Synchronous Data Link Control (SDLC)

The SYSTEMS NETWORK ARCHITECTURE (SNA), as it is defined and implemented by IBM, defines the division of all of the network functions into clearly defined layers. These layers provide many of the same functions as the Open Systems Interconnect (OSI) seven-layered architecture defined by the International Standards Organization (ISO). The SNA layers are not identical to the OSI layers and are not compatible or interoperable.
The DATA LINK CONTROL Layer provides the error-free movement of data between the NETWORK ADDRESSABLE UNITS (NAUs) within a given communication network via the SYNCHRONOUS DATA LINK CONTROL (SDLC) Protocol. The flow of information passes down from the higher layers through the DATA LINK CONTROL Layer and is passed into the PHYSICAL CONTROL Layer. It then passes into the communication links through some type of interface.
All NETWORK ADDRESSABLE UNITS (NAUs) are interconnected by some type of physical or logical link which is defined as the DATA LINK LAYER of the SNA Architecture. The DATA LINK CONTROL LAYER provides the capability to pass information between these associated NAUs using the SDLC Protocol. There are two basic types of links:

I/O CHANNEL COMMUNICATION links are usually hard-wired between co-located nodes (in the same room or facility). They normally operate at a very high data rate and are usually error free.

NON I/O CHANNEL COMMUNICATION links normally require some form of data communications equipment to pass information between nodes. SDLC Protocol is used to ensure error-free performance over these links, which may be either point-to-point (switched or non-switched) or multipoint configurations. Some specific equipment types use the SDLC loop transmission facilities to pass data.

Data Link Control Layer Functions
Communications links provide error-free paths for information to follow via the SDLC protocol. SDLC is a modified subset of the HIGH-LEVEL DATA LINK CONTROL (HDLC) protocol defined by the International Standards Groups CCITT and ISO.
The SDLC protocol is an UNBALANCED procedure that provides for the orderly control of multipoint communications activity in complex networks. This unbalanced procedure is identified as the NORMAL RESPONSE MODE (NRM).
The Data Link Control layer is the lower of the two layers that make up the Path Control Network. The Data Link Control layer communicates upward to the Path Control layer, passing data and control information in BASIC TRANSMISSION UNITS (BTUs).
SDLC is a bit-oriented protocol (BOP). It provides complete transparency, half-duplex [two-way alternate (TWA)] or full-duplex [two-way simultaneous (TWS)] operation, multipoint addressing, flow control, error detection and recovery, and the ability to send more than one message before needing a response.
A set of sequence numbers provides the acknowledgment, flow control, error recovery, and “window” functions. The window function allows the sending station to transmit more than one message before some form of response is received.
The normal method of sequence numbers uses a three-bit counter that rotates the count from 0 through 7 and repeats (Modulo 8). The maximum window size using the normal mode is 7. The extended mode uses a seven bit counter that rotates the count from 0 through 127 and repeats (Modulo 128). The maximum window size using the extended mode is 127.
SDLC is an unbalanced procedure that defines the operation and interaction between a primary station (HOST) and one or more secondary stations (TERMINALS). All of the information that flows between the host and the terminal devices attached to the link is passed using a set of COMMANDS and RESPONSES defined to provide the following functions:







Data Link Layer Activation Flow

Data Link Layer Disconnection

SDLC Frame Formats
All commands and responses are contained within an SDLC FRAME that provides all of the addressing, control, sequence numbering, flow control, and error detection required for reliable transmission of data across a LINK.





SDLC Frame Fields

The LEADING and TRAILING FLAG fields are delimiters which indicate the beginning and end of an SDLC frame. The flag is the only place where the unique 8-bit pattern of 01111110 occurs. The flag pattern is the idle condition on a full-duplex link. On a half-duplex link, the idle condition is all 1 bits (mark).
All non-flag bytes (octets) must obey a rule where, after five consecutive 1 bits are detected in a transmit string, the sending station must insert a bit position with the value of 0
The zero insertion/deletion process (bit stuffing) is performed on the ADDRESS, CONTROL, INFORMATION and FRAME CHECK SEQUENCE (FCS) fields. The receiving station must delete these inserted 0 bits before storing the data in memory. Seven or more consecutive 1 bits are considered an ABORT condition.
The first field after the leading flag is the ADDRESS field. It contains the link address of the secondary station that is sending or receiving the frame. The address field is normally a one-byte (octet) field.
Although not used by most networks, the architecture and protocol has a provision for an extended addressing capability. This 8-bit field provides the ability to identify 256 different secondary addresses. Two of these addresses are reserved for special purposes:
ADDRESS 00 (hexadecimal 00000000) is reserved as the “No Station” address and should not be used.
ADDRESS FF (hexadecimal 11111111) is used as a BROADCAST to identify any and all secondary stations located on a specific link. Not all devices support broadcast.
This field follows the address and is a one-byte field in the normal-mode (Modulo 8) operation. It also also identifies the function or TYPE of frame being transmitted. There are three basic types of frames:
The INFORMATION frame contains SNA headers and data for the higher layers of the Architecture.
The SUPERVISORY frames perform the DATA LINK layer flow control and error recovery.
The UNNUMBERED frames provide the LINK initialization and termination and also define some special frames used for identifying stations, testing functions and the rejection of invalid frames.
The normal mode uses two 3-bit counters in the INFORMATION frame to track the number of the frame that is being sent (Ns) and the number of the frame that is expected to be received next (Nr). The SUPERVISORY frames contain only the Nr counter.
The extended mode (Modulo 128) uses a two-byte CONTROL field in the SUPERVISORY and INFORMATION frames to allow the Nr and Ns counters to be seven bits in length and to rotate the count from 0 through 127, and then repeat. The SUPERVISORY frame contains only the Nr counter.

