IBM SNA Interoperability Concepts
IBM SNA Interoperability Concepts |
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Connections provide the lower-level data path between hardware components on an SNA network. These connections include paths from the mainframe to the FEP and from the FEP to the cluster controller.
FEPs can be connected to mainframes with either an IBM Channel connection or an Open Systems Adapter.
An IBM Channel connects components to a mainframe using a proprietary, high-speed communication link. The channel connection includes a microprocessor within the mainframe that is dedicated to a single purpose, such as directing input/output (I/O) from a mainframe hard disk drive or managing an FEP communications line. Fast and efficient communications through dedicated channel microprocessors represent the native method of connecting to IBM mainframes. Table A.2 describes two common types of IBM Channel connections.
Table A.2 IBM Channel Connections
| Cable Type | Composition | Transmission Rate |
|---|---|---|
| Bus & Tag | Two heavy-gauge multiple lead copper cables with large multiple-pin connectors at each end. | 3.0 or 4.5 megabytes per second. |
| Enterprise System Connectivity (ESCON) | Fiber-optic cable. | Up to 17 megabytes per second. |
An Open System Adapter, placed in the host, provides a direct network connection to Token Ring, Ethernet, and Fiber Distributed Data Interface (FDDI) networks.
Several connection types, also known as links, are available for connecting an FEP to a cluster controller or other components operating lower in the SNA hierarchy. Each connection type uses a different method to gain access to an FEP. Three of the most common links are:
Synchronous Data Link Control (SDLC) Enables a cluster controller to communicate with an FEP over standard (switched) telephone lines or leased (dedicated) telecommunication lines. SDLC has been available for many years and is widely used in the SNA networking environment.
802.2 Data Link Control (DLC) Enables a cluster controller to communicate with an FEP over standard networking topologies, such as Token Ring, Ethernet, or FDDI. Although DLC has not been available as long as SDLC, its efficiency and flexibility are making it very popular for new installations.
X.25/QLLC Enables a cluster controller to communicate with an FEP over standard packet-switching networks. X.25/QLLC is an International Telecommunications Union (ITU) standard for global packet-switching network communications that uses the qualified logical link control (QLLC) protocol, also known as X.25. An X.25 connection is slower than an 802.2-type connection but is comparable to an SDLC connection.
For more information about connection types, see "Interoperability with IBM Host Systems" in this book.