Using the centralized model, LU pools and APPC session pairs can be configured across multiple SNA Server computers in a logical organization called a subdomain to provide load-balancing and hot backup services. Load balancing distributes server loads across all configured servers and prevents one server from being overworked. Hot backup ensures that other SNA Server computers in a subdomain can automatically take over for a failed server. To your users, the switch to a backup computer is transparent. For more information on subdomains, see "Grouping Servers" later in this chapter.
A centralized deployment model is also easier to administer and secure than a branch-based model because all SNA Server computers are located close together. With a branch-based model, SNA expertise is usually required in each branch office to maintain the servers. Changes to mainframe definitions also can be coordinated between host and SNA Server administrators more readily if all personnel are located near one another.
When SNA Server computers are located near to the SNA host, they can be linked by high-speed connections. The channel attachment method provides the highest throughput between SNA Server and a mainframe. In many cases, a channel-attached SNA Server computer can bypass the front-end processor (FEP), thereby minimizing transaction response time. For AS/400 hosts, you can implement high-speed token ring or Ethernet connections.