Information Technology Reference
In-Depth Information
TABLE 12.7
Cable Propagation Delay and Delay Skew Limits
Frequency,
Delay (max.),
Velocity of
Delay skew
MHz
ns/m
propagation (min.), %
(max.), ns
1
5.70
58.5
45
10
5.45
61.1
45
100
5.38
62.0
45
1000
5.04
66.1
20*
*Inferred from Cat 7 proposed specifications, which actually specify to 600 MHz.
What do you do if you have legacy Category 5 cabling that was never tested
for gigabit operation? The answer is provided by the TIA's bulletin, TSB-95,
Additional Transmission Performance Guidelines for 100 ohm 4-pair Category 5
Cabling
, which covers additional specifications for testing installed Cat 5 to gigabit
performance standards. TSB-95 adds requirements for equal level far-end crosstalk
(ELFEXT) and return loss (RL), and includes the propagation delay and delay skew
requirements that were added to TIA-568-A. More information is given in Chapter
15 about testing new and existing cabling, as well as what to do if it fails a test
parameter.
Open offices may present special considerations at gigabit speeds. The addition
of a multiuser outlet or a consolidation point in the middle of the horizontal run
makes the absolute performance characteristics of the wiring components even more
critical. The TIA's recommendations are in TIA-568-C (originally described in TSB-
75), and they are more fully described in Chapter 10, “Open-Office Wiring.”
Planning a Gigabit Installation
Before you can move forward with a Gigabit Ethernet installation, you should plan
your network topology. This process will allow you to determine what network
infrastructure standards you need to provide, including the network cabling and
connecting hardware. The first step is to determine a layout of the network. From
this step you will be able to determine the distances, speeds, and types of media you
will use.
Next, you must determine any special parameters that should be verified, such
as a fiber-optic loss budget for runs longer than 100 m. Finally, you must specify the
components and the installation criteria so that you can properly estimate costs and
bid out the project. Figure 12.5 can be used as a guide to understand the cabling dis-
tance limits for different types of cabling, under IEEE 802.3z and 802.3ab.
