Data Communications Networking Devices - 4th Ed.pdf

204 __________________________________________________________ WIDE AREA NETWORKSFigure 2.21FTP serverUsing a client FTP program to download an access log from a Wndows NTTN3270 program which is a Telnet derivative developed to provide remoteterminal access to IBM mainframes. In this example the TN3270 program wasused to enable a PC to obtain a connection as if it was a 3270 terminal device to anIBM Of®ceVision application running on a S/390 mainframe which provideselectronic mail, calendars, and even access to the Internet.Returning to our examination of Figure 2.20, note that TCP/IP can betransported at the Data Link Layer by a number of popular LANs, to includeEthernet, Fast Ethernet, Token-Ring, and FDDI frames. Due to the considerableeffort expended in the development of LAN adapter cards to support the busstructures used in Apple MacIntosh, IBM PCs and compatible computers, DECAlphas and SUN Microsystem's workstations, and even IBM mainframes, thedevelopment of software based protocol stacks to facilitate the transmission ofTCP/IP on LANs provides the capability to interconnect LAN-based computersto one another, whether they are on the same network and require only thetransmission of frames on a common cable, or on networks separated thousands ofmiles from one another. Thus, TCP/IP represents both a local and a wide areanetwork transmission capability. In the remainder of this section I will review IPand TCP packet headers as well as discuss the use of several related network andtransport layer protocols and higher level protocols implemented over TCP and itsrelated protocol suite.