6100 ADCCP Programming Manual

6100 ADCCP Concepts and Context
Introduction to 6100 ADCCP
1–8 069225 Tandem Computers Incorporated
Figure 1-4. ABM Line-Control Mode
023
Primary
Substation
Secondary
Substation
Combined Station
Combined Station
Secondary
Substation
Primary
Substation
SABM
UA
SABM
UA
Primary
Substation
Secondary
Substation
Combined Station
Combined Station
Secondary
Substation
Primary
Substation
I
I
In Figure 1-4, any primary substations issues a SABM to set up the link The
corresponding secondary substation responds with a UA. After a secondary
substation respond with a UA, the primary and secondary stations can exchange
information without a need for polling. Two-way alternate and two-way
simultaneous operations are supported.
Extended Mode
Each of the modes has a corresponding extended mode. In extended mode, the frame
can have an extended address field, an extended control field, or both. Extended
addressing and extended control are independent; one does not imply the other. An
extended address field allows longer addresses, and therefore supports more stations.
(A network should have only as many stations as it has unique addresses.) An
extended control field allows frames to carry larger sequence numbers, so stations can
transmit more frames before receiving an acknowledgment. Both types of extensions
affect the format of frames exchanged over a link but do not affect the relationship of
the stations on a link. The following subsection, “Frame Formats,” describes the frame
formats of the ADCCP protocol.
Frame Formats In the ADCCP environment, messages are divided into transmission units called
frames. All frames adhere to the same general format but vary depending on whether
the mode is standard or extended and on whether a frame includes application data in
addition to link control information. Figure 1-5 illustrates the general format of an
ADCCP frame.