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What is HDLC (High-level Data Link Control)?
2024-05-08 06:42:31

What is HDLC (High-level Data Link Control)?

HDLC is an acronym that stands for “High-level Data Link Control“. It is a collection of guidelines used to transfer data between network nodes or network points.

What is HDLC (High-level Data Link Control)?

The International Organization for Standardization (ISO) developed HDLC, a synchronous, bit-oriented data link layer protocol. HDLC adheres to ISO/IEC 13239:2002 as the standard. The International Electrotechnical Commission, or ECI for short, is a global authority on electrical and electronic standards that often collaborates with the ISO.

How does HDLC work?

In HDLC, information is arranged into units, or frames, and sent via a network to a location that attests to its safe arrival. The HDLC protocol likewise controls the pace or flow of data transmission.

In Layer 2 of the industry communication reference architecture known as Open Systems Interconnection (OSI), HDLC is one of the most widely utilized internet protocols (IP).

  • The intricate physical layer known as Layer 1 is responsible for both sending and receiving electrical signals.
  • The upper layer, known as Layer 3, is knowledgeable about the network and has access to router tables, which specify where data should be sent or forwarded.

Programming in Layer 3 generates a frame during transmission, which typically includes the source and destination network addresses. Data connection control information is added to a new, bigger frame by HDLC (Layer 2), which wraps the Layer 3 frames.

How is HDLC used in IT networks?

HDLC is predicated on the synchronous data link control (SDLC) protocol developed by IBM, a major player in mainframe computer systems with a huge client base. The HDLC protocol, known as Normal Response Mode, is basically the SDLC protocol.

Several additional industry standards, including frame relay protocols like the ISDN protocol stack known as Link Access Procedure Balanced (LAPB), were derived from HDLC by the networking industry. It serves as the foundation for both Cisco HDLC framing methods, which append protocol fields to the HDLC header, and the framing mechanism that uses Point-to-Point Protocol (PPP) over synchronous lines to connect numerous servers to a WAN (wide area network) internet connection.

In what is known as a multidrop or multipoint network, data is sent from a primary station typically located at the mainframe computer to secondary stations, which may be nearby or distant, over-specialized leased lines. (This is a nonpublic closed network, not the network we often think of. Communication in this setup is often half-duplex.)

HDLC variations are also used for frame relay, a protocol used in both public and private wide area networks (WANs) and public networks that employ the X.25 communications protocol.

The data frame in HDLC version X.25 comprises a packet. (An X.25 network is one in which data packets are transported to their final location by paths chosen based on the network's perceived circumstances as seen by routers and are then reassembled in the correct sequence at the destination.)

Peer-to-peer communication over duplex connections is used by the HDLC X.25 version, where communication may be initiated by either end. The term "Link Access Procedure, Balanced" (LAPB) refers to this particular HDLC mode.

How do HDLC frames work?

It is possible to send HDLC frames across synchronous or asynchronous communication links:

  • Coordinated Framing: Data in synchronous frames is encoded using non-return-to-zero inversion, or NRZI, which means that a 1-bit transmission indicates no change in the signal, and a 0-bit transmission indicates a change. Put differently, each zero informs the receiving modem that it needs to synchronize its clock. Frames may be transmitted across a full-duplex or half-duplex connection in this manner.
  • Incoherent Framing: Nevertheless, bit pattern concerns are superfluous in asynchronous communications. Rather, bit stuffing, or control-octet transparency, is used. Control escape octets use 0x7D values, such as a bit sequence of '10111110' where the least significant bit appears first.

A 16-bit CRC-CCITT or a 32-bit CRC-32 frame check sequence, broadcast across the Information, Control, or Address fields, is the frame check sequence used when delivering HDLC frames (CRC stands for cyclic redundancy check). A receiver may utilize HDLC frames as a channel to apply algorithms for error detection of any mistakes that could have been made during transmission. An error has occurred if the recipient's FCS algorithm differs from the sender's.

Note: CCITT stands for Consultative Committee for International Telegraphy and Telephony. CCITT, an international organization that promotes cooperative standards for telecommunications systems and equipment, is also known as ITU-T (Telecommunication Standardization Sector of the International Telecommunications Union).

