The High-Level Data Link Control (HDLC) protocol is a link layer protocol that is used to provide reliable data transmission over a variety of physical layers.
HDLC is a bit-oriented protocol, which means that it operates at the level of individual bits rather than at the level of characters or bytes.
In HDLC, data is transmitted in the form of frames, which consist of a series of bits that contain the data being transmitted as well as control information.
HDLC (High-Level Data Link Control) is a widely used protocol for establishing and maintaining communication links between devices over a serial link. It was developed in the 1970s by the International Organization for Standardization (ISO) as a standard for point-to-point communication and has since become widely adopted in a variety of applications.
HDLC is implemented in a wide range of hardware and software platforms, including microcontrollers, DSPs, FPGAs, and general-purpose computers. It is often implemented in C or C++, although it can also be implemented in other programming languages or hardware description languages, such as VHDL or Verilog.
Each frame begins with a start flag and ends with a stop flag, which marks the beginning and end of the frame. Between the start and stop flags, the frame contains a number of fields that provide control information, such as a frame check sequence (FCS) that is used for error detection and a frame control field that specifies the type of frame being transmitted.
HDLC also includes a number of mechanisms for ensuring reliable data transmissions, such as positive acknowledgment with retransmission (PAR) and flow control.
PAR involves sending a positive acknowledgment (ACK) message to the sender after a message has been successfully received. If the sender does not receive an ACK, it assumes that the message was not received correctly and retransmits the message.
Flow control involves exchanging messages between the sender and receiver to regulate the flow of data and prevent the receiver from being overwhelmed.
HDLC (High-level Data Link Control) is a networking protocol that is used to transmit data over point-to-point and point-to-multipoint links.
It is defined in a number of technical specifications developed by the Internet Engineering Task Force (IETF), including:
RFC 1662: This specification defines the HDLC framing method, which is used to encapsulate data in frames for transmission over a network. It also defines the HDLC header format and the use of various control fields in the header.
RFC 1547: This specification defines the use of HDLC in the Point-to-Point Protocol (PPP), which is a widely-used protocol for transmitting data over point-to-point links.
RFC 2239: This specification defines the use of HDLC in the Frame Relay Access Device (FRAD) Interface Management Protocol (FIMP), which is used to manage frame relay access devices (FRADs).
RFC 2225: This specification defines the use of HDLC in the Cisco Systems HDLC Protocol, which is a proprietary variant of HDLC developed by Cisco Systems.
These and other technical specifications are important resources for anyone working with HDLC, as they provide detailed information about how HDLC works and how it can be used effectively in different networking environments.
In the High-Level Data Link Control (HDLC) protocol, error detection and correction are implemented using a combination of cyclic redundancy check (CRC) and positive acknowledgment with retransmission (PAR).
CRC is a technique that involves calculating a checksum based on the data bits and appending it to the data. The checksum can be used to detect errors, but it cannot correct the errors.
PAR is a technique that involves sending a positive acknowledgment (ACK) message to the sender after a message has been successfully received. If the sender does not receive an ACK, it assumes that the message was not received correctly and retransmits the message. This allows HDLC to detect and correct errors that occur during transmission.
HDLC also includes a number of other features, such as flow control and frame formatting, that are designed to ensure reliable data transmission over a variety of physical layers.
In satellite communication systems, the layer 2 protocol is typically a link layer protocol such as the Point-to-Point Protocol (PPP). The link layer is responsible for providing reliable data transmission over a physical link, and it is usually the lowest layer in the OSI model that is concerned with end-to-end communication.
PPP is a widely used link layer protocol that is designed to operate over a variety of physical layers, including satellite links. It provides a number of functions that are important in satellite communication systems, such as error detection and correction, compression, and link control.
Other link layer protocols that may be used in satellite communication systems include High-Level Data Link Control (HDLC) and the Frame Relay protocol.