Ethernet Part 1: Gigabit Ethernet

The subject of networking keeps coming up in the e-mail I receive as well as questions at conferences and trade shows. The questions lately have been about Ethernet and some of its variations, so it seems like a perfect time to begin a new series of Help Desk articles on Ethernet. In this series we’ll look at coming, state-of-the-art, and older stuff. We start with a look at Gigabit Ethernet.

There was a day when 10Mbps was considered fast. Then came 100Mbps Fast Ethernet, which despite its failure to meet 100Mbps rates still was faster than the old Ethernet. The last couple of years have seen a lot of buzz about Gigabit Ethernet, and the potential to jam 100Mbps down an Ethernet cable. The buzz was mostly that – buzz – because few Gigabit Ethernet devices were available at reasonable prices, and most didn’t work quite as advertised. Lately, though, Gigabit Ethernet has been coming down in price, hubs are readily available and almost reasonably priced, and the promise of fast speeds for heavily used networks seems real at last.

Gigabit Ethernet runs over any Ethernet media, really, including twisted pair. Of course, the speed over twisted pair isn’t as good as over thick Ethernet cable or fibre optics, but the advantage to twisted pair is that most of our networks have lots of it already strung, ready to be sped up to gigabit speeds. Technically, the specifications for Gigabit Ethernet over twisted pair are called 1000Base-T, which makes sense since 10Base-T and 100Base-T are common terms, and are defined in a supplement to the IEEE 802.3ab standard. You might think it difficult to get 1000Mbps over a twisted pair line, and you would be correct. In order to achieve anything remotely like that speed over twisted pair, some tricks have to be employed.

The first trick is to combing standard signaling techniques with an encoding method that was originally designed for the seldom-used 100Base-T2 and 100Base-T4 standards. The 100Base-T2 (not 100Base-T) system used two pairs of twisted pair Cat 3 cable with a relatively complex encoding method for the signals to be sent. For 1000Base-T, four pairs of Cat 5 cable are used. 100Base-T4 (seldom if ever encountered in the real world) used a method of sending and receiving signals simultaneously over the same pairs of wires. This was adopted by 1000Base-T, too. Finally, from the popular 100-BaseTX fast Ethernet system the signaling standard was adopted for 1000Base-T.

All of the signaling and encoding processes as well as the much higher transmit speeds means that the cable used for twisted pair Gigabit Ethernet are sensitive objects. Cat 5 cables must be used, although Cat 5e is a better choice since it has better signaling carrying capabilities. (There are a number of cables that exceed Cat 5 specifications, and they can all be used.) If you have existing Cat 5 twisted pair, it can be used with Gigabit Ethernet although you may not achieve optimum throughput, depending on the cable quality, length of run, external source of interference, and so on. Building home-made cables is a waste of time with 1000Base-T: the twists have to be maintained all the way to the connector body, and it’s very difficult to construct a cable manually that can meet the Cat 5 specifications. (I pride myself on my cable building abilities, but building 1000Base-T cables made me give up and buy premade cables.) As mentioned, all four pairs have to be in place with the twisted pair cable, so the older four-conductor shortcut cables won’t hack it for Gigabit Ethernet. Maximum cable length for 1000Base-T is 100 meters (328 feet), and there can be no more than two transceivers in the segment from NIC to hub.

Physically, a 1000Base-T network card looks the same externally as a 10 or 100Mbps card. There is usually a single 8-pin RJ45 connector on the back plane with a number of status LEDs to show line conditions. Internally, though, the card is quite different. Both 10 and 100Mbps Ethernet allow for exposed connectors and transceivers, as well as external transceiver cabling. That’s not allowed with 1000Base-T. Instead, the transceiver is built into the interface card (which makes the card more expensive). The same is true of the hub or switch supporting 1000Base-T: a transceiver is used internally for each cable. Most 1000Base-T network cards for PCs use autonegotiation to allow the card to adjust to the fastest supported speed, 10, 100, or 1000Mbps. Gigabit Ethernet over twisted pair does support half-duplex mode, so the potential is there for some serious speed.

Installing a Gigabit Ethernet twisted pair network is no more difficult than any other twisted pair network, except for the care taken with the cables. The NICs install in the same manner as any other network card, and the hubs or switches are familiar pieces of equipment. Over a test 1000Base-T network I set up, I managed to get blazingly fast transfers of files (you ain’t seen nothing like Gigabit Ethernet at full speed – the hard drives can’t keep up with file transfers!). I am certain I didn’t get close to 1000Mbps on my test network but my datascopes couldn’t keep up with the measurement. Installing Gigabit Ethernet is easy, the costs are more reasonable than ever, and most customers will think they’ve gone to heaven when you unleash the speed of this network upgrade on them. In the next installment, Gigabit Ethernet over Fiber Optic cables.