HDLC Frame Types

Three different kinds of HDLC frame architectures are often used. They are listed in the following order:

  • I-frames: Information frames (I-frames) are used to carry error control information together with user data that is sent from the computer network layer. Control fields are also included in I-frames and are used to specify data functions.
  • S-frames: When it is no longer feasible to "piggyback" on transmitted data, supervisory frames (S-frames) are sent, carrying error and flow control data. S-frames do not have information fields because of this.
  • U-frames: Unnumbered frames (U-frames) are used for connection management and all other ancillary functions. Some have information fields, and others don't.

HDLC Encapsulation Protocol

We are aware that every HDLC frame has between six and seven fields, including FCS (Frame Check Sequence) fields, information fields, control fields, and start/end flag fields. The HDLC Protocol standard has six fields. Conversely, Cisco HDLC (cHDLC) has one more protocol field. While the cHDLC protocol supports multi-protocol environments, the standard protocol only supports one protocol. The protocol field in the header allows the identification of distinct protocols, making it feasible to support numerous protocols. Cisco Systems was the creator of SDLC.

  • Address field: The kind of packet that is contained in a cHDLC frame is identified and specified using this field. For Unicast packets, it may be 0*0F, and for Broadcast packets, 0*8F.
  • Control field: This field's default value is zero, or 0*00.
  • Protocol field: To indicate and identify the sort of protocol that is being contained inside a cHDLC frame, this field is particularly necessary. In the case of Internet Protocol, it might be 0x0800.

Verify HDLC Encapsulation

We are aware that Cisco routers often use the HDLC encapsulation approach for serial ports. It won't be mentioned in any of the operating setups as a result. This essentially implies that we cannot even use the show running-config command to confirm HDLC encapsulation. As a result, we need to see and identify the kind of encapsulation in the interface using the display interfaces (Interface) command.

Troubleshoot HDLC Encapsulation

The two kinds of commands listed below may be used to inspect and determine the current state of the serial interface:

  • Show IP interface brief
  • Show interfaces [interface]

One crucial diagnostic tool that helps in diagnosing serial lines is the Show Controllers command. The status of the interface channels and whether or not a cable is connected to the interface are also shown in this command output. The protocol status will be down for a number of reasons related to difficulties that arise during HDLC implementation.

The following are the reasons behind this:

  • Non-Cisco router present at the far end.
  • Use of additional protocols by the remote side router, such as PPP.
  • Incapacity of DCE (Data Circuit-Terminating Equipment) Device to provide clock rate to DTE (Data Terminal Equipment) Device.
  • An issue or problem with the card's internal circuitry.
  • Electrical connections those are unknown.

The following list shows the few serial interface problems:

Serial x is up, Line Protocol is up: This command shows that the line is up and operating as it should. It is optional to take any action.

Serial x is down, Line Protocol is down (DTE mode): This command suggests that something is wrong.

There are several causes for this problem to occur. Below are a few of them:

  • Fault in cable: Swapping out all of the defective cables will fix this problem.
  • Failure of hardware: You may fix this problem by switching the serial line to a different port.

Serial x is up, Line Protocol is down (DTE mode): This command also suggests that something is wrong. A misconfigured local or remote might be the cause of this problem. To fix this issue, use the display interface serial command after placing the modem, CSU (Channel Service Unit), or DSU (Data Service Unit) in local loopback mode. This command shows whether the line protocol has been activated or not.

Advantages of High-Level Data Link Control (HDLC)

