XMODEM is a simple
file transferFile transfer is a generic term for the act of transmitting files over a computer network or the Internet. There are numerous ways and protocols to transfer files over a network. Computers which provide a file transfer service are often called file servers...
protocol developed as a quick hack by
Ward ChristensenWard Christensen, born in West Bend, Wisconsin, U.S., was the founder of the CBBS bulletin board, the first bulletin board system ever brought online. He started development during a blizzard in Chicago, Illinois, and officially established CBBS four weeks later, on February 16, 1978.Christensen...
for use in his 1977
MODEM.ASM terminal program. XMODEM became extremely popular in the early
bulletin board systemA Bulletin Board System, or BBS, is a computer system running software that allows users to connect and log in to the system using a terminal program. Once logged in, a user can perform functions such as uploading and downloading software and data, reading news and bulletins, and exchanging...
(BBS) market, largely because it was so 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.
Chuck ForsbergChuck Forsbergs fame comes in part from developing the YMODEM and ZMODEM file transfer protocols. ZMODEM is a file transfer protocol with error checking and crash recovery. ZMODEM does not wait for positive acknowledgment after each block is sent, but rather sends blocks in rapid succession...
collected a number of these into his
YMODEMYMODEM is a protocol for file transfer used between modems. YMODEM was developed by Chuck Forsberg as the successor to XMODEM and MODEM7, and was first implemented in his CP/M YAM program...
protocol, but poor implementation led to a further fracturing. His later
ZMODEMZMODEM is a file transfer protocol developed by Chuck Forsberg in 1986, in a project funded by Telenet in order to improve file transfers on their X.25 network...
protocol was so superior that it led to the demise of all of these XMODEM derivatives in the early 1990s.
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.
Packet structure
The original XMODEM used a 128-byte data packet, the basic block size used on
CP/MCP/M is an operating system originally created for Intel 8080/85 based microcomputers by Gary Kildall of Digital Research, Inc...
floppy diskA floppy disk is a data storage medium that is composed of a disk of thin, flexible magnetic storage medium encased in a square or rectangular plastic shell. Floppy disks are read and written by a floppy disk drive or FDD, the initials of which should not be confused with "fixed disk drive," which...
s. The packet was prefixed by a simple 3-byte header containing a
<SOHThe C0 and C1 control code sets define control codes for use in text by computer systems that use the ISO/IEC 2022 system of specifying control and graphic characters....
> character, a "block number" from 0-255, and the "inverse" block number—255 minus the block number. Block numbering starts with 1 for the first block sent.
The packet was also suffixed with a single-byte
checksumA checksum or hash sum is a fixed-size datum computed from an arbitrary block of digital data for the purpose of detecting accidental errors that may have been introduced during its transmission or storage. The integrity of the data can be checked at any later time by recomputing the checksum and...
of the data bytes. The checksum was the sum of all bytes in the packet
moduloIn computing, the modulo operation finds the remainder of division of one number by another.Given two numbers, and , a modulo n is the remainder, on division of a by n. For instance, the expression "7 mod 3" would evaluate to 1, while "9 mod 3" would evaluate to 0...
256. The modulo operation was easily computed by discarding all but the eight
least significant bitIn computing, the least significant bit is the bit position in a binary integer giving the units value, that is, determining whether the number is even or odd. The lsb is sometimes referred to as the right-most bit, due to the convention in positional notation of writing less significant digits...
s of the result, or alternatively on an eight bit machine, ignoring
arithmetic overflowThe term arithmetic overflow or simply overflow has the following meanings.# In a computer, the condition that occurs when a calculation produces a result that is greater in magnitude than that which a given register or storage location can store or represent.# In a computer, the amount by which a...
which would produce the same effect automatically. In this way the checksum was restricted to an eight bit quantity which was able to be expressed using a single byte. For example, if the data bytes were 130 and 130, the total of these codes is 260 and the resulting checksum is 4 using this method.
The complete packet was thus 132 bytes long, containing 128 bytes of data, for a total throughput efficiency of about 97%.
