The protocols that modems agree on and use for checking and correcting transmission errors have evolved toward accuracy, speed, and efficiency since 1978 when the Xmodem protocol became a de facto standard. To briefly summarize: in all protocols data is chunked into "blocks" of a certain byte size and sent to the destination modem which checks each block for errors and, depending on the results and the protocol, returns a positive (ACK) or negative (NAK) acknowledgement, the latter usually resulting in a retransmission. The type of checking (checksum or cyclic redundancy checking) and the frequency at which a response is sent vary by protocol. Today, new modems use a V.42 protocol, but the earlier protocols are still in use for older modems.
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|Protocol||Block size||Frequency of response||Type of checking||Other information|
|Xmodem||128 bytes||Every block||checksum||Also called MODEM7|
|Xmodem CRC||128 bytes||Every block||cyclic redundancy checking||Cyclic redundancy checks detect errors that checksum won't|
|Xmodem-1K||1024 bytes||Every block||Cyclic redundancy check||Better for large files|
|WXmodem||128 bytes||Every block but doesn't hold up the sender before accepting the next block||Checksum||More efficient than Xmodem|
|Ymodem||1024 bytes||Every block||Cyclic redundancy check||Includes a batch mode that allows multiple files to be sent with one command|
|Ymodem-g||1024 bytes||Only when an error is detected to blocks sent as a stream and all must arrive successfully||Cyclic redundancy check||Supports batch transfers|
|Zmodem||512 bytes||When a block with an error is detected||Cyclic redundancy check||A transmission can be interrrupted and resumed without retransmitting blocks already sent.|
|Kermit||Adjusts to the computer system||When a block with an error is detected||Checksum||Can resynchonrize transmissions after a line interruption|
|V.42 (LAPM)||128 bytes||Up to 15 blocks (frames) can be sent before a response||Cyclic redundancy check||This takes precedence over V.42 MNP4.|
|V.42 (MNP4)||Varies||When a packet (block) with an error is detected||Cyclic redundancy check||Used when LAPM can't be used by both modems.|