Gigabit Ethernet

Remember when we used to think 10Base-T Ethernet was fast? Although 10Base-T never reached its theoretical limit of 10Mbps, most networks using twisted pair wiring could wring out 4Mbps or so of sustained network traffic. Then, we ran out of patience with 10Base-T and went to 100Base-T. Instead of 100Mbps (the theoretical limit), most Fast Ethernet twisted pair 100Base-T networks could obtain between 40 and 60Mbps, which would suffice for a long time. Well, not really. The simple fact is network people are never happy with the speed of their networks, and while we are locked in to Ethernet and TCP/IP we do want to get even more speed from the network. Hence gigabit Ethernet.

Gigabit Ethernet has been around for a while, but adoption of the network standard has been slow primarily because of the costs involved in upgrading existing equipment. Lately, though, market saturation and dropping prices have helped make gigabit Ethernet economically viable for many networks.

In this article we take a look at what gigabit Ethernet is, what it can do for you, and how you can upgrade or install a network to support gigabit speeds. We’ll look at some real-world numbers from a production network, and let you decide if the performance boost and costs are justified.

What is gigabit Ethernet?

Gigabit Ethernet runs over any Ethernet media, including twisted pair. Naturally, the speed over a twisted pair network isn’t as fast as over a thick Ethernet or fiber optic cables, but the advantage to twisted pair is that most of our networks have lots of it already strung, ready to handle gigabit speeds. Gigabit Ethernet over twisted pair is called 1000Base-T, which makes sense since 10Base-T (10Mbps over twisted pair) and 100Base-T (100Mbps over twisted pair) are common terms. The standards for gigabit Ethernet are defined in a supplement to the IEEE 802.3ab standard, and grow out of the existing Ethernet systems although with a lot of changes to handle the higher speeds and increased collisions of packets.

If you’ve worked with networks in the past, you might think it would be difficult to get 1000Mbps speeds over a lowly twisted pair network cable, and you would be correct. In order to achieve anything remotely like gigabit speeds over twisted pair, some tricks have to be employed. The first trick is to combine standard Ethernet 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 cable with a complex encoding method for the signals to be sent. 100Base-T4 -- seldom encountered in the real world -- used a method of sending and receiving signals simultaneously over the same pairs of wires.) The 100Base-T4 technique for sending and receiving data simultaneously over pairs of wires was adopted by 1000Base-T. Four pairs of Cat 5 cable are used in each 1000Base-T cable (as opposed to only two pairs – four wires – in 10 and 100Base-T). Finally, the popular 100-BaseTX Fast Ethernet system signaling standard was adopted for 1000Base-T. Thus, 1000Base-T is really an amalgam of the best techniques from several other Ethernet standards. It is also quite different from the 10Base-T and 100Base-T systems we know.

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. 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 for gigabit Ethernet, too. If you have a network comprised of existing Cat 5 twisted pair, it can be used with Gigabit Ethernet right away, although you may not achieve optimum throughput depending on the cable quality, length of run, external source of interference, and so on. Forget about building homemade cables for 1000Base-T: the cable strand 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 Cat 5 specifications. The recommended maximum cable length for 1000Base-T is 100 meters (280 feet), and there can be no more than two transceivers in the segment from NIC to hub.

Network components

The gigabit Ethernet network components are much the same as existing Ethernet components, but carry the technology to handle the higher speeds of gigabit Ethernet. For the most part, existing network interface cards (NICs), hubs and switches, and other switching devices all will not work with gigabit Ethernet unless specifically rated for that purpose.

A 1000Base-T network card looks the same externally as a 10 or 100Mbps interface 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, the card is quite different. That’s because both 10 and 100Mbps Ethernet allow for exposed connectors and transceivers, as well as external transceiver cabling but that’s not allowed with 1000Base-T. Instead, the transceiver is built into the interface card, which makes the card much more expensive. The same is true of hubs and switches supporting 1000Base-T where 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 double speed connections.

There is a special configuration of gigabit Ethernet for fiber optic cables, but the costs of fiber optic restrict this network type to those with lots of money and a specific need to avoid twisted pair (usually for shielding purposes). Handling fiber optic cables requires some skill and practice, especially considering the cost of mistakes.

Installing gigabit Ethernet

Installing a Gigabit Ethernet twisted pair network is the same as any other twisted pair network, except for the care taken with the cables. The network interface cards install in the same manner as any other network card, and the hubs or switches are the same except for the new ratings.

To test gigabit Ethernet we equipped our test lab with a dozen Windows 92/2000 machines connected to four servers: two Linux (RedHat 6.2 and Mandrake 7.1), Sun Solaris, and SCO UnixWare 7. We then used scripts to transfer files back and forth between all fourteen machines using FTP, ran simultaneous X sessions from the servers on the Windows machines (using Hummingbird exceed), and scripted HTTP access from all four servers running Apache web servers. We monitored network traffic speed and collisions using in-line network monitoring devices.

From the user’s point of view, gigabit Ethernet is amazing. Transfers from servers are so fast it’s hard to see any lag from request to completion. In many cases, the hard drives couldn’t keep up with the file transfers. On our test network using twisted pair cables, we managed a peak speed of 450Mbps, with average sustained traffic volumes of about 120Mbps. This is a far cry from the theoretical maximum of 1000Mbps, but still a considerable improvement over the 30Mbps of Fast Ethernet!

As mentioned at the start of this article, the costs for gigabit Ethernet are dropping rapidly. When purchasing NICs and hubs in bulk, you can equip a medium network with gigabit equipment for approximately $100 per node. For small networks, the investment won’t pay off unless you are constantly suffering network congestion. For larger networks, congestion is a routine occurrence and gigabit Ethernet can solve the problem easily. As prices continue to fall, it will not be uncommon to see many networks move to gigabit speeds. The users benefit.