Control Field

Control Field: Format (Normal Mode)

The POLL bit in all commands. If the poll bit is set to 1 in a command, a response or INFORMATION frame is being solicited.

The FINAL bit in all responses. If the final bit is set to 1 in a response, it indicates that it is either the only frame sent or the last frame being sent in a series of frames.

Control Field: Supervisory Frame Types

RR – RECEIVE READY: This frame is used as a polling command by the primary station to solicit INFORMATION frames from the secondary station. The RECEIVE READY frame is an ACKNOWLEDGMENT RESPONSE used by the secondary station to indicate that the secondary is now ready to receive INFORMATION frames.
RNR – RECEIVE NOT READY: This frame is used as a flow control command or response to indicate that the station transmitting the RECEIVE NOT READY frame is not able to accept any INFORMATION frames at this time.
REJ – REJECT: This frame is sent by a STATION to indicate that it has received a frame out of the normal sequence. This may indicate the loss of an INFORMATION frame containing USER data.

Control Field: Unnumbered Frame Types

Command Frames

SNRM – SET NORMAL RESPONSE MODE: This command is sent from the primary station to a secondary station to place the SECONDARY in the initialized normal SDLC operating mode. It also resets all counters and timers to 0.
DISC – DISCONNECT: This COMMAND is sent from the primary station to a secondary station to place the SECONDARY station in the off-line disconnected mode.
SIM – SET INITIALIZATION MODE: This command is sent from the primary station to the secondary station to begin the initialization process.

Response Frames

UA – UNNUMBERED ACKNOWLEDGMENT: This response is sent from the secondary station to the primary station in response to a SNRM, DISC, or SIM command.
DM – DISCONNECT MODE: This response is sent from the secondary station to the primary station in response to any command other than SNRM or DISC.
RD – REQUEST DISCONNECT: This response is sent from the secondary to the primary station to request that the SECONDARY station be placed in the off-line or disconnect mode.
RIM – REQUEST INITIALIZATION MODE: this response is sent from the secondary to the primary station to request initialization.
FRMR – FRAME REJECT: This response is sent from the secondary station to the primary station to indicate that an abnormal condition has been detected or that an invalid frame has been received. It contains bits which indicate the reason for the rejection of the frame.

Command or Response Frames

XID – EXCHANGE IDENTIFICATION: This frame may be either a command sent by the primary station or a response sent by the secondary station. It contains information that is used to identify the SECONDARY station. This response is used primarily on the switched (dial-up) communications link.
TEST – TEST: This command is sent from the primary station to the secondary station and may contain some form of a message that may be used to test the Secondary ability to receive data and transmit the data back to the primary station. If the message is larger than the secondary stations receive buffer, the TEST RESPONSE will be returned to the primary station but the response frame will not contain any of the message.
UI – UNNUMBERED INFORMATION: this command allows the primary station to send data to a secondary station and the UNNUMBERED INFORMATION RESPONSE allows the secondary station to send data to the primary station. This mechanism is normally reserved for certain special functions and these frames are not protected by the normal frame-sequence numbering algorithm.

Control Field: Information Frame Types

INFORMATION: This frame contains the information and data relevant to the higher SNA architecture layers. INFO frames consist of several variable-length or optional fields, depending upon the implementation.
The INFO frame contains both the Ns and the Nr fields used for flow control and frame level acknowledgment. The normal INFORMATION field will contain a:
RU – REQUEST or RESPONSE UNIT field (optional) which is normally associated with the user’s data.
The contents of the INFO field are normally called the BASIC TRANSMISSION UNIT (BTU). This consists of the TH and a BASIC INFORMATION UNIT (BIU). The BIU consists of the RH and the RU. The TH and associated BIU is also known as a PATH INFORMATION UNIT (PIU).
The BTU may contain more than one PIU. The entire frame is called BASIC LINK UNIT (BLU).


Control Field: Frame Check Sequence

FRAME CHECK SEQUENCE: This is the last field in any SDLC frame. It is used by the sending and receiving stations to ensure that the message received is identical to the message that was sent.
The FCS field is a two-byte field (16 bits) that contains a CYCLIC REDUNDANCY CHECK (CRC) value that is used to verify the validity of all of the bits contained in the address, control, and information fields. All frames that do not contain a valid FCS must be discarded and ignored. All frames that contain a bit pattern of seven or more consecutive 1 bits must also be discarded as an ABORTED frame.

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