6100 BSC Programming Manual

6100 BSC Concepts and Context
6100 BSC CONCEPTS AND CONTEXT
Using 6100 BSC, a Tandem system communicates reliably with any of
a large number of devices. Although higher-throughput protocols
have been developed since BSC, there is probably none supported
by as many computers, terminals, and other data communication
devices.
As we've already said, 6100 BSC is a point-to-point protocol:
that is, it allows exactly two entities to communicate. Each
communicating entity is called a station. The connection
between stations is called a link.
The rules for interaction between the stations on a link are
determined by several factors, including station types, message
formats, and line configuration options. The rest of this
section discusses these topics, as well as the data link control
procedures of 6100 BSC.
NOTE
Differences between 6100 BSC and the protocol defined
by IBM are indicated in the text.
Station Types
A BSC station can be either a primary or a secondary. You
identify the type of a station in its SYSGEN entry; you can
modify it with CMI or an application request. A link should have
one station of each type, as shown in Figure 1-2.
There are two differences in protocol between a primary and a
secondary station:
• If both stations wish to transmit, the secondary yields to
the primary. The figure demonstrates this convention. The
stations simultaneously bid for the line. Although the
requests take the same form in the primary and the secondary,
the protocol behaves differently for the two types of
stations. In the secondary station, the request completes
with the news: "The other station is bidding." By
convention, the secondary issues a request to acknowledge the
bid and thereby yield the line. In the primary station, the
request to bid has somewhat different results: the primary
repeats the line bid until the secondary yields, or until the
number of retries exhausts the limit defined for the station.
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