Category Archives: QSFP28 transceiver

Can I Use the QSFP+ Optics on QSFP28 Port?

100G Ethernet will have a larger share of network equipment market in 2017, according to Infonetics Research. But we can’t neglect the fact that 100G technology and relevant optics are still under development. Users who plan to layout 100G network for long-hual infrastructures usually met some problems. For example, currently, the qsfp28 optics on the market can only support up to 10 km (QSFP28 100GBASE-LR4) with WDM technology, which means you have to buy the extra expensive WDM devices. For applications beyond 10km, QSFP28 optical transceivers cannot reach it. Therefore, users have to use 40G QSFP+ optics on 100G switches. But here comes a problem, can I use the QSFP+ optics on the QSFP28 port of the 100G switch? If this is okay, can I use the QSFP28 modules on the QSFP+ port? This article discusses the feasibility of this solution and provides a foundational guidance of how to configure the 100G switches.

For Most Switches, QSFP+ Can Be Used on QSFP28 Port

As we all know that QSFP28 transceivers have the same form factor as the QSFP optical transceiver. The former has just 4 electrical lanes that can be used as a 4x10GbE, 4x25GbE, while the latter supports 40G ( 4x10G). So from all of this information, a QSFP28 module breaks out into either 4x25G or 4x10G lanes, which depends on the transceiver used. This is the same case with the SFP28 transceivers that accept SFP+ transceivers and run at the lower 10G speed.

QSFP+ can work on the QSFP28 ports

A 100G QSFP28 port can generally take either a QSFP+ or QSFP28 optics. If the QSFP28 optics support 25G lanes, then it can operate 4x25G breakout, 2x50G breakout or 1x100G (no breakout). The QSFP+ optic supports 10G lanes, so it can run 4x10GE or 1x40GE. If you use the QSFP transceivers in QSFP28 port, keep in mind that you have both single-mode and multimode (SR/LR) optical transceivers and twinax/AOC options that are available.

In all Cases, QSFP28 Optics Cannot Be Used on QSFP+ Port

SFP+ can’t auto-negotiate to support SFP module, similarly QSFP28 modules can not be used on the QSFP port, either. There is the rule about mixing optical transceivers with different speed—it basically comes down to the optic and the port, vice versa. Both ends of the two modules have to match and form factor needs to match as well. Additionally, port speed needs to be equal or greater than the optic used.

How to Configure 100G Switch

For those who are not familiar with how to do the port configuration, you can have a look at the following part.

  • How do you change 100G QSFP ports to support QSFP+ 40GbE transceivers?

Configure the desired speed as 40G:
(config)# interface Ethernet1/1
(config-if-Et1/1)# speed forced 40gfull

  • How do you change 100G QSFP ports to support 4x10GbE mode using a QSFP+ transceiver?

Configure the desired speed as 10G:
(config)# interface Ethernet1/1 – 4
(config-if-Et1/1-4)# speed forced 10000full

  • How do you change 100G QSFP ports from 100GbE mode to 4x25G mode?

Configure the desired speed as 25G:
(config)# interface Ethernet1/1 – 4
(config-if-Et1/1-4)# speed forced 25gfull

  • How do you change 100G QSFP ports back to the default mode?

Configure the port to default mode:
(config)# interface Ethernet1/1-4
(config-if-Et1/1)# no speed

Note that if you have no experience in port configuration, it is advisable for you to consult your switch vendor in advance.


To sum up, QSFP+ modules can be used on the QSFP28 ports, but QSFP28 transceivers cannot transmit 100Gbps on the QSFP+ port. When using the QSFP optics on the QSFP28 port, don’t forget to configure your switch (follow the above instructions). To make sure the smooth network transmission, you need to ensure the connectors on both ends are the same and no manufacturer compatibility issue exists.

100G QSFP28 PSM4 to Address 500m Links in Data Center

100G QSFP28 PSM4 optics is a type of 100G optical transceiver that provides a low-cost solution to long-reach data center optical interconnects. 100G PSM4 (parallel single-mode 4 lane) standard is mainly targeted to data centers that based on a parallel single-mode infrastructure for a link length of 500 m. Compared with the hot-selling 100GBASE-SR4 and 100GBASE-LR4 optics, 100G QSFP28 PSM4 recently wins the popularity among the overall users. This article will provide a complete specification of the 100G QSFP28 PSM4 transceiver and explain the reason why people would need QSFP28 PSM4.

QSFP28 module

QSFP28 PSM4—A Low-Cost but Long-Reach Solution

100G QSFP28 PSM4 is compliant with 100G PSM4 MSA standard, which defines a point-to-point 100 Gb/s link over eight parallel single-mode fibers (4 transmit and 4 receive) up to at least 500 m. PSM4 uses four identical lanes per direction. Each lane carries a 25G optical transmission. The 100G PSM4 standard is now available in QSFP28 and CFP4 form factor. Table 2 shows the diagram of the 100G QSFP28 PSM4 Specification. 100G PSM4 is a low-cost solution. Its cost structure is driven by the cost of the fiber and the high component count. FS.COM offers the Cisco compatible 100G QSFP28 PSM4 at US$750.00.

diagram of QSFP28 PSM4

As you can see in the above image, 100G QSFP28 PSM4 transceiver uses four parallel fibers (lanes) operating in each direction, with transmission distance up to 500 meters. The source of the QSFP28 PSM4 module 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 usually needs the single-mode ribbon cable with an MTP/MPO connector.

Why Do We Need 100G QSFP28 PSM4?

100G PSM4 is the 100G standard that has been launched by multi-source agreement (MSA) to enable 500m links in data center optical interconnects. But as we all know, there are several popular 100G interfaces out there on the market, such as QSFP28 100GBASE-SR4, QSFP28 100GBASE-LR4, QSFP28 100GBASE-CWDM4, and CFP 100GBASE-LR4, etc. So with so many options, why do we still need 100G QSFP28 PSM4?

To better help you make up your mind, you need to figure out the following questions:

Q1: What are the net link budget differences between PSM4, SR4, LR4 and CWDM?
Table 3 displays the detailed information about these 100G standards.

4-wavelength CWDM multiplexer and demultiplexer No need Need No need Need
Connector MPO/MTP connector Two LC connectors MPO/MTP connector Two LC connectors
Reach 500 m 2 km 100 m 10 km

Note: the above diagram excludes the actual loss of each link (it is the ideal situation). In fact, WDM solution are at least 7 db worse link budget than PSM4. For a 2 km connectivity, a CWDM module will have to overcome about 10 db additional losses compared to PSM4. And the 100G LR4 optics at 10 km is 12 db higher total loss than PSM4.

Q2: What power targets are achievable for each, and by extension what form factors?
According to the IEEE data sheet, the WDM solutions cannot reasonably fit inside QSFP thermal envelop, while PSM4 can fit inside the QSFP thermal envelope. That means you would need the extra power for the WDM solution of your network. But if you use the QSFP PSM4, this won’t be a problem.

All in all, a 100G QSFP28 PSM4 transceiver with 500m max reach is a optional choice for customers. Because other 100G optics are either too short for practical application in data center or too long and costly. QSFP28 PSM4 modules are much less expensive than the 10 km, 100GBASE-LR4 module, and support longer distance than 100GBASE-SR4 QSFP28.


QSFP28 PSM4 is the lowest cost solution at under one forth the cost of either WDM alternatives. 100G QSFP28 PSM4 can support a link length of 500 m, which is sufficient for data center interconnect applications. 100G QSFP28 PSM4 also offers the simplest architecture, the most streamlined data path, higher reliability, an easy upgrade path to 100G Ethernet.