Optical Fiber and 10 Gigabit Ethernet - part 4

This is the 4rd page of the whitepaper Optical Fiber and 10 Gigabit Ethernet.
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Key factors to consider in the design of 10 Gigabit Ethernet networks are:

• The network topology, including operating distances, splice losses and numbers of connectors (i.e. the link power budget).
• The fiber cabling type (i.e. single-mode or multimode fiber) and the performance at a specified wavelength. The performance is characterized by channel insertion loss (cabling attenuation), and modal bandwidth(for multimode fiber).
• The use of mode-conditioning patch cords if required. The 1310 nm WWDM solution, 10GBASE-LX4, requires the use of a mode-conditioning patch cord on multimode fiber to achieve its specified range of operating distances.
• The implementation of a cabling design, compatible with LED and laser-based Ethernet network devices, which will allow the integration of current LED based 10 Mbps and 100 Mbps networks and laser-based 1 Gbps and 10 Gbps networks.

When designing individual fiber links, the first step is the characterization of the link power budget. This value (expressed in dB) is specified in the 10GbE standard for each optical interface. Tables for all interfaces are shown in this section. The link power budget is calculated by taking the difference between the minimum transmitter power launched into the fiber, and the minimum receiver sensitivity (Figure 2). The receiver sensitivity is the minimum amount of power that is necessary to maintain the required signal-to-noise ratio over the specified operating conditions. The link power budget determines the amount of total loss due to attenuation and other factors that can be introduced between the transmitter and the receiver.

Figure 2: Link Power Budget

The link power budget is applied to account for the channel insertion loss and power penalty. Channel insertion loss is the key parameter and is defined to address the cable and connector losses (Figure 3). The channel insertion loss consists of the specified cable loss for each operating distance, splice losses and the loss of two connections. A connection consists of a mated pair of optical connectors. An allocation of 1.5 dB is budgeted for connector and splice losses for multimode fiber and 2 dB for single-mode fiber. For 10 Gigabit Ethernet applications a power penalty is allocated to the link power budget. This power penalty takes into account effects such as dispersion that may cause inter-symbol interference and therefore degrade an optical signal.

Figure 3: Fiber Optic Cabling Channel

The 10 Gigabit Ethernet operating distances provided in the tables below are limited by the channel insertion loss, the cable bandwidth for multimode fiber, and the optical transceiver characteristics (i.e., PMD types). 10GBASE-E distances greater than 30 km are considered “engineered links” because to support those distances the attenuation of the cable needs to be less than the maximum specified for standard single-mode fiber (Table 4). Therefore, distances greater than 30 km for installed cabling should be “field-tested” for verification of conformance to the 11 dB (Table 7) channel insertion loss specification. Insertion loss measurements of installed fiber cables are made in accordance with ANSI/TIA/EIA-526-14A/ method B and ANSI/TIA/EIA-526-7/method A-1.

Table 5: 10GBASE-S link power budget as per IEEE Draft P802.3ae/D5.0

* These channel insertion loss numbers are based on a wavelength of 850 nm
** These power penalties are based on a wavelength of 840 nm
 

Table 6: 10GBASE-L link power budget as per IEEE Draft P802.3ae/D5.0

* These channel insertion loss numbers are based on a wavelength of 1310 nm
** These power penalties are based on a wavelength of 1260 nm
 

Table 7: 10GBASE-E link power budget as per IEEE Draft P802.3ae/D5.0

* These channel insertion loss numbers are based on a wavelength of 1550 nm
** These power penalties are based on a wavelength of 1565 nm and other penalties
*** Greater than 30 kilometers distance mandates an "engineerd link" requiring "field testing" for verification of conformance to the 11 dB channel insertion loss specification. Insertion loss measurements of installed fiber cables are made in accordance with ANSI/TIA/EIA-526-14A/method B and EANSI/TIA/EIA-526-7/Method A1
 

Table 8: 10GBASE-LX4 link power budget as per IEEE Draft P802.3ae/D5.0

* These channel insertion loss numbers are based on a wavelength of 1300 nm for multimode and 1310 for single mode. An offset launch pad cord is assumed. The total insertion loss, when including the attenuation of the offset launch patch cord is allowed to be 0.5 dB higher than shown in the table.
** These power penalties are based on a wavelength of 1269 nm and other penalties

Table 9: 10GbE supported fiber and distances

* Commonly referred to as "FDDI Grade Fiber"
** Sometimes referred to as "10 Gigabit Ethernet Multimode Fiber", detailed in TIA-492AAAC
*** 62.5 micron multimode fiber has a model bandwidth of 500 Mhz*km at 1300 nm as opposed to 160 or 200 Mhz*km at 850nm

When designing 10GBASE-E links greater than 30 km (i.e., the cable is not already installed) a cabling link-loss calculation, which is a simple arithmetic process, is used to make sure the combined loss of the cabling components in the link does not exceed the 11 dB channel insertion loss allocated for 10GBASE-E (Table 7). The cabling link-loss is calculated by adding the connector and splice loss to the cable loss. The cable attenuation for the link is calculated by multiplying the link distance by the loss per unit distance specified for the fiber (e.g., dB/km).

