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`High-Level Data Link Control - Wikipedia
`
`High-Level Data Link Control
`
`High-Level Data Link Control (HDLC) is a
`communication protocol used for transmitting data
`between devices
`in
`telecommunication
`and
`networking. Developed by
`the
`International
`Organization for Standardization (ISO), it is defined
`in the standard ISO/IEC 13239:2002.
`
`HDLC ensures reliable data transfer, allowing one
`device to understand data sent by another. It can
`operate with or without a continuous connection
`between devices, making it versatile for various
`network configurations.
`
`High-Level Data Link Control
`Communication protocol
`Abbreviation HDLC
`Purpose
`Data framing
`Developer(s) International Organization for
`Standardization (ISO)
`Introduction 1979
`Based on
`SDLC
`OSI layer
`Data link layer
`
`Originally, HDLC was used in multi-device networks, where one device acted as the master and others as
`slaves, through modes like Normal Response Mode (NRM) and Asynchronous Response Mode (ARM).
`These modes are now rarely used. Currently, HDLC is primarily employed in point-to-point connections,
`such as between routers or network interfaces, using a mode called Asynchronous Balanced Mode (ABM).
`
`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 (Link Access Procedure), while
`ANSI named their essentially identical version ADCCP.
`
`The HDLC specification does not specify the full semantics of the frame fields. This allows other fully
`compliant standards to be derived from it, and 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.
`
`The original ISO standards for HDLC are the following:
`
`ISO 3309-1979 – Frame Structure
`ISO 4335-1979 – Elements of Procedure
`ISO 6159-1980 – Unbalanced Classes of Procedure
`ISO 6256-1981 – Balanced Classes of Procedure
`ISO/IEC 13239:2002, the current standard, replaced all of these specifications.
`
`HDLC was the inspiration for the IEEE 802.2 LLC protocol, and it is the basis for the framing mechanism
`used with the PPP on synchronous lines, as used by many servers to connect to a WAN, most commonly
`the Internet.
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`A similar version is used as the control channel for E-carrier (E1) and SONET multichannel telephone
`lines. Cisco HDLC uses low-level HDLC framing techniques but adds a protocol field to the standard
`HDLC header.
`
`Framing
`
`HDLC frames can be transmitted over synchronous or asynchronous serial communication 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 unique sequence of bits as a frame delimiter, or flag, and
`encoding the data to ensure that the flag sequence is never seen inside a frame. 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.
`
`On both synchronous and asynchronous links, the flag sequence is binary "01111110", or hexadecimal
`0x7E, but the details are quite different.
`
`Synchronous framing
`Because a flag sequence consists of six consecutive 1-bits, other data is coded to ensure that it never
`contains more than five 1-bits in a row. This is done by bit stuffing: any time that five consecutive 1-bits
`appear in the transmitted data, the data is paused and a 0-bit is transmitted.
`
`The receiving device knows that this is being done, and after seeing five 1-bits in a row, a following 0-bit is
`stripped out of the received data. If instead the sixth bit is 1, this is either a flag (if the seventh bit is 0), or
`an error (if the seventh bit is 1). In the latter case, the frame receive procedure is aborted, to be restarted
`when a flag is next seen.
`
`This bit-stuffing serves a second purpose, that of ensuring a sufficient number of signal transitions. On
`synchronous links, the data is NRZI encoded, so that a 0-bit is transmitted as a change in the signal on the
`line, and a 1-bit is sent as no change. Thus, each 0 bit provides an opportunity for a receiving modem to
`synchronize its clock via a phase-locked loop. If there are too many 1-bits in a row, the receiver can lose
`count. Bit-stuffing provides a minimum of one transition per six bit times during transmission of data,
`and one transition per seven bit times during transmission of a flag.
`
`When no frames are being transmitted on a simplex or full-duplex synchronous link, a frame delimiter is
`continuously transmitted on the link. This generates one of two continuous waveforms, depending on the
`initial state:
`
`The HDLC specification allows the 0-bit at the end of a frame delimiter to be shared with the start of the
`next frame delimiter, i.e. "011111101111110". Some hardware does not support this.
