Information Technology Reference
In-Depth Information
Technology Basics
SDLC supports a variety of link types and topologies. It can be used with point-to-point and multipoint
links, bounded and unbounded media, half-duplex and full-duplex transmission facilities, and
circuit-switched and packet-switched networks.
SDLC identifies two types of network nodes:
•
Primary
—Controls the operation of other stations (called secondaries). The primary polls the
secondaries in a predetermined order. Secondaries can then transmit if they have outgoing data. The
primary also sets up and tears down links and manages the link while it is operational.
•
Secondary
—Is controlled by a primary. Secondaries can send information only to the primary, but
they cannot do this unless the primary gives permission.
SDLC primaries and secondaries can be connected in four basic configurations:
•
Point-to-point
—Involves only two nodes, one primary and one secondary.
•
Multipoint
—Involves one primary and multiple secondaries.
•
Loop
—Involves a loop topology, with the primary connected to the first and last secondaries.
Intermediate secondaries pass messages through one another as they respond to the requests of the
primary.
•
Hub go-ahead
—Involves an inbound and an outbound channel. The primary uses the outbound
channel to communicate with the secondaries. The secondaries use the inbound channel to
communicate with the primary. The inbound channel is daisy-chained back to the primary through
each secondary.
Frame Format
The SDLC frame format is shown in Figure 10-1.
