Optical Fiber and 10 Gigabit Ethernet - part 3

This is the 3rd page of the whitepaper Optical Fiber and 10 Gigabit Ethernet.
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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 2, 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.

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.

There are four different types of single-mode fiber in popular use today (as of the writing of this paper, May 2002). They are summarized in Table 3. The ITU-T Series G.652 recommendation, commonly referred to as standard single-mode fiber, represents the majority of the installed base of single-mode fiber. The G.652 recommendation describes both standard single-mode fiber (IEC type B1.1) and low water-peak standard single-mode fiber (IEC type B1.3). The performance data in the 10GbE standard is based upon the use of standard single-mode fiber type B1.1 and B1.3 or in other words the overall G.652 recommendation. This does not however preclude the use of other types of single-mode fiber with 10GBASE-E since their use may potentially enhance the performance of a 10GbE link.

Standard single-mode fiber is essentially a thin core (5-8 microns) of Germanium-doped glass surrounded by a thicker layer of pure glass and is the overwhelming workhorse of the optical communications infrastructure. Nearly any application can be addressed with standard single-mode fiber, but it is optimized to support transmission at 1310 nm. Performance issues with standard single-mode fiber can become more significant as higher data rates (such as 10 Gbps) and longer distances (>40 km) are encountered. Low water-peak standard single-mode fiber (IEC type B1.3) has the same dispersion characteristics as standard single-mode fiber (IEC type B1.1), but has reduced attenuation in the region of the water peak (nominally 1383 nm). As no specification is given for water-peak attenuation in standard single-mode fiber (IEC type B1.1), attenuation in the region of 1383 nm can be significantly higher than that at 1310 nm. By reducing the water impurities introduced in this region during the time of manufacture, low water-peak standard single-mode (IEC type B1.3) fiber provides identical support to standard singlemode fiber, plus can support additional wavelengths between 1360 and 1460 nm.

Note again that the IEEE 802.3ae specification for 10 Gigabit Ethernet assumes standard single-mode fiber (IEC types B1.1 and B1.3) for all single-mode performance specifications. Additional fiber types (e.g., DSF, NZDSF) may offer benefits beyond the constraints of the standard, but are not required to meet any specifications detailed within the 10GbE standard.

Dispersion shifted fiber (DSF) was introduced in the mid 80’s and represents a very small percentage of the installed base of single-mode fiber. The need for DSF was driven by the development of 1550 nm lasers which have much less fiber attenuation than 1310 nm lasers. DSF allowed optical signals to travel significantly farther without the need for regeneration or compensation due to reduced chromatic dispersion characteristics, effectively allowing an optical pulse to maintain its integrity over longer distances. DSF was well suited to meet these needs for single-channel optical transmission systems. However, with the advent of broadband optical amplifiers and wavelength division multiplexing (WDM) the chromatic dispersion characteristics of DSF presented detrimental effects to multiple wavelength signal integrity. As a result, a new type of fiber was needed, namely non-zero dispersion shifted fiber (NZDSF). NZDSF effectively obsoleted DSF, and thus DSF is no longer commercially offered. DSF is not referred to in the IEEE 802.3ae specification.

Cutoff shifted single-mode fiber is designed to allow for extended transmission distances through lower attenuation and the ability to support higher power signals. This fiber is typically used only for transmission in the 1550 nm region due to a high cutoff wavelength around 1500 nm. Due to significant manufacturing complexity, cutoff shifted single-mode fiber is typically much more expensive than other single-mode fiber types. It is commonly found only in submarine applications due to the stringent requirements in such an environment, and is not likely to be encountered in situations where 10 Gigabit Ethernet transport solutions will be deployed. Cutoff shifted fiber is not referred to in the IEEE 802.3ae specification.

Non-zero dispersion shifted fiber (NZDSF) was introduced in the mid 90’s to address issues encountered with multiple wavelength transmission over DSF by maintaining a finite amount of chromatic dispersion across the optical window (typically 1530-1625 nm) commonly exploited by wavelength division multiplexing (WDM). The primary concern addressed by NZDSF is a nonlinear effect known as four wave mixing (FWM). In simple terms, three wavelengths carrying different information can generate signals at another wavelength. In the regularly spaced channel plan of most WDM systems (usually 1.6 nm or less between adjacent wavelengths), the newly generated noise signals can overlap with a wavelength carrying live traffic. NZDSF mitigates this effect by ensuring that all wavelengths in the region of interest (1530-1625 nm) encounter some finite dispersion and thus signals on adjacent wavelengths will not overlap in time for extended periods. Four wave mixing is reduced as the time during which adjacent wavelength signals overlap is shortened. The reduced chromatic dispersion of NZDSF can also reduce the detrimental contributions of other nonlinear effects such as self-phase modulation (SPM) and cross-phase modulation (XPM). NZDSF is optimized for transmission in the 1530-1625 nm window, but can support some 1310 nm configurations with proper consideration given to laser type and systemconfigurations.

