Introduction

HDLC (High Level Data Link Control) is one of the most common data link layer (layer 2) protocols. Many other common layer 2 protocols are heavily based on HDLC, particularly its framing structure. The HDLC protocol is defined by ISO for use on both point-to-point and multipoint (multidrop) data links. It supports full duplex transparent-mode operation and is now extensively used in both multipoint and computer networks.

The rule of HDLC is to ensure that the data has been received without any loss or errors and in the correct order. HDLC has a flow control function which ensures that the data is transmitted as fast as the receiver can receive it.

HDLC Operation Modes

HDLC has three operational modes:

1. Normal Response Mode (NRM)

Normal Response Mode is used in unbalanced configurations, a configuration is which there is only one primary, controlling station. In this mode, slave stations (or secondary) can only transmit when specially instructed by the master (primary station). The link may be point-to-point or multipoint. In the latter case only one primary station is allowed.

2. Asynchronous Response Mode (ARM)

Asynchronous Response Mode is used in unbalanced configurations. It allows a secondary station to initiate a transmission without receiving permission from the primary station. This mode is normally used with point-to-point configurations and full duplex links and allows the secondary station to send frames asynchronously with respect to the primary station.

3. Asynchronous Balanced Mode (ABM)

The Asynchronous Balanced Mode (ABM) is used mainly on full duplex point-to-point links for computer to computer communications and for connections between a computer and a packed switched data network, in this case each station has an equal status and performs the role of both primary and secondary functions. This mode is used in the protocol set known as X.25.

Frame Format

The standards frame of the HDLC protocol handles both data and control messages.

It has the following format:

Flag	Address	Control	Information	FCS	Flag
1 Byte	1-2 Bytes	1 Byte	variable	2 Bytes	1 Byte

Flag - The value of the flag is always (0x7E).

Address field - The address of the secondary station which is sending the frame or the destination of the frame sent by the primary station. The address field contains Service Access Point (6bits), a Command/Response bit to indicate whether the frame relates to information frames (I-frames) being sent from the node or received by the node, and an address extension bit which is usually set to true to indicate that the address is of length one byte. When set to false it indicates an additional byte follows. The address field may be extended to more than one byte by agreement between the involved parties.

Control field – Identifies the type of the frame. In addition, it includes sequence numbers, control features and error tracking according to the frame type.

Information field - Data is transmitted in the information field, which can vary in length depending upon the protocol using the frame. Layer 3 frames are carried in the information field.

FCS - The Frame Check Sequence (FCS) is in most cases - 16 bit wide and serves for errors detection, the generator polynomial used is normally CRC-CCITT: x^16+x^12+x^5+1 .

HDLC uses zero insertion/deletion process (commonly known as bit stuffing) to ensure that the bit pattern of the delimiter flag does not occur in the fields between flags. The HDLC frame is synchronous and therefore relies on the physical layer to provide method of clocking and synchronizing the transmission and reception of frames.

Frame Classes

In the HDLC protocol three classes of frames are used:

1. Unnumbered frames

Unnumbered frames are used for link management. . SNRM and SABM frames, for example, are used both to set up logical link between the primary and the secondary station and to inform the secondary station of the mode of operation to be used. A logical link is subsequently cleared by the primary station sending a DISC frame. The UA frame is used as an acknowledgment to the other frames in this class.

2. Information frames

Information frames are those who carry the actual data. The Information frames can be used to piggyback acknowledgment information relating to the flow of Information frames in the reverse direction when the link is being operated in ABM or ARM.

3. Supervisory frames

Supervisory frames are use for error and flow control. They contain, send and receive sequence numbers. There are four types of supervisory frames but only RR and RNR are used in both NRM and ABM. These frames are used both to indicate the willingness of a secondary station to receive an information frame from the primary station, and for acknowledgment purposes. REJ and SREJ frames are used only in ABM which permits simultaneous two-way communication across a point to point link. The two frames are used to indicate to the other station that a sequence error has occurred, that is an information frame containing an out of sequence N(s) has been received. The SREJ frame is used with a selective repeat transmission procedure, whereas the REJ frame is used with a go back N procedure.

Protocol Operation

The two basic functions in the protocol are link management and data transfer (which includes error and flow control).

Link management

Prior to any kind of transmission (either between two stations connected by a point to point link or between a primary and secondary station connected with a multidrop link) a logical connection between the two communication parties must be established.

A logical connection between the two communication parties is accomplished by the exchange of two unnumbered frames. The procedure for setting a logical connection (In a multidrop link) is given below:

The SNRM is first sent by the primary station with the poll bit set to 1 and the address of the appropriate secondary in the address field. The secondary responds with a UA frame with the final bit set and its own address in the address field. After all the data has been transferred, the link is cleared by the primary sending a DISC frame and the secondary responding with a UA.

Data transfer

In NRM all data (information frames) is transferred under the control of the primary station. The unnumbered poll frame with the P bit set to 1 is normally used by the primary to poll a secondary. If the secondary has no data to transmit, it returns an RNR frame with the F bit set. If data is waiting, it transmits the data, typically as a sequence of information frames.
The two most important aspects associated with the data transfer phase are error control and flow control. Essentially, error control uses a continuous RQ procedure with either a selective repeat or a go back N transmission strategy, while flow controls based on a window mechanism.

References

Data Communications , Computer Networks and Open Systems, by Fred Halsall

Data Communication ICs, High-Level Serial Communications, by Siemens