Normal Response Mode (NRM)—The primary station
on the HDLC link initiates all information transfers with secondary
stations. A secondary station on the link can transmit a response
of one or more information frames only when it receives explicit permission
from the primary station. When the last frame is transmitted, the
secondary station must wait for explicit permission before it can
transmit more frames.
NRM is used most widely for point-to-multipoint links, in which
a single primary station controls many secondary stations.
Asynchronous Response
Mode (ARM)—The secondary station can transmit either data or
control traffic at any time, without explicit permission from the
primary station. The primary station is responsible for error recovery
and link setup, but the secondary station can transmit information
at any time.
ARM is used most commonly with point-to-point links, because
it reduces the overhead on the link by eliminating the need for control
packets.
Asynchronous Balance
Mode (ABM)—All stations are combined stations. Because no other
station can control a combined station, all stations can transmit
information without explicit permission from any other station. ABM
is not a widely used HDLC mode.
