Data Communications Networking Devices - 4th Ed.pdf

2.6 SNA AND APPN _______________________________________________________________ 239Types of physical unitsTable 2.8 lists ®ve types of physical units in an SNA network and theircorresponding node type. In addition, this table contains representative examplesof hardware devices that can operate as a speci®c type of PU. As indicated in Table2.8 the different types of PUs form a hierarchy of hardware classi®cations. At thelowest level, PU type 1 is a single terminal. PU type 2 is a cluster controller which isused to connect many SNA devices onto a common communications circuit. PUtype 4 is a communications controller which is also known as a front-end processor.This device provides communications support for up to several hundred lineterminations, where individual lines in turn can be connected to cluster controllers.At the top of the hardware hierarchy, PU type 5 is a mainframe computer.Table 2.8SNA PU summaryPU type Node Representative hardwarePU type 5 Mainframe S/390, 43XX, 308XPU type 4 Communications controller 3705, 3725, 3720, 3745PU type 3 Not currently de®ned N/APU type 2 Cluster controller 3274, 3276, 3174PU type 1Terminal3180, PC with SNA adapterThe communications controller is also commonly referred to as a front-endprocessor. This device relieves the mainframe of most communications processingfunctions by performing such activities as sampling attached communications linesfor data, buffering the data and passing it to the mainframe as well as performingerror detection and correction procedures. The cluster controller functions similarto a multiplexer or data concentrator by enabling a mixture of up to 64 terminalsand low-speed printers to share a common communications line routed to acommunications controller or directly to the mainframe computer.Multiple domainsFigure 2.43 illustrates a two-domain SNA network. By establishing a physicalconnection between the communications controller in each domain and codingappropriate software for operation on each controller, cross-domain data ¯owbecomes possible. When cross-domain data ¯ow is established terminal devicesconnected to one mainframe gain the capability to access applications operating onthe other mainframe computer.SNA was originally implemented as a networking architecture in which usersestablish sessions with application programs that operate on a mainframe computerwithin the network. Once a session is established a network control program NCP)operating on an IBM communications controller, which in turn is connected to theIBM mainframe, would control the information ¯ow between the user and the