HDLC: A high-level
data link control (HDLC) is a protocol that is a bit-oriented synchronous data
link layer. HDLC ensures the error-free transmission of data to the proper
destinations and controls the data transmission speed. HDLDs can provide both
connection oriented and connectionless services.
Basic characteristics of HGLC protocol:
To satisfy a variety of applications, HDLS defines
three types of stations, tow link configurations, and three data transfer modes
of operation.
The three station types are__
(i) Primary station: Has
the responsibility for controlling the operation of the link. Frames issued by
the primary are called commands.
(ii) Secondary station:
Operates under the control of the primary station. Frames issued by a secondary
are called responses. The primary maintains a separate logical link with each
secondary station the line.
(iii) Combined station:
combines the features of primary and secondary. A combined station may issue
both commands and responses.
The two link configuration are__
(i) Unbalanced configuration: Consists
of one primary and one or more secondary stations and supports both full-duplex
and half-duplex transmission.
(ii) Balanced configuration:
Consists of two combined stations and supports both full-duplex and half-duplex
transmission.
The three data transfer modes are__
(i) Normal response mode (NRM):
Used with an unbalanced configuration. The primary may initiate data transfer
to a secondary, but a secondary may only transmit data in response to a command
from the primary.
(ii) Asynchronous balanced mode (ABM):
Used with a balanced configuration. Either combined station may initiate
transmission without receiving permission from the other combined station.
(iii) Asynchronous response mode (ARM):
Used with an unbalanced configuration. The secondary may initiate transmission
without explicit permission of the primary. The primary still retains
responsibility for the line, including initialization, error recovery, and
logical disconnection.
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