Unveiling 100GBASE-SR4 vs. 100GBASE-BiDi SR: Comprehensive QSFP28 SR Guide

The surge in cloud computing and 5G networks is propelling rapid advancements in data centers, resulting in a heightened need for 100G optical transceiver modules. These modules, constituting a substantial segment of network construction expenses, play a critical role in small, medium, and large-scale data centers. In smaller and medium-sized data centers, over 90% of fiber links are within the 100-meter range, while in larger data centers, more than 70% span less than 100 meters, with over 80% falling under 125 meters. The predominant choice for these links is the 100G multimode module, specifically the QSFP28 100G multimode module, focusing on 100GBASE-SR BD and 100GBASE-SR4.

 

The Overview of 100GBASE-SR4 and 100GBASE-SR BD

 

The 100GBASE-SR4, a widely used 100G multimode module, employs four lanes (4x25Gbps) for transmitting/receiving optical signals, achieving an aggregated data rate of 100Gbps. Designated as "SR" for short reach, the module is optimized for short-distance transmissions, offering a typical range of 70m over OM3 or 100m over OM4 multimode fiber. Conforming to IEEE 802.3BA standards, the 100G QSFP28 SR4 operates as a hot-plugged full-duplex optical module with a center wavelength of 850nm. Its popularity stems from providing four independent send and receive channels, each capable of a 25Gbps rate. It is a key solution for short-distance, high-speed data transmission in 100G Ethernet networks. The following picture shows the working principle of the 100GBASE-SR4 optical module transceiver:

 

 

The 100GBASE-SR BD is the bidirectional counterpart of 100GBASE-SR, utilizing two optical signals in opposite directions on the same multimode fiber with wavelengths of 855nm/908nm. This configuration employs two lanes of 50Gbps in each direction, achieving an aggregated 100Gbps data rate. Unlike the 100G QSFP28 SR4, which necessitates eight optical fibers, the 100G QSFP28 SR BiDi module utilizes WDM technology. This allows each LC port to transmit and receive optical signals of varying wavelengths simultaneously on a multimode fiber. These dual-wavelength VCSEL bi-directional optical interfaces operate with PAM4 at 2 × 50-Gb/s on 855nm/908nm wavelengths. The module's operational range spans up to 70 meters with OM3 fiber, 100 meters with OM4 fiber, and 150 meters with OM5 fiber, showcasing its versatility in multimode fiber systems. The following picture shows the working principle of the 100G QSFP28 SR BiDi optical module transceiver:

 

 

The Difference Between 100GBASE-SR4 vs. 100GBASE-BiDi SR

 

Fiber Count

 

The 100GBASE-SR4 transceiver utilizes 8 fibers, constituting the prevalent multimode fiber cabling structure for short-reach 100G Ethernet. This configuration necessitates four transmit and four receive fibers, typically employed in an MPO/MTP cabling system.

 

In contrast, 100GBASE-BiDi SR transceivers utilize only 2 fibers for bidirectional data transmission, presenting a highly cost-effective solution for 100G network upgrades. Unquestionably, 100GBASE-BiDi requires the least amount of fiber cabling deployment, making it ideal for scenarios where existing multimode fiber is strained or challenging to redeploy with new fiber.

 

Connector

 

The 100GBASE-BiDi SR transceiver employs the conventional LC duplex fiber optic connector, known for its simplicity and ease of use.

 

The 100GBASE-SR4 transceivers utilize MPO/MTP fiber optic connectors, specifically requiring an 8 or 12-fiber MPO/MTP connector.

 

Wavelength

 

For 100GBASE-SR4, both transmit and receive optical signals operate at a wavelength of 850nm, consistent with traditional Ethernet optics. In contrast, 100GBASE-BiDi utilizes distinct wavelengths for transmitting and receiving optical signals within a single fiber. The typical transmission wavelength is 855nm, while the receiving wavelength is 908nm. Consequently, 100GBASE-BiDi optics represent a variation of WDM (wavelength-division multiplexing) optics.

 

Final Cost

 

The overall cost comprises two crucial parts: the expense of 100G transceivers and fiber cabling. Consequently, the final pricing varies depending on the costs of different 100G transceivers and fiber optic cabling, with distinct deployment solutions yielding differing total costs.