`
`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.
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`HDLC transmits bytes of data with the least significant bit first (not to be confused with little-endian
`order, which refers to byte ordering within a multi-byte field).
`
`Asynchronous framing
`When using asynchronous serial communication such as standard RS-232 serial ports, synchronous-style
`bit stuffing is inappropriate for several reasons:
`
`Bit stuffing is not needed to ensure an adequate number of transitions, as start and stop bits provide
`that,
`Because the data is NRZ encoded for transmission, rather than NRZI encoded, the encoded
`waveform is different,
`RS-232 sends bits in groups of 8, making adding single bits very awkward, and
`For the same reason, it is only necessary to specially code flag bytes; it is not necessary to worry
`about the bit pattern straddling multiple bytes.
`Instead asynchronous framing uses "control-octet transparency", also called "byte stuffing" or "octet
`stuffing". The frame boundary octet is 01111110, (0x7E in hexadecimal notation). A "control escape octet",
`has the value 0x7D (bit sequence '10111110', as RS-232 transmits least-significant bit first). 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 byte 0x7E would be transmitted as 0x7D 0x5E ("10111110 01111010").
`Other reserved octet values (such as XON or XOFF) can be escaped in the same way if necessary.
`
`The "abort sequence" 0x7D 0x7E ends a packet with an incomplete byte-stuff sequence, forcing the
`receiver to detect an error. This can be used to abort packet transmission with no chance the partial packet
`will be interpreted as valid by the receiver.
`
`Structure
`
`The contents of an HDLC frame are shown in the following table:
`
`Flag
`8 bits
`
`Address
`8 or more bits
`
`Information
`Control
`8 or 16 bits Variable length, 8×n bits
`
`FCS
`16 or 32 bits
`
`Flag
`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
`data alignments on other than 8-bit boundaries.
`
`The frame check sequence (FCS) is a 16-bit CRC-CCITT or a 32-bit CRC-32 computed over the Address,
`Control, and Information fields. It provides a means by which the receiver can detect errors that may have
`been induced during the transmission of the frame, such as lost bits, flipped bits, and extraneous bits.
`However, given that the algorithms used to calculate the FCS are such that the probability of certain types
`of transmission errors going undetected increases with the length of the data being checked for errors, the
`FCS can implicitly limit the practical size of the frame.
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`If the receiver's calculation of the FCS does not match that of the sender's, indicating that the frame
`contains errors, the receiver can either send a negative acknowledge packet to the sender, or send nothing.
`After either receiving a negative acknowledge packet or timing out waiting for a positive acknowledge
`packet, the sender can retransmit the failed frame.
`
`The FCS was implemented because many early communication links had a relatively high bit error rate,
`and the FCS could readily be computed by simple, fast circuitry or software. More effective forward error
`correction schemes are now widely used by other protocols.
`
`Types of stations (computers) and data transfer modes
`
`Synchronous Data Link Control (SDLC) was originally designed to connect one computer with multiple
`peripherals via a multidrop bus. The original "normal response mode" is a primary-secondary 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 a
`distinction between commands sent by the primary to a secondary, and responses sent by a secondary
`to the primary, but this is not reflected in the encoding; commands and responses are indistinguishable
`except for the difference in the direction in which they are transmitted.
`
`Normal response mode allows the secondary-to-primary link to be shared without contention, because
`it has the primary give the secondaries permission to transmit one at a time. It also allows operation over
`half-duplex communication links, as long as the primary is aware that it may not transmit when it has
`permitted a secondary to do so.
`
`Asynchronous response mode is an HDLC addition[1] for use over full-duplex links. While retaining
`the primary/secondary distinction, it allows the secondary to transmit at any time. Thus, there must be
`some other mechanism to ensure that multiple secondaries do not try to transmit at the same time (or
`only one secondary).
`
`Asynchronous balanced mode adds the concept of a combined terminal which can act as both a
`primary and a secondary. Unfortunately, this mode of operation has some implementation subtleties.
`While the most common frames sent do not care whether they are in a command or response frame, some
`essential ones do (notably most unnumbered frames, and any frame with the P/F bit set), 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).
