Networking

Get IT Done: Make sure your network cabling is ready for gigabit speeds

Assess network specifications for gigabit Ethernet and increased speed


With most LANs running at 100 Mbps, the next step for Ethernet is gigabit. Use of copper gigabit Ethernet is growing, due to the fact that the costs associated with it are quickly dropping and coming within reach of more organizations. If you’re contemplating migrating some of your users to gigabit speeds, you’ll need to determine whether your network cabling is up to the job. I’ll explain what you’ll be looking for in your cabling plant (I’ll focus solely on copper-based networking).

Step 1: Gather your information
To decide whether you’re ready to begin a migration to gigabit networking, you must gather certain information about your current network cabling. You’ll be checking to see whether your cabling meets the gigabit Ethernet requirements on several parameters. If your cabling plant is certified for category 5e or the new category 5 standards, you should be able to run at gigabit speeds with no problems.

If your cabling plant was certified before December 1999, you may need to perform key tests to determine whether your cabling meets the new standards. First, look at the test results from the original cable installations. If the results show that your cabling meets the requirements for gigabit, you’ll likely have no problems. If that is not the case, consider hiring a professional to test and rectify the network under the new standards. People who test cable for a living (such as EEU Professional Testing Services and Network Infrastructure Services) know exactly what to look for.

Another option is to purchase a cable tester and do the testing yourself, although I don’t recommend this method. If you do decide to do the tests yourself, use a level IIE tester that can run a 100-MHz sweep of the network. In this testing, you’ll focus on delay skew, equal level far end crosstalk (ELFEXT), power sum equal level far end crosstalk (PSELFEXT), power sum near end crosstalk (PSNEXT), propagation delay, and return loss (the measure of the ratio of a signal transmitted to the power returned); they’re all critical parameters for gigabit Ethernet. Compare your test results to the specifications in Table A to see if your current network is within the accepted targets for gigabit speeds.

 

 

Table A
Specification Minimum performance targets
Frequency 1-100 MHz
Attenuation 24 dB
NEXT 30.1 dB
Power sum NEXT 27.1
ACR 6.1 dB
Power sum ACR 3.1 dB
ELFEXT 17.4 dB
Power sum ELFEXT 14.4 dB
Return Loss 10 dB
Propagation Delay 548 ns
Delay Skew 50 ns

The specs in Table A are listed for category 5e only. If your current network was certified before December 1999 but meets the above criteria, your network is ready for gigabit speed.

 

 

 

Step 2: Make sure you have the right cable
You must have the proper cabling for gigabit networking. You need a minimum of category 5 (Cat5) cabling to even consider moving to gigabit. If your network is running at 100 Mbps over copper, you already meet this minimum requirement. For existing installations, Cat5 might work, but I strongly recommend against it because it doesn’t meet the required gigabit tests. For new cabling installations, consider category 5e or category 6 cabling.

 

Knowing the difference in cable does not mean that you can just buy the cable, put it in the walls, and punch it down to just anything. You also need to make sure that the punch blocks, wall plates, and patch cables meet category 5 requirements or better. For a jack to meet category 5e standards, it must have a 2-1-3-5-4-6-8-7 pinout configuration instead of the category 5 pinout of 1-2-3-4-5-6-7-8.

Step 3: Know your current cable installation
I’ve seen some older category 5 networks where the professional installers decided that those pesky four extra wires in the cable were getting in the way. Instead of punching them down to the punch block, they simply snipped them off and only punched down wires 1, 2, 3, and 6, which are the only wires used for 10 and 100 Mbps Ethernet.

In many cases, vendors have put together perfectly good category 5 or better patch cables for use in 10 or 100 Mbps applications but only used four of the wires as a way to save on the cost of making the cables. If you try to use this cable for a gigabit application, you will be sorely disappointed; it requires the use of all four pairs, unlike older Ethernet. The pairing in gigabit cabling must be 1-2, 3-6, 4-5, 7-8; the 4-5 and 7-8 pairs aren’t used in 10/100 Ethernet.

It’s very easy to tell if your cabling contractor or patch cable vendor has cut corners in this way. If you look at the RJ-45 connectors with the pin side up on one of your typical patch cables, you should see eight wires—one for each gold pin. If you don’t see wires connected to pins 4, 5, 7, and 8, the patch cable will be unusable for a gigabit application. Likewise, on your patch panels, look at the punch down block for each port. If you only see four wires punched down, your contractor didn’t do his or her job properly. If this is the case, consult the warranty that should have been included with the cabling job and have the contractor correct the problem.

At times, simply looking at the cable will not tell you if all of the wires are properly connected. Everything on your 10/100 Mbps network may be running fine, and you may have looked at the cable and found that all of the pins were connected, but when you attempt to use a gigabit adapter on the cable, it doesn’t work. Why? It very well could be that one of the individual wires on pin 4, 5, 7, or 8 is broken somewhere along the line between the patch panel and the wall outlet. You wouldn’t have noticed this problem with a 100 Mbps network because those wires aren’t used. Detecting this kind of problem before it bites you is a little more difficult than just looking at the cable, but not much more so. To detect such a problem, you’ll have to use a good cable tester. Many of the better cable testers are able to run continuity tests, which can tell you if a particular wire in a cable is broken and the approximate location of the break. Such a tester is invaluable in the migration to gigabit network (as well as to the network administrator’s toolkit).

An invaluable tester
One of my favorite pieces of hardware is the Microtest Microscanner. The Microscanner’s list price is $399 and includes a built-in cable continuity tester that checks all eight individual wires in the cable to make sure that they are not broken. If one is broken, the device can also tell you about how far down the cable the break is so that you can repair it. At the very least, it will tell you which cables are physically incapable of running gigabit Ethernet so that you can plan your rollout accordingly.

Remember the steps
Take a careful, methodical approach to your migration to gigabit networking. Remember to gather all the information you have available about your current network setup, understand the cabling requirements for gigabit, and understand the gigabit specifications and parameters. With this information, you can quickly and painlessly confirm whether your company’s network is ready for the speed of gigabit.

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