180 MultiModem ISI Hybrid Series, ISIHI-2S
Glossary
Workstation: Traditionally a dumb terminal connected to a
host. However, with the advent of LANS and WANS, PCs
that are connected to a LAN are now called workstations
too, even though they are capable of independent process-
ing. A workstation is simply an input/display device through
which a user accesses a resource.
X
X.121: CCITT’s recommendation for unique addressing of
each DTE connected to a communications network through-
out the world. X.121 uses fifteen digits: a one-digit prefix, a
four-digit DNIC number (which identifies the country and
PDN), and a ten-digit national terminal number.
X.21: CCITT’s recommendation for a 15-pin, digital inter-
face. It is not widely accepted, because of the analog loops
still prevalent in data communications. For this reason,
CCITT introduced the X21
bis
standard for use with synchro-
nous modems.
X.25: CCITT’s 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.28: CCITT’s definition of asynchronous commands used
by a local ASCII terminal to configure an X.25 PAD.
X.29: CCITT’s definition of packetized commands sent to
configure a remote PAD via an X.25 link.
XModem: A widely used asynchronous file transfer proto-
col. 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
YModem: An asynchronous file transfer protocol that
improves speed by transmitting 1,024-byte (1K) blocks and
batch file transfer.
Z
ZModem: An 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.