IBM 8265, Руководство по подготовке площадки, Страница: 31 / 164

© Copyright IBM Corp. 1994, 1998
15
Chapter 3. Cabling Recommendations
Part 0
This chapter describes the cabling recommendations for an ATM system.
External and Internal Cabling Recommendations
Horizontal Cabling
The cabling choices provided by the standards include:
•
150-ohm shielded twisted pair (STP) cable (2 pair), 22 gauge, Types 1, 1A, 2, and
2A.
•
100-ohm unshielded twisted pair (UTP) cable, Categories 3, 4, and 5
(2 and 4 pair).
•
100-ohm twisted pair cable with an overall shield (FTP), Categories 3, 4, and 5
(2 and 4 pair).
•
120-ohm twisted pair cable with an overall shield (FTP) - primarily Category 5
(2 and 4 pair).
Note:
This option
is not allowed in the U.S.A. and Canadian Standards.
•
62.5/125-micron optical fiber (50/125-micron cable is an allowed option).
IBM strongly recommends copper cabling for horizontal attachment. (The horizontal
cabling is the set of cabling that runs from the telecommunications closets to offices or
work areas.)
STP cables, carrying on the bulk cable the letter “A” and using enhanced data
connectors (EDCs), meet the new specifications for STP cables in the draft U.S.A.
and international standards. The original STP cable was specified only through 20
MHz. Although there have been minor changes to the specification at frequencies
below 20 MHz, our testing indicates that the overwhelming majority of the original STP
cable meets the performance specifications of the newly specified STP-A cable
through 300 MHz. It should be noted, however, that the transmission characteristics
of the original data connectors for STP do not meet the requirements of the new STP-
A connectors, as specified in the draft U.S.A. and international cabling standards.
The copper cabling with the best transmission capacity is 150-ohm STP-A cabling
(referred to as STP and STP-A by the EIA Standard, and as IBM Types 1, 1A, 2, and
2A within the IBM Cabling System). IBM strongly supports and endorses their use for
horizontal cabling. For new construction we recommend 1A cabling because its
performance is specified to frequencies up to 300 MHz. The most critical transmission
parameters for copper cabling are attenuation, crosstalk, and the ability of the cable to
suppress EMC emissions. For 150-ohm STP cable, the attenuation and crosstalk
values are significantly better than the corresponding values for any of the defined
100- or 120-ohm cables. In addition, use of 150-ohm STP, with its combination of foil
and braided shield, and its very well defined installation procedures, guarantees both
dc and rf shield continuity. dc shield continuity is important for safety considerations.
rf shield continuity provides significant suppression of EMC emissions compared with
unshielded cable. Note that shielded cable may provide no more emission protection
than comparable unshielded cable, unless the shielded cable has the same level of rf
continuity that is built into the components and installation procedures for 150-ohm
STP.