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So, I'm reviewing the different IEEE transmission standards for an exam, and I've noticed that for the 10GBASE-X standards, there's two different sets of fiber-optic standards, 10GBASE-xW and 10GBASE-xR. It's noted that the W standards are for WANs and a bit of googling turns up that they're "SONET capable", but I don't really know what that means.

They have the same ranges, same data rates (or at least, not difference is noted), use the same wavelengths (I mean, yes, SW/SR uses a different wavelength of light than LW/LR, but the W and R ones don't differ), and they're both IEEE 802.3ae specifications.

and importantly, I haven't seen separate specifications for WAN/SONET and LAN fiber-optic transmission standards in the lower speeds---gigabit and 100M fast ether net just have one set of IEEE fiber-optic transmission standards

So why are there separate specifications for the 10 gigabit standards? what does it mean to be "SONET capable" and why did we only need that for 10 gigabit speeds?

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    I think you are trying to compare two completely different protocols: ethernet and SONET. Ethernet is an IEEE protocol (802.3), but SONET is an ITU protocol, originally designed for telephone virtual circuits, but it also often used for data WAN. – Ron Maupin Oct 24 '18 at 16:35
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From IEEE 802.3 Clause 50.1: (emphasis mine)

The WAN Interface Sublayer (WIS) is an optional PHY sublayer that may be used to create a 10GBASE-W PHY that is data-rate and format compatible with the SONET STS-192c transmission format defined by ANSI, as well as the Synchronous Digital Hierarchy (SDH) VC-4-64c container specified by ITU. The purpose of the WIS is to allow 10GBASE-W equipment to generate Ethernet data streams that may be mapped directly to STS-192c or VC-4-64c streams at the PHY level, without requiring MAC or higher-layer processing. The WIS therefore specifies a subset of the logical frame formats in the SONET and SDH standards. In addition, the WIS constrains the effective data throughput at its service interface to the payload capacity of STS-192c / VC-4-64c, i.e., 9.58464 Gb/s. Multiplexed SONET/SDH formats are not supported.

The WIS does not render a 10GBASE-W PHY compliant with either SONET or SDH at any rate or format. A 10GBASE-W interface is not intended to interoperate directly with interfaces that comply with SONET or SDH standards, or other synchronous networks. Such interoperation would require full conformance to the optical, electrical, and logical requirements specified by SONET or SDH, and is outside the scope and intent of this standard. Operation over electrically multiplexed payloads of a transmission network is outside the scope of this standard.

From the perspective of the 10 Gb/s MAC layer, a 10GBASE-W PHY does not appear different (in either the functions or service interface) from a PHY without a WIS, with the exception of sustained data rate. However, a 10GBASE-W interface may interoperate only with another 10GBASE-W interface.

Basically, the 10GBASE-W PHYs can interface seamlessly on top of a SONET network due to their slightly reduced data rate.

The long wave -R PHYs are also designed for WAN application but with Ethernet's native data rate (and they're probably much more common).

There are no separate SONET/SDH-compatible variants for Fast and Gigabit Ethernet because there is (was) no need. You could easily route several GbE links (an many FE links) over an SDH network but 10GE had become too fast to fit without a slight decrease in speed.

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