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Action on the Fiber Front

An addendum adopted at the last meeting of TIA TR 42.1 should be required reading for network designers planning to provision for 10 Gb/s optical fiber backbones.

December 1, 2002  

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For this month’s article I’d like to discuss some new developments in optical fiber application requirements and testing. At the last meeting of TIA TR 42.1 in Scottsdale, Ariz., the subcommittee approved the publication of Addendum 3 to ANSI/TIA/EIA 568-B.1.

This provides information on “Supportable Distances and Channel Attenuation for Optical Fiber Applications by Fiber Type”.

In particular, it supplements the information contained in table E-1 of the 568-B.1. Two new applications (10/100Base-SX and 10G Ethernet) and one new fiber type (850-nm laser-optimized 50/125-m multimode fiber) are now included in Table E.1.

For discussion purposes, I have taken some of the loss values and channel distances and presented it in a graphical format in the chart below for the 10 Gb/s Ethernet application.

It is immediately evident that conventional multimode optical fibers can only support a relatively short distance for the 10GBase-S application using short wavelength (850 nm) VCSEL lasers. The last column is the new 50 m “laser optimized” fiber, a recent addition to the TIA 568 B.3 standard, which can support a much longer backbone distance of 300 meters.

It is also evident that the optical loss budget for a 10 Gb/s Ethernet channel is quite severe (2.3 dB to 2.6 dB) compared to lower speed applications such as 1 Gb/s Ethernet or 622 Mb/s ATM. The loss budget for 1 Gb/s Ethernet (1000Base-SX) is 3.2 dB to 3.9 dB. The loss budget for 622 Mb/s ATM is 4 dB at 850 nm wavelength.

What does this mean for network designers that are planning to provision for 10 Gb/s optical fiber backbones? They need to be aware of these limitations and ensure that the recommended design will meet the criteria for 10 Gb/s operation. What are the options?

For existing applications where the multimode optical fiber (either 62.5 m or 50 m) is already in place and meets the existing TIA 568-B.3 standard, then 10 Gb/s can still be supported for distances up to 300 meters using a more complex technology, i.e. 10GBase-LX4. This technology uses four light sources that are coupled onto the same fiber at a wavelength of ~ 1276, 1300, 1325, 1349 nm;

For new installations up to 300-meter distances, the new 50 m “laser optimized” multimode fiber is the preferred option for 10 Gb/s Ethernet. 10GBase-S is the most economical technology for 10 Gb/s using VCSEL lasers operating at 850 nm;

For new or existing installations greater than 300 meters, single mode fiber is the best option for 10 Gb/s Ethernet. 10GBase-L uses conventional lasers that operate at 1310 nm wavelength and can support distances up to 10 km over singlemode fiber.

What about the tight loss budget for 10GBase-S? Let’s look at a simple channel with two connectors Two mated connector pairs per TIA 568-B.3 have a maximum loss of 2 x 0.75 dB = 1.5 dB loss. Three hundred meters of tight buffer cable per TIA 568 B.3 has a loss of (300 x 3.5/1000) = 1.05 dB loss at 850 nm. The total loss of the channel is (1.5 dB + 1.05) dB = 2.55 dB. That’s not a lot of design flexibility considering that the maximum channel budget for 10 Gb/s Ethernet is only 2.6 dB.

There are a number of factors that can improve the situation significantly. NORDX/CDT has recently completed a set of measurements on connector losses that were presented at the BICSI Quebec conference. The results show that VCSEL testing gives better results between of 0.1 to 0.5 dB compared with LEDs when using an Optical Loss Test Set. This can allow the use of additional connections in the channel.

As you can see, we are ushering in a new era of 10 Gb/s light speed transmission that has it own challenges and its own unique solutions. Are you ready for the challenge?

Paul Kish is Director, IBDN Systems & Standards at NORDX/CDT. He is also vice chair of the TR-42 engineering committee.

TABLE 1: Supportable distances and Channel Attenuation based on IEEE 802.3ae Media Access Control (MAC), Physical Layer, and Management Parameters for 10 Gb/s Operation

Description 62.5 m Fiber 50 m Fiber Unit
Wavelength 850 850 850 850 nm
Modal bandwidth (min) 160 200 500 2000 MHz*km
Operating distance (max) 26 33 82 300 meters
Channel insertion loss 2.6 2.5 2.3 2.6 dB

Disclaimer: The information presented is the author’s view and is not official TIA correspondence.

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