|  | 7.3 | Ethernet and IEEE 802.3
| 7.3.6 | Ethernet 10Base-T media and topologies
In a LAN, where the star topology is used, the networking media is run from a central hub out to each device attached to the network. The physical layout of the star topology resembles spokes radiating from the hub of a wheel. As the graphic shows, a central point of control is used in a star topology. When a star topology is used, communication between devices attached to the local area network is via point-to-point wiring to the central link or hub. All network traffic in a star topology passes through the hub. The hub receives frames on a port, then copies and transmits (repeats) the frame to all of the other ports. The hub can be either active or passive. An active hub connects the networking media as well as regenerates the signal. In Ethernet where hubs act as multiport repeaters, they are sometimes referred to as concentrators. By regenerating the signal, active hubs enable data to travel over greater distances. A passive hub is a device used to connect networking media and does not regenerate a signal. One of the star topology’s advantages is that it is considered the easiest to design and install. This is due to the networking media being run directly out from a central hub to each workstation area. Another advantage is its ease of maintenance since the only area of concentration is located at the hub. In a star topology, the layout used for the networking media is easy to modify and troubleshoot. Workstations can be easily added to a network employing a star topology. If one run of networking media is broken or shorted, then only the device attached at that point is out of commission, the rest of the LAN will remain functional. In short, a star topology means greater reliability. In some ways a star topology's advantages can also be considered disadvantages. For example, while limiting one device per run of networking media can make diagnosis of problems easier, it also increases the amount of networking media required, which adds to the setup costs. And, while the hub can make maintenance easier, it represents a single point of failure (if the hub breaks, everyone's network connection is lost). TIA/EIA-568-A specifies that the physical layout, or topology that is to be used for horizontal cabling, must be a star topology. This means the mechanical termination for each telecommunications outlet/connector is located at the patch panel in the wiring closet. Every outlet is independently and directly wired to the patch panel. The TIA/EIA-568-A specification, for the maximum length of horizontal cabling for unshielded twisted pair cable, is 90 m. The maximum length for patch cords at the telecommunications outlet/connector is 3 m, and the maximum length for patch cords/jumpers at the horizontal cross-connect is 6 m. The maximum distance for a run of horizontal cabling, that extends from the hub to any workstation, is 100 m. (actually 99 m. but it is commonly rounded up to 100 m.) This figure includes the 90 meters for the horizontal cabling, the 3 meters for the patch cords, and the 6 meters for the jumpers at the horizontal cross-connect. Horizontal cabling runs in a star topology radiate out from the hub, much like the spokes of a wheel. This means that a LAN that uses this type of topology could cover an area of a circle with a radius of 100 m. There will be times when the area to be covered by a network will exceed the TIA/EIA-568-A specified maximum length that a simple star topology can accommodate. For example, envision a building where the dimensions are 250 m x 250 m. A simple star topology that adhered to the horizontal cabling standard specified by TIA/EIA-568-A could not provide complete coverage for that building. As indicated in the graphic, workstations E, F, and C are located outside the area that can be covered by a star topology that adheres to TIA/EIA-568-A specifications. As shown, they are not part of the local area network. So users at these workstations wanting to send, share, and receive files, would have to use sneakernet. Because no one wants to return to the days of sneakernet, some cable installers are tempted to solve the problem of a star topology's inadequate coverage by extending the length of the networking media beyond the TIA/EIA-568-A specified maximum length. When signals first leave a transmitting station, they are clean and easily recognizable. However, the longer the cable length, the weaker and more deteriorated the signals become as they pass along the networking media. If a signal travels beyond the specified maximum distance, there is no guarantee that when it reaches a NIC card, the NIC card will be able to read it. If a star topology cannot provide enough coverage for an area to be networked, the network can be extended through the use of internetworking devices that do not result in attenuation of the signal. This resulting topology is designated as an extended star topology. By using repeaters, the distance over which a network can operate is extended. Repeaters take in weakened signals, regenerate and retime them, and send them back out onto the network.