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2.5 Local Network Technology     continued...

As a result of improvements in the technology used to send and receive packets, modern ethernet systems can be simple shielded or unshielded ``twisted pair'' wiring. Many networks are now broken into independent segments connected by routers. Figure 3 shows two local networks. On the left is an older, single-segment ethernet (the black dots represent junctions where two or more cables are connected). A packet broadcast from any point on the network is broadcast to all other parts of the network. On the right is a typical modern ethernet which has several independent segments connected by routers. Packets originating in one segment are examined by the nearest router. If the packet is addressed to another node within the segment, the router ignores it, otherwise the router retransmits the packet to the other routers (and all the other nodes in the segment ignore the packet). The router improves performance by keeping packets destined for nodes within a segment from interfering with local traffic on other segments. Bandwidth within a segment in a modern ethernet can be as high as 100Mbps.

There are several alternatives to Ethernet as the basic technology for local area networks, but they are very recent and standards are still evolving so manufacturers have been slow to commit to any particular method. Fiber Distributed Data Interconnection (FDDI) and Asynchronous Transfer Mode (ATM) are two examples. FDDI is a fiber-optic based system that is a broadcast network, like Ethernet, but with higher bandwidth and other properties. ATM is a point-to-point network with very low overhead. Both are available currently and may become much more widely used in the near future.