Information Technology Reference
In-Depth Information
You can also scale the numbers a little bit to bring them into the reality of your
network. For example, if the magazine clip were only 10 pages, it would need 40
Mbps of network bandwidth to load in 1 minute. Or, it would need 240 Mbps to
load in 10 seconds, which might be much more tolerable. However, do you have a
100 Mbps network now? What impact does devoting 40% of your network's speed
to one transfer have? The generally accepted utilization limit for Ethernet networks
is 60% to 70% utilization. So, the 40 Mbps transfer would eat up about two-thirds
of your network capacity, and the 240 Mbps would chew out about a 30% hunk of
a Gigabit Ethernet pipe.
Installing cabling and components of the proper category is important for future
upgrades in the data rate of your network. However, network data rates and cable
bandwidth do not directly correspond to each other.
Data rates, of course, are given in bits per second, typically in megabits per sec-
ond (Mbps) or even gigabits per second (Gbps). Cabling components (including
cable and connectors) are specified in terms of useful bandwidth in hertz (cycles per
second), typically in megahertz (MHz) or in gigahertz (GHz). Data bits are usually
encoded in a way that reduces the actual bandwidth requirement for a given data
rate. For example, 100 Mbps Fast Ethernet actually requires about 30 MHz of
bandwidth, because of the encoding. Similarly, ATM 155 uses an encoding tech-
nique called CAP-64 to place 155.52 Mbps on the cable at less than 30 MHz.
This is the reason that some data networking standards can operate at much
higher data rates than would seem to be possible, given the cable's nominal band-
width. Thus, we can put a 1 Gbps (1000 Mbps) data rate over cable that is rated
for a 100 MHz bandwidth on each of its four pairs. Now, this is no easy feat. The
interface and digital signal processing circuitry to accomplish this with copper wire
is very sophisticated. Any minor disturbance in the cabling components can cause
such a data link to fail. That is why it is so important to use the proper category of
components for the application.
Table 4.3 shows the improvements that have been made to cable specifications
to enhance the data-carrying capacity. The introduction of Category 5 expanded the
useful operation frequency (bandwidth) from 20 MHz to 100 MHz. TSB-95 added
testing requirements for older Category 5 links to near-5e performance in order to
predict possible operation for Gigabit Ethernet, although the base bandwidth stayed
at 100 MHz.
Category 5e increased the NEXT isolation by 3 dB, incorporated the power-
sum requirements for multiple disturbers (transmission on more than one pair),
