MultiXfer, the application, has been conceived to bring Macintosh users a new file transfer protocol. Over the last ten years people have taken for granted that some protocols like XModem or Kermit were the ones to use to transfer their files. Today, common baud rates commonly reaching 2400 baud, these protocols are no longer so efficient.
XModem and Kermit have a stop-and-wait type of flow control. This means that after each block is sent, the sender has to wait for an acknowledgment in order to resume the transfer. A substantial amount of time is lost in this stop-and-wait process when the two entities are far from each other or when the transfer rate is low. This is why protocols like ZModem or MultiXfer, which use a window concept, dramatically increase the efficiency of file transfers.
MultiXfer protocol does not use new concepts; it just takes those used in some established standards (like CCITT X.25 and ZModem) and joins them together. That is why the MultiXfer protocol is a very efficient protocol, particularly when used for simultaneous upload/download, in which case it outperforms ZModem by as much as 90%.
For those of you who are familiar MCS, you will find similarities in MultiXfer. In fact, the protocol used in MCS is also based on X.25, which is the main inspiration of MultiXfer. However, MultiXfer is more flexible. Let's see why...
MULTIXFER, THE APPLICATION.
MultiXfer is a Mac to Mac serial connection file transfer utility. MultiXfer, the application, can turn your Macintosh into a host computer. With MultiXfer, another Macintosh can call your system, receive and send files or entire folders of your disk remotely. This is accomplished using a convenient interface and a fast protocol that allows simultaneous upload/download along with text interaction.
DESCRIPTION OF FEATURES.
In order to use MultiXfer, you must have two Macintoshes linked together and both running MultiXfer. They can be linked by modems or by a serial cable connecting each other.
When MultiXfer is launched, it is in a terminal emulation mode. In order to be able to talk with a Macintosh peer, MultiXfer must be in the File Transfer Mode. When you select the Connect item under the File menu, a connection procedure takes place in order to get MultiXfer in the File Transfer Mode.
In the File Transfer Mode, you can send a file or folder by selecting the Send item from the File menu. If MultiXfer is already sending a file, the selected file or folder will be added to the Send File Queue. You can remove files from the queue with the Remove button.
You can receive files from the Macintosh peer by selecting the Receive item in the File menu or the Receive button beneath the Host File Catalog List. MultiXfer will notify the peer to send you the file or folder selected in the Host File Catalog List. If MultiXfer notices that the selected file has already been partly transfered, it will initiate the file recovery procedure. This can save you lots of troubles if there is a power failure or if a transfer is interrupted inadvertedly.
In the File Transfer Mode, MultiXfer is always ready to receive a file. If MultiXfer detects that the peer wants to send a file to your Mac, it will receive and store it.
You can abort a transfer at any given time by pressing the Stop button. The Macintosh peer will be notified of this action and the current transfer will be stopped.
You can set the File Transfer Folder of the Macintosh peer to a new folder by using the Change Directory item under the File menu or the Change Directory button beneath the Host File Catalog List. MultiXfer will notify the peer to change to the folder selected in the Host File Catalog List and the new catalog of files will be displayed.
Don't forget that each command can be issued at any point, even if you are already transfering files.
The Parameters menu is used to configure the serial communications port. You can change the baud rate, the number of data and stop bits and the parity. You can also specify if you want local echo of the characters sent through the serial port. Before performing file transfer, you should make sure you have the settings at 8 data bits and no parity.
MultiXfer will give a number extension to a received file which already exists on the current volume. Also, before creating any files on the receiving computer, MultiXfer will check to see if there is enough space to store it. If not, the current transfer will abort and the peer will be notified of the problem.
THE TERMINAL EMULATOR.
The terminal emulator is a VT-52.
If your keyboard does not have the ctrl key, you can use Cmd-Shift instead. Ctrl-S can be generated by Cmd-Shift-S. The break key is Ctrl-"Space".
If you type on the keyboard while MultiXfer is in the File Transfer Mode, the characters will appear normally on the screen (if in the Local Echo Mode). However, they will be sent to the peer only when you hit the Return key.
TECHNICAL NOTES ON THE MULTIXFER PROTOCOL.
Features of the MultiXfer protocol are:
√ Slicing window flow control
√ Multiple channel support
√ Adaptative block length (blocks are 512 to 4096 bytes)
√ File recovery
√ Error checking is CRC CCITT 16 bits
LAST CHANGES.
◊ Efficiency is improved (more than 95% in a one way transfer, up to 180% both sides).
◊ Added a dialog for the connection procedure.
◊ Added send and receive entire folders.
◊ Added file recovery.
◊ All the settings (including positioning of the windows) are recorded.
◊ Throughput is shown at the end of each transfer.
◊ Response to menu selection and list management is improved in the File Transfer Mode.
◊ Modem initialization sequence now works properly.
◊ Proper modified date is now recorded.
◊ Send file queue works properly when files to send originate from different directories.
◊ The receive folder is now the MultiXfer folder instead of peer's transfer file directory.
◊ The About box is much more beautiful (let's see who reads the docs)!
Now, just tell me you don't believe in FreeWare!
POSSIBLE ENHANCEMENTS.
Communication Toolbox support (including AppleTalk support and VT-100 emulator)
Enhancement of the protocol: support for encryption and compression.
Implementation of a compatible MultiXfer protocol under other environments (Unix, Windows) allowing users to transfer files from their Macintosh to other computers.
Implementation of MultiXfer protocol for BBSs. The native code of the protocol could be provided to someone who wanted to integrate the MultiXfer protocol in a BBS program. Besides two way transfer, the use of the "chat" mode could permit users to continue browsing into the BBS while doing file transfers. Operation in this case would be similar to the exchange of file catalogs in the application.
SPECIAL THANKS.
I would like to thank all the beta tester and those of you who have pointed out the weaknesses of MultiXfer and suggested the enhancements that makes MultiXfer easy to use. Special thanks to John Norstad and the Northwestern University for making the Disinfectant source code available.
FINAL WORD.
MultiXfer is FreeWare (my contribution to the Mac software library). I do not give the right to any company or organization to charge a fee for the distribution of this software. MultiXfer is for free distribution only. If you have any comments or suggestions, please send mail to dubuc@IRO.UMontreal.CA or post a letter to:
