High-Level Data Link Control

High-Level Data Link Control (HDLC) is a bit-oriented synchronous data link layer protocol developed by the International Organization for Standardization (ISO). The original ISO standards for HDLC were:

• ISO 3309 — Frame Structure
• ISO 4335 — Elements of Procedure
• ISO 6159 — Unbalanced Classes of Procedure
• ISO 6256 — Balanced Classes of Procedure

The current standard for HDLC is ISO 13239, which replaces all of those standards.

HDLC provides both connection-oriented and connectionless service.

HDLC can be used for point to multipoint connections, but is now used almost exclusively to connect one device to another, using what is known as Asynchronous Balanced Mode (ABM). The original master-slave modes Normal Response Mode (NRM) and Asynchronous Response Mode (ARM) are rarely used.

History

HDLC is based on IBM's SDLC protocol, which is the layer 2 protocol for IBM's Systems Network Architecture (SNA). It was extended and standardized by the ITU as LAP, while ANSI named their essentially identical version ADCCP.

Derivatives have since appeared in innumerable standards. It was adopted into the X.25 protocol stack as LAPB, into the V.42 protocol as LAPM, into the Frame Relay protocol stack as LAPF and into the ISDN protocol stack as LAPD. It was the inspiration for the IEEE 802.2 Logical Link Control protocol, and it is the basis for the framing mechanism used with the Point-to-Point Protocol on synchronous lines, as used by many servers to connect to a wide area network, most commonly the Internet. A mildly different version is also used as the control channel for E-carrier (E1) and SONET multichannel telephone lines. Some vendors, such as Cisco, implemented protocols such as Cisco HDLC that used the low-level HDLC framing techniques but didn't use the standard HDLC header. It has also been used on Tellabs DXX for destination of Trunk.

Framing

HDLC frames can be transmitted over synchronous or asynchronous links. Those links have no mechanism to mark the beginning or end of a frame, so the beginning and end of each frame has to be identified. This is done by using a frame delimiter, or flag, which is a unique sequence of bits that is guaranteed not to be seen inside a frame. This sequence is '01111110', or, in hexadecimal notation, 7E. Each frame begins and ends with a frame delimiter. A frame delimiter at the end of a frame may also mark the start of the next frame. A sequence of 7 or more consecutive 1-bits within a frame will cause the frame to be aborted.

When no frames are being transmitted on a simplex or full-duplex synchronous link, a frame delimiter is continuously transmitted on the link. Using the standard NRZI encoding from bits to line levels (0 bit = transition, 1 bit = no transition), this generates one of two continuous waveforms, depending on the initial state:

This is used by modems to train and synchronize their clocks via phase-locked loops. Some protocols allow the 0-bit at the end of a frame delimiter to be shared with the start of the next frame delimiter, i.e. '011111101111110'.

For half-duplex or multi-drop communication, where several transmitters share a line, a receiver on the line will see continuous idling 1-bits in the inter-frame period when no transmitter is active.

Actual binary data could easily have a sequence of bits that is the same as the flag sequence. So the data's bit sequence must be modified so that it doesn't appear to be a frame delimiter.

Synchronous framing

On synchronous links, this is done with bit stuffing. Any time that 5 consecutive 1-bits appear in the transmitted data, the data is paused and a 0-bit is transmitted. This ensures that no more than 5 consecutive 1-bits will be sent. The receiving device knows this is being done, and after seeing 5 1-bits in a row, a following 0-bit is stripped out of the received data. If the following bit is a 1-bit, the receiver has found a flag.

This also (assuming NRZI with transition for 0 encoding of the output) provides a minimum of one transition per 6 bit times during transmission of data, and one transition per 7 bit times during transmission of flag, so the receiver can stay in sync with the transmitter. Note however, that for this purpose encodings such as 8b/10b encoding are better suited.

HDLC transmits bytes of data with the least significant bit first (little-endian order).

