Ethernet Part 2: Gigabit Ethernet

In the last article we looked at running Gigabit Ethernet over a twisted pair cable (1000Base-T). This time, we look at a faster medium: fiber optic cables. Gigabit Ethernet over Fiber Optics is called 1000Base-X and is specified in the IEEE 802.3z standard (as if you were going to run out and read that standard!).

The 1000Base-X system specifies three components according to the standard: two different fiber optic cables and a copper jumper cable. The copper jumper is almost never used in commercial products, so we’ll assume one of the fiber optic cables will be your choice of medium. The two fiber cables are designed to different specifications, depending on the wavelength of the light used in the fiber cable. 1000Base-SX is the short wavelength cable, while 1000Base-LX is not surprisingly the long wavelength cable. (For completeness, the copper jumper is 1000Base-CX.) The short wavelength systems are usually used where short runs are involved and are much less expensive than long wavelength systems.

The 1000Base-X system is very similar to the X3T11 Fiber Channel system in use for other devices. Fiber Channel has been used for linking multimedia servers in a high-speed network for a while now and is widely accepted. There are some changes to the X3T11 specifications to meet 1000Base-X, primarily in that encoding of the signal is now used to boost throughput (Fiber Channel by itself is good for about 800Mbps, while 1000Base-X is for 1000Mbps).

Physically, 1000Base-X interface cards are much the same size as other Ethernet adapters, except the back plane usually has a duplex fiber optic connector. One fiber optic is used for transmitting, and the second in the duplex setup is for receiving. The fiber transceiver is almost always built into the network interface card and not external to the unit. Typically, 1000Base-X cards support only full duplex modes. The fiber cables in a duplex connector run to a switch or hub designed for either short or long wavelength versions of 1000Base-X. Some high-performance (and expensive) switches and hubs support mixes of short and long wavelengths, providing mixed network connectivity. Alternatively, a device called a Gigabit Interface Converter (GBIC) can be used to convert long and short wavelength versions of 1000Base-X on the same port. To connect two hubs together, a cross-over cable is used.

To confuse the issue, although most fiber vendors use a duplex connector (called a Duplex SC), some versions of 1000Base-X are using a connector called MT-RJ which looks similar to a Firewire (IEEE 1394) connector. The MT-RJ connectors are smaller and more compact, enabling more connections in a set amount of space or card backplane.

Cabling a Gigabit Ethernet Fiber Optic network is a time-consuming task, as the cables usually must be treated with more care than standard twisted pair. It is quite easy to kink a fiber cable and make it unusable, especially at these high transmission rates. Curves in the cables must be eased into with transitions, and stapling the cable to supports requires care to avoid puncturing the cable. The short wavelength 1000Base-SX network can support runs of up to 220meters (about 1,350 feet), which usually suffices for cabling a single building. The longer wavelength 1000Base-LX can have runs up to 5,000meters (about 6,000 feet) which is used for connecting buildings together. Installing Fiber Optic NICs is no more difficult than any other network card, and the hubs and switches are easily set up. Some operating systems have problems with some network cards, especially with Windows NT and Windows 2000, so check manufacturer support carefully before purchasing NICs.

Apart from the care required to cable Gigabit Ethernet Fiber Optic, though, the process of installing a network is routine. Once installed, diagnostics can be used to test the cabling and NICs, and to ensure the drivers are functioning properly. In use, 1000Base-X is noticeably faster than 1000Base-T, with an almost alarming transfer speed of network information. In my tests of transferring 50GB of files, the transfers completed almost as fast as I could time the system and verify the transfer.

So, given both 1000Base-T and 1000Base-X, which should you install for a client. The simplest answer is that if an existing 10Base-T or 100Base-T network exists use that for 1000Base-T. The speed of the 1000Base-T system will be more than adequate for most installations. For a fresh installation, it’s important to balance the customer’s need for transfer speed with the costs. 1000Base-X is quite expensive compared to 1000Base-T, and the performance differences will seldom be worthwhile. For joining high-speed networks together, or for multimedia servers in a cluster, the speed of 1000Base-X may be justified, but for most client machines used in an office, fiber optic is overkill.

The lack of an upgrade between twisted pair and fiber cable makes it difficult to move from one format to another. If future upgrades are to be considered, it’s safe to assume that twisted pair is almost at the limit and offers no real future performance upgrade. Fiber, on the other hand, has loads of potential for faster transfers, and can easily be upgraded as new technologies come along.