Data Communications Networking Devices - 4th Ed.pdf

694 _________________________ WIDE AREA NETWORK DATA CONCENTRATION EQUIPMENTProtocol supportAs previously mentioned, FRADs can support several legacy protocols such asIBM's SNA/SDLC and its older bisynchronous transmission protocol in additionto asynchronous, Ethernet and Token-Ring protocols. For many organizations, akey consideration when selecting a FRAD is its method of support for IBM'sSynchronous NetworkArchitecture SNA). Although SNA represents a legacymainframe-based communications architecture, there are over 50 000 SNA networksin existence. This means that organizations with mainframe networks andLANs that wish to take advantage of the capability of Frame Relay to replace orsupplement expensive SNA leased line networks need to obtain FRADs capable ofsupporting Synchronous Data LinkControl SDLC), the protocol used on SNAnetworks.There are at least four techniques you can consider for transmitting SNA over aFrame Relay network. Those methods are summarized in Table 6.13Table 6.13 Techniques for transporting SNA via a Frame Relay Network SNA/SDLC encapsulation into TCP/IP SNA/SDLC conversion to SNA/LLC and encapsulation into TCP/IP Data Link Switching SNA/SDLC conversion to SNA/LLC2 and encapsulation into Frame Relay based onRFC 1490SNA /SDLC encapsulation into TCP/IPOne of the oldest techniques developed to route SNA traf®c via a Frame Relaynetworkis to ®rst encapsulate it into TCP/IP. Then, TCP/IP is encapsulatedwithin the Frame Relay frame. This results in each frame carrying six bytes ofFrame Relay header overhead, 20 bytes of TCP header overhead, and 20 bytes of IPheader overhead in addition to the SNA header. Due to the high overheadassociated with this technique, it is not widely used.SNA /SDLC conversion to SNA/LLC2Under this SNA transmission technique legacy SNA traf®c is ®rst converted intoSNA/LLC2 and then encapsulated into TCP/IP. Although this technique resultsin the use of Frame Relay, TCP, and IP headers similar to the direct encapsulationof SNA/SDLC into TCP/IP, the conversion to SNA/LLC2 results in theelimination of a signi®cant amount of the polling activity required by SNA. Thisreduces the riskof session timeouts due to the delays resulting from multipleencapsulations.