The Latest on Category Cabling

Oct 1, 2000 12:00 PM, By Paul Rosenberg

Considering all of the changes in cabling designations and the absence of official standards in this industry, staying on top of the latest developments in category cabling is critical.

Nearly everyone involved with data cabling is familiar with Category cabling and knows its classifications continue to change. Since several of these changes formalized in the past few months, it's a good time to take a closer look at these changing cabling designations. While we typically refer to this topic as category cabling, it involves far more than cable. Jacks, patch cords, patch panels, punch-down blocks, and testing adapters must also function correctly and meet specified testing parameters for any system to function as a Cat. "X" system.

Cat. 5 and Gigabit Ethernet. Currently, one of the most important new network types is Gigabit Ethernet, also known by its technical name, 1000Base-T. Unlike the traditional Ethernet protocol (10Base-T) and Fast Ethernet (100Base-T), this new method of transmitting computer signals uses all four pairs of a standard 4-pair cable, which transmits in both directions over each pair. This makes the operation somewhat more difficult, and has left a lot of computer system administrators asking, "Can I run gigabit Ethernet over my existing Cat. 5 system?"

To answer this question, you must perform tests. However, this requires more thorough testing than traditional Cat. 5 applications. Why? Gigabit Ethernet demands more of the cabling system than 10Base-T Ethernet. And although some Cat. 5 installations passed testing upon installation, the systems are not sufficient for Gigabit Ethernet operation.

You can find the new test parameters in a document called TSB95, which describes the field testing of Cat. 5 cabling systems. TSB stands for Telecommunications System Bulletin. While technically not a standard, many industry professionals view this document as such.

New testing requirements for Cat. 5 testing include:

• FEXT (Far End Crosstalk): This is crosstalk (a signal passing from one pair to another through electromagnetic induction) at the far end of a cable run.

• Return loss: Electrical energy reflects back through a cabling system to the source. Variances in the impedance of the system cause these reflections.

• Propagation delay: The time it takes for a signal to transmit over the length of the cable.

• Delay skew: The difference in the time required for a signal to reach the end of a cable on different pairs. In other words, the time delay between two simultaneous signals sent through each pair reach the end of the cable.

A good tester will perform these tests automatically, but be sure your it is built to measure these new requirements - the older ones aren't. So what do you do if your Cat. 5 system will not pass these new testing requirements, but you'd still like to run Gigabit Ethernet?

1. Remove punch-down blocks, and replace with patch panels. Two connector systems are more likely to pass the new test requirements than the very closely spaced punch-down connections. (This option may or may not be sufficient.)

2. Replace your connectors and/or patch cords with Cat. 5e devices. (This option should suffice.)

3. Replace the entire cabling system. (Typically not a cost-effective solution.)

Cat. 5e. Now approved, Enhanced Cat. 5 cabling (Cat. 5e) incorporates the new testing parameters we just covered. Essentially, this new cabling is an improved version of Cat. 5. Just like regular Cat. 5, it is rated for 100 MHz and used in the same applications. Remember, however, TSB95 details testing techniques. Cat. 5e is an entire set of specifications, not only for testing but also for the manufacture and performance of cables, hardware, etc. Cat. 5e cabling and devices are similar to Cat. 5 equipment, however, they're made for more stringent requirements.

See a contradiction yet? We said Cat. 5e systems are rated for 100 MHz, and you can use them for the transmission of Gigabit Ethernet. Obviously, 100 MHz and one Gigabit are quite different levels of signal transmission. (By a factor of 10!) How do you explain this? Cat. 5e carries signals under the Gigabit Ethernet protocol, but not at the implied signal rate of one Gigabit per sec.

Cat. 6. Cat. 6 cabling remains in development. Although you've seen numerous advertisements for Cat. 6 cabling systems, manufacturers designed these products to a "draft" standard, not a formal one. There are several types of Cat. 6 equipment currently on the market that aren't interchangeable. The fact is, cable and tester manufacturers make their products to different bandwidth standards.

The absence of a standard means Cat. 6 systems are now proprietary; this means if you use Cat. 6, you must stick with one make of equipment from end to end. Suppressing crosstalk in RJ45 jacks is extremely difficult, and each of the manufacturers uses slightly different methods to counteract this problem. That means connectors and cables do not mix and match.

In one installation, a contractor put in more than 300 drops of Cat. 6 cabling, and every one failed its test. The problem was the jacks and patch cords came from one manufacturer and the cable from another. When the end user replaced the jacks and patch cords with a supply from the cable manufacturer, 20% of the drops still failed testing. Only after spending a lot of time troubleshooting did all of the drops eventually pass the test. The crazy thing is the manufacturer used this example to promote Cat. 6 devices, by showing how it would support the installer if he or she ran into trouble!

Cat. 7. Cat. 7 cabling is still in the developmental stage. When it becomes a reality, this system will involve larger shielded cables, larger connectors, and reasonably good bandwidth (600 MHz, more or less).

Currently, U.S. standards committees are doing almost nothing on Cat. 7 and focusing on Cat. 6. However, European agencies continue to work on Cat. 7 and will probably have real standards and products in the market considerably ahead of the United States. That's too bad, because Cat. 7 seems to be a technology that has the potential to handle some relatively high bandwidth levels, without pushing the technology so close to the edge that it fails on a regular basis.