Information Technology Reference
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100 or 1000BaseF. This setup has the advantages of 100BaseT star wiring to the
workstations, but the disadvantages of a single backbone bus cabling between the
hubs. If any hub or backbone link connection fails, the entire backbone segment is
likely to fail.
Figure 14.3b shows another alternative. Here, the hubs are connected to a
higher-level backbone hub, via 1000BaseT. Because all hubs repeat the LAN signals
to all ports, the servers can be located anywhere. Some variations of this scheme put
each hub onto a circuit card that mounts in a hub chassis. The hub cards are inter-
connected at the backplane on a high-speed bus that is independent of the topology
of the network.
This bus serves as a type of backbone to all of the hub cards. However, neither
of these really tends to break up the network into smaller, more efficient chunks.
Any network traffic is repeated throughout all of the connected hubs, effectively
limiting the bandwidth of the entire network to a single 1000 Mbps path. Figure
14.3c shows several hubs connected via switching bridges 1 (or routers) to a back-
bone LAN. Notice that servers on the local hubs are available to local workstations
without transiting a bridge. This effectively makes the 1000 Mbps bandwidth of
hub A add to the 1000 Mbps bandwidth of hub B, as long as the user on one hub
has no need to use the server on the other hub. If a user on either hub needs to have
access to a server on the backbone, the user connects across the intervening switch-
ing bridge. Company-wide applications are often placed on servers on the backbone
to give access to any user without putting traffic on other parts of the network.
Thus, a user on hub A can use a backbone server at the same time that a user on
hub B uses a local server. Figure 14.3d shows a variation using a multiport Layer 3
(router) switch.
This type of backbone hierarchy must be considered in planning the location
of telecommunications roomss, equipment roomss, and the cabling between them.
Repeater backbones and bridge backbones can both be supported by using the
wiring standards we have covered in this topic. However, the specific number of
interconnecting cables between telecommunications rooms may be dependent on
the way you lay out your backbone system.
Switch hubs (or stacked switch hubs) are often placed in separate telecommu-
nications rooms. This means that the LAN backbone cabling must be run between
telecommunications rooms to interconnect the switch hubs. These connections
between telecommunications rooms are traditional backbone cabling. The actual
backbone switches may be placed together in the same equipment room, or located
1 Switching hubs function at Layer 2 of the OSI model, as do bridges. Routers and routing switches
are Layer 3 devices.
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