Choosing the right 100G QSFP28 Transceiver

20/06/2017

QSFP28 has become the dominant form factor for 100G networks. The interconnect offers four channels of high-speed differential signals with data rates ranging from 25 Gbps up to potentially 40 Gbps, and meets 100 Gbps Ethernet (4 25 Gbps) and 100 Gbps 4X InfiniBand Enhanced Data Rate (EDR) requirements. There are many products with QSFP28 form factor like the SR4, LR4, PSM4, CWDM4 and AOCs.

Let us take a brief look at the important parameters of 100G QSFP28 transceivers.

QSFP28 AOCs: Active Optical Cables or AOCs are a good fit for short reach data interconnects. Terminated with transceiver style ends, AOCs range from 3m to 100m lengths. It transmits error-free parallel 4 25 Gb/s data over multimode fiber (MMF) cables. Its' small bend radius makes for easy installation and fiber management.

QSFP28 SR4: The QSFP28 SR4 module supports links of 70m over OM3 and 100m over OM4 with MPO connectors. This product is a parallel 100G QSFP28 optical module with 4 independent transmit and receive channels each capable of 25Gb/s operation. The QSFP28 SR4 modules are ideal for rack to rack connections in the datacenter and short reach telecom applications.

QSFP28 LR4: For very long span 100GbE deployment, such as connectivity between two buildings, QSFP28LR4 with duplex LC single-mode fiber cable is the preferred option. The LR4 module uses WDM technologies to achieve 100G transmission over four different wavelengths around 1310nm. It can support distances up to 10km.

QSFP28 ER4 Lite: QSFP28 ER4 Lite is a 100Gb/s transceiver module designed for optical communication applications compliant to Ethernet 100GBASE-ER4 Lite standard. The high performance cooled LAN WDM EA-DFB transmitters and high sensitivity APD receivers provide superior performance for 100Gigabit Ethernet applications up to 25km links without FEC and 32km links with FEC.

QSFP28 CWDM4 and PSM4

The QSFP28 LR4 with 10km reach seemed like overkill for datacenter applications where the longest reach for most applications was about 1 km. The industry was not able to reach a consensus on the intermediate or mid-reaches for datacenter applications (500m to 2km) and so development of two different architectures- WDM and parallel single mode fiber technologies have happened in tandem.

QSFP28 CWDM4: The CWDM4 module uses Mux/Demux technologies with 4 lanes of 25 Gbps optically multiplexed onto and demultiplexed from duplex single-mode fiber. It is centered around the 1310nm band with 20nm channel spacing as defined by the ITU standard. With a reach of 2km, QSFP28 CWDM4 transmits 100G optical signals via a duplex LC interface.

QSFP28 CLR4: Developed by the 100G CLR4 Alliance, CLR4 routes four 25G optical transmissions down a single fiber for reaches of up to 2km. The specifications for CWDM4 and CLR4 are almost the same, the main difference being that CWDM4 requires forward error correction (FEC) to improve signal-to-noise ratio, while CLR4 can be used with or without FEC.

QSFP28 PSM4: The 100G PSM4 specification defines requirements for a point-to-point 100 Gb/s link over eight single mode fibers (4 transmit and 4 receive) of at least 500m, each transmitting at 25Gbps. Four identical and independent lanes are used for each signal direction. PSM4 does not need a MUX/DEMUX for each laser but it does need either a directly modulated DFB laser (DML) or an external modulator for each fiber. With an MTP interface, PSM4 modules can bus 100Gb/s point-to-point over 2km or can be broken out into dual 50Gb/s or quad 25Gb/s links for linking to servers, storage and other subsystems.

From an optical transceiver module structure viewpoint, PSM4 can be more cost effective because of its lower component costs. However, from an infrastructure viewpoint, PSM4 would be more expensive when the link distance is long, mainly because PSM4 uses 8 optical single-mode-fibers while CWDM4 uses only 2 optical single-mode-fibers.

Now that you have the basic information on the different kinds of 100G QSFP28 modules, you can choose the right transceiver for your application. Vitex provides 100G QSFP28 modules for short reach, long reach and intermediate applications. Talk to a Vitex technical specialist at 201-296-0145 for more information.

LINK:	http://vitextech.com/100g-qsfp28-basics/...
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