3.3 Layer 2 Protocol Overview - WAN Protocols
3.3.1 SDLC/HDLC
IBM developed the Synchronous Data Link Control (SDLC) protocol in the mid-1970s for use in Systems Network Architecture (SNA) environments. SDLC was the first link-layer protocol based on synchronous, bit-oriented operation. This section and the next provide a summary of the basic operational characteristics of SDLC and outlines several derivative protocols. 

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- 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, meaning 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—This configuration involves only two nodes, one primary and one secondary. 
  • Multipoint—This configuration involves one primary and multiple secondaries. 
  • Loop—This configuration 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—This configuration 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. 

A typical SDLC-based network configuration is shown in the figure to the left. As illustrated, an IBM establishment controller in a remote site connects to dumb terminals and to a Token Ring network. In a local site, an IBM host connects (via channel-attached techniques) to an IBM front-end processor (FEP), which also can have links to local Token Ring LANs and an SNA backbone. The two sites are connected through an SDLC-based 56-kbps leased line. 

SDLC has been the basis for several other protocols such as High-level Data Link Control (HDLC), Link Access Procedure Balanced (LAPB), Qualified Logical Link Control (QLLC), and IEEE 802.2. Despite the fact that it omits several features used in SDLC, HDLC is generally considered to be a compatible superset of SDLC. LAPB, a subset of HDLC, was created to ensure ongoing compatibility with HDLC, which was modified in the early 1980s. IEEE 802.2 is a modification of HDLC for LAN environments. QLLC is a link-layer protocol defined by IBM that enables SNA data to be transported across X.25 networks.