
system was SEAdog, written by the same author as the then-popular
.arc data compression
In information theory, data compression, source coding, or bit-rate reduction is the process of encoding information using fewer bits than the original representation. Any particular compression is either lossy or lossless. Lossless compressi ...
format. SEAdog included a wide variety of improvements, including
SEAlink, an improved transfer protocol based on the same sliding window concept as WXmodem. It differed from WXmodem mostly in details.
One difference is that SEAlink supported the "zero packet" introduced by TeLink, which is needed in order to operate as a drop-in replacement for TeLink in FidoNet systems where the header was expected.
ACK
s and
NAK
s were extended to three-byte "packets", starting with the
ACK
or
NAK
, then the packet number, then the complement of the packet number, in the same fashion as the original XMODEM packet header. The window size was normally set to six packets.
SEAlink was not expected to operate over X.25 or similar links, and thus did not perform escaping. This was also needed so the zero packet would work properly, as this standard used the
SYN
character that WXmodem had re-purposed. On top of these changes, it added an "Overdrive" mode for half duplex links. This suppressed ACKs for packets that were successfully transferred, in effect making the window of infinite size. This mode was indicated by a flag in the zero block.
SEAlink later added a number of other improvements and was a useful general-purpose protocol. However, it remained rare outside the FidoNet world, and was rarely seen in user-facing software.
XMODEM-1K
Another way to solve the throughput problem is to increase the packet size. Although the fundamental problem of latency remains, the speed at which it becomes a problem is higher. XMODEM-1K with 1024-byte packets was the most popular such solution. In this case, the throughput at 9600 bit/s is 81%, given the same assumptions as above.
XMODEM-1K was an expanded version of XMODEM-CRC, which indicated the longer block size in the ''sender'' by starting a packet with the
character instead of
. Like other backward-compatible XMODEM extensions, it was intended that a -1K transfer could be started with any implementation of XMODEM on the other end, backing off features as required.
XMODEM-1K was originally one of the many improvements to XMODEM introduced by
Chuck Forsberg in his
YMODEM protocol. Forsberg suggested that the various improvements were optional, expecting software authors to implement as many of them as possible. Instead, they generally implemented the bare minimum, leading to a profusion of semi-compatible implementations, and eventually, the splitting out of the name "YMODEM" into "XMODEM-1K" and a variety of YMODEMs. Thus XMODEM-1K actually post-dates YMODEM, but remained fairly common anyway.
NMODEM
NMODEM is a
file transfer
File transfer is the transmission of a computer file through a communication channel from one computer system to another. Typically, file transfer is mediated by a communications protocol. In the history of computing, numerous file transfer protoc ...
protocol developed by L. B. Neal, who released it in 1990. NMODEM is essentially a version of XMODEM-CRC using larger 2048 byte blocks, as opposed to XMODEM's 128 byte blocks.
NMODEM was implemented as a separate program, written in Turbo Pascal 5.0 for the
IBM PC compatible
An IBM PC compatible is any personal computer that is hardware- and software-compatible with the IBM Personal Computer (IBM PC) and its subsequent models. Like the original IBM PC, an IBM PC–compatible computer uses an x86-based central p ...
family of computers. The block size was chosen to match the common cluster size of the
MS-DOS
MS-DOS ( ; acronym for Microsoft Disk Operating System, also known as Microsoft DOS) is an operating system for x86-based personal computers mostly developed by Microsoft. Collectively, MS-DOS, its rebranding as IBM PC DOS, and a few op ...
FAT
In nutrition science, nutrition, biology, and chemistry, fat usually means any ester of fatty acids, or a mixture of such chemical compound, compounds, most commonly those that occur in living beings or in food.
The term often refers specif ...
file system on contemporary
hard drive
A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating hard disk drive platter, pla ...
s, making buffering data for writing simpler.
Protocol spoofing
Over reliable (error-free) connections, it is possible to eliminate latency by "pre-acknowledging" the packets, a technique known more generally as "
protocol spoofing". This is normally accomplished in the link hardware, notably Telebit modems. The modems, when the option was turned on, would notice the XMODEM header and immediately sent an
ACK
. This would cause the sending XMODEM program to immediately send the next packet, making the transfer continuous, like an infinite-sized window. The modems also suppress the
ACK
being sent by the XMODEM software at the far end, thereby freeing up the low-speed return channel.
The system can also be implemented in the protocol itself, and variations of XMODEM offered this feature. In these cases, the receiver would send the
ACK
as soon as the packet started, in the same fashion as the Telebit modems. Since this feature is only an alteration of the receiver-side behavior, it does not require any changes in the protocol on the sender's side.
YMODEM formalized this system.
This concept should be contrasted with the one used in SEAlink, which changes the behavior on both sides of the link. In SEAlink, the receiver stops sending the
ACK
entirely, and the sender changes its behavior to not expect them.
See also
*
BLAST (protocol)
*
Kermit (protocol)
Kermit is a computer file transfer and management protocol and a set of communications software tools primarily used in the early years of personal computing in the 1980s. It provides a consistent approach to file transfer, terminal emulation, ...
References
Citations
Bibliography
*
*
*
*
*
External links
MODEM.ASM original source code, Ward Christensen, October 10, 1977.
XMODEM / YMODEM Protocol Reference by Chuck Forsberg October 10, 1985
XMODEM / YMODEM Protocol Reference by Chuck Forsberg June 18, 1988 (document reformatted October 14, 1988
XMODEM / XMODEM-CRC / WXMODEM File Transfer Protocols synchro.net
adontec.com
{{Modem file transfer protocols
BBS file transfer protocols
Computer-related introductions in 1977