Wide Area Network protocol HDLC stands for the High-Level Data Link Control protocol. It is a data link layer protocol or say layer 2 protocol of the OSI model. HDLC originated from SDLC which was created by IBM in 1970’s. It was developed as layer 2 wide area network protocol. Now, when talking about HDLC one important thing is the HDLC headers. They have no mentioned protocol id information which makes it very difficult to analyse them on the network because there is no information about them to let you know it’s HDLC. So, that means HDLC is vendor specific.
If you have all Cisco routers in your network, you won’t face any problem. They will communicate. If you have all F5 routers again not a problem, they will be able to communicate. The problem comes when you’re running F5 and Cisco because those HDLCs are specific to those particular vendors and they will not talk to each other.
HDLC provides an error-free transmission of data to the correct destination and controls the data transmission speed as well. They used for serial link connections in wide area networks. HDLCs can be both connection-oriented and connectionless services.
A high-level data link control defines rules for data transmission between two network points. Data is organized into units called frames and is sent across the networks. HDLC also manages the speed at which data is transmitted.
HDLC frames are transmitted over synchronous or asynchronous links, which does not make note of the start and end of frames. This is done using a frame delimiter or flag, which contains unique sequence of bits which are not noticeable inside a frame.
There are three types of HDLC frames such as information frames or user data (I-frames), another is supervisory frames or say control data (S-frames) and last one is unnumbered frames (U-frames)
There are 4 common fields within an HDLC frame. These are flag field, address field, control information field and frame check sequence field (FCS).
There are three different configuration types in HDLC such as unbalanced, balanced and symmetrical. Also, there are several different modes of operation for these configurations such as normal response mode, asynchronous response mode and asynchronous balanced mode.
The High-Level Data Link Control has been widely used because it supports many other functionalities such as half-duplex, full-duplex, point to point and multi-point networks. Thus, HDLC is a pretty good WAN protocol.
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