As shown in Table 10 (scenario 1) given a cable attenuation of 0.225 db/km, the cable attenuation for a 40 km link is 9 dB (40 km x 0.225 = 9 dB). Assuming 2 dB for single-mode fiber connector and splice losses the link-loss is 11 dB (9 dB + 2 dB = 11 dB); which is an allowable channel insertion loss for 10GBASE-E (Table 7) and would insure that this link can achieve 40 km. A similar calculation can be done for scenario 2 and 3.

Table 10: 10GBASE-E link-loss calculation examples

* The 10BASE-E channel shall have attenuation between 5 and 11 dB. If required an attenuator can be added to comply with this specification
** This is the maximum fiber attenuation allowed for standerd single mode fiber at 1550 nm as per IEC 60793-2. See table 4 for details.

As with previous generations of Ethernet, 10 Gigabit Ethernet requires a network designer to thoroughly understand the capabilities of his/her fiber infrastructure. With 10GbE new challenges and considerations have emerged such as the effects of chromatic and polarization mode dispersion on signal integrity. In addition, decisions may have to be made regarding whether to use single-mode or multimode fiber. This paper has introduced some basic fiber related concepts and outlined some of the key points to understand and consider when designing a 10 Gigabit Ethernet network.

Reduction in transmitted optical power. Attenuation as a function of distance in optical fiber is logarithmic. Attenuation as a function of optical wavelength is dominated by the degree to which light is scattered by the molecular structure of the optical fiber (“Rayleigh scattering”).

Chromatic dispersion is a measure of the time based broadening which occurs in pulses of light as they propagate along a length of fiber. The spectrum of the optical light pulsed from a transmitter into a fiber includes multiple wavelengths; not just a single wavelength. Chromatic dispersion is caused when different wavelengths of light within the pulse propagate at different velocities. The delay difference between the wavelengths transmitted and those received results in a broadening of the optical pulse. Chromatic dispersion impairs the recovery of the data signal. The wavelength at which the dispersion is minimized (approximately zero) is referred to as the zero-dispersion wavelength, characterized by the symbol lo. Chromatic Dispersion is the most distinguishing difference between the applicable ITU single-mode fiber types. Chromatic dispersion is typically expressed in ps/nm/km (picoseconds of pulse spreading, nanometers of optical wavelength, kilometers of fiber traveled).

At the same center wavelength a broad-spectrum source, like a light emitting diode (LED), will produce much more chromatic dispersion than a narrow spectrum source, like a laser. Chromatic dispersion is the principal dispersion component of singlemode fiber systems, while modal dispersion dominates in laser-based multimode fiber systems.

Above this wavelength, optical signals are single-mode. Signals are multi-mode below the cutoff wavelength. The cutoff wavelength for cabled fiber is lower than that for bare fiber due to mechanical stresses exerted on the fiber by the cabling process. For standard single-mode fiber, the standardized (IEC and ITU) cutoff wavelength for cabled fiber is below 1260 nm. With fiber designed for single-mode applications, transmission at wavelengths below the cutoff is rarely if ever attempted, as bandwidth and distance are significantly reduced and more optimal multimode performance at shorter wavelengths is achieved with fibers designed specifically for that purpose.

The difference in the time delay between modes is called differential mode delay. The tuning of these modes is quantified by thedifferential mode delay measurement.

An amplifier which boosts the optical power of a signal without the need for electrical regeneration. An EDFA, in simple terms, is a length of optical fiber doped with Erbium and “pumped” by a shorter wavelength laser. Information bearing signals transmitted through the doped fiber will have additional energy imparted to them by the excited Erbium, thus increasing their optical power. EDFAs are only effective in the higher wavelength regions (typically 1525-1625 nm).

The generation of light at a new wavelength due to the interaction of transmitted signals at two or more wavelengths. Efficient four wave mixing requires proper phase matching, where signals at adjacent wavelengths are essentially coincident in time.

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 or intermodal distortion. This mainly applies to multimode fiber.

Measure of the highest frequency signal that can be supported over a given distance of multimode fiber, as limited by modal dispersion. Modal bandwidth is typically expressed in MHz*km.

The MFD is used to describe the distribution of the optical power in a fiber by providing an “equivalent” diameter, sometimes referred to as the spot size

Variations in optical properties of an optical fiber as a function of optical power. For example, a high-powered optical pulse can induce changes in the chromatic dispersion of an optical fiber.

Difference in propagation velocity between different optical polarization states. An optical signal can be represented by two orthogonally polarized components, each of which will travel at different velocities due to inherent geometric flaws in a length of optical fiber. Since receivers used in optical communications do not discriminate between different polarization states, the two delayed polarization components will be mixed at the receiving end. This mainly applies to single-mode fiber.

1. “Bellcore’s fiber measurement audit of existing cable plant for use with high bandwidth systems”, J. Peters, A. Dori, and F. Kapron, Proceedings of NFOEC 1997.

2. “PMD assessment of installed fiber plant for 40 gbit/s transmission”, P. Noutsios and S. Poirier, Proceedings of NFOEC 2001.

Links of interest:

• Ethernet Whitepapers
• USA Business & Carrier Metro Ethernet
• Metro Wave Division Multiplexing, DWDM Services and DWDM/ROADM
• Business Phone Systems (external)
• International WAN & DIA Services
• Wholesale SIP Termination (carrier & enterprise)

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• 10Gbps optical fiber
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• GBE fiber attenuator db/km
• GIGA AND 10GIGA ETHERNET DIFFERENCE
• lx4 and mode condiftioning fiber
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Related posts:

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