The 10/100BaseT (and 1000BaseT) Ethernet topology is an active star, rather than the Tapped Bus topology of 10Base5 and 10Base2 . The operation of 100BaseT is identical to 10BaseT. This star topology is quite compatible with the standard home-run method of commercial telephone-style wiring. Although the use of existing telephone wiring was originally the goal of a twisted-pair Ethernet, the standards now recommended that special LAN-certified twisted-pair wire be used exclusively for the LAN network connection. Telephone wiring should be done using a separate cable to be totally in step with the standard
10/100BaseT Hubs. A typical 10/100BaseT installation is shown in above Figure . The center of the star topology is a 10/100BaseT hub. Each workstation (or server) has a 10/100BaseT network adapter port that is connected to the hub over a twisted pair cable. The standard specifies a maximum 90-m distance for each cable leg, plus a total of 10 m for interconnection, both at the workstation and at the hub. Modular cords, jacks, punchdowns, cross-connects, and patches are all allowed. While these wiring devices resemble their telephone counterparts, there are important differences that make them “data grade.” Some of these differences are evident by the performance certifications needed to meet the standards for LAN wiring.
A significant difference between 10/100BaseT and Ethernet coax topologies is the addition of an active hub device. 10/100BaseT still uses the CSMA/CD signaling method. Remember, with the Ethernet coax topologies, any transmission by a station is passively distributed by the coax cable to all the other connected stations. In 10/100BaseT, however, a transmission by a workstation first goes to the hub, which then repeats (retransmits) the signal to all of the other connected stations. Each port thus acts as a transceiver (repeater hop).
A 10/100BaseT hub (or switch) typically has 8, 12, 24, or even more dual speed 10/100BaseT ports. Dual speed hubs usually sense the speed of the connected device, and adjust accordingly. The hub/switch is capable of converting dissimilar speeds between any two or more ports. A typical hub may be a stand-alone unit, or part of a chassis with plug-in hub cards. Older stand-alone hubs usually had one port each for thicknet and thinnet connections, in addition to the 10/100BaseT ports, but these legacy ports are now uncommon. To recognize these legacy ports is easy: The thicknet port has an attachment unit interface (AUI) and the thinnet port has a bayonet naval connector (BNC). (See the Appendix for more on legacy cabling.)
Hubs may be used in combination together to build bigger networks by interconnection through one of the 10/100BaseT ports, or through a thicknet port or a thinnet port if provided with one of these legacy ports. The 10/100BaseT interfaces can be converted to AUI or thinnet interfaces or even fiber-optic links with appropriate transceivers or repeaters.
The active hub gives 10/100BaseT some unique advantages over the coax topologies. Because each hub port is repeated to the rest of the network, each 10/100BaseT port is independent of the others. This means that the cable length of a given port is not affected by the cable length of any other port. It is much simpler to plan and test a 10/100BaseT network, because the cable length for a port must merely be less than the allowable maximum of 100 m (90 m for the horizontal cable,and 10 m for the patch and user cords at either end). As an added benefit, the hub can automatically isolate any port that misbehaves.
10/100BaseT Switches A variation of the 10/100BaseT hub device is called a switched hub. Standard hubs send all packets received from any port to any other port, as you can see from Fig. 2.3. In this way, they share the twisted-pair media among all ports. Undesired collisions can sometimes occur, because there are so many devices sharing the same collision domain. In order to reduce this problem, it is possible to use a simple technique called Layer 2 switching.
A switched hub is a very significant improvement in network interconnection. In most situations (nonbroadcast, nonmulticast), a network intends to send each data packet to one other device on the network. The switch can identify these two stations through their unique Layer 2 media access control (MAC) addresses. Initially, the switch “learns” the MAC addresses of each connected device and notes its associated port number in an internal table. From then on, the switch knows how to send data packets only to the particular switch port that has that device attached.
This technique greatly minimizes traffic going to individual ports, allows two (or more) transmissions to/from two (or more) stations on independent ports to occur simultaneously, and can allow a high-speed uplink to a backbone server to handle many simultaneous data exchanges with workstations. The basic operation of a switched hub is shown in above Figure .
10/100BaseT Advantages. A real advantage to 10/100Base T networks is that workstations may be connected to or disconnected from the hubs (or switches) without interrupting other stations on the network. Hubs usually have status lights that indicate proper connection on the 10BaseT ports and collisions or other error conditions. Standard 10BaseT hubs will automatically isolate (or partition) a port for a wiring reversal, short, or open connection (including no device attached). More sophisticated hubs may also detect more subtle errors and allow for automatic or manual port isolation. Hubs may also be connected to a variety of management tools, from simple network management protocol (SNMP) to sophisticated network hardware managers, such as Hewlett-Packard Openview™ and Sun NetManager™.
The basic interface for 10BaseT is the medium-dependent interface (MDI), a special wiring configuration of the common 8-pin modular jack that is used by some telephone equipment. Two pairs of wires are used: one for the transmit data and one for the receive data. The connections are polarity sensitive. That means that the connection will not operate if the two wires of either pair are reversed. The wiring of the 10BaseT interface is shown in below Figure.
Although 10/100BaseT uses only two wire pairs, it is customary to use 4-pair cable to make the station drops. Several of the 100 Mbps network schemes require all four pairs. In addition, a jack that is wired for all four pairs can support many other types of data and voice connections, which may use some other combination of the eight connector pins.
Some installations use the other two pairs for a telephone connection or for another 10/100BaseT connection. Be wary of telephone installers “stealing” your network wiring pairs for their use. Telephone wiring is often at cross purposes from network wiring. The standards are different, as are the installation practices.