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6.2 Local Area Networks

Figure 16 depicts topologies associated with some LAN technologies available today. The FDDI technology provides a maximum capacity of 100 Mbps and employes a dual-ring topology from which workstations are either connected directly or by bridges. The switching hub allows a variety of LANs, such as Ethernet (i.e., 10BaseX), fast Ethernet (i.e., 100BaseX), token-ring, FDDI, and even ATM to be connected together at the hub. For maximal bandwidth, a single machine can connect directly to a hub port. Alternately, a subnet of several workstations can connect to a single hub port and share the bandwidth. A switching hub is very flexible, but costs a little more if it supports multiple network technologies. The ATM switch offers flexible connectivity, high throughput (155 Mbps or even higher), and consistent quality of service.

Figure 16. Local area network topologies.

Ethernet, token-ring, and FDDI have all been around for some time, especially the first two. Due to mature technology and large installation base, these three technologies have low technology risk.

The improved Ethernet technology (called 100BaseX) and a new polling LAN technology (called 100VG-AnyLan) both look promising for certain applications. Because both provide the same maximum capacity as FDDI but cost less, they are quickly becoming more popular than FDDI and it is possible that one may replace FDDI in the future.

ATM is an emerging technology and should be considered carefully. This technology is designed for quality-on-demand service, so it potentially can replace most technologies used today for data, voice, image, and video. However, it has standardization problems (nearly resolved) and technological problems (of greater concern). The future of ATM looks promising and must be monitored closely.

The primary advantage of the switching hub technology is its ability to host different LAN and/or WAN modules. This technology has been used widely in recent years and should remain current indefinitely. The technology for monolithic (support single technology) hubs is less complex than non-monolithic (support multiple technologies) hubs.

Based on current state of technology and cost-performance trade-off analyses, switching hubs (supporting both Switching-100 and ATM-155) are the suggested technology to apply to a TT&C system implementation. This recommendation is reflected in Figure 16. Switching hubs offer a flexible, scaleable topology that is currently mature, and provide maximal throughput per dollar. In addition, they will support ATM connections in the future, provided that technology matures as expected. ATM offers the advantage of high bandwidth and consistent quality of service. When (and if) it matures, this technology should be incorporated as needed.