100G QSFP28 Optical Transceiver Comparison

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Nowadays,
the trend for 100G Ethernet network is bullish and inevitable. Thus, the
demands for 100G modules are becoming greater and greater that account for a
relatively high proportion of network construction costs. Among various 100G
optical transceivers that are capable of connecting 100G traffic, the 100G
QSFP28 optical transceiver is the most preferred module for smaller
size and lower power consumption. What are 100G QSFP28 optical transceivers?
What are the differences between them? How do we choose 100G QSFP28 optics?
Today, we will make a comprehensive comparison of 100G QSFP28 optical modules
for you.

 

What
Is 100G QSFP28 Optical Transceiver?

The
100G QSFP28 optical transceiver is designed for 100 Gigabit Ethernet, EDR
InfiniBand, or 32G Fibre Channel. It generally has the exact same footprint and
faceplate density as 40G QSFP+. Just as the 40G QSFP+ is implemented using four
10Gbps lanes, the 100G QSFP28 module is implemented with four channels of
high-speed differential signals with data rates ranging from 25Gbps up to
potentially 40Gbps. With an upgrade electrical interface, the QSFP28 100G
transceiver is capable of supporting signals up to 28Gbps. QSFP 100G SR4, QSFP
100G LR4, QSFP 100G PSM4, QSFP 100G CWDM4, and QSFP 100G ER4 are the five
common types of 100G QSFP28 modules designed to meet the different transmission
distance requirements. Compared with 100G CFP/CFP2/CFP4 transceivers, the 100G
QSFP28 module surpasses them with the strong ability to increase density,
decrease power consumption, and decrease the price per bit.

 

Differences
between the 100G QSFP28 Transceiver

In
this section, we will describe the differences between these 100G QSFP28 optics
from the perspectives of standards, laser types, transmission media and
transmission distances.

 

100G
QSFP28 Optical Module Standards

The
100G QSFP28 optical module standards are mainly defined by two key organizations,
namely IEEE and Multi Source Agreement (MSA). The QSFP28 SR4, QSFP28 LR4,
QSFP28 ER4 are defined by the IEEE. How to clarify these three items from their
names? The ‘QSFP’ means the
form factor; ‘28’ represents that each lane has a maximum transmission rate of
28Gbps; ‘SR’ means short reach (100m), ‘LR’ is long reach (10km) and ‘ER’ is
extended reach (40km). ‘4’ represents that the module has 4 lanes. For example,
the QSFP28 LR4 means that it is a long reach 100G module which can transmit
100G signals over four wavelengths with a transmission distance of 10km.

 

100GBASE-SR4
and 100GBASE-LR4 are the most commonly used 100G interface specifications
defined by the IEEE. However, the transmission distance of QSFP28 SR4 is too
short to meet all interconnection requirements and the cost of QSFP28 LR4 is
too high for large data centers. Therefore, MSA brought a solution for
mid-range connectivity to the market and defined the standards of QSFP28 PSM4
and CWDM4. Although the capability of QSFP 100G LR4 completely covers QSFP28
CWDM4, but the QSFP 100G CWDM4 solution is much cheaper and more competitive
than QSFP28 LR4 in the scenario of 2km transmission.

 

The
VCSEL laser features with small size, low power consumption, easy integration,
low price and high coupling efficiency with multimode fibers, which is often
used for QSFP28 SR4 modules. The QSFP28 LR4 and QSFP28 ER4 optics are used for
long-distance transmission (10km or 40km), which need lasers with advantages of
large eye volume, small dispersion, large extinction and long distance. The EML
is designed to meet all those requirements for QSFP 100G LR4 and QSFP 100G ER4
transceivers. The DML can realize the signal modulation through the way of
modulating the injection current of the laser. Since the magnitude of the
injection current changes the refractive index of the active region of the
laser, causing wavelength drift to generate dispersion. Thus, it is difficult
to achieve high-speed signal modulation and it can not transmit for a very long
distance. Thus, the DML laser is suitable for QSFP28 PSM4 and QSFP28 CWDM4
modules with a transmission distance of 500m or 2km.

 

QSFP
100G Transmission Media & Transmission Distance

The
100G QSFP28 optical transceivers are equipped with LC duplex or MTP/MPO-12
connectors, which transmission distances vary from 70m to 40km. The
transmission distances of the QSFP28 optics decide the application scenarios of
them. All the QSFP28 100G transceivers can be applied to direct connection,
interconnection in enterprises and data center networks. The following diagram
shows the specific differences of the QSFP 100G optics.

 

How
to Select the 100G QSFP28 Optical Transceiver?

After
introducing the differences of the various kinds of 100G QSFP28 optical
transceiver, you may have a basic understanding of them. How to choose your
QSFP28 100G modules in the practical application? Here is a selection guide for
you.

 

For
transmission distance between 5m and 100m, you can choose the QSFP28 SR4
optical transceiver module, it can transmits 70m using OM3 fibers or 100m using
OM4 fibers.

 

For
transmission distance between 100m to 2km, you can choose QSFP 100G PSM4 or
QSFP28 CWDM4 modules. The cost of QSFP28 CWDM4 is much higher than the QSFP28
PSM4. However, the QSFP28 CWDM4 requires only two single mode fibers in two-way
transmission which is much less than the eight single mode fibers of QSFP28
PSM4, the total cost of the QSFP 100G PSM4 raises very quickly. In practical
applications, it is necessary to decide whether to use QSFP28 PSM4 or QSFP28
CWDM4 depending on the interconnection distance.

 

For
transmission distance between 10km to 40km, you can choose QSFP 100G LR4 which
has a maximum transmission distance of up to 10 km or QSFP28 ER4 transceivers
which transmission distance can reach up to 40km.