Information Technology Reference
In-Depth Information
capabilities of that central machine also limit both communication
speeds and the number of separate computers allowed. At a con
ceptual level, each machine requires its own wire to the central
machine. In practice, some technology (called
multiplexers
and
concentrators
, in case you care) may allow several circuits to share
the same physical wire, although such technology can help only to
a point.
Ethernet Networks: Rather than rely on a central machine with a
separate wire for each computer, an Ethernet network is based on the
use of a single wire (or bus) running to all machines; that is, the same
cabling system is used for all computers in a local network. This single
cable approach has several nice characteristics. Conceptually, the same
wire or cable is shared by all machines, so separate wiring is not needed
between each machine and a central point. Any computer can send in
formation directly to any other computer without an intermediary, so
communication is not limited by the capabilities of an intermediary.
Hubs Connect Local Computers to Form Small Networks
Although Ethernet systems generally rely on a cable rather than separate wires to
connect computers, in practice, groups of machines may be individually attached to a
single hub to maintain needed electrical properties. These hubs in turn would be con
nected to each other by cables, just like star networks are linked to make long dis
tance phone calls possible. Ethernet networks normally use RJ45, Cat 5 cables; and
communication standards follow 10BaseT, 100BaseT, or 1000BaseT, depending on
electrical interference and computer capabilities. In order to maintain appropriate
electrical connectivity, computers on an Ethernet network plug into a local hub, mak
ing small sections of the network into a star network. Use of the hub, or central ma
chine, also simplifies the difficulties of appropriately connecting the pairs of wires re
lated to sending and receiving signals. Straightthrough cables go from each local
computer to a hub, and the internal circuitry of the hub does the actual connecting of
machines. Hubs also can contain circuitry to check for transmission errors, allowing
corrections to be made quickly when necessary. This enhances overall transmission
speed and reliability.
Although hubs can form a star network in small areas, hubs also can be connected to
yield large networks. Such connection of hubs is called
uplinking
. As with direct commu
nication between computers, the connection of hubs requires that sending and receiving
signals are sent on appropriate pairs of wires. Hubs may handle this administration in
ternally and thus be connected by straightthrough cables. Alternatively, hubs without
this type of internal circuitry normally require crossed RJ45 cables for connections to
other hubs.
