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The Gigabit Ethernet Modal Bandwidth Investigation

   

Contents Next >

The Gigabit Ethernet Modal Bandwidth Investigation

  

10.1. -

The Effective Modal Bandwidth Investigation

  

10.2. -

The Modal Bandwidth Investigation

  

10.3. -

Offset Launch

  

10.4. -

Summary

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Gigabit Ethernet Networking

From: Gigabit Ethernet Networking
Author: David Cunningham; Bill Lane
Publisher: MTP
More Information

10. The Gigabit Ethernet Modal Bandwidth Investigation

The purpose of this chapter is to discuss the resolution of some long-standing and fundamental issues regarding laser-based multimode fiber transmission that were addressed by the Gigabit Ethernet PMD sub-task force, in collaboration with the Telecommunications Industry Association, Fiber Optic 2.2 standards committee (TIA FO 2.2).

Before the telecommunications industry decided to use single-mode fiber, it had seriously considered standardizing on multimode fiber. Multimode fiber was attractive because mechanical tolerances of link components (connectors, splices, laser-coupling optics) are greatly relaxed compared to the single-mode case. However, in the end, two basic technical problems made multimode fiber unsuitable for the telecommunications industry:

The telecommunications industry had identified these problems, but it never developed solutions for them.

Chapter 9, “The Gigabit Ethernet Optical Link Model,” discussed how the Gigabit Ethernet task force resolved the first issue. They were able to make a worst-case power penalty allocation for modal noise by using known worst-case models and extensive experimental testing. The ATM forum also uses this approach.

The basic reason this worst-case approach works for data communication links is that the total connection loss is relatively small. This made it straightforward for the Gigabit Ethernet committee to conduct experiments with mode selective loss much greater than is likely to be encountered in the field. A large proportion of the committee's work during the first 18 months of the Gigabit Ethernet standardization process was spent ensuring that modal noise would not be a problem.

However, during that period, little attention was paid to the second problem—unpredictable bandwidth. This was probably because the industry believed that any form of restricted-mode launch would produce higher bandwidth than the overfilled launch (OFL) bandwidth specified for multimode fiber (see Chapter 6, section 6.3.5 for descriptions of OFL and restricted launches). Therefore, the Gigabit Ethernet task force made the seemingly reasonable assumption that the worst-case bandwidth was the OFL bandwidth specification for each multimode fiber type of interest. In fact, many members of the Gigabit PMD sub-task force were also involved in an investigation led by TIA FO 2.2 regarding the launch dependence on the modal bandwidth of multimode fiber. It was believed that because all laser-based transceivers produce a restricted-mode launch, compared to OFL, all laser-based transceivers would perform much better than predictions based on the OFL bandwidth. It was also hoped that a particularly good (high bandwidth) set of restricted-mode launch types could be identified. These high bandwidth restricted launches could then be used by both transceiver and fiber manufacturers to specify much better performance.

The TIA FO 2.2 committee was conducting a round robin bandwidth evaluation during the initial stages of the Gigabit Ethernet specification. Everyone was confident that the results of the round robin would enable Gigabit Ethernet to specify longer link lengths.

Unfortunately, when the first results from the modal-bandwidth round robin began to be reported, it was clear that there was a major problem. Counter to all expectations, it was discovered that many of the laboratory-defined restricted launches produced lower bandwidth than an OFL. Worse, the effect also occurred with real laser-based transceivers, especially when operating at long wavelengths. Figure 10.1 shows measured bandwidth obtained with an unconditioned 1000BASE-LX transceiver launch and a selection of fibers.

The fibers have an OFL bandwidth spread representative of the range expected in commercial systems. The solid line on the graph represents the expected result for the case where the measured bandwidth equals the OFL bandwidth. The dotted line represents the specified worst-case (minimum) OFL bandwidth specification for operation in the long-wavelength region with the fiber type used.

Clearly, the majority of the bandwidths measured with the transceiver are well below the OFL bandwidth. Shockingly, for the Gigabit Ethernet committee, many results don't even achieve the worst-case (minimum) OFL bandwidth specifications for the fiber type used.

Figure 10.1. Measured Bandwidth Using a 1000BASE-LX Unconditioned Launch Transceiver

NOTE

Two of the fibers used for these tests have OFL bandwidths less than the 500 MHz.km specification. These are only included for completeness, and it is not expected that fibers with such low OFL bandwidth would be encountered by Gigabit Ethernet.

Although Figure 10.1 shows results for only long-wavelength operation, the effect was also observed for operation at short wavelengths. However, the effect was more dramatic with long-wavelength operation.

The initial TIA FO 2.2 results only became available during May 1997. At that time, it was planned to complete draft D3.1 at the July plenary meeting of IEEE 802.3 and to ballot draft D3.1 as the final Gigabit Ethernet specification. The expectation was that Gigabit Ethernet would become an IEEE standard by November 1997.

The rest of this chapter describes the process by which the Gigabit Ethernet committee resolved the bandwidth issue. Part of the solution was to introduce new receiver conformance tests and specifications for laser-based multimode fiber link design into the Gigabit Ethernet standard. Chapter 11, 1000BASE-X: Optical Fiber and Copper PMDs, provides the specifications and details of these new tests.

It is important to understand the committee process so that you can then understand the working environment and pressures that the committee was under. Developing solutions to the bandwidth issue was not conducted in a relaxed academic environment or to academic time scales. Rather, it was done in a standards environment, under public scrutiny, to commercial time scales. It is a great tribute to all concerned that such a difficult technical problem was understood and resolved in time to make Gigabit Ethernet the success itis.

