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C!T ROM 3
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ct-rom iiib.zip
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ct-rom iiib
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WINDOWS
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UTILITY
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MODEM
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ALLOV10A
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ALLOCOMM.TX_
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ALLOCOMM.TX
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1994-05-18
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6KB
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143 lines
.Paragraph
Protocol
A set of rules for the transmission of data. It is the
format by which information is sent through the phone line
system. Error correction routines and compression data are
commom protocols. The newer and better version of transfer
protocol is ZModem.
Baud
The unit of measuring modem signalling speed.The number of signal
changes on the line per second. Most people treat bits per second
(bps) and Baud as being the same, but it is a wrong assumption. Most
current modem send more than one bit per Baud, so the number of bits
per second isn't the same as the baud rate. The bps rate varies from
2400 upto 100 kps or more. It depends not only on the hardware but also
the software and the connection line. Fiber optical cables can reach
extremely high bps rates. Dial phone lines are limited by a modulation
band and noise that typical values of 14400 bps (no compression) are the
top limit.
Parity
A means of checking transfer errors by adding an extra bit each ASCII
character transmitted. Not very reliable and reduces the transfer ratio.
Current error correction protocols have more efficient and reliable routines
making parity checking not needed (set to "None").
Data Bits
Number of bits of data information transfered at a time. The set of
ASCII can be sent using 7 data bits.Normally data bits are set on 8,
allowing a broader range of characters to be transferred.
Stop Bits
Number of bits indicating the end of a data "package" sent and the begining
of the next data "package".
Flow
Serial Port
A connection that allows data to be transmitted bit by bit at a time.The
standard serial port connector is the RS-232 and some variations.Modem
operates as serial devices.Standard PC's generally have 4 serial ports,
COM1-COM4.
Download
Receiving a file on your system from other remote computer or terminal.
Upload
Sending a file from your system to other remote computer or terminal.
Xmodem
Xmodem file transfer protocol (sometimes called MODEM7 or
Xmodem/Checksum) is an error-checking protocol you'll encounter
very frequently. Created in 1978 by Ward Christensen and placed
immediately into the public domain, Xmodem has become a de facto
standard. It can be found on almost all BBSs and online systems
that offer error-checking protocols. And, if a communications
program offers any error-checking protocols at all--even one--
Xmodem will be among them. Xmodem transfers files in 128-byte
blocks. It adds an extra bit--called a checksum--to each block,
which the receiving system uses to calculate whether or not the
block was accurately transmitted. (A complex algorithm, based on
the contents of the block, is used for this calculation).It cannot
be used to communicate at 7 data bits, as it transmits files in 8-bit
format only. If a hardware error-checking protocol or flow control is
in effect, some Xmodem characters can be perceived as control characters,
with unpredictable effects.
Xmodem CRC. CRC (Cyclic Redundancy Check) is an Xmodem
option that modifies how Xmodem checks for errors. CRC adds a
second checksum bit to each block to enhance error checking.
Kermit:
Kermit
Kermit is equally as
popular as Xmodem. Kermit is similar to Xmodem in that it
transfers files in blocks--or packets, as they are referred to in
Kermit. It also resembles Xmodem in its use of a checksum
technique for error checking (a checksum bit--based on packet
contents--is included with each packet).
An interesting feature of Kermit is that its packet
sizes can be changed to accommodate fixed packet sizes on a
remote system, or varying transmission conditions.
Ymodem:
Ymodem operates in a manner very similar to that of Xmodem,
the major difference between the two being the fact that Ymodem
transmits data in 1024-byte (1K) blocks, rather than in 128-byte
blocks. Its major application is transmitting very large files.
Ymodem's large block size increases transfer speed significantly
when few or no errors are encountered.
Zmodem:
Zmodem operates much faster than most other protocols.
Zmodem blocks are 512 bytes in size. The protocol is relatively fast,
even though it uses no buffering, which means a pause for disk
access each time it sends or receives a block. Efficiency rates
of 99% (239 cps @ 2400 bps) are routinely achieved with many
online services' implementations of Zmodem.
Flow
Almost all computer systems uses Data Buffering and Flow Control,
which means that incoming or outgoingdata is temporarily stored in RAM
in what is called a buffer.
Data buffering speeds up file transfer because it eliminates the
number of times a transmission must be paused while the receiving
or sending computer accesses its disk.
At the sending end of a file transfer, enough data is read
from disk to fill a RAM buffer, then sent from RAM. When the
send buffer is empty, the disk is read again. Because the buffer
holds enough data for several seconds of transmission, there is
less disk access.
The reverse occurs at the receiving end, with data being
written to disk only when the receive buffer is filled. The flow
of incoming data is paused when this occurs, and is usually
controlled by XON/XOFF (^Q/^S) protocol.
AT HAYES
A widely accepted standard set of commands stablished by Hayes Microcomputer
for controlling modems.All commands starts with "AT" for calling the modem's
attention.