`
`This means that the address field is not optional, even on point-to-point links where it is not needed to
`disambiguate the peer being talked to. 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
`
`Three fundamental types of HDLC frames may be distinguished:
`
`Information frames, or I-frames, transport user data from the network layer. They can also include flow
`and error control information piggybacked on data.
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`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.
`
`Control field
`The general format of the control field is:
`
`HDLC control fields
`4
`3
`2
`N(S)
`Send sequence no.
`
`1
`
`0
`
`0
`
`I-frame
`
`7
`
`5
`
`6
`N(R)
`Receive sequence no. P/F
`N(R)
`Receive sequence no. P/F
`type
`P/F
`
`type
`
`type
`
`0
`
`1
`
`1 S-frame
`
`1 U-frame
`
`There are also extended (two-byte) forms of I and S frames. Again, the least significant bit (rightmost in
`this table) is sent first.
`
`15
`
`14
`
`13
`
`11
`
`10
`
`12
`N(R)
`Receive sequence no.
`N(R)
`Receive sequence no.
`
`9
`
`Extended HDLC control fields
`8
`7
`6
`5
`4
`3
`N(S)
`Send sequence no.
`
`P/F
`
`2
`
`1
`
`0
`
`0
`
`Extended I-frame
`
`P/F
`
`0
`
`0
`
`0
`
`0
`
`type
`
`0
`
`1 Extended S-frame
`
`P/F bit
`Poll/Final is a single bit with two names. It is called Poll when part of a command (set by the primary
`station to obtain a response from a secondary station), and Final when part of a response (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. (It is equivalent to
`the word "Over" in radio voice procedure.)
`In ARM and ABM, the P bit forces a response. In these modes, the secondary need not wait for a poll
`to transmit, so the final bit may be included in the first response after the poll.
`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
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`efficiency. 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 yet
`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.
`
`N(S), the sequence number of the sent 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 acknowledgment at any time.
`
`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 sub-fields in the control field define
`these functions.
`
`The least significant bit (first transmitted) defines the frame type. 0 means an I-frame. 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 piggybacking is
`impossible or inappropriate, such as when a station does not have data to send. S-frames in HDLC do not
`have information fields, although some HDLC-derived protocols use information fields for "multi-
`selective reject".
`
`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 3-bit sequence number. (Or a 4-bit padding field followed by a 7-bit sequence
`number.)
`
`The first (least significant) 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.
`
`Receive Ready (RR)
`Bit value = 00 (0x00 to match above table type field bit order[2])
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`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)
`Bit value = 01 (0x04 to match above table type field bit order[3])
`Acknowledge some packets but 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)
`Bit value = 10 (0x08 to match above table type field bit order[4])
`Requests immediate retransmission starting with N(R).
`Sent in response to an observed sequence number gap; e.g. after seeing I1/I2/I3/I5, send REJ4.
`Optional to generate; a working implementation may use only RR.
`
`Selective Reject (SREJ)
`Bit value = 11 (0x0c to match above table type field bit order)
`Requests retransmission of only the frame N(R).
`Not supported by all HDLC variants.
`Optional to generate; a working implementation may use only RR, or only RR and REJ.
`
`U-Frames
`Unnumbered frames, or U-frames, are primarily used for link management, although a few are 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 five type bits (2 before P/F bit and 3 bit after P/F
`bit) can create 32 different types of U-frame. In a few cases, the same encoding is used for different things
`as a command and a response.
`
`Mode setting
`The various modes are described in § Link configurations. Briefly, there are two non-operational modes
`(initialization mode and disconnected mode) and three operational modes (normal response,
`asynchronous response, and asynchronous balanced modes) with 3-bit or 7-bit (extended) sequence
`numbers.
`
`Disconnected mode (DM) response
`When the secondary is disconnected (the default state on power-up), it sends this generic response
`to any poll (command frame with the poll flag set) except an acceptable mode setting command. It
`may alternatively give a FRMR response to an unacceptable mode set command.
`Unnumbered acknowledge (UA) response
`This is the secondary's response to an acceptable mode set command, indicating that it is now in
`the requested mode.
