4. 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

As this Cisco article shows, 1000Base-T uses all four twisted pairs of the Category 5 (or higher) cable to create four 250Mbps 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!

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 currently manufactured CAT5 cable should work just fine for 1000Base-T installations, think of using CAT5e as buying insurance against bandwidth problems. If you're on the fence, price out using both CAT5 and CAT5e for your installation, and let your pocketbook be your guide!

One thing you shouldn't buy into, however, is any recommendation that tells you to use CAT6 for a gigabit Ethernet installation. CAT6 was added to the TIA-568 standard in June 2002 and has a 200MHz bandwidth. Despite the fact that vendors would love to sell you their pricier CAT6 wares, you should only consider it if you're going to be running 10Gigabit Ethernet over copper, which frankly any small networker is going to be extremely unlikely to do! And pitches for CAT7 cable? Fuggedaboutit!

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!