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In larger installations, such as Groucho Marx University, Ethernet is usually not the only type of equipment used. At Groucho Marx University, each department's LAN is linked to the campus backbone, which is a fiber optics cable running FDDI (Fiber Distributed Data Interface). FDDI uses an entirely different approach to transmitting data, which basically involves sending around a number of tokens, with a station only being allowed to send a frame if it captures a token. The main advantage of FDDI is a speed of up to 100-Mbps, and a maximum cable length of up to 200-km.
For long-distance network links, a different type of equipment is frequently used, which is based on a standard named X.25. Many so-called Public Data Networks, like Tymnet in the U.S., or Datex-P in Germany, offer this service. X.25 requires special hardware, namely a Packet Assembler/Disassembler or PAD. X.25 defines a set of networking protocols of its own right, but is nevertheless frequently used to connect networks running TCP/IP and other protocols. Since IP packets cannot simply be mapped onto X.25 (and vice versa), they are simply encapsulated in X.25 packets and sent over the network.
Other techniques involve using slow but cheap serial lines for dial-up access. These require yet another protocol for transmission of packets, such as SLIP or PPP, which will be described below.
Thu Mar 7 23:22:06 EST 1996