{"id":341,"date":"2023-06-15T16:01:22","date_gmt":"2023-06-15T08:01:22","guid":{"rendered":"https:\/\/ascentoptics.com\/blog\/?p=341"},"modified":"2026-07-21T16:36:25","modified_gmt":"2026-07-21T08:36:25","slug":"100g-sr4-qsfp28-guide","status":"publish","type":"post","link":"https:\/\/ascentoptics.com\/blog\/100g-sr4-qsfp28-guide\/","title":{"rendered":"100G SR4 Explained: Reach, Fiber, MPO Cabling, Applications, and Compatibility"},"content":{"rendered":"\r\n\r\n\r\n

100G SR4 is one of the most widely deployed optical interfaces for short-reach 100 Gigabit Ethernet. It combines the compact QSFP28<\/a> form factor with four parallel 25G NRZ optical lanes at 850 nm, making it a cost-effective choice for high-density links inside data centers. A standard 100GBASE-SR4 connection reaches up to 70 meters over OM3 multimode fiber or 100 meters over OM4.<\/p>\r\n

This guide explains how a 100G SR4 QSFP28 transceiver works, which cables and connectors it requires, where it is best used, how it compares with other 100G optics, and what to check before deployment.<\/p>\r\n

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What Is a 100G SR4 QSFP28 Transceiver?<\/strong><\/h2>\r\n

A 100G SR4 transceiver<\/a> is a short-reach, parallel-optics module for 100 Gigabit Ethernet over multimode fiber. The name can be read as follows: “100G” indicates a 100 Gb\/s Ethernet interface, “SR” means short reach, and “4” refers to four optical lanes in each transmission direction.<\/p>\r\n

Unlike wavelength-multiplexed modules such as 100G CWDM4 or LR4, 100GBASE-SR4 does not combine multiple wavelengths onto a duplex fiber pair. Instead, it sends four independent 25G NRZ channels over four transmit fibers and receives four channels over four additional fibers. The link therefore uses eight active fibers inside a 12-fiber MPO interface.<\/p>\r\n\r\n\r\n\r\n

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100G SR4 at a Glance<\/strong><\/h3>\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
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Specification<\/strong><\/p>\r\n<\/td>\r\n

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Typical 100GBASE-SR4 Value<\/strong><\/p>\r\n<\/td>\r\n<\/tr>\r\n

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Ethernet specification<\/p>\r\n<\/td>\r\n

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100GBASE-SR4, introduced by IEEE 802.3bm and incorporated into the current IEEE 802.3 standard<\/p>\r\n<\/td>\r\n<\/tr>\r\n

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Form factor<\/p>\r\n<\/td>\r\n

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QSFP28, hot-pluggable<\/p>\r\n<\/td>\r\n<\/tr>\r\n

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Host electrical interface<\/p>\r\n<\/td>\r\n

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Four 25.78125 Gb\/s NRZ lanes, commonly described as 4 x 25G<\/p>\r\n<\/td>\r\n<\/tr>\r\n

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Optical architecture<\/p>\r\n<\/td>\r\n

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Four transmit lanes and four receive lanes in parallel<\/p>\r\n<\/td>\r\n<\/tr>\r\n

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Nominal wavelength<\/p>\r\n<\/td>\r\n

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850 nm near-infrared<\/p>\r\n<\/td>\r\n<\/tr>\r\n

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Fiber type<\/p>\r\n<\/td>\r\n

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OM3 or OM4 multimode fiber<\/p>\r\n<\/td>\r\n<\/tr>\r\n

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Connector<\/p>\r\n<\/td>\r\n

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MTP\/MPO-12; eight fibers are active and four central positions are unused<\/p>\r\n<\/td>\r\n<\/tr>\r\n

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Maximum reach<\/p>\r\n<\/td>\r\n

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70 m over OM3; 100 m over OM4<\/p>\r\n<\/td>\r\n<\/tr>\r\n

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Typical transmitter<\/p>\r\n<\/td>\r\n

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850 nm VCSEL array<\/p>\r\n<\/td>\r\n<\/tr>\r\n

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Typical power and temperature<\/p>\r\n<\/td>\r\n

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Often below 3.5 W and 0 to 70 degrees C for commercial modules; verify the vendor datasheet<\/p>\r\n<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n\r\n\r\n\r\n

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Note: Power consumption, operating temperature, alarm thresholds, and vendor coding are product-specific. Always confirm the actual transceiver datasheet and platform compatibility list.<\/p>\r\n

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How 100GBASE-SR4 Works<\/strong><\/h2>\r\n

On the host side, a switch ASIC or network adapter presents four electrical lanes at approximately 25.78125 Gb\/s per lane. Inside the QSFP28 module, a four-channel VCSEL array converts those electrical signals into four parallel 850 nm optical signals. At the far end, a matching receiver array converts the four optical lanes back into electrical signals and delivers them to the host.<\/p>\r\n

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1.The host sends four 25G-class NRZ electrical lanes to the QSFP28 module.<\/p>\r\n

2. The module drives four 850 nm VCSEL transmitters, one for each optical lane.<\/p>\r\n

3. The MPO-12 cable carries four transmit lanes and four receive lanes in parallel.<\/p>\r\n

4. The receiving module converts the optical signals back to four electrical lanes.<\/p>\r\n

5. The host PCS combines the lanes into a single 100 Gigabit Ethernet interface.<\/p>\r\n

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Because all four optical channels operate in parallel, lane alignment, FEC settings, fiber polarity, and optical loss must all be correct for a stable link.<\/p>\r\n\r\n\r\n\r\n

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MPO-12 Cabling, Fiber Count, and Polarity<\/strong><\/h2>\r\n

Standard 100GBASE-SR4 uses an MTP\/MPO-12 interface rather than a duplex LC connector. Although the connector contains 12 fiber positions, only eight are active: four transmit fibers and four receive fibers. The four center positions are normally unused.<\/p>\r\n

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For a direct SR4-to-SR4 link, verify all of the following:<\/strong><\/h3>\r\n