Optical Transceivers for 10 Gigabit Ethernet

Introduction

As 10 Gigabit Ethernet (10GbE) is introduced into networks the physical limitations and properties of optical fiber introduce new challenges for a network designer. Due to the increased data rate, fiber effects, such as dispersion (intermodal, chromatic or polarization), become a factor in the achievable distances of 10GbE links. This leaves the network designer with new decisions and trade-offs that he/she must understand and overcome.This paper provides an introduction to the world of optical fiber and covers the unique network design issues that 10GbE introduces into an optical fiber network.

In multimode fiber, the time it takes for light to travel through a fiber is different for each mode resulting in a spreading of the pulse at the output of the fiber referred to as intermodal dispersion. The difference in the time delay between the modes is called Differential Mode Delay (DMD). Intermodal dispersion limits multimode fiber bandwidth. This is significant because a fiber’s bandwidth determines its information carrying capacity, i.e., how far a transmission system can operate at a specified bit error rate.

10 Gigabit Ethernet

The IEEE 802.3ae 10 Gigabit Ethernet specification includes a serial interface referred to as 10GBASE-S (the “S” stands for short wavelength) that is designed for 850 nm transmission on multimode fiber. Table 2 provides the wavelength, modal bandwidth, and operating distance for different types of multimode fiber operating at 10 Gbps. Technical issues relating to the use of laser sources with multimode fibers (discussed in the previous section) has significantly limited the operating range of 10GbE over “FDDI grade” fiber. The “FDDI grade” multimode fiber has a modal bandwidth of 160 MHz*km at 850 nm and a modal bandwidth of 500 MHz*km at 1300 nm.

To address the operating range concern, a new multimode fiber specification had to be created for 10GbE to achieve multimode fiber operating distances of 300 m (as specified in the TIA/EIA-568 and ISO/IEC 11801 cabling standards). This new fiber is referred to by some as “10 Gigabit Ethernet multimode fiber” and is an 850 nm, laser-optimized, 50/125 micron fiber with an effective modal bandwidth of 2000 MHz*km and is detailed in TIA-492AAAC. Its key difference, relative to legacy multimode fibers, are the additional requirements for DMD specified in TIA-492AAAC enabled by a new measurement standard for DMD (TIA FOTP-220). As shown in Table 1, this fiber can achieve 300 m of distance with a 10GBASE-S interface. Many leading optical fiber vendors are actively marketing this new multimode fiber for 10GbE applications.

Description	62.5 micron fiber		50 micron fiber			Unit
Wavelength	850	850	850	850	850	nm
Modal bandwidth (min)	160	200	400	500	2000	Mhz km
Operating range	2-26	2-33	2-66	2-82	2-300	m

There are two major factors which will likely drive use of this new “10GbE multimode fiber”:

1. the popularity of short reach (300 m or less) 10GbE applications and
2. the cost of 10GBASE-S interfaces relative to the others.

Evidence of the popularity of low cost, short distance 850 nm multimode Ethernet applications can be found in the number of 1000BASE-SX ports shipped for 1 Gigabit Ethernet. 1000BASE-SX operates up to 550 meters on multimode fiber and has garnered a large percentage of the total number of 1 GbE switch ports shipped. Ultimately the marketplace will determine the popularity of “10GbE multimode fiber”. The alternative is to use single-mode fiber over a 10GBASE-L or 10GBASE-E interface or the 10GBASE-LX4 interface, which supports both single-mode and multimode fiber over distances of 10 km and 300 m, respectively.

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