Data Communications Networking Devices - 4th Ed.pdf

5.3 GATEWAYS __________________________________________________________________ 595Figure 5.35 Using the NetManage Chameleon TN3270 terminal emulation program toobtain TCP/IP access to an IBM SNA hostenabling organizations to operate a more ef®cient and less costly networkinfrastructure.OperationUnder DLSw the connection-oriented protocols of SNAand NetBIOS in the formof Logical Link Control Type 2 LLC2) and Synchronous Data Link ControlSDLC) packets are encapsulated into IP packets. Figure 5.37 illustrates anexample of the manner by which point-to-point SDLC traf®c and LAN-basedLLC2 traf®c are integrated into an IP router-based network. In examining Figure5.37, note that the tunneling effort involves wrapping the IP header around SDLCor LLC2 data. Since such data then becomes encapsulated, as you might expect,another term used to reference the transport of SDLC and LLC2 data in IP packetsis encapsulation.Although the actual tunneling or encapsulation process appears to be relativelysimple to accomplish, in actuality it is a complicated process. The complicationresults from the fact that SNAuses connection-oriented protocols, LLC2 andSDLC, which are based upon positive acknowledgment with retransmissionPAR). As such, if an ACK is not received within a period of time after a sequenceof frames are transmitted, a timer will expire, resulting in the sending station