System information

CHAPTER
Troubleshooting IBM 10-211
10
Troubleshooting IBM
This chapter focuses on connectivity and performance problems associated with bridging and
routing in IBM-based networks. When troubleshooting IBM-based networks, it is important to have
a knowledge of Synchronous Data Link Control (SDLC) and source-route bridging (SRB). The
following sections provide an overview of SDLC and SRB.
SDLC
IBM developed the SDLC protocol in the mid-1970s for use in Systems Network Architecture
(SNA) environments. SDLC was the first of an important new breed of link-layer protocols based on
synchronous, bit-oriented operation. Compared to synchronous character-oriented (for example,
Bisync from IBM) and synchronous byte-count–oriented protocols (for example, Digital Data
Communications Message Protocol [DDCMP] from Digital Equipment Corporation), bit-oriented
synchronous protocols are more efficient, more flexible, and often faster.
After developing SDLC, IBM submitted it to various standards committees. The International
Organization for Standardization (ISO) modified SDLC to create the High-Level Data Link Control
(HDLC) protocol. The International Telecommunication Union Telecommunication Standardization
Sector (ITU-T, formerly CCITT) subsequently modified HDLC to create Link Access Procedure
(LAP), and then Link Access Procedure, Balanced (LAPB). The Institute of Electrical and
Electronic Engineers (IEEE) modified HDLC to create IEEE 802.2. Each of these protocols has
become important in its own domain. SDLC remains the SNA primary link-layer protocol for
wide-area network (WAN) links.
Technology Basics
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—Controls the operation of other stations (called secondaries). The primary polls the
secondaries in a predetermined order. 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—Controlled by a primary. Secondaries can send information only to the primary, but
cannot do this unless the primary gives permission.
SDLC primaries and secondaries can be connected in four basic configurations:
• Point-to-point—Involves only two nodes, one primary and one secondary.