tations, configurations, and three modes of operation. Primary Station A logical primary station is an entity that has primary link control responsibility. It assumes responsibility for organization of data flow and for link level error recovery. Frames issued by the primary station are called commands. Secondary Station A logical secondary station operates under control of a primary station. It has no direct responsibility for control of the link, but instead responds to primary station control. Frames issued by a secondary station are called responses. Combined Station A combined station combines the features of primary and secondary stations. It may issue both commands and responses. Unbalanced Configuration An unbalanced configuration consists of a primary sta- tion and one or more secondary stations. It supports full-duplex and half-duplex operation, point-to-point, and multipoint circuits. An unbalanced configuration is illustrated in Figure 10.24a. Balanced Configuration A balanced configuration consists of two combined stations in which each station has equal and complementary responsibility of the data link. A balanced configuration, shown in Figure 10.24b, operates only in the point-to-point mode and supports full-duplex operation. Modes of Operation With normal response mode (NRM) a primary station initiates data transfer to a secondary station. A secondary station transmits data only in response to a poll from the primary station. This mode of operation applies to an unbalanced configuration. With asynchronous response mode (ARM) a secondary station may DARPA stands for Defense Advanced Research Projects Agency, under the U.S. Department of Defense. 10.10 WHAT ARE DATA PROTOCOLS? Commands Primary Responses Secondary Secondary Commands Combined Combined Responses Figure 10.24 HDLC link configurations. (a) Unbalanced configuration; (b) balanced configuration. initiate transmission without receiving a poll from a primary station. It is useful on a circuit where there is only one active secondary station.