HDLC

Abbreviation can stand for many different things depending upon the perspective it is used in. The term HDLC stands for High-level Data Link Control. It is a transmission protocol that is documented in ISO 3309. As mentioned before HDLC is a transmission protocol, however, it is not made up of single protocol it is made up of a group of protocols or rules for transmitting data between the network points or more commonly known as end points present in the network. In this transmission protocol the data is not sent as such, that is, before its transmission it is organized in to a single unit, known as a frame, and then sent across the connection to the destination that verifies its successful arrival.

Along with this framing feature this HDLC protocol also regulates and monitors the rates at which the data is transmitted. Due to its favorable properties this protocol is extensively used at layer 2 of the industry communication model called the OSI.  The frame structure of the HDLC is that the beginning and the ending of the frame is marked by the flags and the second column or the partition has the destination address while the next column or the interval holds the data that is to be transmitted. The next interval or column in the frame holds the FCS, which is followed by the last interval which has the flag.

In a HDLC session that takes place one station is called a primary station while the other one is named the secondary station and the three different connection modes can be made between these two stations, which will determine that how the primary and secondary stations will interact with one another. The three different types of connections that can be made are Normal unbalanced, asynchronous and asynchronous balanced.