High-level Data Link Control (HDLC) is a bit-oriented link layer protocol. Its most prominent feature is that it can transmit any types of bit stream transparently.
- HDLC supports point-to-point link only and does not support point-to-multipoint link.
- HDLC supports neither IP address negotiation nor authentication. It uses keep-alive messages to check link status.
- HDLC can only be encapsulated on synchronous link. Currently, this protocol is applied on the Serial interface and POS interface.
Configuration in HDLC
HDLC also defines different configurations for the three types of stations. The word configuration refers to the relationship between the hardware devices on a link. Following are the three configurations defined by HDLC.
- Unbalanced Configuration
- Balanced Configuration
- Symmetrical Configuration
Unbalanced Configuration: The unbalanced configuration in an HDLC link consists of a primary station and one or more secondary stations. The unbalanced condition arises because one station controls the other stations. In an unbalanced configuration, any of the following can be used:
Full-Duplex or Half-Duplex operation
Point to Point or Multi-point networks
Figure 1: Unbalanced Configuration
Balanced Configuration: The balanced configuration in an HDLC link consists of two or more combined stations. Each of the stations has equal and complimentary responsibility compared to each other. Balanced configurations can use only the following:
Full – Duplex or Half – Duplex operation
Point to Point networks
An example of a balanced configuration can be found below in Fig.2.
Figure 2: Balanced configuration
Symmetrical Configuration: This third type of configuration is not widely in use today. It consists of two independent point-to-points, unbalanced station configurations as shown in Fig. 3. In this configuration, each station has a primary and secondary status. Each station is logically considered as two stations.
Figure 3: Symmetric Configuration
 “Module 3: Data Link control”, Version 2 CSE IIT, Kharagpur, available online at: http://nptel.ac.in/courses/106105080/pdf/M3L4.pdf
 “Operation Manual – HDLC (Access Volume)”, available online at: http://www.h3c.com.hk/res/200806/03/20080603_627145_06hdlc%20configuration_607199_1285_0.pdf
 “HDLC Configuration”, available online at: http://support.huawei.com/enterprise/docinforeader!loadDocument1.actioncontentId=DOC1000009840&partNo=10032