“Better late than never” probably best describes the latest approved Local Area Network (LAN) standard—ANSI/TIA/EIA-785 100 Mbps Physical Layer Medium Dependent Sublayer and 10 Mbps and 100 Mbps Auto-Negotiation on 850nm Fiber Optics.

This new short-wavelength Fast Ethernet standard, known as 100BASE-SX, offers two primary advantages to the industry—a more cost-effective Fast Ethernet solution and an auto-negotiation solution between 10 and 100 Mbps Ethernet.

10/100 Ethernet has been available over copper for a number of years and is probably the most common application in the marketplace. Unfortunately, this seamless migration strategy has not existed for fiber until now.

The primary technical issue prohibiting a 10/100 solution up until now was the incompatibility of the fiber wavelengths between 10BASE-FL and 100BASE-FX. Ten Mbps Ethernet uses the 850nm wavelength, whereas 100 Mbps Ethernet uses the 1,300nm wavelength. While developing the 100 Mbps solution, IEEE simply adopted the existing Fiber Distributed Data Interface (FDDI) physical medium specifications, which were designed for 2-kilometer campus backbone requirements.

To achieve these distances, the long wavelength was needed for 62.5-micron fiber. Little to no consideration was given to Fiber-to-the-Desktop (FTTD), so a 10/100 solution was not considered a requirement. But now, as more and more FTTD networks being installed with reports of upward of 25 percent of all companies that have some fiber in the horizontal, the situation is changing.

Fortunately, the solution allowed the Telecommunuications Industry Association (TIA) to “kill two birds with one stone”—providing a more cost-effective solution and allowing for auto-negotiation. Short-wavelength (850nm) optics are more cost-effective than 1,300nm optics, so 850nm was the selected wavelength.

The 100BASE-SX standard consists of two solutions—the mandatory short wavelength and the auto-negotiation option. The short wavelength 100BASE-SX solution is very similar to the existing long-wavelength counterpart. The signal encoding is identical, which allows the same silicon to be used in Fast Ethernet. In fact, the same optical drivers and receivers are used as in 10BASE-FL. Currently, short-wavelength optics are roughly a third of the cost of long-wavelength optics, therefore it is believed the total product cost for 100BASE-SX products will be 30 percent lower.

Besides the use of 850nm optics, another difference is that the new standard allows the use of any connector type that meets the performance requirements. This gives the green light for manufacturers to deploy the new small-form factor (SFF) connectors and achieve greater density at a lower overall cost. The new standard supports both 62.5-micron and 50-micron multimode fiber over 300 meters, adequate for FTTD and centralized networks.

Auto-negotiation between 10 and 100 Mbps allows a node to establish the highest-performance link (speed and duplex) with its remote partner. Auto-negotiation allows users to upgrade incrementally, rather than in a single expensive and disruptive step and great advantage once reserved for copper. This feature minimizes the cost of upgrading from 10 to 100 Mbps. The optional auto-negotiation in 100BASE-SX uses essentially the same algorithms as its copper equivalent, 100BASE. The new fiber standard uses a modification of the Fast Link Pulses used in copper.

100BASE-SX uses a pulse that is 1.5 ms long, which is longer than that allowed in 10BASE-FL. This pulse is too short to cause a carrier drop but long enough to be easily distinguishable. This approach enables a one-to-one correspondence between copper and fiber link pulses allowing for ease of conversion between copper and fiber.

Therefore, not only will fiber Network Interface Cards (NIC) and switches become available for 100BASE-SX, but also fiber converters that will allow users to use their existing 10/100 copper products over a fiber cabling network. This allows a fiber-cabling network to successfully support both copper and fiber electronics and provide for an easy migration from copper to fiber.

Additionally, this solution allows for parallel detection of non-negotiating devices and also supports both half- and full-duplex operations.

While the new 100BASE-SX standard (ANSI/TIA/EIA-785) will not provide immediate cost parity with 100BASE-T, it does come much closer than the 100BASE-FL standard. Fortunately, it does remove the ease of migration barrier to deploying fiber to the desk either in a traditional or centralized architecture by providing an auto-negotiation capability.

The standards committee has already started discussing the next step—10/100/1000 Mbps over fiber. Additional information on this subject and FTTD-related topics can be found at the TIA Fiber Optic LAN Section (FOLS) Web site at www.fols.org, specifically the white paper “100BASE-SX Fast Ethernet: A Cost-Effective Migration Path for Fiber in the Horizontal.”

BEAM is director of systems marketing at AMP NETCONNECT Systems. He can be reached at (336) 727-5784 or tebeam@tycoelectonics.com