HDLC Protocol Operation

The two basic functions in the protocol

• Link Management
• Data transfer (which includes error and flow control).

1. 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 a multidrop link) a logical connection between the two communication parties must be established.
2. 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 continues RQ procedure with either a selective repeat or a go back N transmission strategy, while flow controls bases on a window mechanism.

Some standard subsets are listed in the following table.

HDLC Subset		Used for
802.2	Logical Link Control	FDDI, Token Ring, and some Ethernet LAN's
LAP	Link Access Procedure	Early X.25 implementations
LAPB	Link Access Procedure Balanced	Current X.25 implementations
LAPD	Link Access Procedure for the ISDN D Channel	ISDN D channel and frame relay
LAPM	Link Access Procedure for Modems	Error-correcting modems (specified as part of V.42

HDLC Frame Types

The following are the Supervisory Frame Types in HDLC

RR	Information frame acknowledgement and indication to receive more.
REJ	Request for retransmission of all frames after a given sequence number
RNR	Indicates a state of temporary occupation of station (e.g. window full)
SREJ	Request for retransmission of one given frame sequence number.

The following are the Unnumbered Frame Types in HDLC

DISC	Request disconnection
UA	Acknowledgement frame
DM	Response to DISC indicating disconnected mode.
FRMR	Frame reject.
SABM	Initiator for asynchronous balanced mode. No master/slave relationship
SABME	SABM in extended mode.
SARM	Initiator for asynchronous response mode. Semi master/slave relationship
SARME	SAMR in extended mode.
REST	Reset sequence numbers.
CMDR	Command reject.
SNRM	Initiator for normal response mode. Full master/slave relationship
SNRME	SNRM in extended mode.
RD	Request disconnect
RIM	Secondary station request for initialization after disconnection.
SIM	Set initialization mode.
UP	Unnumbered poll.
UI	Unnumbered information. Sends state information/data.
XID	Identification exchange command.

There is one information Frame Type in HDLC

Info	Information frame.

HDLC Frame Classes

In the HDLC protocol, three classes of frames are used:

1. Unnumbered frames are used for link management, for example they are used to set up the logical between the primary station and a secondary station and to inform the secondary station about the mode of operation which is used.
2. 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. 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
4. Asynchronous Response Mode (ARM) 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.
5. Supervisory frames are use for error and flow control. They contain, send and receive sequence numbers.

HDLC Frame Formats

The standard frame of the HDLC protocol handles both data and control messages. It has the following format:

The HDLC frame structure consists of:

1. Flag - The value of the flag is always (0x7E). In order to ensure that the bit pattern of the frame delimiter flag does not appear in the data field of the frame (and therefore cause frame misalignment), a technique known as Bit Stuffing is used by both the transmitter and the receiver.

2. Address field - The first byte of the frame after the header flag is known as the Address Field. HDLC is used on multipoint lines and it can support as many as 256 terminal control units or secondary stations per line. The address field defines the address of the secondary station which is sending the frame or the destination of the frame sent by the primary station.

3. Control Field - The field following the Address Field is called the Control Field and serves to identify the type of the frame. In addition, it includes sequence numbers, control features and error tracking according to the frame type. Every frame holds a one bit field called the Poll/Final bit. In the NRM (Normal Response Mode) mode of HDLC this bit signals which side is 'talking' and provides control over who will speak next and when. When a primary station has finished transmitting a series of frames, it sets the Poll bit, thus giving control to the secondary station. At this time the secondary station may reply to the primary station. When the secondary station finishes transmitting its frames, its sets the Final bit and control returns to the primary station.

4. Modes of operation - HDLC has 3 modes of operation according to the strength of the master/slave relationship. This is determined by a unique frame type specifier. The three modes of operation are:

• Normal Response Mode (NRM) - The primary station initiates the session and full polling is used for all frame transmissions.
• Asynchronous Response Mode (ARM) - This mode is similar to NRM and is signified by the SARM(E) frame. The difference, however, is that secondary stations can transmit freely without waiting for a poll.
• Asynchronous Balanced Mode (ABM) - This mode is totally balanced (i.e. no master/slave relationship) and is signified by the SABM(E) frame. Each station can initialize, supervise, recover from errors and send frames at any time.

5. FCS (Frame Check Sequence) - The Frame Check Sequence (FCS) enables a high level of physical error control by allowing the integrity of the transmitted frame data to be checked. The sequence is first calculated by the transmitter using an algorithm based on the values of all the bits in the frame. The receiver then performs the same calculation on the received frame and compares its value to the CRC.

HDLC (High Level Data Link Control)

HDLC (High Level Data Link Control) is a group of protocols or rules for transmitting data between network points (sometimes called nodes). In HDLC, data is organized into a into (called a frame) and send across a network to a destination that verifies its successful arrival. The HDLC protocol also manages the flow or pacing at which data is sent.

A bit-oriented, synchronous, link layer, data-framing, flow control and error detection and correction protocol. Uses a header with control information and a trailing cyclic redundancy check character (which is usually 16 or 32 bits in length). Implementations are both standard subsets or vendor-specific (such as that used for the 56,000-bits/s interfaces on a vendor's remote bridge or router). IBM calls HDLC as SDLC.

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.

The HDLC protocols 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.