File Transfers

 TeleFinder supports ZMODEM , XMODEM-CRC, and XMODEM-1K file transfers, in addition to a ÒBackgroundÓ protocol using TeleFinderÕs native communications facilities. Select the file transfer protocol you wish to use from TeleFinderÕs Special Menu.

Each of TeleFinderÕs file transfer protocols provide error correction using advanced error detection techniques to insure accurate data transfer.

The Background transfer protocol lets you continue to use the BBS while transfers are in progress. You can also send and receive files at the same time. This is the preferred protocol because of itÕs combination of speed and convenience, but lacks Zmodem's ability to resume.

ZMODEM file transfers have the advantage of 32-bit CRCÕs and file recovery. If a ZMODEM file transfer fails to complete, TeleFinder saves the part of the file already received. You can then complete the transfer either on another call, or during the same call. Just drag the file to the same location to continue the transfer where it left off.

TeleFinder/User supports the ÒstreamingÓ and ÒwindowedÓ ZMODEM variations. Use streaming to get the fastest file transfers. Use windowed when there is a flow control problem. Switching to windowed ZMODEM can be helpful if you notice a large number of retransmits.
The XMODEM file transfer protocol does not require flow control and has less overhead than ZMODEM. Use XMODEM when you have small files to transfer, or when you have problems with flow control. XMODEM does not support file recovery.

X & Z Modem
XModem and ZModem are names of file transfer protocols, which are a set of rules that two computers can use to send a file from one computer to the other.  A protocol will usually provide error detection and correction, by resending the portion that was received in error.  XModem is the first public domain protocol developed for use with personal computers and is one of the most widely supported.  It was developed by Ward Christensen back in 1977.

The ZModem protocol was developed for the public domain under a Telnet contract.  Its goal was to Òalleviate the throughput problems network customers were experiencing with XMODEM and Kermit file transfers.Ó  It was developed by Chuck Forsberg/Omen Technology.  It overcomes many weaknesses of older protocols and adds several useful capabilities. 

ZModem
The ZModem file transfer protocol is arguably one of the best general purpose file transfer protocols for personal computers.  Some of its features include:

¥	various options to let it operate efficiently in a wide range of communication environments.  The most common mode is full streaming, where there is no wait between blocksÑthe receiver only talks when he has an error.
¥	the sequence that starts a file transmission can be detected by the receiving communication software, which can then trigger its receive routines.  This is sometimes referred to as AutoDownload.
¥	Crash Recovery - A file transfer that is interrupted by modem disconnection (or other reasons) can be restarted and it will pick up where it left off.  Note: if the Mac crashes or loses power during a transfer, you may not be able to restart the transfer because the partial file was not closed.
¥	All ZModem transfers are batch transfers:  multiple files can be sent per transfer, each file is sent with its file name, file size and date of last modification.
¥	32 CRC's will error check all blocks: data blocks and control messages.


For receiving files using the ZModem protocol, just start the sender and Telefinder will detect that it should start receiving files with ZModem.

For sending files using ZModem, Telefinder sends a command sequence to put the receiver in a receive ZModem mode.  In the Special menu, ZModem Options will bring up a dialog that includes the command to send to put the other end into ZModem receive. When you select Send ZModem, you will first select what files to send.  When you start the transfer, Telefinder will send the sequence from the ZModem Options and then the transfer will start.

Binary File Type Detection
Telefinder will detect certain types of binary files, and set the appropriate creator and type for the kind of file.  It detects the type by either looking for a specific sequence of bytes at the start of the file (a signature) or by matching the end of the file name (e.g. the extension of an MS-DOS file).  

Text Detection and Conversion
When Telefinder is receiving a file, it inspects the first 128 bytes to determine if it should be treated as a text file.  If all of these bytes are Macintosh printable characters or typical text control characters (return, line feed, tab, etc), then it is considered a text file.  The file type will be set to TEXT and Telefinder will perform End of Line conversion.  If the text has CR/LF at each line end, Telefinder will filter out the LF.  Telefinder currently does not filter any other control characters.

Resume an Interrupted Download
If a download is interrupted by disconnection, Cancel, or whatever (but not a power failure - the file wonÕt be closed properly) it can restart where it left off using ZModem.

MacBinary II
Telefinder supports the original MacBinary format for the complete transfer of Macintosh files and their additional file information.  The MacBinary II standard (dated July 1987) is also supported.  The file transfer status dialog will display either ÔMacBinaryÕ or ÔMacBinary IIÕ when that format is being used.

Transfer Efficiency Calculations
I donÕt know if there is an accepted procedure for transfer efficiency calculation, so I will just explain how I am currently calculating efficiency.  All timings use the Macintosh TickCount, which has a resolution of 1/60th of a second.

For receiving files using ZModem, the clock starts when Òblock zeroÓ (the block with the file name and other information) is received.  This block zero is not the same as the MacBinary header.  For XModem file receives, the clock starts just before the initial NAK (or whatever) is sent.  The clock stops when all file data is received, written and the file is closed.

For sending files using ZModem, the clock starts after block zero is sent.  For XModem, it starts when Telefinder starts waiting for the initial NAK.  The clock stops on ZModem file sends when the ZRINIT response is received for the ZEOF message.  For YModem and XModem, it stops when the ACK response is received for the EOT message.

The CPS figure (characters per second) is the total file bytes (does not include block zero) divided by seconds.  The efficiency figure is CPS times 10 bits per data byte divided by BPS (bits per second, sometimes called baud rate).

Hardware Handshake
If you have a high speed modem or one with compression and error control (like MNP, V.42 and V.42bis), you may want to use hardware handshaking to get the most efficient file transfers.  This requires a correctly wired cable and additional modem configuration.  See the section on High Speed Modems.

Compressed Files

The files available for download on BBSÕs and online services are almost always in a compressed format.  There are several ÒarchiveÓ programs that will compress one or more files into an Òarchive file.Ó  In addition to reducing the time to transfer the file, it provides a means to make a package of several related files, so that anyone who downloads a package will get any necessary files.  For example, the Telefinder archive file that I use contains the program, this documentation file, and several other useful files.  Packaging them together in one archive file will make sure that a downloader will get all the files.

Here are the file extensions for the common archive formats.  A file extension (which is more common with non-Mac operating systems) appears at the end of the file name and provides some indication of the type of file.  I prefer .sit, .sea or .image files to other types but will accept any listed here with the exception of .tar or .Z files.  I will erase those files, they are too much trouble.

.cpt	Compactor format -- use Compact Pro or Extractor
.hqx	BinHex format -- use BinHex 4.0, BinHex DA, any of the StuffIt programs, Compact Pro, or mcvert (for Unix).
.sit	StuffIt format -- use any of the StuffIt programs.
.sea 	Self Extracting Archive.  Double-clicking on this should give you the uncompressed file.
.pit 	PackIt format, not used much any more -- use PackIt or any of the StuffIt programs.
.image	DiskCopy images of a floppy disk.  This is the format used for the System software on many services.  Use the DiskCopy application to translate these back onto disks.  You can also use Drop Disk to mount these images. Shrinkwrap will also make and mount images.
.Z	Unix 'compress' files.  Uncompress with the Unix command 'uncompress' or on the Mac with MacCompress (available from sumex).
.tar	Unix tar (tape archive) file.
.dd	DiskDoubler format.  Use DiskDoubler or DDExpand.