 

100GBASE-SR4 transceivers offer the most economical pricing and represent the prevalent form factor for 100G QSFP28 transceivers. They are well-suited for data center deployments with short-range requirements and cost considerations. However, it's important to note that 100GBASE-SR4 optics require 8 fiber cables, making deployment challenging for somewhat longer distances. This option is recommended when abundant fiber resources are available and the deployment distance remains within manageable limits.

 

In contrast, 100GBASE-BiDi offers a versatile and cost-effective 100G Ethernet solution, requiring only two fiber cables for short reach and easily integrating into existing infrastructures. Despite the higher price of 100GBASE-BiDi SR transceivers compared to 100GBASE-SR4, this option is optimal for network upgrades when existing infrastructure is under strain.

 

In summary, considering fiber resources and cabling costs, 100GBASE-SR BD cabling demands only 1/4 of the fiber resources required by 100GBASE-SR4, leading to significant capital expenditure savings. However, the higher cost of 100GBASE-SR BD modules offsets these savings. Therefore, when evaluating the overall cost of optical module + fiber, choosing 100GBASE-SR BD may not necessarily result in lower costs than using 100GBASE-SR4.

 

Constructing Data Center Cabling with 100G QSFP28 BiDi SR1.2

 

The surge in mission-critical workloads, such as artificial intelligence (AI) and machine learning, intensifies computing and traffic demands in data center structures. This necessitates transitioning to denser and faster connections within data center ridges, backbones, and between ridge/rack top switches and servers. Upgrading from 10G to 40G or 100G traditionally involves converting LC duplex cabling to MPO cabling, leading to increased operating costs. Simplifying wiring and reducing capital expenditure can be achieved using the BiDi solution, exemplified by the 100G QSFP28 BiDi SR1.2 optical module.

 

Similar to its 40G SR BD module, the 100G QSFP28 BiDi SR1.2 enables data center operators to reuse their existing duplex LC fiber infrastructure when upgrading from 10G SR or 40G BiDi to 100G. This facilitates a flexible, step-by-step upgrade of network devices within the data center.

 

The 100G QSFP28 BiDi SR1.2 is a Quad Small Form-factor Pluggable (QSFP28) BiDi optical module, supporting 100Gbps and conforming to IEEE802.3bm 100GBASE-SR4 standards. Employing dual-core duplex LC multimode optical fiber, the module utilizes 4×25G NRZ modulation at the electrical end, converting to 2x50Gbps PAM4 modulation at the optical end. This results in a total bandwidth of 100Gb, with PAM4 technology supporting signal transmission at 50Gb data rates and 25Gbaud rates.

 

400G to 100G Spine Leaf Architecture

 

Deploying 400Gbps SR4.2 in the spine layer, the network backbone, facilitates connection to the leaf node through a breakout model. Alternatively, utilizing QSFP-100G-SR1.2 BiDi in both spine and leaf nodes provides an option to upgrade spine nodes to 400G, catering to growing bandwidth demands.

 

 

Switch-to-server Connectivity

 

Leveraging the flexibility of pluggable transceivers over preassembled cables, operators can opt for QSFP-100G-SR1.2 BiDi for top-of-rack connectivity to servers or deploy 400G in the top-of-rack (TOR) and downstream connect to servers using 1 x QDD-400G-SR4.2 to 4 x QSFP-100G-SR1.2 in a breakout model.

 

 

Conclusion

 

In conclusion, the choice between 100GBASE-SR BD and 100GBASE-SR4 hinges on factors like cost and infrastructure considerations. Despite the higher overall application cost of 100GBASE-SR BD, it is often preferred due to its ability to seamlessly upgrade from 10G/25G networks, utilizing existing LC duplex multimode fiber infrastructure. The advantages of 100GBASE-SR BiDi, including cost reduction, smooth upgrades, and future network readiness, make it a strategic choice. Conversely, 100GBASE-SR4's appeal lies in its ability to connect a single 100G port to up to four 25G ports, and its lower total price, catering to specific network requirements and budget constraints. QSFPTEK can provide 100GBASE-SR BD and 100GBASE-SR4 optic fiber transceivers with good quality and a 5-year warranty. Welcome to contact us via [email protected]