25G SFP28 Cable – Best for TOR Server Connection?

During the past few years, there is a dramatic increase in the demand for bandwidth requirement for our communications. Whether be in a communication service provider or in a public or private data centers, a development in connectivity that can cater higher speed, bandwidth is needed. That is why last July 2014, an industry consortium was formed to create a new Ethernet connectivity standard in data centers. This standard was called 25 Gigabit Ethernet or 25Base-T, developed by IEEE 802-3 task force P802.3by. This standard was derived from the 100GB Ethernet Standard, however, its operation works as a single lane connection with 25Gbps that can be run on fibers or coppers. Last June 2016, this technology was commercially released using new transceiver interfaces called SFP28 (1x25Gbps) and QSFP28 (4x25Gbps). This article will discuss the transceiver form factor 25G SFP28.

The SFP28 Transceiver Form Factor

The QSFP28 transceiver form factor was constructed in four parallel 25Gpbs data lanes allowing a maximum rate of 100Gbps, the SFP28 offers a single 25Gbps data lane. The physical structure of the 25G SFP28 transceiver is the same with the popular SFP and SFP+. This characteristic provides a flexible solution due to the fact that 100 Gigabit can also be divided individually in to four 25Gbps connections. SFP28 uses a 28Gbps lane (25Gbps + error correction) specifically used for top-of-rack (TOR) switch to server connectivity. Moreover, despite transceiver variants the 25G SFP28 form factor is also available in both copper (Direct Attach Cable - DAC) and fiber optic (Active Optical Cable - AOC) cable variants.

Direct Attach Cable and Active Optical Cable SFP28 Solutions

The DAC copper cable version is manufactured in a single fixed-configuration module which means the copper cables are directly attached to an 25G SFP28 transceiver module. This version is ideal to be used for short distances ranging today from 1 to 5 Meter. On the other hand, the optical fiber AOC version functions in either an 850nm that utilizes a pair of multi-mode fiber. This type, as well as the transceiver variant, works to a maximum distance of up to 100 Meters. The single-mode type can work on up to 10 kilometers at 1310nm.

Benefits of 25G SFP28 Cable Connections

The development of 25G SFP28 connections has provided a wide range benefits especially in a web-scale data center environment where the trend goes towards a single port server due to the costs.

Primarily, it gives way to efficiently utilize data and switch port density. The reason for this is that, existing 100G port can be used as a 4x25G with as QSFP28 to SFP28 DAC or AOC break out cable instead of using for different ports. For example, a 25Gbe transceiver can provide 2.5 times more data than the popular 10G DAC / AOC SFP+ solution and can provide greater port density.

Moreover, it provided an extremely efficient increase in speed to server to top-of-rack (TOR) especially when using the Direct Attached Copper assembly. It also simplifies development of interoperability specifications and systems due to the fact that it is backward compatible and gives an easier upgrade path from an existing 10G ToR server configuration.

Furthermore, using 25G SFP28 for ToR servers are more economical. This is because it can provide higher port densities, fewer ToR switches and cables are needed. It allows a more cost-effective alternative top-of-rack server connection that connects via point-to-point links. It enables also End of Row (EoR) or Middle of Row (MoR) by using structured cabling. As a result, it reduces the capital expense in the construction cost compared to other configurations like the 40GbE.

The Future of SFP28 Connections in a Network

Ultimately, the 25G SFP28 cable solutions feature a reduced power and smaller footprint requirement for data centers because it limits the power per port to under 3W.

Due to this benefits that the 25G SFP28 cable assembly provides, it is forecasted that it will be popular in the years to come. It is believed that the dominant next generation server connection is toward the 25Gbps speed in server and in the near future, there will be more equipment that will use the 25G SFP28 DAC or AOC cable assembly solutions.