Asynchronous framing

When using asynchronous serial communication such as standard RS-232 serial ports, bits are sent in groups of 8, and bit-stuffing is inconvenient. Instead they use "control-octet transparency", also called "byte stuffing" or "octet stuffing". The frame boundary octet is 01111110, (7E in hexadecimal notation). A "control escape octet", has the bit sequence '01111101', (7D hexadecimal). If either of these two octets appears in the transmitted data, an escape octet is sent, followed by the original data octet with bit 5 inverted. For example, the data sequence "01111110" (7E hex) would be transmitted as "01111101 01011110" ("7D 5E" hex). Other reserved octet values (such as XON or XOFF) can be escaped in the same way if necessary.

Structure

The contents of an HDLC frame, including the flag, are

Flag	Address	Control	Information	FCS	Flag
8 bits	8 or more bits	8 or 16 bits	Variable length, 0 or more bits	16 or 32 bits	8 bits

Note that the end flag of one frame may be (but does not have to be) the beginning (start) flag of the next frame.

Data is usually sent in multiples of 8 bits, but only some variants require this; others theoretically permit odd data sizes.

The FCS is the Frame Check Sequence, a cyclic redundancy check computed over the entire frame, including the 'Address', 'Control' and 'Information' fields. The calculation is designed to detect errors in the transmission of the frame — lost bits, flipped bits, extraneous bits — so that the frame can be dropped by the receiver if an error is detected. It is this method of detecting errors that can set an upper bound on the size of the data portion of the frame. Essentially, the longer the length of the data portion of the frame becomes, the harder it is to guarantee that certain types of transmission errors will be found. The Frame Check Sequence is either a 16-bit CRC-CCITT or a 32-bit CRC-32.

The FCS is needed to detect transmission errors. When HDLC was designed, long-haul digital media were designed for telephone systems, which only need a bit error rate (BER) less than or equal to one erroneous bit per 1×10⁺⁵ bits transfered (BER = 1×10⁻⁵). Digital data for computers normally requires a bit error rate less than or equal to one erroneous bit per 1×10⁺¹² bits transfered (BER = 1×10⁻¹²). By checking the FCS, the receiver can discover bad data. If the data is ok, it sends an "acknowledge" packet back to the sender. The sender can then send the next frame. If the receiver sends a "negative acknowledge" or simply drops the bad frame, the sender either receives the negative acknowledge, or runs into its time limit while waiting for the acknowledge. It then retransmits the failed frame. Modern optical networks have reliability substantially better than a BER of 1×10⁻¹⁵, but that simply makes HDLC even more reliable.

Types of Stations (Computers), and Data Transfer Modes

SDLC was originally designed to connect one computer with multiple peripherals. The original "normal response mode" is a master-slave mode where the computer (or primary terminal) gives each peripheral (secondary terminal) permission to speak in turn. Because all communication is either to or from the primary terminal, frames include only one address, that of the secondary terminal; the primary terminal is not assigned an address. There is also a strong distinction between commands sent by the primary to a secondary, and responses sent by a secondary to the primary. Commands and responses are in fact indistinguishable; the only difference is the direction in which they are transmitted.

Normal response mode allows operation over half-duplex communication links, as long as the primary is aware that it may not transmit when it has given permission to a secondary.

Asynchronous response mode is an HDLC addition for use over full-duplex links. While retaining the primary/secondary distinction, it allows the secondary to transmit at any time.

Asynchronous balanced mode added the concept of a combined terminal which can act as both a primary and a secondary. There are some subtleties about this mode of operation; while many features of the protocol do not care whether they are in a command or response frame, some do, and the address field of a received frame must be examined to determine whether it contains a command (the address received is ours) or a response (the address received is that of the other terminal).

Some HDLC variants extend the address field to include both source and destination addresses, or an explicit command/response bit.

HDLC Operations, and Frame Types

There are three fundamental types of HDLC frames.

• Information frames, or I-frames, transport user data from the network layer. In addition they can also include flow and error control information piggybacked on data.
• Supervisory Frames, or S-frames, are used for flow and error control whenever piggybacking is impossible or inappropriate, such as when a station does not have data to send. S-frames do not have information fields.
• Unnumbered frames, or U-frames, are used for various miscellaneous purposes, including link management. Some U-frames contain an information field, depending on the type.

