Over the years, SNA has evolved to accommodate new technologies and adapted to the changes in data communication.

Today, there are two implementations of SNA: subarea networking and Advanced Peer-to-Peer Networking (APPN):

• Subarea networking

  Subarea networking was the initial implementation of SNA that defined mainframe-based hierarchical networks in which every resource and SNA route had to be predefined. In the initial implementation of SNA, adding resources or changing SNA routes necessitated the shutdown of parts of the network.

• Advanced Peer-to-Peer Networking (APPN)

  To address the deficiency of the static nature of subarea SNA, IBM introduced an SNA-based peer network, with no hierarchical relations, and with dynamic definition of resources.

  At a later stage, APPN was enhanced with the introduction of High Performance Routing (HPR) and SNA/IP, which, as its name implies, is a high performance routing protocol that can be optionally exploited by APPN.

Neither subarea networking nor APPN resolved a weakness related to the loss of an SNA session when a resource along the session route fails. Besides improving routing performance, HPR provides non-disruptive re-routing of the SNA session to an available alternate route. HPR also enables the integration of SNA into IP-based backbones.

Hierarchical systems are organized in the shape of pyramid, with each row of objects linked directly to objects beneath it. SNA subarea, besides implementing the model of a hierarchical system, is centrally managed from the top of the pyramid.

Network resources in SNA are managed (that is, known and operated) from a central point of control that is aware of all the activity in the network, whether a resource is operational, and the connectivity status of the resource. The resources can send reports on their status to the control point. Based on networking and organizational requirements, a hierarchical network can be divided into sub-networks, where every sub-network has a control point with its controlled resources.

We can use an airport control tower as an example to explain the centrally-managed approach. All airplanes in the control tower sphere of control (a sub-network) are controlled and report to the control tower. The control tower also "operates" the resources (airplanes and runways) by granting landing and takeoff authorization.

In a peer network, every resource is self-contained and controls its own resources. Most of the time a networking resource in a peer network is not aware of its network peers, and learns about their existence when it starts to communicate with the peer resources.

We can use a Windows workstation as an example. We define only the local network of the workstation. The workstation can connect and exchange data with every resource it is authorized to access, as long as the physical path is available.

A national real estate franchise is good illustration of a peer network. Every local real estate office maintains the listing in its area and is not aware of the information stored in other offices. If a customer who plans to relocate asks for service from the local office, the office will call (connect to) the office in the city his customer plans to move to and get the listing from the remote location. If the customer had not made this request, the local office would not be aware of the remote office, and would learn about the remote office only when there was a need to access data that was stored remotely.

By now, you are probably asking yourself why SNA initially followed the hierarchical path and TCP/IP, which was developed at the same time, is a peer protocol. Well, the answer is that the goals of the protocols were different. TCP/IP was developed to provide collaboration between computers and data sharing. SNA was developed for central control.

In the 1980s, TCP/IP was used extensively by scientists who wanted to share research papers and ideas stored on their campus computers with academic staff around the world. IBM designed SNA for business data processing applications. The hierarchical topology of SNA matches the organizational structure of businesses and enterprises. The most common example is a bank where the tellers in the branch require access to the bank's central database. The same paradigm is also true for the insurance and retail industry. Also, businesses that have regional offices connected to a corporate site can implement the hierarchical network model.