IEEE P802.3bs Task Force adopts serial 100G for 400 Gigabit Ethernet

Members of the IEEE P802.3bs Task Force agreed during the recent IEEE 802.1/IEEE 802.3 Joint Interim Meeting to adopt a single-mode fiber PMD based on four wavelengths of 100 Gbps each, according to the meeting's host, The Ethernet Alliance. The decision follows months of debate regarding whether serial 100G technology was sufficiently mature to be included in the 400 Gigabit Ethernet specification.

Task Force members voted to adopt a 4x100-Gbps PMD based on PAM-4 modulation for single-mode fiber applications at reaches of 500 m. The PMD would use four fibers, similarly to the way the PSM4 MSA addresses 100 Gigabit Ethernet applications via four 25-Gbps transmissions over individual fibers.

The group also voted to adopt an 8x50-Gbps approach for reaches of at least 10 km. This PMD also calls for PAM-4 modulation, but over a single fiber.

As described in a recent article for Lightwave authored by IEEE P802.3bs Task Force Chairman John D'Ambrosia, the task force members were charged with developing specification for four applications scenarios:

1. 100 m over multimode fiber
2. 500 m over single-mode fiber
3. 2 km over single-mode fiber
4. 10 km over single-mode fiber.
   

The Task Force members quickly agreed to a 16x25-Gbps approach using NRZ format for the multimode fiber application. However, agreement on the single-mode specifications proved a thornier proposition. In particular, battle lines formed regarding data rate – 50 Gbps or 100 Gbps per wavelength or fiber – and modulation format. NRZ, PAM-4, and discrete multi-tone (DMT) formats were debated.

The question of whether serial 100G was ready for inclusion in an IEEE standard in particular appeared to raise passions (see "Is serial 100G ready for 400 Gigabit Ethernet standardization?"). The fact that the task force has adopted 4x100G at 500 m and 8x50 Gbps at 10 km either represents a nice compromise or sets up a final showdown among the two data-rate camps at 2 km. The Ethernet Alliance press announcement did not reveal the status of specifications for that requirement.

The decisions reached at 500 m and 10 km also opens the question of whether proponents of DMT will seek to form an MSA to offer the technology to the market independently.

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