High-Level Data Link Control(HDLC)

HDLC (High-Level Data Link Control) is a data link layer protocol used for communication between two devices in a network. It provides reliable, error-free data transmission over point-to-point or multi-point links.

HDLC is a bit-oriented protocol, meaning it operates on individual bits rather than bytes or frames. It uses a frame structure to encapsulate the data being transmitted, with each frame consisting of a header, a data field, and a trailer.

In High-Level Data Link Control (HDLC) protocol, there are three types of stations that can be used:

1. Primary Station: The primary station is responsible for initiating and controlling data transfer on the data link. It is the station that starts the exchange of frames, and it can send commands and requests to secondary stations. In an HDLC network, there can only be one primary station.
2. Secondary Station: Secondary stations are devices that receive commands or requests from the primary station and respond accordingly. They cannot initiate data transfer on their own, but can only respond to requests from the primary station. There can be multiple secondary stations in an HDLC network.
3. Combined Station: A combined station is a device that can function as both a primary and secondary station. It can initiate data transfer on its own, as well as respond to requests from other stations. A combined station is used when a device needs to communicate with other devices in a network but also needs to have control over the data flow.

In High-Level Data Link Control (HDLC) protocol, there are three modes of data transfer:

1. • Normal Response Mode (NRM): In NRM, data is transmitted in a half-duplex mode, which means that only one device can transmit data at a time. The device that is currently transmitting data has control of the data link, and the other device must wait until the transmission is complete before it can transmit data. NRM is commonly used in point-to-point communication systems.
2. 
   • Asynchronous Balanced Mode (ABM): In ABM, data can be transmitted in a full-duplex mode, which means that both devices can transmit data simultaneously. ABM is commonly used in multipoint communication systems, such as Local Area Networks (LANs). In ABM, each device on the network is assigned a unique address, and data is transmitted to a specific device based on its address.  
   • Asynchronous Response Mode (ARM): In ARM, one device is designated as the primary station and the other devices are designated as secondary stations. The primary station has control of the data link and can initiate data transfer, while the secondary stations can only respond to requests from the primary station. ARM is commonly used in polling-based systems, where the primary station polls each secondary station in turn to determine if it has data to send.

HDLC also provides error detection and correction using a technique called cyclic redundancy check (CRC). It detects and discards any frames that have errors or are corrupted during transmission.

The frame format of HDLC:

The HDLC (High-Level Data Link Control) protocol uses a frame structure to encapsulate the data being transmitted. The frame structure consists of a header, a data field, and a trailer. The frame format of HDLC is standardized and follows a specific format, as described below:

1. Flag: The frame begins and ends with a flag byte (01111110 in binary), which indicates the start and end of the frame. This flag byte is used to synchronize the sender and receiver.
2. Address: The address field is used to specify the address of the receiver device. It can be either a single octet or multiple octets.
3. Control: The control field specifies the type of frame being transmitted, such as a data frame, supervisory frame, or unnumbered frame.
4. Payload/Information: The information field contains the data being transmitted. The length of this field can vary depending on the size of the data being transmitted.
5. FCS: The frame check sequence (FCS) field contains a checksum value that is calculated by the sender based on the data being transmitted. The receiver calculates its own checksum and compares it with the checksum in the FCS field to check for errors.
6. Flag: The frame ends with a flag byte (01111110 in binary), indicating the end of the frame.

The format of the HDLC frame can vary depending on the type of frame being transmitted. For example, in a supervisory frame, the control field may contain additional information such as the status of the communication link.

Advantages of HDLC:

1. Efficient data transmission: HDLC provides reliable, error-free data transmission over point-to-point or multi-point links.
2. Simple protocol: The protocol is simple and easy to implement, requiring less complex hardware and software.
3. Flexibility: HDLC can operate in different modes, providing flexibility in data transmission.
4. Robust error control: The CRC error detection and correction technique used in HDLC provides robust error control.

Disadvantages of HDLC:

1. Limited functionality: HDLC provides limited functionality and may not be suitable for complex networks.
2. Limited scalability: HDLC is limited to point-to-point or multi-point links and may not be scalable for larger networks.
3. Compatibility issues: HDLC may have compatibility issues with other protocols, which can cause problems in interoperability between different networks.

In summary, HDLC is a reliable and efficient protocol for data transmission in point-to-point or multi-point links. However, it may have limited functionality and scalability and may not be compatible with other protocols.