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Xmodem

X-Modem is a character-oriented transport protocol from 1978 that forms the de facto standard for error-tolerantdata transmission

and is supported by various terminal programs. In X-modem

data transmission,data is transferred in blocks and transmitted from the sending modem

to the receiving modem.Each data block has a block size of 128 bytes, where each block consists of a Start of Header (SOH), a sequential number from 0 to 255, 128 data bytes and a checksum for the data bytes. Each data block is acknowledged with an acknowledgement flag (ACK) or by a negative acknowledgement (NAK). These control characters are transmitted from the receiving data station

to the sending one. If the transmission contains errors, the receiving station uses the NAK flag to request the sending station to retransmit the data block. There are more powerful variants of the X-Modem transmission protocol that can transmit up to 1024 bytes and use cyclic block checking (CRC-16) in error control. In addition, there are similar protocols Y-Modem, Z-Modem as well as Kermit and V.42.

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Englisch: Xmodem
Updated at: 01.12.2017
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Translations: DE