The file was marked "complete" with a
<EOTIn telecommunication, an end-of-transmission character is a transmission control character used to indicate the conclusion of a transmission that may have included one or more texts and any associated message headings....
> character sent after the last block. This packet was not in a block, but sent alone as a single character. 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
<ACKFor teleprinters, Acknowledge character is a transmission control character transmitted by the receiving station as an affirmative response to the sending station....
> 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
<NAKNaK is a sodium - potassium alloy usually liquid at room temperature. Various commercial grades are available. NaK is highly reactive with air or water and must be handled with special precautions...
>. If a
<NAKNaK is a sodium - potassium alloy usually liquid at room temperature. Various commercial grades are available. NaK is highly reactive with air or water and must be handled with special precautions...
> 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.
Transfers were sender-driven, the receiver acting passively on data sent to it. However, the transfer was actually
started by the receiver sending a single
. This was a side-effect of the way the user interacted with the sending machine, generally by navigating to the requested file, asking the sender to transfer it, then using a command in their local software to start receiving. Since the delay until the user could invoke the command was unknown, XMODEM instead made the receiver trigger the transfer when it was ready.
Minor problems
XMODEM was written for
CP/MCP/M is an operating system originally created for Intel 8080/85 based microcomputers by Gary Kildall of Digital Research, Inc...
machines, and bears several marks of that
operating systemAn operating system is an interface between hardware and user which is responsible for the management and coordination of activities and the sharing of the resources of the computer that acts as a host for computing applications run on the machine. As a host, one of the purposes of an operating...
. 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-DOSMS-DOS is an operating system developed by Microsoft. It was the most commonly used member of the DOS family of operating systems and was the main operating system for personal computers during the 1980s. It was preceded by M-DOS , designed and copyrighted by Microsoft in 1979...
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, that 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. Typically this meant the user would navigate on the sender's system to select the file they wanted, and then invoke the transfer from their end using a command in their terminal emulator.
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
as normal at the start of a transfer. When the
'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. An XMODEM file transfer starts with the receiver sending a single
character to the sender, which then starts sending packets of 128-bytes of data prefixed with a
. MODEM7 changed this behavior only slightly, by sending the filename, in 8.3 filename format, before the first data packet. For a non-aware XMODEM implementation this data would simply be ignored while it waited for the
to arrive. With "aware" software, the file could be saved with that name. Transfers could continue with another
, each file being saved under the name being sent to the receiver.
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 JenningsTom Jennings is the creator of FidoNet, the first message and file networking system for BBSes...
, author of the original
FidoNetFidoNet is a worldwide computer network that is used for communication between bulletin board systems. It was most popular in the early 1990s, prior to the introduction of easy and affordable access to the Internet...
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
timestampA timestamp is a sequence of characters, denoting the date and/or time at which a certain event occurred. This data is usually presented in a consistent format, allowing for easy comparison of two different records and tracking progress over time; the practice of recording timestamps in a...
, which were placed in a regular 128 byte XMODEM block. Whereas a normal XMODEM transfer would start with the sender sending "block 1", the TeLink header packet was labeled "block 0".
Again, 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 be sure it was being
'ed because it did not understand the "block 0", or because there was a transmission error. However, TeLink was generally limited to FidoNet software, which demanded it, so it was safe to assume the receiver implemented it as well.
The basic "block 0" system became a standard in the FidoNet community, and was re-used by a number of future protocols like
SEAlinkSEAlink is a file transfer protocol that is backward compatible with XMODEM but features a sliding window system for improved throughput. SEAlink was written in 1986 as a part of the SEAdog FidoNet mailer written by System Enhancement Associates, creators of the famous ARC program...
and
YMODEMYMODEM is a protocol for file transfer used between modems. YMODEM was developed by Chuck Forsberg as the successor to XMODEM and MODEM7, and was first implemented in his CP/M YAM program...
.
