Editing XMODEM (section)

{{Short description|File transfer protocol}}
{{More footnotes|date=January 2009}}
{{Infobox networking protocol
|       title = XMODEM
|        logo = 
|    logo alt = 
|       image = 
|   image alt = 
|     caption = 
|    is stack = 
|     purpose = file transfer protocol
|   developer = [[Ward Christensen]]<ref>[https://books.google.com/books?id=9eJxx_ZGKngC&dq=%22Ward+Christensen%22&pg=PA451 Telecommunications: XMODEM: A Standard Is Born], By Alfred Glossbrenner, PC Mag, 17 April 1984, Page 451-452, ''... but the protocol itself was long ago placed in the public domain by its creator, Chicagoan Ward Christensen. Since its introduction in 1978, XMODEM ...''</ref><ref>[https://books.google.com/books?id=EzAEAAAAMBAJ&dq=%22Ward+Christensen%22&pg=PA26 In Focus: History lesson: Ward Christensen's free free-exchange software], By Michael Swaine, InfoWorld, 1 Nov 1982, Page 26</ref>
|        date = {{Start date and age|  1977  |  |  }}<!--Fill in: Year (4 digits), month and day (2 digits)-->
|    based on = 
|  influenced = [[YMODEM]], many others
|    osilayer = 
|       ports = 
|        rfcs = 
|    hardware = [[modem]]s
}}
'''XMODEM''' is a simple [[file transfer]] protocol developed as a quick [[Hacker (hobbyist)|hack]] 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.<ref>Ward Christensen, [http://www.bbsdocumentary.com/software/AAA/AAA/CBBS/memories.txt "Memories"], 25 November 1992</ref><ref name="meeks198902">{{Cite magazine |last=Meeks |first=Brock |date=February 1989 |title=The ABCs of X-, Y-, and ZMODEM |url=https://archive.org/details/eu_BYTE-1989-02_OCR/page/n217/mode/2up?view=theater |access-date=2024-10-08 |magazine=BYTE |pages=163–166}}</ref>

XMODEM, like most file transfer protocols, breaks up the original data into a series of "[[Packet (information technology)|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]] (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.{{r|meeks198902}} Christensen believed his original XMODEM to be "the single most modified program in computing history".<ref>{{cite web|url=http://www.well.com/user/hlr/vcbook/vcbook4.html|title=The Virtual Community}}</ref>

[[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.