Envoy ACP/XF Configuration and Management Manual

Introduction to EnvoyACP/XF
EnvoyACP/XF Configuration and Management Manual—429232-003
2-2
Protocols Supported
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Advanced Data Communications Control Procedures (ADCCP). Developed by the
American National Standards Institute (ANSI), this mode enables a choice of either
Normal Response Mode (NRM) or Asynchronous Balanced Mode (ABM).
•
Synchronous Data Link Control (SDLC). In this mode, one station is configured as
the primary and the other station as the secondary. The primary station initiates
data transfer and is in control during the exchange of messages. The primary
station notifies each secondary station when the secondary station can transmit
data and when to expect to receive data. The secondary station can transmit data
only when permitted to do so by the primary station. On a point-to-point line, the
primary station polls the secondary station; on a multipoint line, the supervisor
station polls the tributaries in a user-specified sequence.
•
High-Level Data Link Control (HDLC). In this mode, two combined stations
communicate on a point-to-point link. A combined station is divided into two logical
substations: a primary that communicates with the remote secondary substation,
and a secondary that communicates with the remote primary substation. Either or
both stations issue commands to set up or dissolve the link. During data
transmission, the stations function as peers.
Figure 2-2 shows the relationship between the bit-synchronous protocols.
The ADCCP NRM, ABM, and HDLC modes have corresponding extended modes.
Extended modes differ from standard modes in two important respects. First, extended
modes can have longer control fields. Longer control fields allow frames to carry larger
sequence numbers so stations can transmit more frames before receiving an
acknowledgment. Second, extended modes allow longer address fields and thus can
support more stations. (A network can have only as many stations as it has unique
addresses.) Both of these differences affect the format of frames exchanged over the
link.
Figure 2-2. Bit-Synchronous Data Link Protocols
VST004.vsd
HDLC
ABM
ADCCP
NRM
SDLC