All of the APPC communications from the Windows NT system to the AS/400 is handled by Microsoft's SNA Server, another BackOffice component. The SNA Server is an essential component in the Data Distributor migration solution, and it will not work without SNA Server.
Using SNA Server for the communications services offers several benefits to a Data Distributor user. First, the configuration for APPC communications is maintained within SNA Server's administrator rather than within Data Distributor. All of the communications configuration needed within the Data Distributor is to specify LU names for the local system and the remote AS/400.
Since the Data Distributor is using the SNA Server 32-bit client, the actual SNA Server service may be running on the same system or on any other system that can be reached via network. This offers tremendous flexibility in that the server may be moved at any time for any reason, without needing to change anything in the Data Distributor configuration. If the SNA Server is moved to another system for performance reasons, the only noticeable difference for the Data Distributor is the improved performance.
Another benefit of using SNA Server is that there can be dynamic load-leveling and a hot backup. If those situations where reliability is paramount, this feature, which is built into SNA Server, is essential. In addition, SNA Server has a backup controller in the same manner as Windows NT's domain controller and backup controller. In this way, if the primary SNA administrator system goes down, the backup has all configurations current and assumes the role of primary.
An additional benefit of using SNA Server is that it provides media and protocol independence. The SNA Server can maintain links to the same system using an FDDI fiber optic link and an SDLC modem link, and this will be invisible to the Data Distributor (except for the slight difference in performance!). Similarly, if the Data Distributor is running on a different system than the SNA Server, the network protocol between the systems will be the native network protocol in use, such as NetBEUI or TCP/IP rather than the APPC coming from the AS/400.