Going down to the wire with copper gigabit

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 per second 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,” said Tim Hadden, project manager for Cache Valley Electric Inc., a company that installs cable for businesses in Utah, Colorado and California.

“When wiring a building for 10/100 [Mbps], the cabling was more forgiving,” Hadden said. “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 said.

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 500Mbps 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 said.

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 said. Krone said 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 said.

Cat 6 cabling itself can be thicker – sometimes 50 per cent 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.

“The size of Cat 6 is the biggest issue with it,” said 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.”