SDLC: Synchronous Data Link Control by IBM

The Synchronous Data Link Control (SDLC) protocol, an IBM data link layer protocol for use in the Systems Network Architecture (SNA) environment.

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. SDLC supports a variety of link types and topologies. It can be used with point-to-point and multipoint links, bounded and unbounded media, half-duplex and full-duplex transmission facilities, and circuit-switched and packet-switched networks.

SDLC identifies two types of network nodes: primary and secondary. Primary nodes control the operation of other stations, called secondaries. The primary polls the secondaries in a predetermined order, and secondaries can then transmit if they have outgoing data. The primary also sets up and tears down links and manages the link while it is operational. Secondary nodes are controlled by a primary, which means that secondaries can send information to the primary only if the primary grants permission.

SDLC primaries and secondaries can be connected in four basic configurations:

• Point-to-point- Involves only two nodes, one primary and one secondary.
• Multipoint- Involves one primary and multiple secondaries.
• Loop- Involves a loop topology, with the primary connected to the first and last secondaries. Intermediate secondaries pass messages through one another as they respond to the requests of the primary.

• Hub go-ahead- Involves an inbound and an outbound channel. The primary uses the outbound channel to communicate with the secondaries. The secondaries use the inbound channel to communicate with the primary. The inbound channel is daisy-chained back to the primary through each secondary.

SDLC has a few derivatives which are adopted in different environment:

• HDLC, an ISO protocol for x.25 network
• LAPB, an ITU-T protocol used in the ISDN network
• LAPF, an ITU-T protocol used in the Frame Relay network   
• IEEE 802.2, often referred to as LLC and has three types, used in the local area network
• QLLC, used to transport SNA data across X.25 networks

• Flag- Initiates and terminates error checking.
• Address- Contains the SDLC address of the secondary station, which indicates whether the frame comes from the primary or secondary.
• Control- Employs three different formats, depending on the type of SDLC frame used:
• Information (I) frame- Carries upper-layer information and some control information.
• Supervisory (S) frame- Provides control information. An S frame can request and suspend transmission, report on status, and acknowledge receipt of I frames. S frames do not have an information field.
• Unnumbered (U) frame- Supports control purposes and is not sequenced. A U frame can be used to initialize secondaries. Depending on the function of the U frame, its control field is 1 or 2 bytes. Some U frames have an information field.
• Data- Contains a path information unit (PIU) or exchange identification (XID) information.
• Frame check sequence (FCS)- Precedes the ending flag delimiter and is usually a cyclic redundancy check (CRC) calculation remainder.