TIA/EIA-785 short-wavelength FE standard updated

The TIA/EIA-785 standard for 100Base-SX was developed to support 10-Mbit/sec Ethernet and 100-Mbit/sec Fast Ethernet (FE) at the more economical 850-nm short-wavelength over multimode fiber (see Lightwave, August 2001, p. 29). Prior to the new standard, the only operating wavelength for 100-Mbit/sec FE (100Base-FX) was 1300 nm, incompatible with earlier 10-Mbit/sec Ethernet systems (10Base-FL) operating at 850 nm. Since the 100-Mbit/sec FE standard was originally intended as a backbone technology, the discrepancy in operating wavelengths was not an issue. But as 100-Mbit/sec fiber Ethernet began to be deployed to the desktop, the lack of interoperability and migration path slowed the deployment of fiber networks.

Ratified in May 2001, TIA/EIA-785 eliminated these interoperability and migration issues by supporting data communications between legacy 10-Mbit/sec fiber LAN optoelectronics and a new generation of less expensive short-wavelength

10/100-Mbit/sec-capable products (parallel detection) or between two link endpoints comprising 100Base-SX-capable products (auto-negotiation). With a minimum 300-m link length specified in the standard, conforming products also support horizontal cabling subsystems as specified in the TIA/EIA-568B.1 building cabling standard, whether using the traditional hierarchical star architecture or the newer centralized fiber-optic cabling systems.

The TIA F.O. 2.2 committee (renamed F.O. 4.2) recently completed work on Addendum #1 to TIA/EIA-785, to eliminate the receiver bandwidth specification. Extensive simulation and link testing performed by the F.O. 2.2 working group confirmed that the inclusion of receiver bandwidth was technically redundant with another specification: that of stressed receive sensitivity.

Eliminating this specification removes unwarranted limitations in transceiver designs, potentially expanding the available transceiver and manufacturer choices. Since the transceiver is one of the more costly components composing a fiber port, this amendment is expected to further lower the cost of fiber-to-the-desk networks.

John Struhar is a distinguished member of technical staff at OFS (Norcross, GA) and chairman of the Telecommunications Industry Association's Fiber Optics LAN Section. He can be reached at 770-798-3281 or jstruhar@ofsoptics.com.

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