Make your own free website on

HDLC - A Technical Overview

High-Level Data Link Control, also know as HDLC, is a bit oriented, switched and non-switched protocol. It is a data link control protocol, and falls within layer 2, the Data Link Layer, of the Open Systems Interface(OSI) model as shown in figure 1.

Figure 1

HDLC is a protocol developed by the International Organization for Standardization(ISO). It falls under the ISO standards ISO 3309 and ISO 4335. It has found itself being used throughout the world. It has been so widely implemented because it supports both half duplex and full duplex communication lines, point to point(peer to peer) and multi-point networks, and switched or non-switched channels. The procedures outlined in HDLC are designed to permit synchronous, code-transparent data transmission. Other benefits of HDLC are that the control information is always in the same position, and specific bit patterns used for control differ dramatically from those in representing data, which reduces the chance of errors.

It has also led to many subsets. Two subsets widely in use are Synchronous Data Link Control(SDLC) and Link Access Procedure-Balanced(LAP-B).

This technical overview will be concerned with the following aspects of HDLC:


HDLC specifies the following three types of stations for data link control:


Within a network using HDLC as it's data link protocol, if a configuration is used in which there is a primary station, it is used as the controlling station on the link. It has the responsibility of controlling all other stations on the link(usually secondary stations). Despite this important aspect of being on the link, the primary station is also responsible for the organization of data flow on the link. It also takes care of error recovery at the data link level(layer 2 of the OSI model).


If the data link protocol being used is HDLC, and a primary station is present, a secondary station must also be present on the data link. The secondary station is under the control of the primary station. It has no ability, or direct responsibility for controlling the link. It is only activated when requested by the primary station. It only responds to the primary station. The secondary station's frames are called responses. It can only send response frames when requested by the primary station.


A combined station is a combination of a primary and secondary station. On the link, all combined stations are able to send and receive commands and responses without any permission from any other stations on the link. Each combined station is in full control of itself, and does not rely on any other stations on the link. No other stations can control any combined station.

HDLC also defines three types of configurations for the three types of stations:


The unbalanced configuration in an HDLC link consists of a primary station and one or more secondary stations. The unbalanced occurs because one stations controls the other stations. In a unbalanced configurations, any of the following can be used:

An example of an unbalanced configuration can be found below in figure 2.a


The balanced configuration in an HDLC link consists of two or more combined stations. Each of the stations have equal and complimentary responsibility compared to each other. Balanced configurations can used only the following:

An example of a balanced configuration can be found below in figure 2.b

Figure 2.a An unbalanced configuration

Figure 2.b A balanced configuration


This third type of configuration is not widely in use today. It consists of two independent point to point, unbalanced station configurations. In this configurations, each station has a primary and secondary status. Each station is logically considered as two stations.

HDLC Operational Modes

HDLC offers three different modes of operation. These three modes of operations are:

Normal Response Mode

This is the mode in which the primary station initiates transfers to the secondary station. The secondary station can only transmit a response when, and only when, it is instructed to do so by the primary station. In other words, the secondary station must receive explicit permission from the primary station to transfer a response. After receiving permission from the primary station, the secondary station initiates it's transmission. This transmission from the secondary station to the primary station may be much more than just an acknowledgment of a frame. It may in fact be more than one information frame. Once the last frame is transmitted by the secondary station, it must wait once again from explicit permission to transfer anything, from the primary station. Normal Response Mode is only used within an unbalanced configuration.

Asynchronous Response Mode

In this mode, the primary station doesn't initiate transfers to the secondary station. In fact, the secondary station does not have to wait to receive explicit permission from the primary station to transfer any frames. The frames may be more than just acknowledgment frames. They may contain data, or control information regarding the status of the secondary station. This mode can reduce overhead on the link, as no frames need to be transferred in order to give the secondary station permission to initiate a transfer. However some limitations do exist. Due to the fact that this mode is Asynchronous, the secondary station must wait until it detects and idle channel before it can transfer any frames. This is when the ARM link is operating at half-duplex. If the ARM link is operating at full-duplex, the secondary station can transmit at any time. In this mode, the primary station still retains responsibility for error recovery, link setup, and link disconnection.

Asynchronous Balanced Mode

This mode uses combined stations. There is no need for permission on the part of any station in this mode. This is because combined stations do not require any sort of instructions to perform any task on the link.

Normal Response Mode is used most frequently in multi-point lines, where the primary station controls the link. Asynchronous Response Mode is better for point to point links, as it reduces overhead. Asynchronous Balanced Mode is not used widely today.

The "asynchronous" in both ARM and ABM does not refer to the format of the data on the link. It refers to the fact that any given station can transfer frames without explicit permission or instruction from any other station.

HDLC Non-Operational Modes

HDLC also defines three non-operational modes. These three non-operational modes are:

The two disconnected modes(NDM and ADM) differ from the operational modes in that the secondary station is logically disconnected from the link(note the secondary station is not physically disconnected from the link). The IM mode is different from the operations modes in that the secondary station's data link control program is in need of regeneration or it is in need of an exchange of parameters to be used in an operational mode.

HDLC Frame Structure

HDLC uses the term "frame" to indicate an entity of data(or a protocol data unit) transmitted from one station to another. Figure 3 below is a graphical representation of a HDLC frame with an information field.

Figure 3 An HDLC frame with an information field.

Field Name

Size(in bits)

Flag Field( F )

8 bits

Address Field( A )

8 bits

Control Field( C )

8 or 16 bits

Information Field( I )

Variable; Not used in some frames

Frame Check Sequence( FCS )

16 or 32 bits

Closing Flag Field( F )

8 bits


Every frame on the link must begin and end with a flag sequence field(F). Stations attached to the data link must continually listen for a flag sequence. The flag sequence is an octet looking like 01111110. Flags are continuously transmitted on the link between frames to keep the link active. Two other bit sequences are used in HDLC as signals for the stations on the link. These two bit sequences are:

The time between the transmission of actual frames is called the interframe time fill. The interframe time fill is accomplished by transmitting continuous flags between frames. The flags may be in 8 bit multiples.

