Data transfer

 

There are three general-purpose file transfer utilities

 

1.      Xmodem needs a 256 byte buffer, is public domain, sends down 8-bit characters and has good error detection. It operates on half duplex.

2.      Zmodem can transmit very large files and is reliable. It operates on full duplex.

3.      CompuServ-B is provided on the CompuServ Information Service.

 

Communication packages should have the ability to set communication parameters such as baud rate, number of data bits, number of stop bits and type of parity checking. It should be able to emulate a dumb terminal.

 

Data can be compressed using software. This allows a smaller file to be sent or stored. For the file to be useful it needs to be 'uncompressed'. Two types of data compression are repeating character compression (replaces any repeated bytes with a count) and common word compression (replaces common words with one or two bytes)

PKZIP is a shareware program. The files are zipped to make them smaller then unzipped to resore them. Many files can be zipped into one file.

Stuffit is a compression program for Macintosh computers.

 

A modem can compress information before sending it and decompress it on receiving information. V.42bis and MNP-5 are data compression hardware facilities.

 

Terms

FILE TRANSFER PROTOCOL                   CHECKSUM CRC               PK ZIP

DATA COMPRESSION                               X-MODEM     MNP-5            STUFFIT

COMPRESSION RATIO                              Z-MODEM     V-42 bis         PACKET        Y-MODEM

Theory Exercises
P340 Q1 to Q4