With the rapid adoption in the use of the Internet around the world there has been an acceleration in the development and deployment of advanced optical communication systems using new technologies and high-speed interconnects. To meet the needs of users, the transmissions rates of optical communication systems and interconnects have increased significantly from 10Gigabit per second (Gbps), to 25Gbps and on to 100Gbps, 200Gbps, and beyond 400Gbps systems. The photonics (fibre optics and electronics) industry has also moved rapidly to reduce the physical size, power, and cost of the increase in data rates and is where we will discuss form factors such as QSFP56 (or 4 x 50-56Gbps), which is a member of the QSFP family and used in 200G applications – typically Datacentres (Datacenter, DCs) or shorter reach interconnects.
What is QSFP56?
Before explaining the QSFP56, let us take a look at SFP and QSFP form factors. The commonly available SFP was created for high end networking demands and stands for Small Form-Factor Pluggable – also known as a gigabit interface converter. SFP is a popular transceiver due to its small size, allowing it to be used in tight networking spaces, converting electrical signals to optical and vice versa (typically in duplex mode). QSFP stands for Quad Small Form-Factor Pluggable and was developed after SFP. QSFP modules denote four lanes for up to 4 wavelengths, meaning the transceiver can provide higher bandwidth capacity compared with the previous single-channel SFP modules. QSFP+ modules then arose for use with 40G modules and QSFP28 (4 x 25-28Gbps) for 100G applications.
QSFP56 was developed after QSFP28 and is called a Quad 50 Gigabits Small Form-Factor Pluggable. QSFP56 transceivers are designed to be used with 200G Ethernet, due to denoting 4×50-56Gb/s. QSFP56 modules are classified
into CR, SR, DR, FR and LR (depending on the reach and optical fibre type). This enables error free transmission over a single mode fibre (SMF) or multi-mode fibre (MMF).
The various form factors and nomenclature was developed through the collaboration of several industry organisations and is also called Multi-Source Agreement (MSA). The industry organisations work closely with the standards bodies (IEEE and ITU-T) to ensure the systems are interoperable and the terminology is consistent.
What is a 200G Transceiver and what are the types?
A 200G transceiver is a pluggable module that allows for transmission speeds of up to 200G overactive optical cables (AOCs), MMF and SMF. Currently there are 2 types of 200G optical modules, QSFP56 and QSFP-DD.
First, as mentioned QSFP56 is an evolution of QSFP+ and QSFP28, being a Quad 50 Gigabits Small Form-Factor Pluggable and designed specifically for 200G. QSFP56 compared to QSFP+ and QSFP28 supports 4 x 50Gbps channels which achieves higher network speed and transmission rates. Another important difference is that QSFP56 changes from Non-Return to Zero (NRZ, or binary code) encoding to Pulse Amplitude Modulation 4-level (PAM-4) encoding. The 2 main packages of QSFP56 optical modules are SR4 (Short Reach – typically 100m links over MMF) and FR4 (typically 2km links over SMF).
The next type of 200G transceiver is QSFP-DD, which stands for Quad Small Form-Factor Pluggable Double-Density. These modules are ‘double-density’ meaning that the transceiver supports double the number of high-speed electrical interfaces compared to QSFP28. With NRZ modulation technology, data rates of 25G x 8 channels can then be achieved for 200G network speeds. Comparatively, with PAM-4 modulation technology, data rates of 50G x 4 channels can be achieved, enabling 500G network speeds.
QSFP56 is more commonly used in 200G transceiver packages due to being designed specifically for these network speeds.