TECHNICAL BULLETIN #18 - Rev 1.10 (9/03/96)

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TITLE   : Glossary of Common File Transfer Terms
PRODUCT : Procomm Plus
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ASCII
ASCII (American Standard Code for Information Interchange) is the standard set
of characters used by mini and micro computers.  Each ASCII character uses the
first seven digits of the eight digit binary number called a byte, to form a
single alpha-numeric character.  Each binary digit in a byte is referred to as
a bit, the smallest unit of data storage in a computer.

AUTODOWNLOAD
Autodownloading refers to the ability of an application to automatically
receive a file from a host machine.  This feature allows you to send files to
computers without a host mode function, and avoids the need for operators at
both ends.

BATCH TRANSFERS
Batch transfer of files refers to the ability of a protocol to send multiple
files sequentially.  DOWNLOAD Downloading a file involves all the procedures
required to receive a file from a host computer and store it on your local
computer.

ERROR CORRECTION
The ability to fix detected errors in data by retransmitting the sections of
information containing errors.

ERROR DETECTION
Most file transfer protocols include a way to determine if there were errors
in the transmission.  A protocol's error detection capability is the method
used to test for missing or erroneous data in each packet transferred from the
host system.  There are two types of software error detection in use today,
Checksum and Cyclical Redundancy Check.

* CHECKSUM error detection adds the value of all characters in a data packet
  and places the result in one or two characters at the end of the packet. The
  receiver checks the data packet for transmission errors by performing a
  checksum on the data and comparing the result to the value in the error
  detection byte.

* CYCLIC REDUNDACY  CHECK (CRC) uses up to three bytes for error checking.
  The mathematical equation that it performs is more complex, but more
  reliably detects transmission errors. CRC is used in most modern file
  transfer protocols.

FLOW CONTROL
Flow control maintains a smooth exchange of data between the computer and the
modem or between two computers.  There are two types of flow control, software
and hardware.

* SOFTWARE FLOW CONTROL uses ASCII control characters to regulate the
  exchange of data between two computers.

* HARDWARE FLOW CONTROL uses electrical signals in the cable and the modem's
  memory buffers to control the data exchange between the computer and the
  modem.

PROTOCOL
A file transfer protocol is an agreed-upon set of rules and procedures for the
transmission of data between computers.  Each computer participating in the
data transfer must use the same protocol to transfer the file.  There are two
types of file transfers protocols:

* BINARY  PROTOCOLS send and receive all possible characters, both printable
  and non-printable.  This protocol type is used to transmit programs,
  compressed files, database files and many word processing documents.
  Examples are KERMIT and ZMODEM, which transmit information in packets or
  blocks that are read by the receiving computer. Each packet is numbered and
  provided with error correcting bits, allowing the host system to verify the
  accuracy of the transmitted data.

* TEXT PROTOCOLS, such as ASCII and Raw ASCII, are used to send files
  containing printable characters (and some printer-related control
  characters) only.  These transfers do not use packets to format the
  information; instead they "type" the data into the host. ASCII file
  transfers are simple dumps of information, with no error correcting
  capabilities,  using software flow control to regulate the flow of data. An
  RASCII file transfer use no flow control, and can be set to send all of the
  first 127 ASCII characters, whereas an ASCII transfer will only send the
  bell, backspace, carriage return, line feed, form feed, DC4, and ASCII
  Characters 32-127.

TRAMSMISSON METHODS
There are three main methods for sending and receiving a file:  ack-nak,
sliding windows, and data streaming.  -In ack-nak mode, data is transmitted in
"packets", blocks of data, each of which contains, at least, header
information, packet number, and error correction data enclosing the file data.
The packet method of transmitting data sends a single packet of information,
waiting to receive confirmation usually in the form of an ACK character) that
no errors occurred, before sending another data packet.

|--------------|---------------|------|-------------------------|
|PACKET HEADER | PACKET NUMBER | DATA | ERROR DETECTING BYTE(s) |
|______________|_______________|______|_________________________|

* The sliding windows method, which requires a full duplex connection, sends
  a fixed number of data packets before waiting to receive confirmation that
  those packets were correct. Full duplex communication channels handle data
  transmission in both directions simultaneously, whereas half duplex channels
  communicate in only one direction at a time.

* Streaming protocols send the entire file in sequential packets before
  waiting for an acknowledgment that the data was error free. Depending on the
  protocol, the data may be corrected or the transfer aborted if errors occur
  during a transfer.

UPLOAD
Uploading means sending a file from your local computer to a host system.

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           Copyright (c) 1996 Quarterdeck  All rights reserved.
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