The general format of the control field is:

HDLC control fields

7	6	5	4	3	2	1	0
N(R) Receive sequence no.			P/F	N(S) Send sequence no.			0	I-frame
N(R) Receive sequence no.			P/F	type		0	1	S-frame
type			P/F	type		1	1	U-frame

There are also extended (2-byte) forms of I and S frames. Again, the least significant bit (rightmost in this table) is sent first.

Extended HDLC control fields

15	14	13	12	11	10	9	8		7	6	5	4	3	2	1	0
N(R) Receive sequence no.							P/F		N(S) Send sequence no.							0	Extended I-frame
N(R) Receive sequence no.							P/F		0	0	0	0	type		0	1	Extended S-frame

The P/F bit

Poll/Final is a single bit with two names. It is called Poll when set by the primary station to obtain a response from a secondary station, and Final when set by the secondary station to indicate a response or the end of transmission. In all other cases, the bit is clear.

The bit is used as a token that is passed back and forth between the stations. Only one token should exist at a time. The secondary only sends a Final when it has received a Poll from the primary. The primary only sends a Poll when it has received a Final back from the secondary, or after a timeout indicating that the bit has been lost.

• In NRM, possession of the poll token also grants the addressed secondary permission to transmit. The secondary sets the F-bit in its last response frame to give up permission to transmit.
• In ARM and ABM, the P/F bits force a response. In these modes, the secondary need not wait for a poll to transmit, so need not wait to respond with a final bit.
• If no response is received to a P bit in a reasonable period of time, the primary station times out and sends P again.
• The P/F bit is at the heart of the basic checkpoint retransmission scheme that is required to implement HDLC; all other variants (such as the REJ S-frame) are optional and only serve to increase effeiciency. Whenever a station receives a P/F bit, it may assume that any frames that it sent before it last transmitted the P/F bit and not yet acknowledged will never arrive, and so should be retransmitted.

When operating as a combined station, it is important to maintain the distinction between P and F bits, because there may be two checkpoint cycles operating simultaneously. A P bit arriving in a command from the remote station is not in response to our P bit; only an F bit arriving in a response is.

N(R), the receive sequence number

Both I and S frames contain a receive sequence number N(R). N(R) provides a positive acknowledgement for the receipt of I-frames from the other side of the link. Its value is always the first frame not received; it acknowledges that all frames with N(S) values up to N(R)-1 (modulo 8 or modulo 128) have been received and indicates the N(S) of the next frame it expects to receive.

N(R) operates the same way whether it is part of a command or response. A combined station only has one sequence number space.

I-Frames (user data)

Information frames, or I-frames, transport user data from the network layer. In addition they also include flow and error control information piggybacked on data. The subfields in the control field define these functions.

The least significant bit (first transmitted) defines the frame type. 0 means an I-frame.

N(S) defines the sequence number of send frame. This is incremented for successive I-frames, modulo 8 or modulo 128. Depending on the number of bits in the sequence number, up to 7 or 127 I-frames may be awaiting acknowledgement at any time.

The P/F and N(R) fields operate as described above. Except for the interpretation of the P/F field, there is no difference between a command I frame and a response I frame; when P/F is 0, the two forms are exactly equivalent.

S-Frames (control)

Supervisory Frames, or S-frames, are used for flow and error control whenever piggypacking is impossible or inappropriate, such as when a station does not have data to send. S-frames do not have information fields.

The S-frame control field includes a leading "10" indicating that it is an S-frame. This is followed by a 2-bit type, a poll/final bit, and a sequence number. If 7-bit sequence numbers are used, there is also a 4-bit padding field.

The first 2 bits mean it is an S-frame. All S frames include a P/F bit and a receive sequence number as described above. Except for the interpretation of the P/F field, there is no difference between a command S frame and a response S frame; when P/F is 0, the two forms are exactly equivalent.

The 2-bit type field encodes the type of S frame.

Receive Ready (RR)

• Indicate that the sender is ready to receive more data (cancels the effect of a previous RNR).
• Send this packet if you need to send a packet but have no I frame to send.
• A primary station can send this with the P-bit set to solicit data from a secondary station.
• A secondary terminal can use this with the F-bit set to respond to a poll if it has no data to send.

