Decoding 25G Ethernet And Its Optics

Most us of us may quite familiar with 10G, 40G and 100G Ethernet network, but we know little about 25G Ethernet, or even never heard about it. In the race of 10G, 40G and 100G Ethernet, it seems that 25G Ethernet lagged far behind. However, it is a good choice for network connectivity indeed. So what is 25G Ethernet and its benefits? And what about its cables and optical modules? The detailed information about 25G Ethernet would be provided in this article.

Overview on 25G Ethernet
Researched and developed by IEEE group, 25G Gigabit Ethernet is a standard for Ethernet connectivity. Compared with 10G, 40G and 100G Ethernet, 25G and 50G Ethernet were the latest developed Ethernet specification. 25G Ethernet Consortium was formed in July 2014 to support the specification of single-lane 25G Ethernet. It is an open organization to all third parties who wish to participate as members to enable the transmission of Ethernet frames at 25 or 50 Gigabit per seconds (Gbps) and to promote the standardization and improvement of the interfaces for applicable products.

25G Ethernet Specification
Based on single lane 25 Gb/s technology, the Consortium standard provides specifications for implementation, enabling Ethernet users to deploy cost effective Ethernet technologies. It is a technology that aimed at using (a) one 25 Gb/s lane to connect a single MAC operating at 25 GB/s by means of a Clause 49 PCS. The following figure shows how 25 Gb/s ports can be enabled.


Figure1: 25Gb/s SerDes Single Lane

For the configurations shown in Figure 1, 25Gb/s interfaces represent themselves as networking ports. They can auto-negotiate link speed by proprietary bit locations in extended capabilities registers.

Features and advantages of 25G Ethernet
  • Technology
    25G Ethernet technology is designed for application and infrastructure efficiency that results in a better data center ROI.
  • Robustness and reliability
    The 25G Ethernet technology is robust and reliable, and the products of 25G Ethernet are deigned to the highest standards and built to last. Weakness in one area can cascade and cause challenges that impact the entire system design and limit product reliability. The consequences would be too ghastly to contemplate.
  • Performance
    Industry analysts are predicting that 25GbE Ethernet will have the fastest adoption ramp ever for a new Ethernet technology. To make this forecast a reality requires not only 25 GbE technology that is both manufacturable and offers better features and capabilities, but also provides a true performance advantage.
  • Ease of adoption
    The IEEE group and 25G Ethernet Consortium have worked hard to ensure that 25G Ethernet offers a seamless upgrade to 10G Ethernet environments, with backwards compatibility that use the same 10GbE LC fiber cabling that has already been deployed in the data center.
  • Price
    Although 25GbE Ethernet is slightly more expensive than the 10GbE pricing, it still delivers a reasonale price. In fact the 25GbE pricing is very competitive, with only a 30% to 40% premium over 10GbE, and this premium is expected to come down over time. Overall, 25G Ethernet still offers a cost-effective solution for network connectivity.
25G SFP28 Optics
In general, there are three kinds of SFP28 optics, 25G SFP28 AOC and DAC cables as well as 25G SFP28 transceivers.

25G SFP28 AOC and DAC cable
The 25G SFP28 active optical cable is designed to overcome the bandwidth limitation of traditional copper cable so as to meet the high bandwidth in data center. It is hot pluggable and preterminated twin axial cable/fiber cable. The transmission distance can be 70 m by active optical cable with OM3 fiber. It is applicable to 25G and 10G Ethernet, data center and in-rack connection. For SFP28 DAC cable, there are two types: 25G SFP28 to SFP28 DAC and 100G QSFP28 to four SFP28 breakout DAC. These copper cables provide a cost-effective solution for connection with Top of Rack switches.


Figure2: 25G SFP28 Active Optical Cable

25G SFP28 transceivers
Basically, there are two kinds of 25G SFP28 transceiver, 25G SFP28 SR and 25G SFP28 LR optical transceiver. The 25G SFP28 SR designed to transfer data over short distance while the 25G SFP28 LR transceiver set for long distance transmission. The maximum transmission length of 25G SFP28 SR is 100 m over 50μm multimode optical fiber. The transmission distance of 25G SFP28 LR optical transceiver can be up to 10 km over single-mode fiber. Both the transceivers can be applied to data center, 10G&25G Ethernet, high-speed storage area networks and in-rack connection.


Figure3: 25G SFP28 Transceiver

Conclusion
25G Ethernet is taking off fast. It has a handful of very powerful data center and cloud vendors and suppliers-Google, Microsoft, Broadcom, Arista, Mellanox and Cisco. The 25G technology already exists in 100G will drive better cost efficiencies than 40G as well, saving cloud providers and enterprise data centers capital and operating expense. 

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