IEEE Std 802.3by™ ”Standard for Ethernet Amendment 2: Media Access Control Parameters, Physical Layers and Management Parameters for 25 Gb/s Operation”  was developed by the IEEE P802.3by 25 Gb/s Ethernet Task Force and approved by the IEEE-SA Standards Board on June 30, 2016.  25 Gb/s was identified as a cost-optimized step on the migration path to 400 Gb/s speeds and maximizes the efficiency of server to first-level equipment (i.e. Top-of-Rack, access layer, and leaf) connections. This amendment defines 25 Gb/s Physical Layer (PHY) specifications for operation over up to 100 m of multimode optical fiber cabling and up to 5 m of a single lane (two pairs) of twinaxial cable and attachment interfaces supporting 25 Gb/s transmission in electrical backplanes.  The recognized direct attach twinaxial cable assembly form factors are SFP28 to SFP28, QSFP28 to QSFP28, and QSFP28 to 4 x SFP28.

The five 25 Gb/s Ethernet PHY specifications are:

25GBASE-CR:  25 Gb/s using 25GBASE-R encoding over one lane of twinaxial copper cable

25GBASE-CR-S: Equivalent to 25GBASE-CR without support for the RS-FEC sublayer

25GBASE-KR: 25 Gb/s using 25GBASE-R encoding over one lane of an electrical backplane

25GBASE-KR-S: Equivalent to 25GBASE-KR without support for the RS-FEC sublayer

25GBASE-SR: 25 Gb/s using 25GBASE-R encoding over one lane (i.e. 2 fibers total) of multimode fiber with reach up to at least 100 m

Goals and Objectives for 25 Gb/s Ethernet operation:

  • Support a MAC data rate of 25 Gb/s
  • Support full-duplex operation only
  • Preserve the Ethernet frame format utilizing the Ethernet MAC
  • Preserve minimum and maximum FrameSize of current IEEE 802.3 Standard
  • Support a BER of better than or equal to 10-12 at the MAC/PLS service interface (or the frame loss ratio equivalent)
  • Support optional Energy-Efficient Ethernet operation
  • Define a single-lane 25 Gb/s PHY for operation over a printed circuit board backplane consistent with channels specified in IEEE Std 802.3bj-2014 Clause 93
  • Define a single-lane PHY for operation over links consistent with copper twin axial cables, with lengths up to at least 3m
  • Define a single-lane PHY for operation over links consistent with copper twin axial cables, with lengths up to at least 5m
  • Define a single-lane PHY for operation over MMF consistent with IEEE P802.3bm Clause 95
  • Provide appropriate support for OTN

The IEEE 802.3 25 Gb/s Ethernet Call-For-Interest Consensus Presentation can be found here: http://www.ieee802.org/3/cfi/0714_1/CFI_01_0714.pdf

The Project Authorization Request (PAR), approved on December 10, 2014, can be found here: http://www.ieee802.org/3/by/P802_3by_PAR_approved_121214.pdf

 

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