SNA (Systems Network Architecture)

IBM’s mainframe-network infrastructure is referred to as Systems Network Architecture (SNA). SNA was the prevailing enterprise network infrastructure. It was developed in the 1970s at IBM with an overall architecture similar to the Open System Interconnection (OSI) reference model. A mainframe running Advanced Communication Facility/Virtual Telecommunication Access Method (ACF/VTAM) serves as the center of an SNA network. ACF/VTAM is responsible for establishing all network sessions and for activating and deactivating network resources. SNA explicitly defines all resources, which eliminates the need for broadcast traffic and reduces header size.

As Transmission Control Protocol/Internet Protocol (TCP/IP) (and other desktop protocols such as the Novell Internetwork Packet Exchange (IPX)) gained acceptance n enterprise networks, companies faced having to support multiple network infrastructures. The desire for a common set of protocols grew. TCP/IP eventually became the protocol of choice for convergence mainly due to certain technical benefits and the open nature of the protocols, which facilitated support from multiple vendors. Additionally, with the wide acceptance of the Internet and corporate intranets, companies wanted their traditional non-IP infrastructure devices to communicate with the new IP devices.

Larger companies had, and continue to have, large investments in SNA equipment. Although the maintenance and annual upkeep for these devices is relatively more expensive as they grow older, companies want to bridge the two worlds of SNA and TCP/IP together.

Why Are Companies Migrating Their SNA Networks?
Until recently, all midsize and large corporations, government institutions, and educational organizations ran their applications on IBM (and compatible) mainframes and midrange systems that ran primarily on SNA networks. SNA networks provided the following benefits:

High availability �' Well-designed SNA networks provided services around the clock and experienced little disruption in service: 99.99 percent or better availability.
Predictable response times �' Built around high transaction rates, SNA networks could provide predictable and consistent response times.
Secure �' SNA networks, by design, were difficult to hack into, thus providing secure access to mission-critical applications.

Despite these benefits, TCP/I which was considerably weaker in the preceding SNA characteristics, became popular because it addressed the following SNA weaknesses:

Lack of dynamic definition and configuration of resources �' You had to predefine everything in an SNA network. Although IBM addressed these issues later with its Advanced Peer-to-Peer Networking (APPN), TCP/IP had already taken hold with its more flexible and dynamic nature.
Proprietary protocols �' IBM defined, owned, and managed SNA. At the end of the 1980s, IBM customers began demanding support for protocols based on open standards. Rather than adopt an existing open standard, IBM attempted to introduce its own protocols into the public domain (APPN). However, APPN was more expensive and available on fewer platforms than TCP/IP.

Eventually, IBM’s attempt to provide its own standards lost out to TCP/IP.