... communications packages. The Xmodem protocol transfers data in blocks of 128 bytes, giving each transmitted frame a sequential block number; this number is used to identify the location of errors. A checksum (see Block Check Characters) is attached to each block to check transmission errors. Its ability to find and correct errors makes it suitable for the transfer of files, which must retain their integrity, such as program files. Zmodem Is one of the most advanced protocols, being much faster than Xmodem. Its error correction controls are absolutely reliable. CCITT V42bis This protocol includes a data compression (through encoding, data is reduced in volume) technique and error detection and correction. Both the sending and receiving modem must possess the error correction facility. Implementation Top Down implementation is one way to achieve implementation of software. This involves coding, testing and debugging each module in turn starting with the top level and working down. This means that each ...

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