Editing Systems Network Architecture (section)

==Advantages and disadvantages==
{{procon|date=November 2012}}

SNA removed link control from the application program and placed it in the NCP. This had the following advantages and disadvantages:<!--Examine: Are these just (dis)advantages of placing LC into NCP, or is this meant to be a general comparison?-->

===Advantages===
*Localization of problems in the telecommunications network was easier because a relatively small amount of software actually dealt with communication links. There was a single error reporting system.
*Adding communication capability to an application program was much easier because the formidable area of link control software that typically requires interrupt processors and software timers was relegated to system software and [[IBM Network Control Program|NCP]].
*With the advent of [[Advanced Peer-to-Peer Networking]] (APPN), routing functionality was the responsibility of the computer as opposed to the router (as with TCP/IP networks). Each computer maintained a list of Nodes that defined the forwarding mechanisms. A centralized node type known as a Network Node maintained Global tables of all other node types. APPN stopped the need to maintain [[Advanced Program-to-Program Communication]] (APPC) routing tables that explicitly defined endpoint to endpoint connectivity. APPN sessions would route to endpoints through other allowed node types until it found the destination. This is similar to the way that routers for the [[Internet Protocol]] and the Netware [[Internetwork Packet Exchange]] protocol function. (APPN is also sometimes referred to PU2.1 or Physical Unit 2.1. APPC, also sometime referred to LU6.2 or Logical Unit 6.2, was the only protocol defined to APPN networks, but was originally one of many protocols supported by VTAM/NCP, along with LU0, LU1, LU2 (3270 Terminal), and LU3. APPC was primarily used between CICS environments, as well as database services, because it contact protocols for 2-phase commit processing). Physical Units were PU5 (VTAM), PU4 (37xx), PU2 (Cluster Controller). A PU5 was the most capable and considered the primary on all communication. Other PU devices requested a connection from the PU5 and the PU5 could establish the connection or not. The other PU types could only be secondary to the PU5. A PU2.1 added the ability to a PU2.1 to connect to another PU2.1 in a peer-to-peer environment.<ref>IBM Systems Network Architecture and APPN PU2.1 References Guides</ref>)

===Disadvantages===
*Connection to non-SNA networks was difficult. An application that needed access to some communication scheme not supported in the current version of SNA would have faced obstacles. Before IBM included [[X.25]] support (NPSI) in SNA, connecting to an X.25 network would have been awkward. Conversion between X.25 and SNA protocols could have been provided either by NCP software modifications or by an external [[protocol converter]].
*A sheaf of alternate pathways between every pair of nodes in a network had to be predesigned and stored centrally. Choice among these pathways by SNA was rigid and did not take advantage of current link loads for optimum speed.
*SNA network installation and maintenance are complicated and SNA network products are (or were) expensive. Attempts to reduce SNA network complexity by adding [[IBM Advanced Peer-to-Peer Networking]] functionality were not really successful, if only because the migration from traditional SNA to SNA/APPN was very complex, without providing much additional value, at least initially. SNA software licences (VTAM) cost as much as $10,000 a month for high-end systems. And SNA [[IBM 3745]] Communications Controllers typically cost over $100K. [[TCP/IP]] was still seen as unfit for commercial applications e.g. in the finance industry until the late 1980s, but rapidly took over in the 1990s due to its peer-to-peer networking and packet communication technology.
*SNA's connection based architecture invoked huge state machine logic to keep track of everything. APPN added a new dimension to state logic with its concept of differing node types. While it was solid when everything was running correctly, there was still a need for manual intervention. Simple things like watching the Control Point sessions had to be done manually. APPN wasn't without issues; in the early days many shops abandoned it due to issues found in APPN support. Over time, however, many of the issues were worked out but not before TCP/IP became increasingly popular in the early 1990s, which marked the beginning of the end for SNA.