Receive Not Ready (RNR)

• Acknowledge some packets and request no more be sent until further notice.
• Can be used like RR with P bit set to solicit the status of a secondary station.
• Can be used like RR with F bit set to respond to a poll if the station is busy.

Reject (REJ)

• Requests immediate retransmission starting with N(R).
• Sent in response to an observed sequence number gap. After seeing I1/I2/I3/I5, send REJ4.
• Optional to generate; a working implementation can use only RR.

Selective Reject (SREJ)

• Requests retransmission of only the frame N(r).
• Not supported by all HDLC variants.
• Optional to generate; a working implementation can use only RR, or only RR and REJ.

U-Frames

Unnumbered frames, or U-frames, are used for link management, and can also be used to transfer user data. They exchange session management and control information between connected devices, and some U-frames contain an information field, used for system management information or user data.

The first 2 bits (11) mean it is a U-frame. The 5 type bits (2 before P/F bit and 3 bit after P/F bit) can create 32 different types of U-frame

• Mode settings (SNRM, SNRME, SARM, SARME, SABM, SABME, UA, DM, RIM, SIM, RD, DISC)
• Information Transfer (UP, UI)
• Recovery (FRMR, RSET)
  • Invalid Control Field
  • Data Field Too Long
  • Data field not allowed with received Frame Type
  • Invalid Receive Count
• Miscellaneous (XID, TEST)

Link Configurations

Link configurations can be categorized as being either:

• Unbalanced, which consists of one primary terminal, and one or more secondary terminals.
• Balanced, which consists of two peer terminals.

The three link configurations are:

• Normal Response Mode (NRM) is an unbalanced configuration in which only the primary terminal may initiate data transfer. The secondary terminal transmits data only in response to commands from the primary terminal. The primary terminal polls the secondary terminal(s) to determine whether they have data to transmit, and then selects one to transmit.
• Asynchronous Response Mode (ARM) is an unbalanced configuration in which secondary terminals may transmit without permission from the primary terminal. However, the primary terminal still retains responsibility for line initialization, error recovery, and logical disconnect.
• Asynchronous Balanced Mode (ABM) is a balanced configuration in which either station may initiate the transmission.

An additional link configuration is Disconnected mode. This is the mode that a secondary station is in before it is initialized by the primary, or when it is explicitly disconnected. In this mode, the secondary responds to almost every frame other than a mode set command with a "Disconnected mode" response. The purpose of this mode is to allow the primary to reliably detect a secondary being powered off or otherwise reset.

HDLC Command and response repertoire

• Commands (BALA, I, RR, RNR, (SNRM or SARM or SABM) DISC
• Responses (I, RR, RNR, UA, DM, FRMR)

Basic Operations

• Initialization can be requested by either side. When the six-mode set-command is issued. This command:
  • Signals the other side that initialization is requested
  • Specifies the mode, NRM, ABM, ARM
  • Specifies whether 3 or 7 bit sequence numbers are in use.

The HDLC module on the other end transmits (UA) frame when the request is accepted. And if the request is rejected it sends (DM) disconnect mode frame.

Functional Extensions (Options)

• For Switched Circuits
  • Commands: ADD - XID
  • Responses: ADD - XID, RD
• For 2-way Simultaneous commands & responses are ADD - REJ
• For Single Frame Retransmission commands & responses: ADD - SREJ
• For Information Commands & Responses: ADD - Ul
• For Initialization
  • Commands: ADD - SIM
  • Responses: ADD - RIM
• For Group Polling
  • Commands: ADD - UP
• Extended Addressing
• Delete Response I Frames
• Delete Command I Frames
• Extended Numbering
• For Mode Reset (ABM only) Commands are: ADD - RSET
• Data Link Test Commands & Responses are: ADD - TEST
• Request Disconnect. Responses are ADD - RD
• 32-bit FCS

