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ASCII
This is a 7 bit protocol, and consists of characters which make up the upper- and lowercase alphabet, symbols, numbers and special control characters.
There are no error checking capabilities, and is good for sending a burst transmission of raw / data uninterrupted.
When the receiving computer needs to stop the data reception, it sends a Control S character to the sending computer, and then when it is ready to receive again, it transmits a Control Q character. This is usually done when the computer needs to proccess the information it is receiving.
This protocol uses XON/XOFF handshaking (ie. XON/XOFF flow control must be enabled on both receive and transmit computers).
With this protocol, only one file can be transmitted at a time, and the file name is not transmitted with the data, therefore, a name must be given for it at the receiving modem.
Kermit / Super Kermit
Kermit was developed to meet the needs of file transfer between different computers, from home computers to main frames.
Kermit uses variable packet sizes to a maximum of 1024 bytes. Various number of files can be transmitted simultaneously with this protocol, and so the file names for each file is transmitted with the data.
With Super Kermit (Windowed Kermit), multiple packets are sent at one time, known as a transmission window. This protocol is usefull for transfering data over data networks like Telnet and Tymnet, which have long transmission delays. With this type of network, two communication delays are interjected for each packet sent.
Error correction is done, and the receiving computer will aknowledge receipt and ask for re-transmission of bad packets, only once all the packets within a window have been received, thereby reducing the delay time.
CompuServe is the only public service to supports Kermit.
XModem (XModem 7 / XModem Checksum / Christenson)
XModem Checksum sends data packets of 128 bytes. A simple 8 bit data checksum is added to each packet and is checked at the receiving end, whereby it can request that bad packets be re-sent.
The data format is set to 8 bits, 1 stop bit, no parity. Only one file can be sent at time using Xmodem, and the file name is not transmitted with the data.
XModem CRC
This enhances the original XModem by replacing the 8 bit checksum with a 16 bit Cyclic redundancy Check (CRC), thereby improving error detection.
With the 8 bit checksum, it is possible for 1 in 256 bad packets to a valid checksum and therfore going undetected. With CRC, only 1 in 700 billion bad packets will genrate a valid CRC, an assurance of 99.9984% that any transmission errors will be detected.
Only one file can be sent at time using Xmodem, and the file name is not transmitted with the data.
XModem 1K
XModem improves the speed of file transfers by increasing the 128 packet size to 1024 bytes (1Kbyte), and will maintain it there as long as no transmission errors are detected.
Only one file can be sent at time using Xmodem, and the file name is not transmitted with the data.
YModem
This is also called YModem Batch, YModem 1K and YModem CRC. This protocol is the same as XModem except that with YModem, you can send and receive multiple files, and the file names are included with the data.
YModem-G
YModem-G performs the same error checking as YModem, except that it won't perform error recovery. To use this protocol, an error-correction modem is required ( MNP 2-4 or V.42 ).
ZModem
This is the newest, most practical and most reliable protocol to use. It allows multiple file transfers and includes file names, sizes and creation dates. If the transfer was interrupted, it allows you to resume transmission where it got cut off.
The data block sizes varies depending on line conditions. Both 32-bit and 16-bit CRC error-checking are available using ZModem.
Since this is a new protocol, not all sites will have implememented ZModem identically, and problems might be experienced.