HDLC is a code-transparent protocol. It does not rely on a specific code for interpretation of line control. This means that if a bit at position N in an octet has a specific meaning, regardless of the other bits in the same octet. If an octet has a bit sequence of 01111110, but is not a flag field, HLDC uses a technique called bit-stuffing to differentiate this bit sequence from a flag field. Once the transmitter detects that it is sending 5 consecutive 1's, in inserts a 0 bit to prevent a "phony" flag.

At the receiving end, the receiving station inspects the incoming frame. If it detects 5 consecutive 1's it looks at the next bit. If it is a 0, it pulls it out. If it is a 1, it looks at the 8th bit. If the 8th bit is a 0, it know an abort or idle signal has been sent. It then proceeds to inspect the following bits to determine appropriate action. This is the manner in which HDLC achieves code-transparency. HDLC is not concerned with any specific bit code inside the data stream. It is only concerned with keeping flags unique.


The address field(A) identifies the primary or secondary stations involvement in the frame transmission or reception. Each station on the link has a unique address. In an unbalanced configuration, the A field in both commands and responses refers to the secondary station. In a balanced configuration, the command frame contains the destination station address and the response frame has the sending station's address.


HDLC uses the control field(C) to determine how to control the communications process. This field contains the commands, responses and sequences numbers used to maintain the data flow accountability of the link, defines the functions of the frame and initiates the logic to control the movement of traffic between sending and receiving stations. There three control field formats:

The frame is used to transmit end-user data between two


The control field performs control functions such as acknowledgment of frames, requests for re-transmission, and requests for temporary suspension of frames being transmitted. Its use depends on the operational mode being used.

This control field format is also used for control purposes. It is used to perform link initialization, link disconnection and other link control functions.


The 5th bit position in the control field is called the poll/final bit, or p/f bit. It can only be recognized when it is set to 1. If it is set to 0, it is ignored. The poll/final bit is used to provide dialogue between the primary station and secondary station. The primary station uses P=1 to acquire a status response from the secondary station. The P bit signifies a poll. The secondary station responds to the P bit by transmitting a data or status frame to the primary station with the P/F bit set to F=1. The F bit can also be used to signal the end of a transmission from the secondary station under Normal Response Mode.


This field is not always in a HDLC frame. It is only present when the Information Transfer Format is being used in the control field. The information field contains the actually data the sender is transmitting to the receiver.


This field contains a 16 bit, or 32 bit cyclic redundancy check. It is used for error detection.


The set of commands and responses in HDLC is summarized in table 1.


The functions of the information command and response is to transfer sequentially numbered frames, each containing an information field, across the data link.


Supervisory(S) commands and responses are used to perform numbered supervisory functions such as acknowledgment, polling, temporary suspension of information transfer, or error recovery. Frames with the S format control field cannot contain an information field. A primary station may use the S format command frame with the P bit set to 1 to request a response from a secondary station regarding its status. Supervisory Format commands and responses are as follows:


The unnumbered format commands and responses are used to extend the number of data link control functions. The unnumbered format frames have 5 modifier bits which allow for up to 32 additional commands and 32 additional response functions. Below, 13 command functions, and 8 response functions are described.

TABLE 1 HDLC Commands and Responses

Information Transfer

Information Transfer

Format Commands

Format Responses

I - Information I - Information

Supervisory Format

Supervisory Format



RR - Receive ready RR - Receive ready
RNR - Receive not ready RNR - Receive not ready
REJ - Reject REJ - Reject
SREJ - Selective reject SREJ - Selective reject

Unnumbered Format

Unnumbered Format



SNRM - Set Normal Response Mode UA - Unnumbered Acknowledgment
SARM - Set Asynchronous Response Mode DM - Disconnected Mode
SABM - Set Asynchronous Balanced Mode RIM - Request Initialization Mode
DISC - Disconnect RD - Request Disconnect
SNRME - Set Normal Response Mode Extended UI - Unnumbered Information
SARME - Set Asynchronous Response Mode Extended XID - Exchange Identification
SABME - Set Asynchronous Balanced Mode Extended FRMR - Frame Reject
SIM - Set Initialization Mode TEST - Test
UP - Unnumbered Poll
UI - Unnumbered Information
XID - Exchange identification
RSET - Reset
TEST - Test


Many other data link protocols have been derived from HDLC. However, some of them reach beyond the scope of HDLC. Two other popular offsets of HDLC are Synchronous Data Link Control(SDLC), and Link Access Protocol, Balanced(LAP-B). SDLC is used and developed by IBM. It is used in a variety of terminal to computer applications. It is also part of IBM's SNA communication architecture. LAP-B was developed by the ITU-T. It is derived mainly from the asynchronous response mode(ARM) of HDLC. It is commonly used for attaching devices to packet-switched networks.



  1. Black, Ulyess. Data Networks: Concepts, Theory and Practice, Prentice-Hall, United States, 1989
  2. Freeman, Roger L. Practical Data Communications, John Wiley & Sons, Inc., United States, 1995
  3. Freeman, Roger L. Reference Manual For Telecommunications Engineering, John Wiley & Sons, Inc., Canada, 1994
  4. Folts, Harold C. McGraw-Hill's Compilation of Open Systems Standards Vol. 2, McGraw-Hill
  5. Held, Gilbert, Data Communications Networking Devices, John Wiley & Sons, Inc., England, 1992

Internet Web Sites

  1. Notes, Images and other Miscellania