100G QSFP28 optics are now mainstream technology, but as the speed of transceivers increased, and to meet customer demand for more cost effective transceivers, different solutions appeared that can still lead to confusion when choosing the right module to use. Here we will look at two technologies, optical multiplexing and FEC to highlight some of the difference between transceivers supplied by L2tek.
One of the first choices to make when choosing 100G QSFP28 modules is whether you will be using MTP/MPO multi-fibre cabling or Dual LC connections.
100G QSFP modules actually carry the data in 4 duplex lanes at 28Gbps. Typically in MTP/MPO systems these are carried on 8 fibres (the middle fibres in a 12 fibre cable)
Importantly there is no internal multiplexing of the optical path, something we’ll come on to next, each signal is sent along a separate fibre within the MTP/MPO cable.
Short range -SR4 -ESR4 and Long range -PSM4 use this connection type, one on multi-mode fibre giving between 100m and 300m range, the other using single mode for 2km/10km range. It’s important to select the right fibre type for your installation.
Please note that the two types shown are not compatible with each other. The same type must be used on each end of the link and the fibre type has to be correct.
Example Gigalight 100G QSFP28 part numbers for 100G QSFP-SR4 GQS-MPO101-SR4C (100m) GQS-MPO101-ESR4C (300m)
The next type of device to consider is a module with integrated optical multiplexer/de-multiplexer, which enables the use of LC duplex cables.
Each device contains a 4 way optical multiplexer on both the transmit and receive paths
Four different wavelengths are used to send the 4 x 28Gbps channels down a single fibre on the Tx and Rx fibres
Dual LC single mode duplex fibre patch cables can be used
Long range -LR4 (10km) and extended range -ER4 (40km) and -ZR4 (80km) are examples of Dual LC QSFPs using Lan WDM wavelengths
Medium range -CWDM4 and -CLR4 with ranges of 2km or 10km are examples using CWDM wavelengths
Please note that Lan WDM and CWDM parts use different wavelengths and are not compatible with each other.
LWDM vs CWDM
As can be seen in the graphs below, the wavelengths for -LR4 -ER4 – ZR4 have a narrower distribution and spacing compared to the 20nm spacing on CWDM devices. Using CWDM wavelengths generally reduces cost of components, especially lasers, but again the two types of QSFP are not interchangeable and users should make sure the same type of device is being used on both ends of the link.
Example Gigalight 100G QSFP28 part numbers for 100G QSFP-LR4 GQS-SPO101-LR4C (10km) 100G -CWDM4 GQS-SPO101-CIR4C (2km) 100G-CLR4 GQS-SPO101-CCR4C (2km) GQS-SPO111-ER4C (40km)
Finally a quick look at Forward Error Correction FEC
From the part numbers above it can be seen that from a top level specification both the -CWDM4 and -CLR4 achieve the same range of 2km (10km versions are also available). The key difference is that the -CWDM4 can only achieve this range if Forward Error Correction FEC is used, whilst the -CLR4 can achieve the range without FEC. FEC involves the sending of extra information which can be used to correct bit errors on the link. As Bit Error Rate is likely to increase with range, FEC effectively increases the range possible with any module.
It is important to know that FEC is not a function in the QSFP module itself, but is implemented on an Ethernet switch port as and when required. Because FEC adds in extra information changing the data stream, the same FEC algorithm has to be used on the receive end to prevent the data appearing to be scrambled. i.e. the ports at both ends of the link have to have FEC either on or off.
We hope that this information is helpful when you come to choose the right QSFP module for your application. Please feel free to contact the team et L2tek if you would like further information or support when choosing parts. And remember, all our transceivers are available either direct with L2tek or through our online store @ Videobits.L2tek Transceivers Videobits Transceivers