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 Mahr, which used a 16-bit
CRCA cyclic redundancy check is a non-secure hash function designed to detect accidental changes to raw computer data, and is commonly used in digital networks and storage devices such as hard disk drives. A CRC-enabled device calculates a short, fixed-length binary sequence, known as the CRC code or...
in place of the 8-bit checksum. CRC's 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 bytes long 99.9969%, and even higher for longer data.
XMODEM-CRC was designed to be backwardly compatible with XMODEM. To do this, the receiver simply 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. Ironically, any software that
did support -CRC in their basic XMODEM transfer, as it was intended, surreptitiously suggested the user should not attempt to use -CRC. To the average user, XMODEM-CRC was essentially a "second protocol", and treated as such.
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 just over 3.5 seconds to send (132 bytes * 8 bits per byte / 300 bits per second). If it then took 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 3.7 seconds, just over 92% throughput.
As modem speeds increase, the fixed delay needed to send the
/
grows in proportion to time needed to send the packet. For instance, at 2400 bit/s the packets took only 0.44 seconds to send, so if the
/
still took 0.2 seconds to make it back (this is
latency, not throughput), the throughput has fallen to under 60%. At 9600 bit/s it is under 30% – 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 a further fracturing of the XMODEM landscape in the user's terminal emulator. In the end, dozens of versions of XMODEM would emerge.
SEAlink
One of the first "third party" mailers for the
FidoNetFidoNet is a worldwide computer network that is used for communication between bulletin board systems. It was most popular in the early 1990s, prior to the introduction of easy and affordable access to the Internet...
system was
SEAdog, written by the same author as the then-popular .arc
data compressionIn computer science and information theory, data compression or source coding is the process of encoding information using fewer bits than an unencoded representation would use, through use of specific encoding schemes.As with any communication, compressed data communication only works when both...
format. SEAdog included a wide variety of improvements, including
SEAlinkSEAlink is a file transfer protocol that is backward compatible with XMODEM but features a sliding window system for improved throughput. SEAlink was written in 1986 as a part of the SEAdog FidoNet mailer written by System Enhancement Associates, creators of the famous ARC program...
, an improved transfer protocol.
SEAlink used a method known as sliding windows to avoid the inter-packet delay. To do this, the protocol did not wait for the
/
to arrive, and immediately moved onto the next packet. It was only after some number of packets had been sent, the "window", that the protocol would stop and wait. In order for this to work, SEAlink needed to known
which packet the receiver was
/
ing, which it did by appending the packet number to the
or
character.
SEAlink later added a number of other improvements, and was generally a useful protocol. However it remained rare, and was typically only found in FidoNet mailers.
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 actually one of the many improvements to XMODEM introduced by
Chuck ForsbergChuck Forsbergs fame comes in part from developing the YMODEM and ZMODEM file transfer protocols. ZMODEM is a file transfer protocol with error checking and crash recovery. ZMODEM does not wait for positive acknowledgment after each block is sent, but rather sends blocks in rapid succession...
in his
YMODEMYMODEM is a protocol for file transfer used between modems. YMODEM was developed by Chuck Forsberg as the successor to XMODEM and MODEM7, and was first implemented in his CP/M YAM program...
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.
A backwards compatible extensions of XMODEM with 32k and 64k block lengths was created by Adontec for better performance on high-speed error free connections like ISDN or TCP/IP networks.
Pre-acknowledge
Over reliable (error-free) connections, the receiver could eliminate the latency issue by "pre-acknowledging" the packets. The receiver would already send ACK while the packet was still being transmitted. This effectively breaks error-correction since a packet is always acknowledged regardless of its integrity (which can only be checked after it has been completely received).
Since this feature is only an alteration of the receiver-side behaviour, it does not require any changes in the protocol or on the sender's side.
Pre-acknowledge was also possible for
YMODEMYMODEM is a protocol for file transfer used between modems. YMODEM was developed by Chuck Forsberg as the successor to XMODEM and MODEM7, and was first implemented in his CP/M YAM program...
. It was made obsolete by variants such as YMODEM-g or
ZMODEMZMODEM is a file transfer protocol developed by Chuck Forsberg in 1986, in a project funded by Telenet in order to improve file transfers on their X.25 network...
.