In SNA Server terms, a host connection is the data communications path between SNA Server and a host system. For a mainframe, the connection corresponds to a physical unit (PU) definition in VTAM. On the AS/400, the connection corresponds to an APPC controller definition.
For each physical adapter or connection, an appropriate link service is installed and configured within SNA Server. The link service is a Windows NT service or device driver that is used to control server-to-host communication adapters supported by SNA Server. Once configured, the link service is available for use not only on the configured SNA Server, but on any SNA Server in the subdomain using the Distributed Link Services feature of SNA Server. More information on using Distributed Link Services can be found in the SNA Server Administration guide.
Once link services are configured, connections are then created over a defined link service. Using a host connection, a client PC on a LAN can communicate with the mainframe system. For some link services, it is possible to define multiple connections over a single host link.
In SNA terms, the combination of a connection and the link service it uses is equivalent to a physical unit (PU). In a SNA networks, SNA Server provides PU 2 or PU 2.1 functionality similar to that of a IBM cluster controller.
There are several factors that should be taken into account when determining a physical connection method to your mainframe:
In general, having enough future capacity to support additional connections to your host system is recommended. High-speed token ring connections are the best choice for an all-purpose connection to a host. For greater performance, channel type connections are recommended for mainframe environments.
For some link services, multiple host connections are possible using a single adapter, most notably DLC and channel-type link services. SNA Server supports up to 250 host connections per server; up to four instances of SNA Server are supported on a single computer.