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ith the growth in data center size along with the increased data rates of optical interconnects, there comes a need for low cost solutions capable of at least 500m reach. The 100G PSM4 specification is targeted to service the need on a parallel single-mode infrastructure. It provides a low cost solution to long reach data center optical interconnects. This tutorial will introduce the 100G PSM4 standard and 100GBase-Psm4 QSFP28.
100G PSM4 Standard
Specified by MSA, the 100G PSM4 standard defines requirements for a point-to-point 100G link over eight single-mode fibers up to at least 500 meters. Four identical and independent lanes are used for each signal direction. It aims to promote the creation and adoption of parallel single-mode 4-lane (PSM4) approaches to 100G in the data center. The PSM4 enables 500m data center reach links that will support key customer requirements. Thus there is a need for PSM4 optical transceivers to fill the requirement for low-cost 100G connections at reaches of 500 meters in applications that fall in between the IEEE multi-wavelength 10km 100GBASE-LR4 single-mode fiber approach and its multimode-fiber based 100GBASE-SR10 short reach specifications.
100GBASE-PSM4 QSFP28 Transceiver Basics
Defined by the 100G PSM4 MSA, the 100GBASE-PSM4 transceiver uses four parallel fibers (lanes) operating in each direction, with each lane carrying a 25G optical transmission. The source is a single uncooled distributed feedback (DFB) laser operating at 1310 nm. It needs either a directly modulated DFB laser (DML) or an external modulator for each fiber. The 100GBASE-PSM4 transceiver sends the signal down an eight-fiber cable with an MPO connector with transmission distance up to 500 meters. It is designed with form factor, optical/electrical connection and digital diagnostic interface according to the QSFP28 MSA. An optical fiber ribbon cable with an MTP/MPO connector can be plugged into the QSFP28 module receptacle.
The picture above shows the functional block diagram of the 100GBASE-PSM4 QSFP28 transceiver. The transmitter module accepts electrical input signals compatible with common mode logic (CML) levels. All input data signals are differential and internally terminated. The receiver module converts parallel optical input signals via a photo detector array into parallel electrical output signals. The receiver module outputs electrical signals are also voltage compatible with CML levels. All data signals are differential and support a data rates up to 25 Gbps per channel.
100GBASE-PSM4 QSFP28 Transceiver Deployment Considerations
The 100GBASE-PSM4 QSFP28 transceiver is technically straightforward to implement and is likely to be the most economical of the QSFP28 interfaces. But while the PSM transceiver cost will likely be cheap, parallel fiber is expensive so that the module price advantage is eroded by the longer link. A data center manager will consider using the PSM4-based on such factors as the parallel single-mode fiber already deployed or whether the PSM4 is deemed the most cost-effective solution for their requirements. The PSM4’s 25G channels can be used as individual lower speed breakout links.
From an optical transceiver module structure viewpoint, PSM4 can be very cost-effective. It uses a single uncooled CW laser which splits its output power into four integrated silicon modulators. And its array-fiber coupling to an MTP connector is relatively simple. While from an infrastructure viewpoint, PSM4 would be expensive when the link distance is long, mainly due to the fact that PSM4 uses 8 optical single-mode fibers.
Summary
100GBASE-PSM4 QSFP28 transceiver has its own strengths and weakness. It starts with a low transceiver cost, but as the link distance increases, its total cost climbs up very fast. Based on the specifications of PSM4 MSA, the technology has to be limited to 500 meters, which can actually cover the majority of today’s data center needs. Gelink provides high quality 100GBASE-PSM4 QSFP28 transceiver which has been tested on corresponding equipment before shipping. If you need to buy 100GBASE-PSM4 QSFP28 transceiver, Gelink is an excellent option.
