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Whether using radio or infrared light frequencies, wireless LANs are a relatively immature technology which is yet to be widely accepted. Acquisition costs are not particularly low when compared to wired LANs, although reconfiguration costs are virtually nonexistent. Additionally, wireless offers the advantage of portability. On the negative side, bandwidth and throughput are limited. Error performance and security are issues of some significance. Additionally, costs are not insignificant, with the access hubs averaging about $2,000 and the workstation cards about $600 [9-6], [9-9], and [9-10]
The physical topology (layout) of a LAN is in the form of a Bus, Ring, or Star. (trains, ovals, planets, and constellations are not defined; vendors promoting such topologies should be avoided at all costs!)
Bus Topologies
As shown in Figure 9.2, bus topologies are multipoint electrical circuits, that can be implemented using coax, UTP, or STP. Data transmission is bidirectional, with the attached devices transmitting in both directions. While generally operating at a raw data rate of 10 Mbps, actual throughput is much less. Bus networks employ a decentralized method of media access control known as CSMA (Carrier Sense Multiple Access), that allows the attached devices to make independent decisions relative to media access and initiation of transmission. This approach results in data collisions and requires frequent retransmission. Bus networks are specified in the IEEE 802.3 standard, and generally have a maximum specified length of 1.5 miles (2.5 Km). Ethernet is based on a bus topology. A tree topology is a variation on the bus theme, with multiple branches off the trunk of the central bus. Bus networks also suffer from the vulnerability of the busshould the bus be compromised, the entire network is compromised. Similarly, tree networks are dependent on the integrity of the root bus [9-2].
Figure 9.2 Ethernet bus topology.
Ring Networks
Figure 9.3 shows a network laid out in a physical ring, or closed loop, configuration. Information travels around the ring in only one direction, with each attached station or node serving as a repeater [9-2]. Rings generally are coax or fiber (FDDI) in nature, operating at raw transmission rates of 4, 16, 20 or 100+ Mbps. Rings are deterministic in nature, employing token-passing as the method of media access control to ensure the ability of all nodes to access the network within a predetermined time interval. Priority access is recognized. A master control station is responsible for controlling access to the transmission medium. Backup control stations assumes responsibility in the event of a failure of the master. Throughput is very close to raw bandwidth, as data collisions do not occur in such a carefully controlled environment. On the negative side, the failure of a single node can compromise the entire network. Electrical ring networks are specified in the IEEE 802.5 standard (FDDI is an ANSI specification). Token-Passing Ring, IBM Token Ring, and FDDI all are based on ring topologies.
Figure 9.3 Ring topology.
Star Topologies
Figure 9.4 shows a network consisting of a central node, hub, or switch, to which all other devices are attached directly, generally via UTP or STP. Transmission rates vary with AT&Ts StarLAN operating at 1 to 10 Mbps, and both 100Base-T and 100VG-AnyLAN at 100 Mbps. The primary advantage of a star is that a disruptive or failed station can be isolated, thereby eliminating any negative effect it may have on LAN performance. Additionally, each node has access to the full bandwidth of the LAN, at least in a LAN switch environment. The primary disadvantage is that a hub failure is catastrophic; because all connectivity is provided through the central hub, its failure affects the entire LAN. Examples of star configurations include AT&Ts StarLAN and DataKit, 100Base-T, 100VG-AnyLAN, PBXs, and Centrex CO-LANs.
Figure 9.4 100Base-T Star topology.
Physical Versus Logical Topology
This refers to the fact that the network may operate logically as though the physical layout were different. An Ethernet bus LAN might be configured physically as a star in a 10/100Base-T implementation. The bus still exists, but protected under the skin of the central hub to which all stations are connected via UTP; the internal bus is known as a collapsed backbone. In this fashion, the network gains the logical advantages of the Ethernet protocol, as well as the physical advantages of a UTP-based star. Similarly, a ring network might operate logically as a ring, but be supported physically by a collapsed backbone bus.
There exist two LAN transmission options, Baseband and Broadband. Baseband LANs, which are the most prevalent by far, are single-channel systems which support a single transmission at any given time. Broadband LANs, which are most unusual, support multiple transmissions via multiple frequency channels.
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