{"id":101718,"date":"2026-10-20T10:57:18","date_gmt":"2026-10-20T10:57:18","guid":{"rendered":"https:\/\/optics.carritech.com\/?p=101718"},"modified":"2026-10-20T11:00:28","modified_gmt":"2026-10-20T11:00:28","slug":"40g-qsfp-transceivers","status":"publish","type":"post","link":"https:\/\/optics.carritech.com\/es\/40g-qsfp-transceivers\/","title":{"rendered":"40G QSFP+ transceivers: the generation nobody is upgrading from"},"content":{"rendered":"

Forty gigabit Ethernet was meant to be a stepping stone. It arrived in 2010, it was overtaken by 100G within five years, and every roadmap published since has treated it as a transitional speed on the way somewhere else. Sixteen years later there are 40G QSFP+ ports carrying production traffic in campus cores, firewall clusters, storage front-ends, spine-leaf fabrics and transport platforms all over Europe, and most of them are not going anywhere.<\/p>\n\n\n\n

That is because a 40G port is usually four 10G lanes bonded together, and four 10G lanes is still a perfectly sensible amount of bandwidth for an uplink. The chassis works. The optics work. The fibre is in the ground. What has changed is the supply: the original vendors have moved their attention to 400G and 800G, and 40G optics are getting harder and more expensive to buy from them every year.<\/p>\n\n\n\n

This guide covers what the 40G QSFP+ variants actually are, which cabling each one needs, the MPO trap that catches people out, how breakout to 4 × 10G works, why FEC does not come into it, and which modules fit which platform.<\/p>\n\n\n\n

What QSFP+ is<\/h2>\n\n\n\n

QSFP+ is the "quad small form-factor pluggable plus" package: four lanes of 10G in a module roughly 72 mm long and 18 mm wide, managed over I²C, drawing between about 1.5 W and 3.5 W depending on the reach. It is defined mechanically and electrically in the SFF-8436 specification, now maintained through the SFF specifications at SNIA<\/a>, and the Ethernet PMDs that run over it come from IEEE 802.3ba<\/a> and its successors.<\/p>\n\n\n\n

Two properties matter more than any datasheet line. First, QSFP+ is mechanically and electrically compatible with QSFP28 cages<\/strong> — a 40G module will generally work in a 100G port, which is why 40G optics keep appearing in equipment bought years after 40G stopped being specified. Second, the four lanes can be broken out<\/strong>, so one QSFP+ port can become four independent 10G interfaces. Together those two facts explain most of 40G’s longevity.<\/p>\n\n\n\n

The 40G QSFP+ variants, compared<\/h2>\n\n\n\n

Eight optical variants cover essentially all 40G deployment. The names follow the Ethernet convention, with two widely-used additions that are not IEEE standards at all.<\/p>\n\n\n\n

\n
Variante<\/th>Longitud de onda<\/th>Fibre and connector<\/th>Llegue a<\/th>Typical power<\/th><\/tr><\/thead>\n
SR4<\/strong><\/td>850 nm × 4 lanes<\/td>OM3\/OM4 multimode, MPO-12<\/td>100 m (OM3) \/ 150 m (OM4)<\/td>≤ 1.5 W<\/td><\/tr>
BiDi (SRBD)<\/strong><\/td>850 \/ 900 nm<\/td>OM3\/OM4 multimode, duplex LC<\/strong><\/td>100 m (OM3) \/ 150 m (OM4)<\/td>~2.5 W<\/td><\/tr>
PSM4 \/ PLR4<\/strong><\/td>1310 nm × 4 lanes<\/td>Single-mode, MPO ribbon<\/td>up to 10 km<\/td>≤ 3.5 W<\/td><\/tr>
LR4<\/strong><\/td>1310 nm CWDM × 4<\/td>Single-mode, duplex LC<\/td>10 km<\/td>≤ 3.5 W<\/td><\/tr>
ER4<\/strong><\/td>1310 nm CWDM × 4<\/td>Single-mode, duplex LC<\/td>30–40 km<\/td>≤ 3.5 W<\/td><\/tr>
ZR4<\/strong><\/td>1310 nm LAN-WDM × 4<\/td>Single-mode, duplex LC<\/td>80 km<\/td>2.5 W typ.<\/td><\/tr>
ZR4+ \/ EZR4<\/strong><\/td>1550 nm<\/td>Single-mode, duplex LC<\/td>100 km<\/td>≤ 3.5 W<\/td><\/tr><\/tbody>\n<\/table><\/figure>\n\n\n\n

The two non-standard entries are the interesting ones. BiDi<\/strong> is a vendor-originated design, not an IEEE PMD, and it exists for one reason covered in the next section. PSM4 and PLR4<\/strong> are parallel single-mode designs that put each 10G lane on its own fibre pair rather than multiplexing four wavelengths onto one pair — cheaper optics, more fibre. Everything else on that list is a standard PMD: SR4 and LR4 came with 802.3ba<\/a> in 2010, and ER4 was added later, in the 2015 amendment that also tidied up the 100G single-mode interfaces.<\/p>\n\n\n\n

The MPO trap, and why BiDi exists<\/h2>\n\n\n\n

A 40GBASE-SR4 module does not use a duplex LC patch lead. It uses a 12-fibre MPO ribbon, of which it uses eight: four fibres transmitting, four receiving, with the middle four unused. That is a different cabling type, a different polarity convention, a different cleaning regime and a different set of failure modes from everything else in a typical 10G estate.<\/p>\n\n\n\n

Which is why a large number of 40G links that should have been SR4 are BiDi instead. A 40G BiDi module carries two 20G channels in each direction over a single pair of multimode fibres using two wavelengths — 850 nm and 900 nm — so it plugs into the duplex LC multimode patch leads already installed for 10G<\/strong>. Same reach, same fibre, no re-cabling, no ribbon, no polarity exercise.<\/p>\n\n\n\n

Three things to know before you choose BiDi over SR4:<\/p>\n\n\n\n