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HDLC (High-level Data Link Control) is a group of protocols or rules for transmitting data between network points (sometimes called nodes). In more technical terms, HDLC is a bit-oriented, synchronous data link layer protocol created by the International Organization for Standardization (ISO).HDLC is a group of data link (Layer 2) protocols used to transmit synchronous data packets between point-to-point nodes. Data is organized into addressable frames. This format has been used for other multipoint-to-multipoint protocols, and inspired the HDLC-like framing protocol described in RFC 1662.The HDLC protocol is a general purpose protocol which operates at the data link layer of the OSI reference model. The protocol uses the services of a physical layer, and provides either a best effort or reliable communications path between the transmitter and receiver (i.e. with acknowledged data transfer).
| BISYNC Features | HDLC Features |
|---|---|
| It supports both point to point and point to multipoint configuration. | It also supports both point to point and point to multipoint configuration. |
| It follows stop and wait flow control protocol. | It follows sliding window flow control protocol. |
High-Level Data Link Control (HDLC)
Images related to the topicHigh-Level Data Link Control (HDLC)
In previous articles, we have seen the sling window and stop-wait protocols of the data link layer. Now we will understand the HDLC (High Level Data Link Control) protocol in the DLL. High-Level Data Link Control (HDLC) is a bit-oriented protocol for communication over point-to-point and multipoint links and is based on the ARQ mechanism described in the previous article. It is the most wely used data link layer protocol. It has the advantages of flexibility, adaptability, reliability and operational efficiency. HDLC will have two transmission modes, Normal Response Mode (NRM) and Asynchronous Balanced Mode (ABM). In Normal Response Mode (NRM), the station configuration is unbalanced. We have only one master and several slaves. The master can send commands and the slave can only reply. In Asynchronous Balanced Mode (ABM), the configuration is balanced. The connection is point-to-point, and each station can act as a primary and secondary (acting as a peer) station. The following three types of stations are defined for the HDLC protocol. Master: The master is responsible for data connection management. The frames sent by the master are called commands. Secondary station: The secondary station operates under the control of the primary station. A frame sent by a slave is called a reply. Combined station: Combined station can be used as master station or as auxiliary station. So it can send commands and responses.
Frames in HDLC
To prove the necessary flexibility to support all possible options in the modes and configurations just described, HDLC defines three types of frames, i. H. Information frames (I-frames), surveillance frames (S-frames), and unnumbered frames (V-frames). I-frames are used to transmit user data and to control (piggyback) information about the user data. S-frames are only used to transmit control information. V-frames are reserved for system management.
Operating modes for data transfer in HDLC
HDLC allows both synchronous and asynchronous communication modes. The following operating modes are available for data transfer: Normal Response Mode (NRM): This is a synchronous communication mode. This mode is suitable for point-to-point and point-to-multipoint configurations. Asynchronous Response Mode (ARM): This mode is used for communication between master and slave. As the name suggests, it is an asynchronous mode (communication. Asynchronous Balanced Mode (ABM): This mode is suitable for point-to-point communication between two combined stations. Since these two stations are combined stations, connection management functions can be performed.
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Frame Structure in HDLC
Flags field: The flags field of an HDLC frame is an 8-bit sequence with a bit pattern of 01111110 that entifies the start and end of the frame. Address Field: The second field of the HDLC frame contains the address of the slave. The address field consists of 8 bits, so it can address 256 addresses. Control field: The control field is a 1 or 2 byte segment of the frame used for flow and error control. The interpretation of the bits in this field depends on the frame type. The control field usually consists of 8 bits, but the number of bits can be extended to 16 bits. It contains different data for each different type of frame, ie. H. I-frames, S-frames, and V-frames. Information field: The information field contains user data or management information from the network layer. It consists of the user’s data bits and is completely transparent. FCS Field: Frame Check Sequence (FCS) is basically an HDLC error detection field. It can contain 2 or 4 bytes. This is a 16-bit field used to detect errors in address, control and information fields. The data field of the
Transparency In HDLC
HDLC frame can carry textual and non-textual information. Examples of non-text information are audio, video, graphics, etc. If the data field of the HDLC frame contains the pattern 01111110 reserved for the flags field, the receiver treats this sequence as an end flag.