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`Set ... mode (SNRM, SARM, SABM) command
`Place the secondary in the specified mode, with 3-bit sequence numbers (1-byte control field). The
`secondary acknowledges with UA. If the secondary does not implement the mode, it responds with
`DM or FRMR.
`Set ... mode extended (SNRME, SARME, SABME) command
`Place the secondary in the specified mode, with 7-bit sequence numbers (2-byte control field).
`Set mode (SM) command
`Generic mode set, new in ISO/IEC 13239, using an information field to select parameters. ISO/IEC
`13239 added many additional options to HDLC, including 15- and 31-bit sequence numbers, which
`can only be selected with this command.
`Disconnect (DISC) command
`This command causes the secondary to acknowledge with UA and disconnect (enter disconnected
`mode). Any unacknowledged frames are lost.
`Request disconnect (RD) response
`This response requests the primary to send a DISC command. The primary should do so promptly,
`but may delay long enough to ensure all pending frames are acknowledged.
`Set initialization mode (SIM) command
`This rarely-implemented command is used to perform some secondary-specific initialization, such as
`downloading firmware. What happens in initialization mode is not otherwise specified in the HDLC
`standard.
`Request initialization mode (RIM) response
`This requests the primary to send SIM and initialize the secondary. It sent in lieu of DM if the
`secondary requires initialization.
`
`Information transfer
`These frames may be used as part of normal information transfer.
`
`Unnumbered information (UI)
`This frame (command or response) communicates user data, but without acknowledgement or
`retransmission in case of error.
`UI with header check (UIH)
`This frame (command or response), a ISO/IEC 13239 addition and rarely used, is like UI but also
`excludes CRC protection. Only a configurable-length prefix ("header") of the frame is covered by the
`CRC polynomial; errors in the rest of the frame are not detected.
`Unnumbered poll (UP) command
`This command solicits a response from the secondary. With the poll bit set, it acts like any other poll
`frame, without the acknowledgement that must be included in I or S frame. With the poll bit clear, it
`has a special meaning in normal response mode: the secondary may respond, even though it has
`not received the poll bit. This is rarely used in HDLC, but was used in the original IBM SDLC as a
`substitute for the lack of asynchronous response mode; where the communication channel could
`handle simultaneous responses, the primary would periodically send UP to the broadcast address to
`collect any pending responses.
`
`Error Recovery
`
`Frame reject (FRMR) response
`The FRMR response contains a description of the unacceptable frame, in a standardized format.
`The first 1 or 2 bytes are a copy of the rejected control field, the next 1 or 2 contain the secondary's
`current send and receive sequence numbers (and a flag indicating that the frame was a response,
`applicable only in balanced mode), and the following 4 or 5 bits are error flags indicating the reason
`for the rejection. The secondary repeats the same FRMR response to every poll until the error is
`cleared by a mode set command or RSET. The error flags are:
`W: the frame type (control field) is not understood or not implemented.
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`X: the frame type is not understood with a non-empty information field, but one was present.
`Y: the frame included an information field that is larger than the secondary can accept.
`Z: the frame included an invalid receive sequence number N(R), one which is not between the
`previously received value and the highest sequence number transmitted. (This error cannot be
`cleared by receiving RSET, but can be cleared by sending RSET.)
`V: the frame included an invalid send sequence number N(S), greater than the last number
`acknowledged plus the transmit window size. This error is only possible if a transmit window size
`smaller than the maximum has been negotiated.
`The error flags are normally padded with 0 bits to an 8-bit boundary, but HDLC permits frames
`which are not a multiple of a byte long.
`
`Reset (RSET) command
`The RSET command causes a secondary to reset its receive sequence number so the next
`expected frame is sequence number 0. This is a possible alternative to sending a new mode set
`command, which resets both sequence numbers. It is acknowledged with UA, like a mode set
`command.
