Assessing the Risk

Some network managers might ask what are the odds that Category 5 links in their cable plant won't pass the return loss and ELFEXT tests. The 1000BASE-T Task Force (and the cabling companies) estimate that less than 10% of the installed base of Category 5 cable was improperly installed, i.e., not in accordance with the installation instructions contained in ANSI/TIA/EIA568-A (1995). This improperly installed cable may not support 1000BASE-T. It is critical to observe that such cable also won't support 100BASE-TX. That being said, such substandard links are not a product of the Category 5 cable itself, but rather a product of the connectors in the link. Solutions to these connector problems are discussed below.

Testing for Return Loss and ELFEXT

How do network managers test their cable plant for return loss and ELFEXT? Many vendors are offering new testers or software updates to retrofit older, standard Category 5 cable test equipment. These updated, or new testers will allow measurement of these parameters.

ANSI/TIA/EIA568-A and Testing

The ANSI/TIA/EIA568-A specification does not currently specify testing for return loss, ELFEXT and PSELFEXT. When this specification was last revised in 1995, these issues were not well understood. When dealing with the problem in 1998, the TIA 41.8.1 Task Force responsible for 568A elected to define current Category 5 cabling practices and the new return loss and Far-End Crosstalk measures as Enhanced Category 5. The return loss and Far-End Crosstalk measures for Category 5 cabling will be specified in Technical Service Bulletin ANSI/TIA/EIA-TSB-95.

On the other hand, ISO/IEC11801--the International and European cabling standard--is being modified to add return loss and ELFEXT measures to the specifications for Category 5 cabling and will not define a separate Enhanced Category 5 cabling.

Correcting Problem Cable Installations

If the cabling link doesn't pass Category 5 transmission performance tests and the new return loss and Far-End Crosstalk tests, the problem is most likely in the connectors or patch cable rather than the horizontal cable. ANSI/TIA/EIA568A-1995 allows four connectors in a Category 5 link (see Figure 1 below). Connectors are the major causes of both return loss and far end crosstalk, and cable connector vendors have made significant improvements in connector performance since Category 5 connecting hardware was first introduced. Figure 1 shows a Category 5 UTP horizontal cabling system as per ANSI/TIA/EIA-568-A (1995).

ANSTI/TIA/EIA TSB-95 (1998) defines five corrective actions that can be taken to improve return loss and Far-End Crosstalk performance. (The link should be re-tested after each option is implemented.)

• If the link has a cross-connect, reconfigure the cross-connect as an interconnect.
• Replace the transition point connector with a transition point connector that meets the Enhanced Category 5 specification.
• Replace the work area outlet with an interconnect that meets the Enhanced Category 5 specification.
• Replace the interconnect with an interconnect that meets the Enhanced Category 5 specification.
• Replace the patch cord with a cord constructed from patch cable that meets the Enhanced Category 5 specification to correct for return loss failures at low frequencies (<20 MHz).

Figure 2 shows the same Category 5 horizontal cabling system shown in Figure 1 after the five options described above have been implemented.

Specifications for New Installations

Network designers pulling new cabling might want to consider Enhanced Category 5 or Category 6 to get better signal margin. But to get best performance network designers need to make sure the cabling is properly installed. However, they should be aware that the standards defining performance, installation, and testing of Enhanced Category 5 and Category 6 cabling are still in development. In addition, network designers should be aware that as of late 1998 Category 6 connectors provided by different vendors are not interoperable.

Topology Rules

The rules should be the same as those used for 100BASE-TX. Category 5 link lengths are limited to 100 meters by the TIA/EIA-568-A cabling standard. Half-duplex collision domains should be the same as 100BASE-TX; however, each half-duplex collision domain can support only one half-duplex repeater.