Xmodem

This is a simple file transfer protocol that was developed in the late 1970's. Data is divided into blocks. Each block is composed of

·        Start of header character-SOH, ASCII 1

·        One byte block number-starting at 0 and incrementing to 255, then 'wrapping' back and starting at zero again

·        The one's complement of the block number

·        128 byte data block size

·        checksum byte-a single byte calculated by adding together the 128 data bytes and ignoring the carry in the addition. The receiving computer compares the received checksum with the calculated checksum. If they match, the receiving computer sends an 'OK' to the sender and the sender transmits another block. If the checksums do not match a 'NOT OK' is sent back to the sender and the block is retransmitted. The process continues until all files have been transferred.

 

A one byte checksum is not adequate in detecting all errors an extension to the Xmodem known as Xmodem-crc was devised. That is, a cyclic redundancy check is applied to the block. The binary numbers in the block are treated as one huge binary number and this is divided by a number, ie CRC-16 the binary number is divided by 16. The remainder is transmitted and compared with one calculated by the receiver.

 

The drawbacks of Xmodem

·        Transmission is slow because of 'acknowledge'(ACK) and 'not acknowledge'(NAK) protocol.

·        It does not work well with packet-handling systems.

·        A two-byte Cyclic Redundancy Computation (CRC) is needed to overcome multiple errors  that can occur in a noisy line.

·        Original block of data was 128 bytes. Longer blocks 1k are still small.

·        Designed for connections where each end of the connection is totally occupied in processing the data.