`
`Peer discovery
`
`Exchange identification (XID)
`An XID command includes an information field specifying the primary's capabilities; the secondary
`responds with an XID response specifying its capabilities. This is normally done before sending a
`mode set command. Systems Network Architecture defined one format for the information field, in
`which the most significant bit of the first byte is clear (0), but HDLC implementations normally
`implement the variant defined in ISO 8885, which has the most significant bit of the first byte set (1).
`TEST
`A TEST command is simply a ping command for debugging purposes. The payload of the TEST
`command is returned in the TEST response.
`
`Defined in other standards
`There are several U frames which are not part of HDLC, but defined in other related standards.
`
`Nonreserved (NR0, NR1, NR2, NR3)
`The "nonreserved" commands and responses are guaranteed by the HDLC standard to be available
`for other uses.
`Ack connectionless (AC0, AC1)
`These are defined in the IEEE 802.2 logical link control standard.
`Configure (CFGR)
`This command was defined in SDLC for debugging. It had a 1-byte payload which specified a non-
`standard test mode for the secondary. Even numbers disabled the mode, while odd numbers
`enabled it. A payload of 0 disabled all test modes. The secondary normally acknowledged a
`configure command by echoing it in response.
`Beacon (BCN) response
`This response was defined in SDLC to indicate a communications failure. A secondary which
`received no frames at all for a long time would begin sending a stream of beacon responses,
`allowing a unidirectional fault to be located. Note that ISO/IEC 13239 assigns UIH the same
`encoding as BCN.
`
`Link configurations
`
`Link configurations can be categorized as being either:
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`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 terminals transmit data only in response to commands from the
`primary terminal. The primary terminal polls each secondary terminal to give it an opportunity to
`transmit any data it has.
`Asynchronous Response Mode (ARM) is an unbalanced configuration in which secondary terminals
`may transmit without permission from the primary terminal. However, there is still a distinguished
`primary terminal which retains responsibility for line initialization, error recovery, and logical
`disconnect.
`Asynchronous Balanced Mode (ABM) is a balanced configuration in which either station may initialize,
`supervise, recover from errors, and send frames at any time. There is no master/slave relationship.
`The DTE (data terminal equipment) and DCE (data circuit-terminating equipment) are treated as
`equals. The initiator for Asynchronous Balanced Mode sends an SABM.
`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
`
`The minimal set required for operation are:
`
`Commands: I, RR, RNR, DISC, and one of SNRM, SARM or SABM
`Responses: I, RR, RNR, UA, DM, FRMR
`
`Basic operations
`Initialization can be requested by either side. When the primary sends one of the six mode-set
`commands, it:
`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. 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
`
`https://en.wikipedia.org/wiki/High-Level_Data_Link_Control
`
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`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 and response repertoire
`
`Type Of
`Frame
`
`Name
`
`Command/
`Response
`
`Description
`
`Info
`
`Information(I)
`
`Supervisory
`(S)
`
`Receive
`Ready (RR)
`
`Receive Not
`Ready (RNR)
`
`C/R
`
`C/R
`
`C/R
`
`Reject (REJ)
`
`C/R
`
`User exchange data
`
`Positive
`Acknowledgement
`
`Positive
`Acknowledgement
`
`Negative
`Acknowledgement
`
`Ready to
`receive I-frame
`N(R)
`Not ready to
`receive
`Retransmit
`starting with
`N(R)
`
`Selective
`Reject
`(SREJ)
`
`C/R
`
`Negative
`Acknowledgement
`
`Retransmit only
`N(R)
`
`C-Field Format
`5
`4
`3
`2
`P/F N(S)
`
`P/F
`
`P/F
`
`P/F
`
`P/F
`
`0
`
`0
`
`1
`
`1
`
`0
`
`1
`
`0
`
`1
`
`1
`
`0
`
`0
`
`0
`
`0
`
`0
`0
`
`1
`
`1
`
`1
`
`1
`
`6
`7
`N(R)
`
`N(R)
`
`N(R)
`
`N(R)
`
`N(R)
`
`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 command 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.