  • Standardization and Interoperability: An international standard called HDLC guarantees compatibility between gadgets made by various manufacturers. This standardization encourages data connection layer communication to be conducted consistently and globally.
  • Efficiency: HDLC is renowned for managing data transport tasks with efficiency. It optimizes the communication process by using a bit-oriented strategy to minimize overhead and a range of frame kinds, including information, acknowledgment, and control frames.
  • Error Detection and Correction: Mechanisms for error detection and repair are included in HDLC. The integrity of the transmitted data is ensured by the receiver's ability to recognize and reject frames containing mistakes via the implementation of a Frame Check Sequence (FCS).
  • Flow Control: To govern the pace at which data is sent between the sender and the recipient, HDLC offers flow control techniques. This guarantees that the receiving device can handle incoming data at a reasonable speed and helps avoid data overload.
  • Robustness: HDLC's resilience is partly attributed to its mistake detection and repair techniques. It ensures the dependability of data transfer even under less-than-ideal network circumstances by recovering from transmission faults.
  • Compatibility with Multiple Networks: Due to its adaptability, HDLC may be used with both synchronous and asynchronous connections on different kinds of networks. Its versatility makes it more useful in a variety of networking settings.
  • Suitability for Reliable Data Transfer: Applications needing dependable data transport, such as file transfers, where data integrity is essential, are well suited for HDLC. The characteristics that identify and fix errors aid in preserving the correctness of the information that is conveyed.
  • Addressing and Multiplexing: Address fields in HDLC frames enable multiplexing when many devices use the same communication channel. With the help of many linked devices, this capability makes it possible to identify the intended receiver.

Disadvantages of High-Level Data Link Control (HDLC)

  • Limited Flexibility: Although standardization of HDLC facilitates interoperability, it may also restrict its versatility. For certain specialized applications or situations that call for unique protocols, it may not be appropriate.
  • Complexity: It may be difficult to implement HDLC, especially for those who are unfamiliar with the protocol. It may not be easy to set up and debug because of its bit-oriented structure and variety of frame kinds.
  • Lack of Encryption: There are no built-in encryption methods in HDLC. Additional security measures must be put in place when secure communication is a top concern, which might make the system more complicated overall.
  • Limited Support for Multicast and Broadcast: HDLC may not be the most effective option in multicast or broadcast applications when a single signal has to reach numerous receivers at once, even if it allows multipoint setups.
  • Difficulty in Debugging: HDLC implementations may be difficult to debug and troubleshoot because of their bit-oriented design and range of frame types. Issues may need to be identified and resolved with specific expertise and equipment.
  • Overhead: Even though HDLC has less overhead than some other protocols, it still requires some extra bits for error-checking, control, and addressing. This cost is a drawback in scenarios where bandwidth is an important consideration.
  • Not Ideal for Broadcast Networks: The main applications for HDLC are multipoint and point-to-point setups. For broadcast networks, where many devices must concurrently receive the same message, there may be better options.

Applications of High-Level Data Link Control (HDLC)

  • WAN Connectivity: Wide area network (WAN) settings often employ HDLC. It ensures effective and error-free data transfer across long-distance lines by offering a dependable and standardized protocol.
  • Point-to-Point Communication: Because HDLC was originally intended to be a point-to-point protocol, it works effectively in situations where direct communication between two devices is req9uired. This could occur when leased lines or routers are connected between two sites.
  • Frame Relay Networks: A popular packet-switching system called Frame Relay often uses HDLC encapsulation. Because of its effectiveness with frames, HDLC may be used in Frame Relay networks, where data is sent in frames of varying sizes.
  • Multipoint Configurations: HDLC enables multipoint setups, which facilitates communication between several devices over a single connection. This makes it useful in situations when a shared communication channel is required for several devices.
  • Embedded Systems and IoT: HDLC may be used to facilitate device-to-device communication in certain embedded systems and Internet of Things (IoT) applications where standardization and interoperability are critical.
  • Satellite Communication: In satellite communication systems, where dependable and effective data transport is crucial, HDLC is used. The error detection and repair features of the protocol contribute to the integrity of data delivered across potentially noisy satellite connections.
  • Telecommunication Networks: Applications for HDLC may be found in a variety of wired and wireless communications networks. Its uniform methodology enables smooth communication among various network components.
  • Industrial Automation and Control Systems: Because HDLC is standardized and reliable, it may be used in industrial settings, particularly in supervisory control and data acquisition (SCADA) systems, to facilitate communication between sensors, PLCs, and other control devices.
  • Legacy Systems Integration: When integrating new systems with outdated hardware, HDLC is often used. It is a feasible solution for guaranteeing compatibility and communication across more traditional and current technologies because of its extensive acceptance and support.
  • Voice over IP (VoIP) Networks: Some VoIP systems use HDLC to encapsulate voice data during transmission. Although other VoIP protocols, such as Real-time Transport Protocol (RTP), are more widely used, HDLC's dependability properties may make it useful in certain situations.