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Also known as: Zmodem Z-Modem is an error-correction protocol used in the transfer of serial data files between modems. Z-Modem is a faster successor to Y-Modem and has better error checking capability. Modems using the Z-Modem protocol send data in 512-byte blocks or packets, and the error detection method is cyclic redundancy check (CRC). Like Y-Modem, Z-Modem has the capability of handling batch transmission (sending a number of files with one command) and variable block sizes of data. Z-Modem is also similar to Y-Modem-G, because it sends data in a continuous stream, and does not require a positive acknowledgement (ACK) for successfully transferred blocks of data. However, if an error is detected in a received block, a negative acknowledgement (NAK) is returned to the sender and the block resent. Z-Modem differs from the earlier protocols in that it is able to recover after a "crash", i.e. when a transmission is cancelled or for some other reason interrupted, the transmission may be restarted without resending the previously transferred blocks of data