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File Transfer Protocols

Many communications applications need to transfer files or other large amounts of data from one machine to another. This could be accomplished by having the sender call PutBlock repeatedly and the receiver call GetBlock correspondingly. However, the application would have a tremendous amount of detail work still to do. It would need logic to transfer file name and size information, to check for and recover from transmission errors, to handle file I/O, etc., etc.

That’s why Async Professional provides standard, tested, reliable, high performance file transfer protocols. The term “protocol” means that both sides of the communication link behave in a clear, well-defined manner following agreed-upon rules. The rules vary among the different protocols and some protocols offer more control and features than others. At a minimum, each protocol handles file I/O and serial port I/O and checks for errors. Some protocols also include error correcting logic, multi-file transfers, and automatic recovery after partial file transfers.

Async Professional offers the most widely used industry standard file transfer protocols, as shown in Table 14.1.

Available Async Professional file transfer protocols
Protocol	Component	Description
Xmodem	TApdProtocol	128 byte blocks with checksum block checking. See ASC for more information.
XmodemCRC	TApdProtocol	128 byte blocks with CRC block checking. See ASC for more information.
Xmodem1K	TApdProtocol	1024 byte blocks with CRC block checking. See ASC for more information.
Xmodem1KG	TApdProtocol	Streaming Xmodem1K. See ASC for more information.
Ymodem	TApdProtocol	1024 byte blocks, batch. See ASC for more information.
YmodemG	TApdProtocol	Streaming Ymodem. See ASC for more information.
Zmodem	TApdProtocol	1024 byte blocks, batch, streaming, restartable. See TApdPr for more information.
Kermit	TApdProtocol	80 byte blocks, batch, with long blocks and windowing. See ASC for more information.
ASCII	TApdProtocol	ASCII stream with inter-character and inter-line delays. See ASC for more information.
FTP	TApdFTPClient	An internet file transfer protocol. See FTP for more information.

Three related classes are also included in the file transfer protocol implementation. TApdAbstractStatus defines a mechanism by which the protocol can report its status (percent completion, transfer rate, etc.) to the user. TApdProtocolStatus derives from TApdAbstractStatus to present protocol status in a particular style. TApdProtocolLog is a small component that writes to a log file the status of each file transferred by an associated TApdProtocol component.

This document maintained by the Async Professional Project.