The basic choice is between optical fiber and copper. Until recently, 10 gigabit rates could be achieved only by using optical fiber, but now standards have been established permitting the use of twisted pair. Fiber and copper can be used in the same data center with the choice for each connection based on the distance between devices to be connected. To further complicate the decision, several options are available for both fiber and copper.
Fiber options include multi-mode or single-mode fiber. Multi-mode is limited to shorter distances than single-mode. Because multi-mode cable is thicker than single-mode, portions of the light beam follow different paths as they bounce back and forth between the walls of the fiber. The result is that the signal reaching the other end of the cable is distorted. The amount of distortion increases with the length of the cable. The light beam follows a single path through thinner single-mode cable, so the amount of distortion is much lower.
Multi-mode cable was installed in the 1980s to support FDDI at 100 Mbps. This cable was 62.5 microns thick. The initial 10GBase-SR fiber interface standard was limited to 26-meter connection distances using this cable. Removing and replacing installed cable is expensive, so the IEEE developed 10GBase-LRM, also known as IEEE 802.3aq. It uses a circuit that compensates for distortion. The result is that the new standard can drive signals 220 meters over 62.5 micron cable.
Cable production techniques have improved. Newly manufactured multi-mode cable is only 50 microns thick. The 10GBase-SR interface can support 300-meter connection distances with new, thinner multi-mode cable.
Single-mode fiber can carry signals up to 40 km, so it is used to connect to wide-area networks. It is more difficult to connect because of the thinness of the cable, and it requires a more expensive laser light source than multi-mode does. The result is that multi-mode is used where the connection length does not require single-mode.
10GBase-CX4, standardized by the IEEE as 802.3ak in 2002, was the first available copper interconnect for 10 GbE. Similar to copper InfiniBand cable, 10GBase-CX4 uses four cables, each carrying two-and-a-half gigabits of data. It is limited to approximately 10 to 15 meters but provides an extremely cost-effective method to connect equipment within that distance.
Small Form-Factor Pluggable Copper (SFP+CU) provides another low-cost, low-energy interconnect. It is limited to approximately 10 meters.
IEEE 802.3an, more widely known as 10GBase-T, was completed in 2006 and is expected to become the most widely used 10 GbE interconnect method. It utilizes familiar twisted pair cable and RJ-45 connectors.
Current interface chips consume more power, produce more heat and introduce greater latency than fiber or earlier copper interfaces. The same problems were encountered with initial Gigabit Ethernet twisted pair interfaces, so it is expected that second (and later) generation chips will improve. In the meantime, these issues have slowed 10GBase-T adoption.
In many cases, existing twisted pair cable must be replaced. Although some installations have been successful running 10 GbE over installed Category 5 cable, this is not recommended. Even if testing indicates that the cable can carry 10 megabit traffic, intermittent problems may occur in the future.
Category 6, 6a UTP or 6a F/UTP (foil wrapped UTP) is recommended. Category 6 cable is limited to 55 meters, while 6a or 6a F/UTP can support 100-meter connections. Category 6 and 6a cables are thicker than Category 5 to reduce the possibility of alien crosstalk -- that is, crosstalk between adjacent cables. Thicker cable means that conductors in adjacent cables are farther apart, reducing the possibility of crosstalk. Foil wrapping virtually eliminates the possibility of alien crosstalk for Category 6a F/UTP.
Replacing existing cable may not be as simple as pulling out one cable and putting in another. Increased cable thickness means that Category 6 cable takes up more space in cable trays. Additional trays may be necessary if there isn't enough room in the existing tray. Care must also be taken to avoid piling too many cables on top of one another. The weight can squeeze cables on the bottom and reduce the spacing between conductors.
Evaluate labor costs carefully. Fiber has been considered more expensive to install because care must be taken to avoid tight bends and install connectors . Twisted pair at 10 gigabit rates also requires extra care, compared with lower-bandwidth twisted pair, when attaching connectors and arranging cable in trays. Installation costs for twisted pair can increase significantly if additional trays must be installed.
Plan for the higher data rates
100 GbE is on the way. The IEEE 802.3ba committee began working on 100 GbE in 2007 and expects to complete the work in 2010. The goal is to support 40-km connections over single-mode fiber, 100 meters over multi-mode, but only 10 meters over copper. Consider installing fiber now, even if copper appears the best choice for current needs.
About the author:
David B. Jacobs of The Jacobs Group has more than 20 years of networking industry experience. He has managed leading-edge software development projects and consulted to Fortune 500 companies as well as software startups.
This was first published in April 2009