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Complete Connect / Optical Transceivers / QSFP, QSFP+ and QSFP28. What’s the Difference?
QSFP+ 40G BASE Optical Transceivers

QSFP, QSFP+ and QSFP28. What’s the Difference?

There is a wide choice of abbreviations to try and understand when it comes to the many interface and data rate types of fibre optic transceivers that are available today. Over the next few weeks, the team at Complete Connect will try to help you get a better understanding of what these mean.

Today we are focusing on QSFP, QSFP+ and QSFP28. They look very similar in terms of their physical appearance but the data rates that they transmit at are quite different.

QSFP

This abbreviation stands for Quad Small Form Factor Pluggable.
This type of cable support Infiniband, Fibre Channel, Ethernet and Sonet/SDH standards as well as other types of proprietary interconnects.
They are available for use in either singlemode or multimode applications and is able to support 4 independent channels that can individually transmit at up to 1.25 Gbp/s per channel for an aggregate combined speed of 4.3Gbp/s. However, due to technology advancements these type of optics are seldom used.

QSFP+

This abbreviation stands for Quad Small Form Factor Pluggable Plus. This interface type supports Ethernet, Fibre Channel and Infiniband at 10Gbps per channel up to a maximum of 4 channels (Quad) for a combined aggregate data rate of 40Gbps or 56Gbps. Like the QSFP interface, this QSFP+ version also supports transmission over both singlemode and multimode infrastructures to a maximum distance of 40km.
Line cards / ports that accept QSFP+ modules are typically used in one of two ways.
  1. Connecting two switches together with 40G or 56G connectivity
  2. Connecting one switch transmitting / receiving a 40G data rate to 4 devices each working at 10G

Direct Connect QSFP+ to QSFP+

The most common methods for creating a QSFP+ to QSFP+ link is to use either a Direct Attach Copper cable or two QSFP+ transceivers and a fibre cable. For short distances either method is possible but for distances over 10m only the 2nd method works.

Direct Connect one QSFP+ Port to 4 four SFP+ Ports

A second  common requirement for network managers is to be able to connect one QSFP+ 40G port to multiple 10G ports. Both Direct Attach Copper (DAC) cables and QSFP+ transceivers with fibre cables offer this as an option. The fibre cable required with QSFP+ transceivers is an MTP – LC fibre breakout cable. Again for short distances, DAC breakout cables offer the most cost effective connection but these cables are limited to transmission distances of around 10m. QSFP+ transceivers and MTP breakout cables offer network connectivity for distances up to 400m.
For distances over 400m QSFP+ transceivers and singlemode fibre are required. Two options are available; LR4 duplex networking or PLR4 (PMS4) parallel networking.

QSFP28

The QSFP28 is also based on the QUAD interface. The QSFP28 ramps up the data speeds to 100G by using 28Gbps per channel. Unlike the older 100G CFP, CFP2 and CFP4 interfaces, the newer QSFP28 interface offers a much smaller form type. Due to its size this interface is becoming the standard interface choice for 100G transmission.
The QSFP28 interface is highly flexible and can be used in the following combinations.
  • 100Gbp/s
  • 2 X 50Gbp/s
  • 4 X 25Gbp/s (SFP28)

Direct Connect QSFP28 to QSFP28

Direct Connect one QSFP28 to four SFP28 Ports

Like the QSFP+ interface above there are several breakout cable options that can be considered, depending on the application. For very short distances (less than 10m) direct attach copper breakout cables can be used as well as QSFP28/SFP28 transceivers and multimode fibre cables. Over 10m QSFP28/SFP28 transceivers multimode fibre cables are the best option up to 150m and then over this distance QSFP28/SFP28 transceivers and singlemode fibre cables are required.
It is important to note that typically a QSFP28 module cannot be broken down in to 10G channels, however it is backwards compatible in that if used in a QSFP+ port, it would allow a breakdown into 4 X 10G SFP+ channels.

If you need a little more help and advice with any of our range of QSFP optics or fibre connectivity cables, then you can talk to a person via our chat (bottom right)

Pros and Cons of using Direct Attach Copper (DAC) Cables versus Fibre and Transceivers

Costs

Costs increase as you go from using DAC cables to transceivers and fibre.
Costs increase as you go from using transceivers over multimode to transceivers over singlemode.

Connection Distance

Distance increases as you go from using DAC cables to transceivers and fibre.
Distance increases as you go from using transceivers over multimode to transceivers over singlemode.
40G transmission distances

Flexibility

DAC cables are fixed length so typically are fixed to one application.
A QSFP+ or QSFP28 DAC breakout cable will typically only be able to connect to SFP+ or SFP28 ports in the same rack
Transceivers can be disconnected to the fibre cables and reused with other different length fibre cables.
QSFP+ or QSFP28 transceivers can also connect to SFP+ or SFP28 transceivers split across different racks.

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