6100 ADCCP Programming Manual

6100 ADCCP Concepts and Context
Introduction to 6100 ADCCP
1–4 069225 Tandem Computers Incorporated
6100 ADCCP Concepts
and Context
The purpose of the ADCCP protocol is to provide efficient, reliable communication
between computers, terminals, and other devices connected by communication lines.
Each communicating entity is called a station; the connection between stations is called
a link.
In most networks, the stations vary in communications features, range of commands,
and communications authority. For example, if a mainframe is connected to a string of
terminals, the mainframe normally controls the terminals. Even if devices on a link
have the same communications features (for example, two mainframes are connected),
it may be desirable to let one device dominate the other with one device initiating the
communication, and the other responding.
The rules for interaction among the stations on a link are determined by three factors:
Station types
Line-control modes
Line-configuration options
The first two factors define the relationship between or among the stations.
Configuration options further define the protocol. The next few subsections discuss
each of these topics, as well as the frame types and formats characteristic of ADCCP.
Station Types Station type indicates the relative status of stations on a link. ADCCP recognizes three
types:
Primary station Controls communication on a data link. A primary station
establishes and ends communication sessions, performs error
recovery, and, in many cases determines when other stations
may transmit or receive data. A link can have only one primary
station.
Secondary station Communicates under direction of a primary station. A link can
have one or more secondary stations.
Combined station Acts as a primary and a secondary station. Logically, a
combined station consists of two substations: a primary
substation and a secondary substation.
Figure 1-1 shows the ways these kinds of stations can be combined. The first
configuration has a primary station connected to a secondary station. The second
configuration has a primary station connected to a string of secondary stations. The
third configuration has two combined stations connected to one another.