You may have heard plenty of buzz over the past few months regarding 25 gigabit Ethernet. It’s no surprise considering that several big data center and cloud computing providers like Google, Microsoft, Broadcom, Arista and Mellanox formed the 25 Gigabit Ethernet Consortium earlier this year.

Shorter thereafter, IEEE formed a 25G Ethernet study group. And just a couple of weeks ago, Broadcom announced the availability of a new high-density 25G Ethernet switch for cloud-scale data centers.

Why 25G? It simply makes sense from a technology, cost, scalability and flexibility perspective. Let’s take a closer look.

We’re Already There

Existing 100G standards such as 100GBASE-LR4 and 100GBASE-ER4 already are comprised of four lanes of 25G over singlemode fiber using course wavelength division multiplexing (CWDM) technology. The 100GBASE-CR4 and 100GBASE-KR4 standards for 4 X 25G over twinax and backplane were also recently ratified, and the 100GBASE-SR4 standard with 4 X 25G per lane over multimode fiber is also well underway.

As mentioned in one of my blogs earlier this year, 100 gigabit QSFP28 SR4 optical transceiver modules are already available for this application. With the same footprint as the 40G QSFP+ for four 10G lanes, the QSFP28 essentially offers 250% more density. It also supports direct-attach copper (DAC) cable assemblies, active optical assemblies and transceivers.

With the 4 X 25G per lane technology already available for 100G, the process to go to a single 25G lane is a simple one that requires very minor changes and significantly reduces cost compared to 40G using four 10G lanes.

cant ignore cat 6a bannerIdeal for the Cloud

One of the key drivers for 25G is cloud computing. Not only are cloud providers looking to decrease cost and improve density, but improvements in server speeds has rendered 10G no longer fast enough.

Many of these environments have standardized on a top of rack (ToR) topology, and keeping up with these connections at 10G requires more switches in the rack and therefore higher cost.


25G also works from a scalability standpoint. With uplinks migrating to 100G using the same type of technology, it makes sense to increase the switch-server speed at the edge to 25G. With cost benefits over 40G and the technology already gaining traction, a recent five-year forecast by Dell’Oro Group predicts that 25G will take over Ethernet server port sales by 2018.

Beyond Top of Rack

While 25G over twinax DAC assemblies will fulfill the ToR server environments where a distance of 3 to 5 meters is more than adequate, there is also the need for longer distances to support middle of row (MoR) topologies to about 15m and end of row (EoR) to 30m. That’s where a future 25GBASE-T application over balanced twisted-pair copper cabling has potential to fill the gap.

At the September meeting of IEEE 802.3, the Task Force developing 40GBASE-T (P802.3bq) discussed the technical feasibility and the benefits of 25GBASE-T relative to 40GBASE-T. As a result, a Call for Interest (CFI) is scheduled for the next IEEE 802.3 meeting in November to form a study group to explore 25 Gb/s BASE-T Ethernet and potentially extend the work of P802.3bq Task Force to include it. Some considerations will include maintaining backwards compatibility with 10GBASE-T and forward compatibility with 40GBASE-T through autonegotiation, as well as keeping the power per port low and supporting Energy Efficient Ethernet.

When it comes to home run applications where servers and storage devices are consolidated into separate areas of a data center, the distances required increase to about 50 to 100m. Multimode fiber technology based on four lanes of 25G that is used in the upcoming 100GBASE-SR4 standard supports 100m distances over OM4 fiber. A single-lane 25G multimode fiber PHY could be a significant solution to support home run applications and an overall broader range of 25G architectures.

Stay tuned

While standards groups are actively focusing on 25G Ethernet over twisted-pair, twinax and fiber, it’s still too soon to tell how it will shape up. Regardless, 25G Ethernet has broad market potential in the server interconnect world because it’s both economically and technically feasible—on a variety of media. In other words, 25G Ethernet is definitely coming—and it’s coming fast. The experts at Belden will keep you posted as the situation develops.