HDLC Command/Response Repertoire

Type Of Frame	Name	Command/ Response	Description	Info	C-Field Format
					7	6	5	4	3	2	1	0
Information(I)		C/R	User exchange data		N(R)			P/F	N(S)			0
Supervisory (S)	Receive Ready (RR)	C/R	Positive Acknowledgement	Ready to receive I-frame N(R)	N(R)			P/F	0	0	0	1
	Receive Not Ready (RNR)	C/R	Positive Acknowledgement	Not ready to receive	N(R)			P/F	0	1	0	1
	Reject (REJ)	C/R	Negative Acknowledgement	Retransmit starting with N(R)	N(R)			P/F	1	0	0	1
	Selective Reject (SREJ)	C/R	Negative Acknowledgement	Retransmit only N(R)	N(R)			P/F	1	1	0	1

Unnumbered Frames

Unnumbered frames are identified by the low two bits being 1. With the P/F flag, that leaves 5 bits as a frame type. Even though fewer than 32 values are in use, some types have different meanings depending on the direction they are sent: as a request or as a response. The relationship between the DISC (disconnect) command and the RD (request disconnect) response seems clear enough, but the reason for making SARM command numerically equal to the DM response is obscure.

Name	Command/ Response	Description	Info	C-Field Format
				7	6	5	4	3	2	1	0
Set normal response SNRM	C	Set mode	Use 3 bit sequence number	1	0	0	P	0	0	1	1
Set normal response extended mode SNRME	C	Set mode; extended	Use 7 bit sequence number	1	1	0	P	1	1	1	1
Set asynchronous response SARM	C	Set mode	Use 3 bit sequence number	0	0	0	P	1	1	1	1
Set asynchronous response extended mode SARME	C	Set mode; extended	Use 7 bit sequence number	0	1	0	P	1	1	1	1
Set asynchronous balanced mode SABM	C	Set mode	Use 3 bit sequence number	0	0	1	P	1	1	1	1
Set asynchronous balanced extended mode SABME	C	Set mode; extended	Use 7 bit sequence number	0	1	1	P	1	1	1	1
Set initialization mode SIM	C	Initialize link control function	in the addressed station	0	0	0	P	0	1	1	1
Disconnect DISC	C	Terminate logical link connection	Future I and S frames return DM	0	1	0	P	0	0	1	1
Unnumbered Acknowledgment UA	R	Acknowledge acceptance	of one of the set-mode commands.	0	1	1	F	0	0	1	1
Disconnect Mode DM	R	Responder in Disconnect Mode	mode set required	0	0	0	F	1	1	1	1
Request Disconnect RD	R	Solicitation for DISC Command		0	1	0	F	0	0	1	1
Request Initialization Mode RIM	R	Initialization needed	Request for SIM command	0	0	0	F	0	1	1	1
Unnumbered Information UI	C/R	Unacknowledged data	has a payload	0	0	0	P/F	0	0	1	1
Unnumbered Poll (UP)	C	Used to solicit	control information	0	0	1	P	0	0	1	1
Reset RSET	C	Used for recovery	Resets N(R) but not N(S)	1	0	0	P	1	1	1	1
Exchange Identification XID	C/R	Used to Request/	Report capabilities	1	0	1	P/F	1	1	1	1
Test TEST	C/R	Exchange identical information	fields for testing	1	1	1	P/F	0	0	1	1
Frame Reject FRMR	R	Report receipt	of unacceptable frame	1	1	0	F	0	1	1	1

The UI, XID and TEST frames contain a payload, and can be used as both commands and responses.

• A UI frame contains user information, but unlike an I frame it is not acknowledged or retransmitted if lost.
• The XID frame is used to exchange terminal capabilities. IBM Systems Network Architecture defined one format, but the variant defined in ISO 8885 is more commonly used. A primary advertisies its capabilities with an XID command, and a secondary returns an XID response.
• The TEST frame is simply a ping command for debugging purposes. The payload of the TEST command is returned in the TEST response.

See also

• PPP, SLIP

External links

• RFC 2687, Proposed Standard, PPP in a Real-time Oriented HDLC-like Framing
• RFC 1662, standard 51, PPP in HDLC-like Framing
• HDLC information page
• Data Communication Lectures of Manfred Lindner - Part HDLC

Bibliography

• Computer Communications (course notes) by Chaim Zieglier PhD, Brooklyn College.
• Data and Computer communications, by Willam Stallings. (Seventh Edition, Prentice Hall)