Bush__The_Essential_Physics_for_Medical_Imaging - Biomedical ...

sic (nonswitched) forms of Ethernet, the transmmmg node "broadcasts" eachpacket. All other nodes receive the packet and read its destination address, but onlythe destination node stores the information in memory. 1OBase-T Ethernet is physicallyconfigured in a star topology, with each node connected to a central hub byunshielded twisted pair (UTP) telephone wiring. However, 1OBase-T Ethernetbehaves logically as a bus topology, because the hub broadcasts the packets to allnodes. 10Base-5, 1OBase-2, and 1OBase-T Ethernet support data transfer rates upto 10 Mbps. Fast Ethernet (l OOBase-TX) is like 1OBase-T, except that it permitsdata transfer rates up to 100 Mbps over high-grade Category 5 UTP wiring.A 1OBase-T or Fast Ethernet network can be converted to a switched Ethernetnetwork by replacing the hub with an Ethernet switch. The switch, in contrast to ahub, does not broadcast the packets to all nodes. Instead, it reads the address oneach packet and forwards the packet only to the destination node. In this way, theswitch permits several pairs of nodes to simultaneously communicate at the fullbandwidth (either 10 or 100 Mbps) of the network. There are also optical fiber versionsof Ethernet and Fast Ethernet. Gigabit Ethernet, providing data transfer ratesup to 1 Gbps, has been introduced.Fiber Distributed Data Interface and AsynchronousTransfer ModeFiber distributed data interface (FDDI) is a token-passing network that uses opticalfiber cable configured in a ring to transfer information at rates up to 100 Mbps.FDDI can have a second ring transmitting in the opposite direction, permitting thenetwork to function with a break in either ring.Asynchronous transfer mode (ATM) is a very fast protocol that transmits smallpackets of uniform size. ATM can be used for LANs and over lines leased from atelecommunications company to connect distant facilities. ATM typically transmitsinformation at 155 Mbps, but data transfer rates can range from 25 to more than600 Mbps. ATM can use either twisted pair cables carrying electrical signals or opticalfiber cables.Although FDDI or ATM can be used to connect individual nodes on a LANsegment, they are more commonly used as high-capacity data paths called "backbones"between LAN segments in larger and more complex networks. Gigabit Ethernetis also likely to be used for backbones.An extended LAN connects facilities, such as the various buildings of a medical center,over a larger area than can be served by a single LAN segment. An extendedLAN may be formed by connecting LAN segments with devices such as bridges.Network backbones of high-bandwidth media such as FDDI or ATM may be usedto carry heavy information traffic between LAN segments. LANs at distances thatcannot be served by the network backbone are usually connected by communicationschannels provided by a telecommunications company.Repeaters and bridges can be used to extend or connect LANs. A repeater connectstwo LANs of the same type, thereby extending the distance that can be served.A repeater merely transmits all traffic on each segment of a LAN to the other segment.Because all traffic is repeated, even when the transmitting and destination