Specifications
MultiModem ISI Hybrid Series, ISIHI-2S/2U 183
Glossary
country and PDN), and a ten-digit national terminal
number.
X.21CCITTs recommendation for a 15-pin, digital
interface. It is not widely accepted, because of the analog
loops still prevalent in data communications. For this
reason, CCITT introduced the X21bis standard for use with
synchronous modems.
X.25CCITTs definition of a three-level packet-switching
protocol to be used between packet-mode DTEs and
network DCEs. X.25 corresponds with the lower three/four
layers of the seven layer OSI model.
X.28CCITTs definition of asynchronous commands used
by a local ASCII terminal to configure an X.25 PAD.
X.29CCITTs definition of packetized commands sent to
configure a remote PAD via an X.25 link.
XModemA widely used asynchronous file transfer
protocol. Programs typically use both the older version,
checksum, and the new version, CRC method, to detect
errors. If CRC is not present at the other end of a file
transfer, then it will use checksum.
Y
YModemAn asynchronous file transfer protocol that
improves speed by transmitting 1,024-byte (1K) blocks and
batch file transfer.
Z
ZModemAn asynchronous file transfer protocol that is
more efficient than XModem. It sends file name, date and
size first, and responds well to changing line conditions
due to its variable length blocks. It uses CRC error
correction and is effective in delay-induced satellite
transmission.