The IEEE 802.3ae specification makes a brief reference to NZDSF as follows: “It is believed that for 10GBASE-E, type B4 (NZDSF) fiber with positive dispersion may be substituted for B1.1 or B1.3 (standard single-mode fiber). A link using B4 (NZDSF) fiber with negative dispersion should be validated for compliance at TP3”.

Standard single-mode fiber can address nearly any application, depending on the level of cost and complexity that an operator is willing to employ. The latter issues become more significant as higher data rates, different wavelengths, and/or longer distances are adopted.

For short fiber spans, optical transmission at 1310 nm remains an appealing option due to the price and availability of lasers at this wavelength. Several factors drive consideration of transmission at higher wavelengths, however. At higher data rates, requirements on receiver sensitivity typically grow more stringent, requiring higher received optical powers to maintain low error rates. Due to relatively high fiber attenuation at 1310 nm (see Table 4), maximum allowable transmission distances are reduced at 1310 nm compared to 1550 nm. At extended distances, which exceed the allowable sensitivities of optical receivers, signals in the 1550 nm region can be optically amplified (usually with an EDFA) whereas optical amplification is not commonly available at 1310 nm. As a result, 1310 nm transmission requires electrical regeneration, which is fundamentally more expensive than optical amplification.

Optical pulses carrying digital information comprise a finite spectrum of wavelengths (not just one infinitely narrow wavelength).Since different wavelengths travel at different velocities in an optical fiber, the individual components of a single pulse will spread as the pulse propagates. Eventually, adjacent optical pulses will overlap with one another and the signal will become excessively degraded. At 1310 nm, attenuation will degrade a signal transmitted over standard single-mode fiber before chromatic dispersion becomes a problem. As a result chromatic dispersion is not an issue for 10 Gbps data rate transmission at 1310 nm over standard single-mode fiber. However, at 1550 nm, increased chromatic dispersion in standard single-mode fiber becomes the significant limiting factor, typically limiting 10 Gigabit Ethernet transmission to 40 km, although this specification is also dependent on the choice of transmitter. Beyond the dispersion limited distance of standard single-mode fiber, a signal requires either electrical regeneration or some means of optical dispersion compensation. DSF and NZDSF have reduced chromatic dispersion in the 1550 nm region, thus extending the allowable distance before regeneration or optical dispersion compensation would otherwise be required.

A routinely cited potential impact on 10 Gbps applications is the influence of Polarization Mode Dispersion (PMD) introduced by some installed fiber infrastructures. PMD effectively separates an optical signal into two identical signals, which propagate down a fiber at different speeds. If the two components are significantly separated when a signal is finally received, the encoded information can be considerably deteriorated. Most optical fibers that comply to the current G.652 (standard single-mode fiber) and G.655 (non-zero dispersion shifted fiber) standards are suitable for 10 Gbps transmission in WAN-size applications. However, there are potential issues with older infrastructures, particularly those that contain fiber installed prior to the 90’s. Some optical fiber manufactured prior to this time had acceptable PMD characteristics, although the lack of PMD performance requirements in an industry standard allowed for significant variation between vendors and their various manufacturing techniques. In fact, the necessity for standardization was precipitated in large part by the discovery of very poor PMD performance with fiber manufactured by one major supplier. Although standardization of PMD largely solved the problem, a significant amount of fiber installed prior to the early ‘90s remains unlit and poses potential problems with 10 Gbps deployment. The situation is significant enough to warrant several major carriers to require PMD testing on any network link being considered for 10 Gbps transport. PMD remains a significant focus in optical fiber development as ultra-high data rates (40 Gbps and above) are considered.

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Related posts:

1. Optical Fiber and 10 Gigabit Ethernet - part 4
2. Optical Fiber and 10 Gigabit Ethernet
3. Optical Fiber and 10 Gigabit Ethernet - part 2