Data Communications Networking Devices - 4th Ed.pdf

308 ________________________________________________________ LOCAL AREA NETWORKSstandard operates at 1 Mbps and uses unshielded twisted-pair UTP) wiring in astar topology; all stations are wired to a hub, which is known as a multiple access unitMAU). To avoid confusion with the term `medium access unit', we will refer tothis wiring concentrator as a hub.Each station in a 1BASE-5 network contains a network interface card NIC),cabled via UTP on a point-to-point basis to a hub port. The hub is responsible forrepeating signals and detecting collisions.The maximum cabling distance from a station to a hub is 250 meters; up to ®vehubs can be cascaded together to produce a maximum network span of 2500meters. The highest level hub is known as the header hub and is responsible forbroadcasting news of collisions to all other hubs in the network. These hubs, whichare known as intermediate hubs, are responsible for reporting all collisions to theheader hub.AT&T's 1 Mbps StarLan network, along with other 1BASE-5 systems, initiallyreceived a degree of acceptance for use in small organizations. However, theintroduction of 10BASE-T, which provided an operating rate 10 times thatobtainable under 1BASE-5, severely limited the further acceptance of 1BASE-5networks.10BASE-TIn the late 1980s, a committee of the IEEE recognized the requirement oforganizations to transmit Ethernet at a 10 Mbps operating rate over low-cost andreadily available unshielded twisted-pair cable. Although several vendors hadalready introduced equipment that permitted Ethernet signaling via UTP cabling,such equipment was based on proprietary designs and was not interperable. Thus, anew task of the IEEE was to develop a standards for 802.3 networks operating at 10Mbps using UTP cable. The resulting standard was approved by the IEEE as802.3i in September 1990, and is more commonly known as 10BASE-T.The 10BASE-T standard supports an operating rate of 10 Mbps at a distance ofup to 100 meters 328 feet) over UTP Category 3 cable without the use of arepeater. The UTP cable requires two pairs of twisted wire. One pair is used fortransmitting, while the other pair is used for receiving. Each pair of wires istwisted together, and each twist is 180 degrees. Any electromagnetic interferenceEMI) or radio frequency interference RFI) is therefore received 180 degrees outof phase; this theoretically cancels out EMI and RFI noise while leaving thenetwork signal. In reality, the wire between twists acts as an antenna and receives adegree of noise.Network componentsA 10BASE-T network can be constructed with network interface cards, UTPcable, and one or more hubs. Each NIC is installed in the expansion slot of acomputer and wired on a point-to-point basis to a hub port. When all of the portson a hub are used, one hub can be connected to another to expand the network,resulting in a physical star, logical bus network structure.