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Getting down to the wire with copper Gigabit

Mar 24, 20035 mins

Some organizations building out copper-based Gigabit Ethernet networks are finding that when it comes to squeezing the most out of 1000Base-T, the devil is in the details. And the details have a lot to do with cable.

Some organizations building out copper-based Gigabit Ethernet networks are finding that when it comes to squeezing the most out of 1000Base-T, the devil is in the details. And the details have a lot to do with cable.

While most 1000Base-T switch vendors say their products are certified for use with Category 5, 5e and the newer, sturdier Category 6 cable, users and others say your megabits/sec might vary depending on the type of cable used and how it is installed.

“You have to be a lot more careful with the cable if you’re installing it for Gigabit Ethernet,” says Tim Hadden, project manager for Cache Valley Electric, a company that installs cable for businesses in Utah, Colorado and California.

“When wiring a building for 10/100 [M bit/sec], the cabling was more forgiving,” Hadden says. “You could kink it and bend it and still get full performance.” But when planning for Gigabit Ethernet, “you have to be much more careful with the little things, the termination of the wires and how cables fit into the jacks,” he says.

Experts say that there are many cable-related issues that can slow copper-based Gigabit Ethernet. A big one is delay skew – the mis-synchronization of signals over different wire pairs. Gigabit Ethernet uses all eight wires, or four twisted pairs, in a Cat 5/6 cable to transmit data. The technology provides a 475-nanosecond window for fragments of a packet to arrive from the four pairs of copper wires at a given endpoint.

This window is required because cable makers twist the wire pairs to cut down on cross-talk, or signal interference, among wire pairs. The twisting results in wires of different length causing the signals to arrive at slightly different intervals. If signals miss the window, the packet is dropped and must be sent again. These retransmissions can choke a Gigabit pipe down to 500M bit/sec worth of throughput.

“As more customers migrate to Gigabit, especially to the desktop where the cable runs are longer, these issues will come up more often,” Hadden says.

Some cable vendors have released products in the last quarter aimed at addressing some issues that can slow high-speed LAN links.

Ortronics this month announced patch panel and cable products designed to monitor cable performance. Running through the shielding of its cables is an extra wire, which hooks into a separate port on patch panels. The patch panels can be monitored by hooking a serial port on the panels into a separate IP-based network appliance. This appliance runs software that can monitor the status of patch panel links and can let users access a Web-based monitoring application to view the link status of any patch panel connection.

Also this quarter, Krone released its AirES 2 line of Cat 5e and Cat 6 cable products that use air pockets instead of insulating materials to insulate the cable, making the cable smaller.

This reduced size allows for less variance in lengths of twisted pairs, which results in less delay skew, the company says. Krone says its cabling products can shorten delay skew to about 18 nanoseconds, allowing less packet loss and higher Gigabit Ethernet throughput. The air pockets make the cable one-third smaller than most commercial Cat 5e and Cat 6 products, the company says.

Putting it to the test

Many users have put Gigabit Ethernet over copper to the test, as approximately 4.8 million 1000Base-T ports were shipped last year, according to Gartner. Most of those ports are on backbone switches tied directly to server links via short patch cables.

Sequenom, a San Diego genetics research firm, is one company buying into copper Gigabit. It has 90% Cat 6 cabling deployed throughout its newly built headquarters, where it uses copper to connect servers and a few desktops with Gigabit Ethernet.

“We haven’t had any problems with Gigabit over copper since moving into the new building,” says Mats Vilhelmsson, IT manager for Sequenom.

While a new building with Cat 6 can be more than adequate for Gigabit, the cabling itself can be thicker – sometimes 50% wider in diameter – than Cat 5 or 5e – because of extra shielding and insulation. That means fewer cables will fit into conduits and ducts, a fact that has to be considered before deployment.

Copper Gigabit gotchas

These cable-related issues can hurt performance of 1000Base-T connections, especially longer runs such as Gigabit desktop links.
Delay skew: The mis-synchronization of signals among the four wire pairs used in Gigabit Ethernet can lead to packet drops and slower bandwidth.
Attenuation: While the IEEE says 1000Base-T can reach up to 300 feet over copper wiring, the signal on a cable could weaken depending on the wiring used.
Distance: While fiber-based Gigabit connections can reach for up to 10 miles, 1000Base-T has a limit of about 300 feet. If copper Gigabit links are stretched near the limit, kinking or sharp bends in a Cat 5 or Cat 6 cabling can impede performance.

“The size of Cat 6 is the biggest issue with it,” says an IT manager from a large pharmaceutical firm. “If you’ve had Cat 5 in the past and you want to switch to Cat 6, you’re just not going to get as many drops down to the desktops as you had before.”

The manager says he is in the process of upgrading the corporate backbone to 10G Ethernet, with fiber Gigabit connections to the corporate servers. With Gigabit to the desktop being a future need, especially with some of the research staff, he says his approach is to buy best and “not skimp.”

“Cabling is only 15% to 20% of your total datacom and telecom infrastructure costs,” he says. “There’s no reason to try to pinch pennies with cabling; that’s for when you buy the electronics.”