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.

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:

• Stations and Configurations
• Operational Modes
• Non-Operational Modes
• Frame Structure
• Commands and Responses
• HDLC Subsets(SDLC and LAPB)

HDLC STATIONS AND CONFIGURATIONS

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

• Primary Station
• Secondary Station
• Combined Station

PRIMARY STATION

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).

SECONDARY STATION

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.

COMBINED 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:

• Unbalanced Configuration
• Balanced Configuration
• Symmetrical Configuration

UNBALANCED CONFIGURATION

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:

• Full - Duplex or Half - Duplex operation
• Point to Point or Multi-point networks

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

BALANCED CONFIGURATION

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:

• Full - Duplex or Half - Duplex operation
• Point to Point networks

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

SYMMETRICAL 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(NRM)
• Asynchronous Response Mode(ARM)
• Asynchronous Balanced Mode(ABM)

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:

• Normal Disconnected Mode(NDM)
• Asynchronous Disconnected Mode(ADM)
• Initialization Mode(IM)

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.

THE FLAG FIELD

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:

• Seven 1's, but less than 15 signals an abort signal. The stations on the link know there is a problem on the link.
• 15 or more 1's indicate that the channel is in an idle state.

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 8^th bit. If the 8^th 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

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.

THE CONTROL FIELD

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:

• Information Transfer Format

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

devices.

• Supervisory Format

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.

• Unnumbered Format

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 POLL/FINAL BIT(P/F)

The 5^th 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.

THE INFORMATION FIELD

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.

THE FRAME CHECK SEQUENCE FIELD

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

HDLC COMMANDS AND RESPONSES

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

INFORMATION TRANSFER FORMAT COMMAND AND RESPONSE

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

SUPERVISORY FORMAT COMMANDS AND RESPONSES

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:

• Receive Ready(RR) is used by the primary or secondary station to indicate that it is ready to receive an information frame and/or acknowledge previously received frames.
• Receive Not Ready(RNR) is used to indicate that the primary or secondary station is not ready to receive any information frames or acknowledgments.
• Reject(REJ) is used to request the retransmission of frames.
• Selective Reject(SREJ) is used by a station to request retransmission of specific frames. An SREJ must be transmitted for each erroneous frame; each frame is treated as a separate error. Only one SREJ can remain outstanding on the link at any one time.

UNNUMBERED FORMAT COMMANDS RESPONSES

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.

• Set Normal Response Mode(SNRM) places the secondary station into NRM. NRM does not allow the secondary station to send any unsolicited frames. Hence the primary station has control of the link.
• Set Asynchronous Response Mode(SARM) allows a secondary station to transmit frames without a poll from the primary station.
• Set Asynchronous Balanced Mode(SABM) sets the operational mode of the link to ABM.
• Disconnect(DISC) places the secondary station in to a disconnected mode.
• Set Normal Response Mode Extended(SNRME) increases the size of the control field to 2 octets instead of one in NRM. This is used for extended sequencing. The same applies for SARME and SABME.
• Set Initialization Mode(SIM) is used to cause the secondary station to initiate a station-specific procedure(s) to initialize its data link level control functions.
• Unnumbered Poll(UP) polls a stations without regard to sequencing or acknowledgment.
• Unnumbered Information(UI) is used to send information to a secondary station.
• Exchange Identification(XID) is used to cause the secondary station to identify itself and provide the primary station identifications characteristics of itself.
• Reset(RSET) is used to reset the receive state variable in the addressed station.
• Test(TEST) is used to cause the addressed secondary station to respond with a TEST response at the first response opportunity. It performs a basic test of the data link control.
• Unnumbered Acknowledgment(UA) is used by the secondary station to acknowledge the receipt and acceptance of an SNRM, SARM, SABM, SNRME, SARME, SABME, RSET, SIM, or DISC commands.
• Disconnected Mode(DM) is transmitted from a secondary station to indicate it is in disconnected mode(non-operational mode.)
• Request Initialization Mode(RIM) is a request from a secondary station for initialization to a primary station. Once the secondary station sends RIM, it can only respond to SIM, DSIC, TEST or XID commands.
• Request Disconnect(RD) is sent by the secondary station to inform the primary station that it wishes to disconnect from the link and go into a non-operational mode(NDM or ADM).
• Frame Reject(FRMR) is used by the secondary station in an operation mode to report that a condition has occurred in transmission of a frame and retransmission of the frame will not correct the condition.

TABLE 1 HDLC Commands and Responses

HDLC SUBSETS

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.

Texts

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

http://www.cs.indiana.edu/l/www/classes/a247-whit/misc.html