Is it possible to use SFP transceiver in a SFP+ slot

2020-06-01

Fiber Optic Network is the latest technology in the world of the Internet. It’s a high speed and reliable network solution to provide a stable Internet connection. For a Fiber Optic Network to function flawlessly it needs optical cables and optical transceivers to be fitted in the networking equipment. The optical transceivers are the devices that convert the electrical signal into an optical light and send it down the optical cable. The optical cables then transmit the optical light on the other side of the Optical network into the other transceiver which converts the optical light into an electrical input.

There are many types of optical transceivers from various manufacturers. From SFP and SFP+, up to QSFP and QSFP+ used mainly for 40GB and 100GB Ethernet. However there are standards existing that these Optical Transceivers need to meet for them to be used in a network equipment. The most used and known Optical Transceivers, which are used in the day to day operations, are SFP and SFP+ types. SFP stands for Small Factor Pluggable and these are small and compact transceivers. Both transceivers have the Digital Diagnostics Monitoring (DDM) feature for easier error detection and maintenance. However there is a fundamental difference in their functioning and capabilities. The biggest difference is that the SFP+ transceiver support up to 16 GB Ethernet in a fiber channel solution, unlike the SFP transceiver which supports up to 4GB Ethernet.

A common question that can be run upon in the world of optical networks is if it’s possible to use a SFP transceiver in a SFP+ transceiver slot. This question entails many other questions: can the SFP+ negotiate the speed down to 1GB/s, will the SFP transceiver fit in the SFP+ slot, can 1GB and 10GB speeds exist on the same link? Even though these questions and their answers depend on the manufacturers (and can be found in their datasheets), we will try to give an answer to most of these questions.

The first and main thing is that most of the time the SFP transceiver will be plugged in a SFP+ slot with ease and will fit perfectly. However the other way around it won’t be possible. Most of the time SFP+ transceivers won’t be able to fit into a SFP slot. This is mainly a power availability issue, SFP+ transceivers use more power than SFP transceivers. When a SFP transceiver is plugged into a SFP+ slot, the transceiver won’t automatically detect this and amend the speed to 1GB, however the transceiver will function at 1GB due to its nature (SFP+ 1000BASE-EX). The transceiver will hard-code this speed unless it’s changed with some kind of configuration or the switch is reloaded. With this in mind we need to know that a SFP+ transceiver will almost never support speeds below 1GB/s and that’s why SFP transceivers with less than 1GB speeds will not fit in the SFP+ slot.

Unlike the copper SFP transceivers which have the option to auto negotiate the speed of the link, optical SFP (figure 1) and SFP+ (figure 2) transceivers don’t have this feature. This means with plugging SFP transceiver in a SFP+ slot, one side will function at 1GB and the other will remain at the speed already set. If no other action is taken besides the plugging of the SFP into the SFP+ port, the link will not work. This is also the issue when using Twinax-cables, it still won’t auto negotiate the speed.

When mixing these two transceivers for the same optical link, even though this is not advisable, the main thing to do is to set the speed settings on both sides equally to 1GB/s. This will ensure both sides of the link function at the same speed and the connection would be successful.

Both GBIC-SHOP BlueOptics© SFP (figure 1) and SFP+ (figure 2) transceivers come with 5 year warranty and lifetime support. They are tested and developed to withstand high and low temperatures and they fulfill all MSA and industrial standards for quality and reliability.