How you can use QSFP28 transceivers for 100G data center connectivity

To offer maximum channel count and lowest cost per bit, Ethernet switch vendors strive to maximize the density of optical transceiver ports on their equipment. This is achieved by choosing the smallest 100G form factor transceivers available and results in a cost optimized solution for connecting switches together in a rack or data center. For this reason, 100G switch vendors’ preferred choice is the QSFP28 form factor which is the smallest module with the lowest power consumption capable of handling 100G traffic. They also have the same physical size as the QSFP+ which means that switch vendors can achieve a 2.5x increase in traffic throughput without front panel redesign.

Short range QSFP28-SR4 versions support connections up to 100 metres over multimode fiber offering a cost-efficient option. Problem is that they use a more expensive non-standard MPO (multi push on/pull off cable) connector.  Long range QSFP28-LR4 versions support single mode fiber with the benefit of using standard LC connectors and supporting distances up to 10km.

MPO connector type used for short distance 850nm 100G connectivity

LR transceiver has internal multiplexer that transports 4 x 25G lanes over an LC fiber

For data handling and transfer within the rack and the data center, the QSFP28 is in most cases the perfect fit. The problem arises when 100G traffic needs to be transported over longer distances or as part of a DWDM network, for example 100G data center connectivity between different geographical locations. Here the switch manufacturers’ gain becomes the end users’ loss.

It’s not as if there is a shortage of 100G form factors though. The CFP for example, is a real work horse with DWDM Coherent technology enabling multi-channel long distance connectivity >1000km. The problem is these units are large and far too big to be used in an Ethernet switch. Even if the CFP2 is smaller, and the CFP4 smaller still, the size and power is still too high for the switch vendors to stomach. Yes they could offer DWDM support for the few links where DWDM is required, but the increased power consumption and significantly reduced port count results in 100G switches which are simply no longer cost effective or of interest.

The challenge of long distance connectivity

So, the QSFP28 may be the smallest 100G transceiver, but it does not have support for connectivity between data centers. Even if recent developments have brought a QSFP28 version with DWDM support, this is still limited to <2km.  The switch vendors’ low powered, high capacity solutions for intra data center traffic therefore doesn’t help when traffic needs to be transported further afield to other sites or data centers.

To handle 100G data center connectivity requirements, organisations are in many cases forced to consider full blown telecom grade DWDM platforms. Platforms that take the output of the QSFP28 transceiver and run it through a complex web of transponders, amplifiers, signal conditioning, multiplexers and network management. Platforms designed for the Telco world, not the Datacom world, and that are usually far too complex or over specified for these relatively simple traffic demands. Consequently the capex and opex for these systems is also too high.  A dilemma when considering that DWDM is typically only required for a small percentage of the connections to a standard Ethernet switch.

What is actually needed is a solution that allows best in breed 100G switches and routers to be used with maximum port count, lowest power consumption and lowest cost per bit. And for those occasions where the traffic from these switches and routers needs to be connected over longer distances or a DWDM network, a solution that is not overly complicated or going to break the bank. A solution that is simple to install and operate and cost effective to own and run. All the things that a Telco system is not. Something that can work and integrate in harmony with the Datacom switches. That allows the user to leverage the full benefits of QSFP28 transceivers in the Datacom equipment and yet still receive the full benefits of a Telco system for traffic transportation between sites. A solution that offers best of both worlds.

The Smartoptics DCP family is built for exactly for this purpose. To keep 100G data center connectivity flexible and affordable on the occasions where DWDM is required. The DCP-101 is a modular 100G DWDM transponder designed to simplify 100G networking and build capacity, without adding unnecessary cost or hardware. Low powered stand-alone modules that can be placed close to the 100G switches whose traffic they need to connect on a DWDM or dark fiber network. The DCP-101 also accepts the low cost QSFP28 copper cable assembly (DAC) which means that 100G cabling and transceiver costs can