| 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 primaries and secondaries can be connected in four basic configurations:
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.
|