`
`https://en.wikipedia.org/wiki/High-Level_Data_Link_Control
`
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`High-Level Data Link Control - Wikipedia
`
`Name
`
`Command/
`Response
`
`Description
`
`Info
`
`Set normal response
`mode SNRM
`
`SNRM extended SNRME
`
`Set asynchronous
`response mode SARM
`
`SARM extended SARME
`
`C
`
`C
`
`C
`
`C
`
`Set mode
`
`Set mode; extended
`
`Set mode
`
`Set mode; extended
`
`Set mode
`
`C-Field Format
`5
`4
`3
`2
`
`0
`
`0
`
`0
`
`0
`
`1
`
`P
`
`P
`
`P
`
`P
`
`P
`
`0
`
`1
`
`1
`
`1
`
`1
`
`0
`
`1
`
`1
`
`1
`
`1
`
`1
`
`1
`
`1
`
`1
`
`1
`
`1
`
`0
`
`1
`
`1
`
`1
`
`1
`
`1
`
`7
`
`1
`
`1
`
`0
`
`0
`
`0
`
`6
`
`0
`
`1
`
`0
`
`1
`
`0
`
`Use 3 bit
`sequence number
`Use 7 bit
`sequence number
`Use 3 bit
`sequence number
`Use 7 bit
`sequence number
`Use 3 bit
`sequence number
`Use 7 bit
`sequence number
`New in ISO 13239
`Set mode, generic
`Initialize link control function in the
`addressed station
`
`Set mode; extended
`
`Initialization needed
`
`Request for SIM
`command
`Future I and S
`frames return DM
`
`0
`
`1
`
`0
`
`0
`
`0
`
`0
`
`1
`
`1
`
`0
`
`0
`
`1
`
`1
`
`1
`
`0
`
`0
`
`0
`
`0
`
`0
`
`P
`
`P
`
`P
`
`F
`
`P
`
`F
`
`1
`
`0
`
`0
`
`0
`
`0
`
`0
`
`1
`
`0
`
`1
`
`1
`
`0
`
`0
`
`1
`
`1
`
`1
`
`1
`
`1
`
`1
`
`1
`
`1
`
`1
`
`1
`
`1
`
`1
`
`Set asynchronous
`balanced mode SABM
`
`SABM extended SABME
`
`Set Mode SM
`Set initialization mode
`SIM
`Request initialization
`mode RIM
`
`Disconnect DISC
`
`C
`
`C
`
`C
`
`C
`
`R
`
`C
`
`R
`
`Request disconnect RD
`
`Unnumbered
`acknowledgment UA
`
`Disconnect mode DM
`
`Unnumbered information
`UI
`UI with header check UIH
`Unnumbered poll UP
`
`Reset RSET
`
`Terminate logical link
`connection
`Solicitation for DISC
`Command
`Acknowledge acceptance of one of the set-
`mode commands.
`Responder in
`disconnected mode
`
`Mode set required
`
`Unacknowledged data
`
`Has a payload
`
`New in ISO 13239
`Unacknowledged data
`Used to solicit control information
`Resets N(R) but
`not N(S)
`
`Used for recovery
`
`Used to Request/Report capabilities
`
`R
`
`R
`
`C/R
`
`C/R
`C
`
`C
`
`C/R
`
`0
`
`0
`
`0
`
`1
`0
`
`1
`
`1
`
`1
`
`0
`
`0
`
`1
`0
`
`0
`
`0
`
`1
`
`0
`
`F
`
`F
`
`0 P/F
`
`1 P/F
`1
`P
`
`0
`
`P
`
`1 P/F
`
`0
`
`1
`
`0
`
`1
`0
`
`1
`
`1
`
`0
`
`1
`
`0
`
`1
`0
`
`1
`
`1
`
`1
`
`1
`
`1
`
`1
`1
`
`1
`
`1
`
`1
`
`1
`
`1
`
`1
`1
`
`1
`
`1
`
`Exchange identification
`XID
`
`Test TEST
`
`Frame reject FRMR
`
`Nonreserved 0 NR0
`
`Nonreserved 1 NR1
`
`Nonreserved 2 NR2
`
`Nonreserved 3 NR3
`
`C/R
`
`R
`
`C/R
`
`C/R
`
`C/R
`
`C/R
`
`Not standardized
`
`Not st