10.1. The Effective Modal Bandwidth Investigation

The poor performance of laser-based multimode fiber links called the November 1997 completion date for Gigabit Ethernet into question. Obviously, between May and July 1997, the Gigabit Ethernet community put considerable pressure on the PMD sub-taskforce with a view to forcing a solution to the bandwidth issue. The response to this pressure was the formation of an ad hoc committee chaired by Dave Smith (Honeywell). It was known as the Effective Modal Bandwidth Investigation (EMBI). Between May and July1997, the EMBI met weekly (sometimes biweekly) by phone and also had several face-to-face meetings. The members of the EMBI performed many theoretical and experimental studies and then pooled their results so that a reasonable solution could be found by the July plenary meeting. The final results of the EMBI investigation were as follows:

  • Less than 5 percent of fibers were observed to have bandwidth less than 160 MHz.km at short wavelengths with restricted-mode launch.

  • As many as 30 percent of fibers were observed to have bandwidth less than 500 MHz.km for operation at long wavelength with restricted-mode launches.

Table 10.1 summarizes some key bandwidth results reported by the EMBI.

Table 10.1. Range of Modal Banddwidths Observed by the EMBL for 62μm MMF <entry>aLimited by measurement capabilities</entry>
Minimum
Observed
Bandwidth
(MHz.km)
Mean
Observed
Bandwidth
(MHz.km)
Maximum
Observed
Bandwidth
(MHz.km)
SX140400>2,400a
LX250800>2,400a

aLimited by measurement capabilities

10.1.1. Gigabit Ethernet Link Model Worst-Case Operating Ranges

The EMBI also concluded that the Gigabit Ethernet link model (see Chapter 9), as it existed then, could be used to predict worst-case operating ranges. However, instead of using the OFL bandwidth specification for each fiber type, a new value based on the performance of restricted-mode launch should be used. Based on the measurements conducted by the EMBI, Gigabit Ethernet chose the worst-case modal bandwidth (WCMB) values for each link type shown in Table 10.2.

NOTE

Multimode fiber is abbreviated to MMF in tables and graphs throughout this chapter.

Table 10.2. Worst-Case Modal Bandwidth Values (MHz.km) used forDraft D3.1
62.5 μm MMF50 μm MMF
SX160a500a
LX250375

a Very few cases were observed to have bandwidth less than OFL bandwidth, so the OFL bandwidth was used.

Based on these worst-case effective modal bandwidths, the Gigabit Ethernet link model (with jitter due to duty cycle distortion [DCD] equal to zero) was used to calculate the operating ranges for D3.1 shown in Table 10.3.

Table 10.3. Operating Ranges of Draft D3.1
62.5 μm MMF50 μm MMF
SX260 m550 m
LX440 m550 m

10.1.2. Gigabit Ethernet Jitter Budget Problems

At the July plenary meeting of IEEE 802.3, some members of the committee thought that the standard should not go to its final sponsor group (IEEE 802) ballot. They felt that Gigabit Ethernet had not met the objective of supporting the installed base of 62.5 μm MMF within the building backbone and proposed that more work should be done with the goal of achieving a 550 m operating distance on 62.5 μm MMF. In the end, it was decided that the ability to achieve 550 m on 50 μm MMF was sufficient, and draft D3.1 went to sponsor group (IEEE 802) ballot. The members of the Gigabit Ethernet PMD, TIA FO 2.2, and the EMBI were greatly relieved that the panic was over and the standard was done. We all believed we had done the best job possible and that by reducing the bandwidth used to calculate operating distances, we had picked a “better safe than sorry” solution.

Between July and the September interim meeting of Gigabit Ethernet, lightning struck the PMD group again. Results of both laboratory and customer testing indicated that the D3.1 draft had not sufficiently solved the modal bandwidth problem!

Digital Equipment Corporation (DEC) reported at the September interim meeting that there was a jitter budget problem that D3.1 had not addressed. They reported that the additional jitter was due to excessive differential mode delay (DMD) (see Chapter 6, section 6.3.5.1) due to the restricted-mode launch of laser-based transceivers. Furthermore, it was pointed out that the Gigabit Ethernet link model does not take jitter into account. To illustrate their point, the contributors from DEC showed measured step responses similar to that of Figure 10.2.

Figure 10.2. Step Response for a 500 m Length of Fiber Exhibiting Severe DMD

This step response was obtained using an unconditioned 1000BASE-LX transceiver and a 500 m length of fiber having severe DMD. Due to the DMD characteristics of the fiber, the unconditioned laser launch excites mainly two mode groups with a DMD of 1,800ps/km. This bimodal DMD causes the step response to have two transition regions separated by a plateau. Jitter is very clearly a problem for this transceiver-fiber combination, as can be seen from the eye diagram in Figure 10.3.

Figure 10.3. Eye Diagram for a 500 m Length of Fiber Exhibiting Severe DMD

During the London meeting, Hewlett-Packard presented a graph of lowest measured modal bandwidth, or worst-case modal bandwidth (WCMB), as it was known (see Figure 10.4). The WCMB estimated from the peak-to-peak DMD data clearly indicated a strong correlation between peak-to-peak DMD and WCMB.

Figure 10.4. Correlation between WCMB and Peak-to-Peak DMD Data
   

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