Cabling for Gigabit

As I briefly mentioned in the Introduction, one of the key requirements for the 1000Base-T standard was that it work with existing Category 5 (CAT 5) cabling. After all, what good was a copper-over-Ethernet standard if it couldn't run on most of the copper that's already in the walls, network closets, and ceilings of corporate America? So the good news is unless your network was installed in 1996 or earlier (the standard was revised during 1995), you're probably ready to rock when you decide it's time to move up to gigabit speeds.

But before you move on to the next section, there are some issues that you need to be aware of:

Four pairs required
1000Base-T uses all four twisted pairs of the Category 5 (or higher) cable to create four 250 Mbps channels. (A different encoding scheme - 5 level PAM - is also used so that the signals stay within the 100MHz bandwidth rating of CAT5 cabling.) These two features result in the ability to achieve full gigabit bandwidth while using existing CAT5 cabling.

But since 10/100Base-T required only two of the four CAT-5 cable pairs, some people didn't bother to connect the unused pairs when doing their own cabling. In other cases, the unused pairs might have been used for telephone or Power-Over-Ethernet (POE). Fortunately, gigabit Network Interface Cards (NICs) and switches are smart enough to automatically fall back to 100Base-T operation if they determine that all four pairs are not available for use. So your network will still work with gigabit switches and adapters, but you won't be getting your money's worth!

I can't tell you the hours I have wasted trying to track down why I wasn't getting a gigabit connection, only to find that I was using a patch cable with only two pairs connected! Check your connector ends carefully to make sure that you see wires crimped in all eight positions! If you find any two-pair-only cables, do yourself a favor now and throw them out!

No bad crimps and no cheap jacks
Another problem that do-it-yourself cablers may find is that poor crimps and inexpensive wall jacks can lower the maximum achievable bandwidth of their gigabit network. These issues can cause impedance mismatches which cause Return Loss, which can cause lower bandwidth. Although you can try "shotgunning" (blindly changing things) the problem if you run into bandwidth problems, you'll need to either have a qualified network installer test your network for Return Loss and Crosstalk to qualify it for gigabit operation, or just live with the lower speed.

Length and topology limitations
1000Base-T is limited to the same 100 Meter maximum length as 10/100Base-T. This rule also means that there is a 200 Meter maximum distance (Network Diameter) from one computer to another that is connected to the same switch.

In general, 1000Base-T network topology rules are the same as for 100Base-T, except that only one repeater is allowed per LAN segment (or to be more exact per "half-duplex collision domain"). But since gigabit Ethernet products don't support half-duplex operation, the last point can be safely ignored. Basically, if your LAN is working with 100Base-T, you shouldn't have to rearrange things for gigabit operation.

For new network installations, the general recommendation is to use CAT5e cable. Although CAT5 and CAT5e cabling both have a 100MHz bandwidth, CAT5e cable is manufactured so that additional parameters that are important for high frequency data signals are better controlled.

TIP: See these Belden white papers for a clearly written explanation of what specs are involved in CAT5e cabling:

- Installation Effects Upon Alien Crosstalk and Equal Level Far End Crosstalk

- Impedance/Return Loss

TIP: This page has a handy twisted pair cabling CAT table.

Although CAT5e cable should work just fine for gigabit-capable networks, many people like to buy "insurance" by using CAT 6 cabling instead. (CAT6 was added to the TIA-568 standard in June 2002 and has a 200MHz bandwidth.) It's your money, but CAT6 is really needed only if you are going to run a 10G network. If you're going to run the cable, however, make sure you also use CAT6 connectors.

If you really have money burning a hole in your pocket, a better way to spend it would be to hire a network installation pro with gigabit Ethernet experience to either install your gigabit Ethernet cabling, or test your existing installation to qualify it for gigabit operation. I definitely recommend hiring the pro if you're installing CAT6, since things like cable bend radius, and number and quality of connectors are essential for achieving the high throughput that you're paying for!

My experience, at least with home installations, is that you are more likely to hit a bandwidth wall due to the devices on the network, not the cabling. So let's look at that next.

Tags: gigabit, LAN, Tutorial,

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