Envoy ACP/XF Application Programming Manual

Glossary
EnvoyACP/XF Application Programming Manual–132179
Glossary-3
full-duplex
full-duplex. A method of serial communications in which the data flow between two points
can occur in both directions simultaneously.
Guardian. An environment available for interactive or programmatic use with the Tandem
NonStop Kernel. Processes that run in the Guardian environment use the Guardian
system procedure calls as their application program interface.
half-duplex. A method of serial communications in which the data flow between two points
can occur in only one direction at a time.
HDLC. High-level Data-Link Control. The use of a specified series of bits to control data
links in accordance with the International Standards for HDLC: ISO 3309 Frame
Structure and ISO 4335 Elements of Procedures. This data communications protocol is
bit-synchronized as opposed to byte-synchronized and uses asynchronous balanced
mode (ABM) communications between stations.
information field. The field in a bit-synchronous frame that contains data or additional
control information. If present, the information field follows the control field and varies
in length.
I-frame. See information frame.
information frame. A frame format used to pass data between two stations.
interchange circuit. A channel that provides a path for information flow between the server
and remote modem.
IOP. Input/output Process. EnvoyACP/XF is an IOP. It is an input-output process controlling
a bit-synchronous communications line.
ITS. Information Transfer State. A station in ITS is fully operational and capable of
transmitting and receiving I-frames, S-frames, or U-frames.
LDS. Logical Disconnect State. A station in LDS cannot transmit or accept I-frames,
UI-frames, or S-response frames. The secondary station or secondary substation (of a
combined station) is limited to accepting one of the mode-setting commands,
transmitting a DM or RIM response at each response opportunity, and responding to an
XID command.
MCW. Message control word. The first 2 bytes (one word) of the application buffer, used by
EnvoyACP/XF to build the control and address fields of the frame.
modulo. Refers to the maximum number of frames that can be transmitted before an
acknowledgment is received; it is the maximum number of frames that can be handled
by the link-level sequence numbers. Since the link-level sequence numbers are contained
in the control field, the size of the modulo is determined by the size of the control field.
With standard 1-octet control fields, the maximum is modulo-8. With extended control
fields, the maximum is modulo-128. See also window.