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I'm campaigning for a comment submitted barely in time for the May meeting. The 10 Gb/s upstream wavelength is currently defined as 1260 to 1280 nm. See for example, Table 91-6 and many following references. This wavelength is also specified for the for 10/1GBASE–PRX–D3 (Table 91-7). According to my expert, this wavelength is dangerous because single mode fiber becomes overmoded at about 1260 nm, resulting in a second propagation mode at a different velocity. In addition, this wavelength will be overlapped by the normal 1310 nm 1 Gb/s upstream optical signal. This is acknowledged in the proposed document:
91A.3.2 Upstream wavelength allocation
The 1.25 GBd upstream transmission uses the 1270 – 1360 nm wavelength band, as specified in Clause 60. The 10.3125 GBd upstream transmission uses the 1260 – 1280 nm wavelength band, as specified in Clause 92. The two wavelength bands overlap, thus WDM channel multiplexign {sic} cannot be used to separate the two data channels.
An OLT supporting both upstream channels must use TDMA techniques to avoid collisions between transmissions originating from different ONUs.
We presume that the reason for specifying a separate wavelength band is to afford the opportunity to build a mixed-mode OLT receiver (capable of receiving both 1 Gb/s and 10 Gb/s) that is optimized for 10 Gb/s. However, this would imply a WDM at the OLT receiver, whose loss could rob a good bit of the improvement to be gained. This is acknowledged in Section 91A.5. Furthermore, this implies tight standards for the ONT 10 Gb/s laser, which could add cost. Since we have already acknowledged that the OLT will need to use TDMA to separate the 1 Gb/s from the 10 Gb/s upstream, we propose that the 1260-1280 nm upstream requirement be dropped, and that upstream transmitters for both speeds use the same wavelength band.
Figure 91A-5 shows some dual-rate receiver architectures that seem practical and which can provide optimization for each speed without having to use dual receivers. Implicit in these receivers is the ability to not only switch bias, but to also switch low pass filtering - since you have electrical amplification ahead of the filters, you can even leave both filters connected to the source and their respective loads all the time, so that the transient response of the filter is minimized.
This seems a better solution than using dual wavelengths, with the potential issues of the 1260 - 1280 nm band.
Thanks,
jim
Jim Farmer, K4BSE
Chief Technical Officer,
Wave7 Optics, Inc.
An Enablence Technologies Company
1075 Windward Ridge Parkway
Alpharetta, GA 30005 USA
678-339-1045
678-640-0860 (cell)
jim.farmer@w7optics.com
www.wave7optics.com