The Quad Small Form-factor Pluggable (QSFP) is a compact, hot-pluggable transceiver. The data rates are from 4×1 Gb/s for QSFP and 4×10 Gbit/s for QSFP+ and to the highest rate of 4×28 Gbit/s known as QSFP28 used for 100 Gbit/s links.
The QSFP28 standard is designed to carry 100 Gigabit Ethernet, EDR InfiniBand or 32G Fibre Channel. This transceiver type is also used with direct-attach breakout cables to adapt a single 100GbE port to four independent 25 gigabit ethernet ports (QSFP28-to-4x-SFP28). Sometimes this transceiver type is also referred to as “QSFP100” or “100G QSFP” for sake of simplicity.
QSFP28 transceiver not only have the same physical size as the QSFP+ used for 40G traffic, but the lowest power consumption among those that are capable of handling 100G traffic.
Basically, there are two types of transceivers: QSFP28-SR4 and QSFP28-LR4.
QSFP28-SR4 transceivers is specially designed to support connections of up to 100 meters over multimode fiber. This approach is similar to using AOC cables, but here it is possible to use structured cabling. They use more expensive non-standard MPO (multi push-on/pull-off cable) connectors which cancel out some of the cost savings of the transceiver.
QSFP28-LR4 versions support connections up to 10km over single-mode fiber. They use standard LC connectors and the existing structured LC cabling.
QSFP28 Cable Assemblies
QSFP28 cable (DAC or AOC cables) is the more convenient, low-cost method of connecting 100G equipment. Using cable assemblies removes many of the problems associated with dirty connectors. DAC is suitable for applications within 15m and AOC up to 70m. AOC cable assemblies provide similar performance to discrete transceivers and fiber cables.
Active Direct Attach Copper Cable
Active copper cables are designed in the same cable type as the passive one, but they contain low power circuitry in the connector to boost the signal and are driven from the port without additional power requirements. The active version provides a low cost alternative to optical transceivers, and are generally used for end of row or middle of row data center architectures for interconnect distances of up to 15 meters.
The main difference between active DAC and passive DAC is that there is a driving chip in the design of active DAC.
Active Optical Cable
Active optical cable (AOC) incorporates active electrical and optical components. It can achieve longer distance than the copper assemblies. In general, active optical cable can reach more than 100m via multimode fiber. Compared to direct attach copper cable, AOC (eg. Cisco SFP-10G-AOC10M) weighs less and can support longer transmission distance. It is immune to electromagnetic energy since the optical fiber is dielectric (not able to conduct electric current). And it is an alternative to optical transceivers and it can eliminate the separable interface between transceiver module and optical cable. However, it costs more than copper cable. 100GbE QSFP28 AOC is composed of an OM4 multimode cable connecting two QSFP28 connectors on each end. Using the same port as transceiver optics, direct attach cables can support Ethernet, Infiniband and Fibre Channel but with independent protocols. In general, direct attach cable assemblies are divided into three families—direct attach passive copper cable, direct attach active copper cable and active optical cable (AOC).
Advantages of Active Optical Cables
The AOC assemblies provide the lowest total cost solution for data centers by having the key advantages as following:
- Low weight for high port count architectures;
- Small bend radius for easy installations;
- Low power consumption enabling a greener environment.
For the 100G longer distance, the CFP and CFP2 offer DWDM Coherent technology and enable multi-channel long distance connectivity of more than 1000km. One thing we can’t miss is that the CFP is too big to be used in an Ethernet switch in volume.
Fan-out cable or breakout cable is considered as one of the the latest enabling technologies to help increase port densities and lower costs. Taking one (large bandwidth) physical interface and breaking it out into several (smaller bandwidth) interfaces, it has been highly recommended to be used in network migration. Breakout cables are also possible on most 100GbE QSFP+ ports where each of the 4 optical lines are broken out to 4 individual 25GbE or 10GbE interfaces. This solution requires either the deployment of a breakout cable that has 4 physical 25G / 10G endpoints, or the use of a breakout mux where an SR4 optic with MPO / MTP cable is deployed.