Envoy ACP/XF Configuration and Management Manual

Introduction to EnvoyACP/XF
EnvoyACP/XF Configuration and Management Manual—429232-003
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Point-to-Point Data Link
SDLC and HDLC are subsets of the broader ADCCP protocol standard:
Table 2-1 summarizes the various combinations of protocols, line configurations, and
station types supported by EnvoyACP/XF.
EnvoyACP/XF helps to control and maintain station-to-station activity. It handles the
data-link level of communications, such as activating the link by using a Set
Asynchronous Balanced Mode (SABM) command for HDLC or a Set Normal Response
Mode (SNRM) for SDLC.
Point-to-Point Data Link
The simplest networks consist of one or more computers and/or terminals connected
by individual communications lines to a remote computer. Each such machine-to-
machine connection is a point-to-point data link, and each computer or terminal is a
station in the overall network.
In a point-to-point data link, the local station and the remote station are connected by
either a switched or a nonswitched line.
In a switched-line configuration, the electrical connection between the two stations
exists over the public data network (PDN). A switched line sometimes is called a dial-
SDLC Developed by IBM and a de facto standard within the field of
bit-synchronous data communications, SDLC is analogous to ADCCP-
NRM where a primary station is in control of the data link.
Like ADCCP, SDLC can use extended control fields. However, the SDLC
protocol does not support extended address fields (see the IBM
Synchronous Data Link Control Information Manual).
HDLC Developed by the International Standards Organization (ISO), HDLC is
analogous to ADCCP-ABM where each station serves as a combined
station. ABM can be used only with a single, point-to-point data link.
Like ADCCP, HDLC also supports the extended mode for control and
address fields (see the ISO International Standard 3309-1979).
Table 2-1. EnvoyACP/XF Protocol Capabilities
Protocol Line Configuration Station Type
ADCCP-NRM Point-to-Point Primary or Secondary
Multipoint Supervisor or Tributary
SDLC Point-to-Point Primary or Secondary
Multipoint Supervisor or Tributary
ADCCP-ABM Point-to-Point Combined*
HDLC Point-to-Point Combined*
* A combined station is one station that has both primary and secondary functionality: the primary portion of one
station communicates with the secondary portion of the other station.