High-Level Data Link Control (HDLC) is a widely used data link layer protocol that provides reliable and efficient communication over a variety of networks. HDLC was first standardized by the International Organization for Standardization (ISO) in 1979, and it has since been adopted by a number of other organizations, including the American National Standards Institute (ANSI) and the International Telecommunication Union (ITU).
HDLC is a bit-oriented protocol that uses a frame structure to transmit data over a network. The HDLC frame consists of several fields, including a flag field, an address field, a control field, a data field, and a cyclic redundancy check (CRC) field.
The flag field is a unique bit pattern that marks the beginning and end of each HDLC frame. The address field identifies the destination node on the network and can be either a 1-byte or a 2-byte field. The control field contains information about the type of frame being sent and any control information required for that frame.
The data field contains the actual data being transmitted over the network and can range in size from 0 to 4096 bytes. The CRC field is used for error detection and is calculated over the entire frame, including the flag, address, control, and data fields.
HDLC supports three different modes of operation: normal response mode (NRM), asynchronous balanced mode (ABM), and synchronous balanced mode (SBM).
In NRM, one node on the network acts as the primary station, while all other nodes act as secondary stations. The primary station controls the flow of data and sends commands to the secondary stations to indicate when they can transmit data.
In ABM, all nodes on the network are equal, and any node can initiate data transfer. This mode is useful for point-to-point connections where both nodes have equal control over the data flow.
In SBM, two nodes on the network are equal and can initiate data transfer. This mode is used for synchronous communication and is often used in high-speed data networks.
HDLC has several advantages over other data link layer protocols. It is a reliable and efficient protocol that provides error detection and correction, flow control, and retransmission of lost or corrupted data. It also supports a wide range of network topologies and can be used over both synchronous and asynchronous connections.
However, HDLC has some limitations. It is a complex protocol that can be difficult to implement and configure. It also lacks some of the features found in other data link layer protocols, such as the ability to prioritize traffic.
In conclusion, HDLC is a widely used data link layer protocol that provides reliable and efficient communication over a variety of networks. It uses a bit-oriented frame structure and supports three different modes of operation. Although it has some limitations, HDLC remains a popular choice for many network applications due to its reliability and flexibility.
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