Bit Stuffing in HDLC
Bit padding is used to overcome the lack of data transparency. In HDLC, transparency is achieved by ensuring that the unique flag sequence (01111110) does not appear in the address, control, information and FCS fields. After five consecutive ‘1’s appear anywhere after the on flag and before the off flag, an extra ‘0’ bit is inserted at the transmitter. On the receiving end, the additional “0” bits are removed after five consecutive “1”s. This technique is called “zero stuffing” or bit stuffing. For the three operating conditions, bit stuffing is not performed. First, if the bit sequence is really a sign, second, if the transmission is aborted, and third, if the channel is le. Point-to-Point Protocol (PPP) While HDLC is a common protocol that can be used in point-to-point and multipoint configurations, one of the most fundamental protocols for point-to-point access is the Point-to-Point Protocol. Point Protocol (PPP). PPP defines the basic format of frames to be exchanged between devices. PPP defines how network layer data is encapsulated in data link frames. PPP also proves configuration of network addresses. This is especially useful when home users need a temporary network address to connect to the Internet. PPP is a data link layer protocol. However, it uses many other protocols to do the following: Connection establishment. Authentication of relevant parties. Carry network layer data. The PPP protocol can run over a variety of point-to-point transmission links, such as ADSL and SONET. Many Internet users use this protocol to connect their home computers to their Internet Service Prover (ISP) servers. PPP is used to control and manage data transmission. This protocol is designed for users who want to access the Internet by connecting their computer system to an Internet service prover’s computer over a telephone line. In the next article, we will learn more about how the MAC layer works. Stay tuned..report this ad
What is the use of HDLC protocol?
HDLC is a group of data link (Layer 2) protocols used to transmit synchronous data packets between point-to-point nodes. Data is organized into addressable frames. This format has been used for other multipoint-to-multipoint protocols, and inspired the HDLC-like framing protocol described in RFC 1662.
What is HDLC protocol explain the features of HDLC?
| BISYNC Features | HDLC Features |
|---|---|
| It supports both point to point and point to multipoint configuration. | It also supports both point to point and point to multipoint configuration. |
| It follows stop and wait flow control protocol. | It follows sliding window flow control protocol. |
Is HDLC application layer protocol?
The HDLC protocol is a general purpose protocol which operates at the data link layer of the OSI reference model. The protocol uses the services of a physical layer, and provides either a best effort or reliable communications path between the transmitter and receiver (i.e. with acknowledged data transfer).
Which protocol adapted the standards of HDLC protocol?
High-level Data Link Control (HDLC) was derived from Synchronous Data Link Control (SDLC) and was developed by the International Organization for Standardization (ISO) for point-to-point communication. It was later adapted by the International Telecommunication Union (ITU) for X. 25 Link Access protocol.
Why do we need HDLC?
Importance of SDLC
SDLC allows developers to analyze the requirements. It helps in reducing unnecessary costs during development. During the initial phases, developers can estimate the costs and predict costly mistakes. It enables developers to design and build high-quality software products.
What is the data link layer?
The data link layer is the protocol layer in a program that handles the moving of data into and out of a physical link in a network. The data link layer is Layer 2 in the Open Systems Interconnection (OSI) architecture model for a set of telecommunication protocols.
What is HDLC explain the frame format of HDLC protocol with its diagram?
High-Level Data Link Control (HDLC) generally uses term “frame” to indicate and represent an entity of data or a protocol of data unit often transmitted or transferred from one station to another station. Each and every frame on link should begin and end with Flag Sequence Field (F).
When and where is HDLC applied?
HDLC can be used for point-to-multipoint connections via the original master-slave modes Normal Response Mode (NRM) and Asynchronous Response Mode (ARM), but they are now rarely used; it is now used almost exclusively to connect one device to another, using Asynchronous Balanced Mode (ABM).
What is HDLC How does it performs flow control?
HDLC uses synchronous serial transmission to provide error-free communication between two points. HDLC defines a Layer 2 framing structure that allows for flow control and error control through the use of acknowledgments. Each frame has the same format, whether it is a data frame or a control frame.
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