100Base-SX paves path for all-fiber net
 
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Ethernet remains by far the most widely understood and deployed LAN protocol. With many networks still running 10Base-FL, administrators are looking for simple, efficient migration strategies that will allow them to upgrade their networks incrementally as users need higher speeds. The proposed 100Base-SX standard will help them find that path.
While optical fiber has become the cabling medium of choice for network backbones and risers, the decision has not been as clear to the desktop. There have been three main obstacles as network managers look to upgrade to Fast Ethernet.
- 10Base-FL and 100Base-FX are specified at incompatible optical wavelengths, 850nm and 1300nm, respectively.
- Since 10Base-FL and 100Base-FX are incompatible, no autonegotiation scheme was developed for optical systems.
- The initial costs of deploying a multimode-fiber-based network are higher than the costs for copper.
When the existing standards were written, there was little market demand for fiber to the desktop. However, as speeds have increased, net managers have been confronted with an installed copper network that is not up to the task. Given optical fiber's inherent bandwidth and immunity to interference, it is a good choice for high-speed networks, provided the issues highlighted above are resolved.
A group of Telecommunications Industry Association (TIA) members has recognized the need for an optical-fiber-based network with a migration path from Ethernet to Fast Ethernet.
The resulting development of 100Base-SX was driven by the needs of users who want the benefits of optical fiber and the future protection afforded by a standards-based network. The proposed 100Base-SX standard calls for 850nm wavelength optics that are compatible with 10Base-FL and reasonably priced. In addition, the proposed standard will provide fiber customers with a means to upgrade to Fast Ethernet speeds today and install a fiber infrastructure that will support Gigabit Ethernet, thus making future upgrades easy and affordable.
The 100Base-SX standard provides for:
- Operation over at least 300 meters (full duplex) on Fiber Distributed Data Interface grade 62.5/125 micron multimode fiber.
- Interoperability with the installed base of 10Base-FL.
- Low-cost electronics, meaning the LED devices used in 100Base-SX are the same cost-effective devices used in 10Base-FL.
The communication techniques spelled out in the proposed standard are designed to autonegotiate between current 10Base-FL and new 100Base-SX systems, while offering detection of non-negotiating devices.
The requirement to support a minimum link distance of 300 meters - as opposed to the 100-meter constraint for copper - lets system designers use centralized fiber-optic cabling systems that provide service to areas where equipment rooms do not exist and cannot be added. It also allows hub and switch equipment to be centralized, which can reduce total network costs and simplify administration.
And by installing multimode fiber, users can upgrade their networks in the future without recabling. This significantly reduces the lifetime cost of a network.
More than 25 companies have banded together to promote 100Base-SX. Several have already introduced prestandard products, with more under development. Interoperability among these devices was demonstrated last fall at NetWorld+Interop in Atlanta.
The first ballot for the proposed 100Base-SX standard, SP-4360, was completed by a TIA committee at the end of February 1999.
At this point, no significant technical issues with the proposal remain. The TIA expects to send the draft standard out for a second ballot soon, with ratification expected in the fourth quarter of this year.
Related Links
TIA's Fiber Optics LAN Section
100BASE-SX: A New Proposed Standard For Fast Fiber Ethernet
By Yoseph Linde, chairman of LANart.
