Ymodem

Ymodem is an enhanced Xmodem. The enhancements are

·        CRC-16 error checking

·        1024 bytes data block size

·        transmission of the file name within the first block of transmission

·        ability to perform batch file transmission, ie multiple files are set up then transferred one after the other

·        Used with modems that do error correction in hardware or when two computers are hardwired.

 

Zmodem

Zmodem is an enhanced Ymodem. The enhancements are

·        8 bit data can be transmitted over a 7 bit data link by encoding control characters.

·        A host computer will continually transmit without waiting for acknowledgement.

·        Transfers can be automatically remotely initiated.

·        If transmission is interrupted at reconnection the transmission will begin from where it left off.

 

CompuServe B

This is not quite as fast as Zmodem. It ensures the accuracy of the transmitted file by comparing each line of data with the original. It is protected from many types of interference.

 

Parameters

Bits per second equals the number of bits which pass a particular point in a second in time. The most common setting is 7 or 8.

In asynchronous transmission every character is surrounded by start and stop bits. Most systems use one start bit and two stop bits.

The parity bit is an additional data bit used for error checking. Odd parity means that in an eight bit transmission the number of one's is odd. The data is contained in 7 bits and the eighth bit is adjusted to make the total number of bits odd.

Word size is often two bytes of eight bit data, ie 16 bits.

Block size refers to the amount of data which can be bundled together. For example, Xmodem block is 128 bytes and Zmodem is 1024 bytes.

Handshaking is needed to determine the flow control or what to do when the buffer is full.