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.The node smeters (2,000 feet) of an active hub orNIC must be connected directly to thea node, or within 30 meters (100 feet)T-connector; that is, drop cable is notof a passive hub.allowed.Passive hubs can be used only betweenT-connectors must be at least 1 metera node and an active hub; two passive(3.25 feet) apart on the cable.hubs cannot be next to each other.AAt most, eight nodes can be connected passive hub must be within 30 metersin a series (with no intervening hubs).(100 feet) of an active hub and within30 meters (100 feet) of a node.Both ends of a cable segment must beterminated with either a BNC termina- Nodes can be attached anywhere ontor or an active hub (or link).the network, provided the node iswithin the required distance of anThe cabling cannot loop back on itself.active or passive hub: within 610For example, the cable cannot go frommeters (2,000 feet) of an active huban active hub through other hubs andor within 30 meters (100 feet) of aeventually connect back into the origi-passive hub.nal hub.Unused hub ports must be terminatedLow-Impedance ARCneton a passive hub and should be termi-nated on an active hub.Low-impedance ARCnet networks use a startopology, in which passive hubs serve to col-The cabling cannot loop back on itself.lect nodes, as illustrated in the figure Lay-For example, the cable cannot go fromout for a low-impedance ARCnet network. an active hub through other hubs andEach passive hub is connected to an activeeventually connect back into the origi-hub.Active hubs can be linked with eachnal hub.other, and they can also be linked directly ARCnet (Attached Resource Computer Network) 51LAYOUT FOR A LOW-IMPEDANCE ARCNET NETWORKabcdefghijklmnopqrstuvwxyz 52 ARCnet (Attached Resource Computer Network)IBM Type 1 (shielded) cable:Mixed-Impedance ARCnet200 meters (660 feet)A mixed ARCnet network is one thatThe maximum cable length for theincludes both high- and low-impedanceentire network is 6,000 meters (20,000components in the same network, asfeet)illustrated in the figure Layout of amixed-impedance ARCnet network, withThe maximum number of cable seg-low-impedance and high-impedance com-ments in a series is three.If UTP cableponents. In this type of network, all theis used, the series of segments can be atrestrictions for both impedance levelsmost about 130 meters (430 feet); formust be observed.coaxial cable, the maximum length isPerhaps the most important constraintabout 300 meters (990 feet).for a mixed-impedance ARCnet is that high-Each cable segment must be termi-impedance NICs can be used in place ofnated at both ends by being connectedlow-impedance cards, but the reverse is notto an active hub or terminator.possible.Because of this restriction, it is cru-cial that you keep track of what kind of NICAn ARCnet network can have a maxi-is in each node.mum of 255 nodes.Each active hubcounts as a node.Restrictions on ARCnet NetworksAt most, 10 nodes are allowed in aThe following restrictions apply to bothseries when UTP cable is used; 8 nodeshigh- and low-impedance ARCnet networks:if coaxial cable is used.The maximum length of a cable seg-The maximum distance between anyment depends on the type of cable.Thetwo nodes on the network is deter-general restriction is that the signalmined by the constraint that no ARC-attenuation must be less than 11 dBnet signal can have a propagationover the entire cable segment at a fre-delay of more than 31 microseconds.quency of 5 MHz.In practice, thisThe total propagation delay is deter-leads to the following maximummined by adding the propagationdistances:delays in all the devices (nodes, hubs,and cable) connecting the nodes.Net-Coaxial cable: 450 600 meters (1,500work components generally have2,000 feet)propagation delays of less than 0.5UTP and IBM Type 3 (unshielded) cable:microseconds, and much less in some100 meters (330 feet)cases. ARCnet (Attached Resource Computer Network) 53LAYOUT OF A MIXED-IMPEDANCE ARCNET NETWORK,aWITH LOW-IMPEDANCE AND HIGH-IMPEDANCE COMPONENTSbcdefghijklmnopqrstuvwxyz 54 ARCnet (Attached Resource Computer Network)small networks, the throughput is lessARCnet Advantagesthan 65 percent of maximum, and thisARCnet has the following advantages:value decreases as more nodes areadded to the network.Components are relatively inexpen-sive.Street prices for basic ARCnetThe network administrator must man-NICs usually are less than those forually set a unique address by adjustingEthernet or Token Ring NICs.switches on every NIC in the network
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