Cutting-Edge Connectivity: Inside the 100GBASE-SR4 and 100GBASE-BiDi SR QSFP28 Technologies

The rise of cloud computing and the expansion of 5G networks are driving significant advancements in data center technology, necessitating an increased demand for 100G optical transceiver modules. These modules, which account for a significant portion of the costs associated with network infrastructure, are essential for the efficient operation of data centers of all sizes. In small to medium-sized data centers, over 90% of fiber optic links are contained within distances of 100 meters. In contrast, in larger data centers, more than 70% of these links are under 100 meters, with in excess of 80% not exceeding 125 meters. The 100G multimode module, especially the QSFP28 100G multimode type, has become the preferred option for such connections, with a focus on models like the 100GBASE-SR BiDi and the 100GBASE-SR4.

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

The 100GBASE SR4 is a common 100G multimode module that utilizes four transmission lanes at 25Gbps each, resulting in a combined bandwidth of 100Gbps. It is labeled "SR" for short reach and is engineered for brief transmission spans, typically offering 70 meters of connectivity over OM3 fiber or 100 meters over OM4 fiber. This module adheres to the IEEE 802.3BA standards and functions as a plug-and-play, full-duplex optical unit with an 850nm central wavelength. Its adoption is widespread because it provides four separate channels for both sending and receiving data, with each capable of 25Gbps. It stands as a go-to choice for near-field, high-data-rate communications in 100G Ethernet frameworks. A diagram here might illustrate the operational mechanism of the 100GBASE-SR4 optical module transceiver.

The 100GBASE-SR BiDi module represents the bidirectional variant of the 100GBASE-SR, deploying a single multimode fiber for two-way optical signal traffic at wavelengths of 855nm and 908nm. This model achieves a 100Gbps data rate using two channels that transmit and receive at 50Gbps in both directions. Contrasting with the 100G QSFP28 SR4, which requires eight fiber strands, the 100GBASE-SR BiDi module leverages WDM (Wavelength Division Multiplexing) technology to enable bi-directional communications through a single LC connector, where each connector transmits and receives optical signals at different wavelengths simultaneously over a single multimode fiber. These dual-wavelength VCSEL bi-directional transceivers use PAM4 modulation to carry 2 × 50-Gb/s on 855nm/908nm wavelengths, with an operational span reaching up to 70m on OM3, 100m on OM4, and 150m on OM5 fiber, proving its adaptability in various multimode fiber environments. An accompanying diagram could detail the function of the 100G QSFP28 SR BiDi optical module transceiver.

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

Fiber Count

The 100GBASE-SR4 transceiver makes use of an 8-fiber setup, which stands as the standard multimode fiber cabling approach for short-distance 100G Ethernet connectivity. This setup demands the use of four fibers for transmission and another four for receiving signals, often connected through an MPO/MTP cabling framework. On the other hand, the 100GBASE-BiDi SR transceivers require just two fibers to manage bidirectional data flow, offering a cost-efficient alternative for upgrading to a 100G network infrastructure. It is evident that 100GBASE-BiDi is the most efficient in terms of fiber cabling needs, making it an optimal choice in situations where the existing multimode fiber resource is limited or where laying new fibers is impractical.

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.

How to Choose 100GBASE-SR4 and 100GBASE-BiDi SR

When choosing between 100GBASE-SR4 modules and 100GBASE-BiDi SR modules, one must consider several key factors to determine which type of module is best suited for your network needs. Here are some of the main points to consider when making your decision:

Reach Requirements

The standard transmission distance for 100GBASE-SR4 is typically 70 to 100 meters when using OM4 multimode fiber, and this distance might be reduced to 50 meters for OM3 fiber. For 100GBASE-BiDi SR, it can often provide a transmission distance of 150 meters with OM4 multimode fiber, offering longer connection distances than 100GBASE-SR4, with slightly reduced distances when using OM3 fiber.

Fiber Type and Core Count

For 100GBASE-SR4, it requires 8 fiber cores (4 transmit + 4 receive) and utilizes parallel fiber communication. Regarding 100GBASE-BiDi SR, it only needs 2 fiber cores for bidirectional transmission and utilizes wavelength division multiplexing technology to transmit in both directions on a single fiber.

Port Density and Cost

In the case of 100GBASE-SR4, more port space on switch panels is needed due to the utilization of multiple fiber cores. For 100GBASE-BiDi SR, the requirement for just two fibers makes it more effective in terms of port occupation and fiber savings.

Compatibility and Scalability

Check which type of module is supported by your network equipment, such as switches and routers, and whether they are compatible with your existing fiber cabling infrastructure. Consider which module would be easier to scale and maintain when planning for future network expansion and upgrades.

Conclusion

Ultimately, the decision to opt for either 100GBASE-SR BiDi or 100GBASE-SR4 is influenced by considerations of cost and existing infrastructure. Although 100GBASE-SR BiDi commands a higher overall deployment cost, it is often favored for its compatibility with the upgrade from 10G/25G networks while leveraging current LC duplex multimode fiber setups. The benefits offered by 100GBASE-SR BiDi, such as cost efficiency, ease in transitioning, and readiness for future network expansions, render it an advantageous strategic option. On the other hand, 100GBASE-SR4 is attractive for its feature allowing a single 100G port to interface with up to four 25G ports, as well as its more affordable total cost, serving particular networking needs and budgetary limitations.