CS5865 Laboratory #1 Notes File Transfer Protocols
1 CS5865 Laboratory #1 Notes File Transfer Protocols - An Overview Dr. B.J. Kurz XMODEM (Ward Christensen, 1970), one of the earliest ‘industry standard’ file transfer protocols in the public domain. HDX operation, character (byte)-oriented, fixed-length 128-byte blocks, 8-bit characters (bytes), 8-bit BCC = sum-mod256, stop-wait ARQ, block counts, binary data capability. Poor error detection capability, typically 95%. data block format: SOH/count/1’s compl. count/ user data/BCC control block format: ACK or NAK (or C), EOT Several later versions of XMODEM are in use: XMODEM-CRC same as regular XMODEM above except uses 8-bit BCC = CRC-8 (some implementations use 16-bit BCC = CRC-16). Better error detection capability, typically 99.97% (for CRC-8). Some versions negotiate the error character generation method: the sender first tries sum- mod256 BCC, then switches to CRC-8 BCC after three unsuccessful attempts indicated by NAKs from the receiver. XMODEM-1K same as XMODEM-CRC except uses 1024-byte blocks for better throughput (fewer line- turn-arounds), but slightly degraded error detection performance. WXMODEM a sliding-window version of XMODEM. Group size is fixed as 4 blocks. Much improved throughput. FDX capable lines needed if continous-ARQ error recovery used. YMODEM (Chuck Forsberg) a variation of XMODEM-1K. Supports two block lengths of 128 bytes and 1024 bytes, uses 2-byte CRC-16, excellent error detection capability, typically 99.99%. KERMIT (Frank de Cruz, Columbia Univ. Comp. Center, N.Y), most popular file transfer protocol, copyrighted, over 200 implementations for various systems. This is a comprehensive 2 remote communications package including terminal emulators, storage-to-storage transfer, not only a file transfer protocol.
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