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XMODEM is a simple file transfer protocol that was widely used for transferring files between computers over a serial connection. It was developed by Ward Christensen in the late 1970s and became one of the most popular protocols for file transfer in the early days of personal computers.

Here are some key features of XMODEM:

  1. Reliability: XMODEM was designed to ensure reliable file transfers over noisy and error-prone serial connections. It incorporated error detection and correction mechanisms to handle data corruption during transmission.
  2. Block-based Transmission: XMODEM divides the file into blocks of fixed size (usually 128 bytes) for transmission. Each block is numbered and includes a checksum to verify the integrity of the data.
  3. Simple Acknowledgment Mechanism: After each block is sent, the receiver acknowledges its successful reception. If an acknowledgment is not received within a specified timeout period, the sender retransmits the block.
  4. Basic Error Correction: XMODEM uses a basic form of error correction called “Go-Back-N” protocol. If the receiver detects an error in a block, it requests the sender to retransmit the entire batch of blocks starting from the one with the error.
  5. Limited Error Recovery: XMODEM has limited error recovery capabilities. It does not support selective retransmission of individual blocks but instead requires the retransmission of entire batches.
  6. ASCII Mode: XMODEM supports an ASCII mode that allows for the transfer of text files, converting end-of-line characters between platforms (e.g., between Windows and Unix).

While XMODEM was widely used in the past, it has some limitations compared to modern file transfer protocols. It operates at a relatively low speed and does not support features like encryption, compression, or efficient error recovery. As technology progressed, more advanced protocols like YMODEM, ZMODEM, and later protocols such as FTP, TFTP, and SCP emerged, providing improved performance, security, and functionality for file transfers. Nonetheless, XMODEM played a significant role in the early days of computer communications and contributed to the development of subsequent file transfer protocols.

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