Data Communications Networking Devices - 4th Ed.pdf

2.6 SNA AND APPN _______________________________________________________________ 237to control terminals, controllers, processors and data links in the network. Thoseservices include initializing nodes and managing the data ¯ow between devices toinclude formatting and synchronizing data. In programmable devices, such asmainframe computers and communications controllers, the PU is normally implementedin software. In less intelligent devices, such as cluster controllers andterminals, the PU is typically implemented in read- only memory. In an SNAnetwork each PU operates under the control of an SSCP. The PU can be consideredto function as an entry point between the network and one or more logical units.The logical unitThe third type of network-addressable unit in an SNA network is the logical unit,known by its abbreviation as the LU which represents a software module locatedin a devices memory. The LU is the interface or point of access between the enduserand an SNA network. Through the LU an end-user gains access to networkresources and transmits and receives data over the network. Each PU can have oneor more LUs, with each LU having a distinct address.Multiple session capabilityAs an example of the communications capability of SNA, consider an end-userwith an IBM PC and an SDLC communications adapter who establishes aconnection to an IBM mainframe computer. The IBM PC is PU, with its displayand printer considered to be LUs. After communications is established the PCuser could direct a ®le to his or her printer by establishing an LU-to-LU sessionbetween the mainframe and printer while using the PC as an interactive terminalrunning an application program as a second LU-to-LU session. Thus, the transferof data between PUs can represent a series of multiplexed LU-to-LU sessions,enabling multiple activities to occur concurrently.SNA network structureThe structure of an SNA network can be considered to represent a hierarchy inwhich each device controls a speci®c part of the network and operates under thecontrol of a device at the next higher level. The highest level in an SNA network isrepresented by a host or mainframe computer which executes a software moduleknown as a communications access method. At the next lower level are one or morecommunications controllers, IBM's term for a front-end processor. Eachcommunications controller executes a Network Control Program NCP) whichde®nes the operation of devices connected to the controllers, their PUs and LUs,operating rate, data code, and other communications-related functions such as themaximum packet size that can be transmitted. Connected to communicationscontrollers are cluster controllers, IBM's term for a control unit. Thus, the thirdlevel in a SNA network can be considered to be represented by cluster controllers.