Data Communications Networking Devices - 4th Ed.pdf

6.7 FRONT-END PROCESSORS _____________________________________________________ 703Figure 6.52 Using a 3745 communications controller permits Token-Ring and Ethernetconnectivity while extending channel and link attachment capabilitiesEthernet LANs. When coupled with its increased serial line support capability, the3745 becomes a true communications hub in an enterprise mainframe-centric basednetwork. Figure 6.52 illustrates a possible network con®guration based upon theuse of several 3745 communications controllers to provide SNA and LAN communicationsaccess to multiple host computers at two corporate data centers.In examining Figure 6.52, note that while two 3745s are channel-attached to hostcomputers, a third communications controller operates as a remote controller,concentrating traf®c from legacy terminal devices connected to a control unit aswell as from workstations connected to Ethernet and Token-Ring networks forforwarding to other 3745s. These in turn provide access to host computers. Alsonote that the 3745s can run conventional SNA, which requires all sessions to berouted via a host or the more modern APPN in which sessions can be establishedbetween peers without requiring access to a host. Under APPN, it becomespossible, for example, for a station on an Ethernet LAN at one location to access aserver at a different location.Communications structureThe key to the communications capability of the 3745 is its communicationssubsystem structure. Figure 6.53 illustrates in schematic form the structure of the