Quick Link II Fax  (250-3027B)   Protocols - Windows   Faxback Doc. # 3226

ASCII

The ASCII protocol is a seven-bit protocol that consists of the 128
characters that make up the upper- and lower- case alphabet, numbers,
characters available on a standard keyboard, and certain special control
characters.

The ASCII protocol has no error-checking capabilities.  ASCII protocol
uses XON/XOFF handshaking and required XON/XOFF flow control to be enabled
at both communicating devices.  By default, Quick Link II Fax has XON/XOFF
flow control disabled.  When the computer receiving data needs to stop the
data transmission temporarily to process the received data, it sends a
transmission temporarily to process the received data, it sends a Ctrl-S
(^S) character to stop the sending computer from sending data.  When the
receiving computer has processed the received data and is ready to accept
more, it sends the sending computer a Ctrl-Q (^Q) character to resume
data transmission.

Since there are no additional error checking characters sent during the
transmission, ASCII is good for sending a burst transmission of raw
text/data uninterrupted.

Kermit/SuperKermit

Quick Link II Fax provides both the standard Kermit as well as the
powerful Super Kermit (Windowed Kermit) protocol.  It was developed to
meet the needs for file transfer between a number of different types of
computers, including mainframes, mini computers and personal computers.
Unlike Xmodem and Ymodem, Kermit uses variable packet sizes, with a
maximum size of 1024 bytes.  Like Ymodem, Kermit provides for batch file
transfers.

SuperKermit is especially useful when transferring data over data networks
such as Telenet or Tymnet.  These networks have long communications delays
that can significantly degrade the performance of file transfers.  For
each packet transmitted, two communications delays will be interjected
into the transfer.  SuperKermit addresses this problem by sending multiple
packets at one time, also known as a transmission window.  All error
correction is still performed, however the receiving computer does not
acknowledge the receipt of the data or asks for re-transmission of any bad
packets until all packets in the window have been received.  This process
results in a dramatic reduction in the delay time.

Of the public services, only CompuServe supports Kermit.  If you select
Super Kermit and the remote computer only supports Kermit, Quick Link II
Fax will drop back to Kermit.

Xmodem

Quick Link II Fax provides three different Xmodem protocols: Xmodem 1K,
Xmodem CRC and the original Xmodem.

In order for Xmodem to work, the data format must be set to 8 data bits, 1
stop bit, and no parity.  If you did not begin the communications session
with this setting, Quick Link II Fax will automatically correct this for
you.  Xmodem Checksum sends the data in packets of 128 bytes.  Therefore
a 1K file (1,024 bytes) would be transmitted in 8 data packets.  A simple
data checksum is added to each packet and is checked on the receiving end
of the transfer.  If the receiving Xmodem detects a bad packet, it can
request the packet again giving a good level of error recovery.

Xmodem CRC

In an effort to guard against undetected errors the original Xmodem was
enhanced by replacing the 8 bit checksum with a 16 bit Cyclic Redundancy
Check (CRC).  This change provides a 99.9984% assurance of detecting any
transmission errors.  With the checksum method, it is possible for 1 out
of 256 bad packets to have a valid checksum, and thus go undetected.  With
the CRC method only 1 out of 700 billion bad packets will generate a valid
CRC.  The CRC method also transmits 128 byte blocks or packets of data.
If you select Xmodem CRC and the other system does not support it Quick
Link II Fax will automatically switch over to the standard Xmodem: to the
user both methods appear to operate identically.

Xmodem 1K

This method of Xmodem replaces the original 128 byte packets with packets
of 1,024 bytes when possible.  Assuming that no transmission errors are
detected, this method of Xmodem will enlarge the packet size to 1K and
maintain that packet size for as long as possible.  Enlarging the packet
size will improve the speed of the file transfer.  If you attempt to
receive a file using Xmodem 1K and the other side only supports Xmodem
CRC, Quick Link II Fax will fall back to Xmodem CRC.

Ymodem

The Ymodem protocol (also called Ymodem Batch, Ymodem IK, and Ymodem CRC)
is very similar to the Xmodem 1k, with two major differences - with Ymodem
you can automatically receive or send multiple files in one session, and
file names are included with the transfer.  In general, Ymodem is very
fast and very safe and preferable to Xmodem, even for single files.

Ymodem-G

Ymodem-G is a file transfer protocol that provides the same error checking
as Ymodem, but it will not perform any error checking.  Therefore, Ymodem
requires an error-correcting modem or fax/modem with either MNP 2-4 and/or
V.42.  If both ends of the transmission meet the requirements, transfers
will be exceptionally fast.

Data communications setup options

In addition to permitting Quick Link II Fax to communicate with the myriad
of standard data communication configurations, the SETUP menu contains the
options to customize Quick Link II Fax to your system.

(br/all-11/4/94)