Data Communications Networking Devices - 4th Ed.pdf

250 __________________________________________________________ WIDE AREA NETWORKSFigure 2.49A more complex APPN network with multiple paths to some nodesRoute selectionUnder APPN, routing is based upon network nodes maintaining a `route additionresistance' value set up by network administrators and `class of service' of data to berouted. Class-of-service routing enables different types of data to be routed viapaths optimized for batch, interactive, batch-secure and interactive-secure. APPNuses eight values that are de®ned for each network link that are used in conjunctionwith the class of service to select an appropriate route. Value de®ned for each linkinclude propagation delay, cost per byte, cost for connect time, effective capacity,and security. Using values de®ned for the links, a batch session might be routed on apath with high capacity and low cost, while an interactive session would probably beplaced on a terrestrial link instead of a satellite link to minimize propagation delay.APPN can be considered to represent a considerable enhancement to SNA as itprovides ef®cient routing services that bypass the requirement of SNA date to ¯owin a hierarchical manner. However, APPN is similar to SNA in the fact that suchnetworks do not have true network addresses, making pure routing betweendifferent networks dif®cult. In addition, the structure of APPN is similar to SNAwith respect to its basic network layer operations which are illustrated in Figure2.50. In examining Figure 2.50 note that APPN's network layer can be consideredto represent APPC which converts LU service requests into frames for transport atthe data link layer. Although SNA was originally limited to LLC Type 2 LLC2)and SDLC transmission via a variety of physical layer interfaces, a number ofconversion devices have been developed which extend both SNA and APPNtransmission to frame relay. In addition, other products enable SNA and APPNdata to be transported under a different network layer, a technique referred to as