25G Transceiver FAQ

Written by Craig Pasek, Product Manager, Transceiver Modules Group

I’ve posted a few blogs about 25G transceivers (here, here, and here). Here’s an FAQ to address questions that have arisen from the significant uptick in the use of 25G transceivers, which is driven by high bandwidth demands in enterprise, data center and service provider applications.

Q: Why is the use of 25G increasing?

Many network operators have chosen 25G instead of multiple 10G’s, because 25G provides 2.5x bandwidth of the 10G in the same familiar SFP form factor at approximately the same power. This has enabled network equipment manufacturers to provide higher bandwidth connectivity. Rack mountable switches and routers populated with 12 ports, 24 ports, and 48 ports on a single 1 RU faceplate are common for SFP.

Q: What is the cost per bit of 25G?

25G provides 2.5x the bandwidth of 10G at a slight increase in cost. The result is nearly a 50% reduction in the cost per bit.

Q: Is 25G standardized?

The IEEE (Institute of Electrical and Electronic Engineers) has standardized 25G. See IEEE802.3by and IEEE802.3cc for the details.

Q: Which popular 25G transceivers are available today ?

MMF (Multi Mode Fiber):
SFP-10/25G-CSR-S (up to 400m on OM4)
SFP-25G-SR-S (up to 100m on OM4)

SMF (Single Mode Fiber):
SFP-10/25G-LR (up to 10km)

Cables:
SFP-25G-AOCxxM (up to 10m)
SFP-H25G-CUxM (up to 5m)

Q: What is SFP28?

SFP28 is the standardized pluggable form factor for 25G transceivers. It has the same mechanical dimensions as 10G SFP+ and 1G SFP. The electrical interface of 25G was envisioned to operate up to 28Gbps to accommodate overhead for a 25Gbps signal.  Today most 25G transceivers operate at a 25.78125Gbps nominal data rate. The standards body that defines SFP28 is SFF (Small Form Factor Committee).

Q: What is SR?

SR is Short Reach, and generally refers to transceivers that operate over MMF up to a few 100 meters.

Q: What is LR?

LR is Long Reach, and generally refers to transceivers that operate over SMF at up to 10km.

Q: What does “AOC” mean?

AOC is Active Optical Cable.  These cables are generally 1 to 10 meters in length and have active components that connect SFP28 hosts at 25G.

Q: What does “DAC” mean?

DAC is Direct Attached Cable. These cables are generally 1 to 5 meters in length and are passive.

Q: What does “10/25G” mean?

These are dual-rate transceivers that support both 10G and 25G rates.

Q: Explain “CSR”

“Cisco Short Reach” is Cisco technology that enables reach of 300/400m over OM3/4 MMF.

Note: The IEEE standard 25G SR’s reach is only up to 70/100m over OM3/4 MMF.

Q: What does “breakout” mode mean?

Breakout enables multilane QSFP transceivers to interoperate with single lane SFP transceivers.

For example, up to 4x SFP-25G-SR-S can interoperate with a QSFP-100G-SR4-S using a MMF breakout cable; 4x SFP-10/25G-LR-S interoperates with a QSFP-4X10G-LR-S (at 10G) and a QSFP-100G-PSM4-S (at 25G – with the distance limited to 500m) using SMF breakout cable; and 4x SFP-10/25G-CSR-S interoperates with a QSFP-40G-SR4 (at 10G – with the distance limited to 100/150m over OM3/4) or a QSFP-100G-SR4-S (at 25G – with the distance limited to 70/100m over OM3/4) using a MMF breakout cable.

Q: What new technology is in 25G transceivers?

25G transceivers have CDR (Clock Data Recovery) circuits and generally require FEC (Forward Error Correction).

Q: When is FEC needed?

FEC decreases the BER to 10-12. For 25G, RS-FEC is used when the pre-FEC BER is above 5×10-5 and FC-FEC is used when the pre-FEC BER is above 10-8.  Additional details can be found in IEEE802.3 Clause 74 for FC-FEC and Clause 91 for RS-FEC.

Q: What is BER?

BER refers to Bit Error Rate.

Q: What Cisco host platforms support 25G transceivers ?

See the Cisco transceiver Compatibility Matrix: https://tmgmatrix.cisco.com/home

Q: Where can additional information about Cisco 25G transceivers be found ?

See the Cisco 25G datasheet: https://www.cisco.com/c/en/us/products/collateral/interfaces-modules/transceiver-modules/datasheet-c78-736950.html