Ymodem协议.docVIP

Ymodem协议 - 1 - XMODEM/YMODEM PROTOCOL REFERENCE A compendium of documents describing the XMODEM and YMODEM File Transfer Protocols Edited by Chuck Forsberg Please distribute as widely as possible. Questions to Chuck Forsberg Omen Technology Inc 17505-V Sauvie Island Road Portland Oregon 97231 Voice: 503-621-3406 Modem (Telegodzilla): 503-621-3746 Speed 1200,300 Compuserve: 70715,131 UUCP: ...!tektronix!reed!omen!caf - 2 - 1. ROSETTA STONE Here are some definitions which reflect the current vernacular in the computer media. The attempt here is identify the file transfer protocol rather than specific programs. XMODEM refers to the original 1979 file transfer etiquette introduced by Ward Christensens 1979 MODEM2 program. Its also called the MODEM or MODEM2 protocol. Some who are unaware of MODEM7s unusual batch file mode call it MODEM7. Other aliases include CP/M Userss Group and TERM II FTP 3. This protocol is supported by every serious communications program because of its universality, simplicity, and reasonable performance. XMODEM/CRC replaces XMODEMs 1 byte checksum with a two byte Cyclical Redunancy Check (CRC-16), giving modern error detection protection. YMODEM refers to the XMODEM/CRC protocol with the throughput and/or batch transmission enhancements described below. 2. YET ANOTHER PROTOCOL? Since its development half a decade ago, the Ward Christensen modem protocol has enabled a wide variety of computer systems to interchange data. There is hardly a communications program that doesnt at least claim to support this protocol. Recent advances in computing, modems and networking have revealed a number of weaknesses in the original protocol: + The short block length caused throughput to suffer when used with timesharing systems, packet switched networks, satellite circuits, an