With the rise of 100G and 400G networking, it is easy to assume that 40G technologies should already be obsolete. In practice, many legacy data centers continue to rely heavily on 40G SR4 optical modules. This is not due to a lack of awareness or reluctance to upgrade, but rather the result of practical design decisions, existing infrastructure, and cost considerations that still make 40G SR4 a viable solution today.
Designed for an Earlier Data Center Era
40G SR4 was originally designed for data centers built around multimode fiber and parallel optics. It uses four transmit and four receive lanes over MPO fiber, which aligned well with the cabling practices of its time. Many legacy data centers were deployed with large amounts of OM3 and OM4 fiber, as well as MPO-based trunk cabling, specifically to support 40G architectures.
Replacing this infrastructure is not trivial. Cabling often represents a significant portion of the total network investment, and in many cases, it continues to meet performance requirements. As long as the fiber plant remains reliable, 40G SR4 can operate efficiently without forcing a costly physical redesign.
Cost Efficiency in Existing Environments
Another key reason 40G SR4 remains popular is cost. In legacy environments where switches, line cards, and server NICs are already built around 40G, replacing optics with higher-speed alternatives does not always deliver immediate value. Upgrading to 100G often requires not only new transceivers, but also new switches, breakout cabling, and in some cases, additional training for operations staff.
For organizations focused on stability rather than maximum bandwidth, maintaining 40G SR4 links is often the more economical choice. It allows data centers to extend the useful life of existing hardware while allocating budget to areas with higher business impact.
Operational Familiarity and Stability
Legacy data centers tend to prioritize predictability and uptime. Over the years, 40G SR4 has proven to be a stable and well-understood technology. Network teams are familiar with its behavior, troubleshooting methods, and operational limits. Spare parts are readily available, and interoperability issues are well documented.
This operational maturity reduces risk. In contrast, introducing new architectures can increase complexity, especially in environments that do not require higher bandwidth immediately. For many organizations, the reliability of a known solution outweighs the potential benefits of upgrading.
A Bridge to Future Architectures
While 40G SR4 may no longer be the preferred choice for new greenfield deployments, it continues to serve as a bridge technology in brownfield environments. Many data centers operate in a hybrid state, where 40G coexists alongside 100G and even 400G links. In these scenarios, 40G SR4 plays a supporting role, handling workloads that do not demand higher speeds.
This gradual transition approach allows organizations to modernize at their own pace, rather than forcing disruptive, large-scale upgrades.
Conclusion
40G SR4 has not been fully replaced because it continues to meet the needs of many legacy data centers. Its compatibility with existing infrastructure, cost efficiency, operational stability, and role in gradual network evolution all contribute to its ongoing relevance. Rather than being obsolete, 40G SR4 has simply shifted from a leading technology to a dependable workhorse, one that still plays an important role in today’s data center networks.
