Information Technology Reference
In-Depth Information
HDLC is a standard developed by the ISO to provide a basis for the data link layer for
point-to-point and multi-drop connections. It can transfer data either in a simplex,
half-duplex, or full-duplex mode. Frames are generally limited to 256 bytes in length and a
single control field performs most data link control functions.
I.2 HDLC protocol
In HDLC, a node is either defined as a primary station or a secondary station. A primary sta-
tion controls the flow of information and issues commands to secondary stations. The secon-
dary station then sends back responses to the primary. A primary station with one or more
secondary stations is known as unbalanced configuration.
HDLC allows for point-to-point and multi-drop. In point-to-point communications a pri-
mary station communicates with a single secondary station. For multi-drop, one primary sta-
tion communications with many secondary stations.
In point-to-point communications it is possible for a station be operate as a primary and a
secondary station. At any time, one of the stations can be a primary and the other the secon-
dary. Thus, commands and responses flow back and forth over the transmission link. This is
known as a balanced configuration, or combined stations.
I.2.1 HDLC modes of operation
HDLC has three modes of operation. Unbalanced configurations can use the normal response
mode (NRM). Secondary stations can only transmit when specifically instructed by the pri-
mary station. When used as a point-to-point or multi-drop configuration only one primary
station is used. Figure I.2 shows a multi-drop NRM configuration.
Unbalanced configurations can also use the asynchronous response mode (ARM). It dif-
fers from NRM in that the secondary is allowed to communicate with the primary without
receiving permission from the primary.
Secondary
station
1
Commands
Commands
Primary/
Secondary
station
Primary/
Secondary
station
Primary
station
Responses
Responses
Asynchronous
balanced
mode (ABM)
Normal response
mode (NRM)
Secondary
station N
Figure I.2
NRM and ABM mode
 
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