What Are SFP, SFP+, SFP28, QSFP+, QSFP28 and QSFP-DD and OSFP

What Are The Differences Between SFP, SFP+, SFP28, QSFP+, QSFP28 and QSFP-DD and OSFP ?
SFP, SFP+, SFP28, QSFP+ and QSFP28 are different optical transceiver types. And all of them are hot-pluggable network interface modules used for connecting a network switch and other networking devices (such as a server or media converter) for data transmission. Then, SFP vs SFP+, SFP28 vs SFP+, QSFP vs QSFP28, what are the differences? Is QSFP28 compatible with QSFP+? Can I use SFP28 transceiver in SFP+ ports? This post will offer explanations.

Cable Form Factors and Connector Types
Before figuring out the difference in SFP vs SFP+, SFP28 vs SFP+, or QSFP vs QSFP28, it’s necessary to know what SFP, SFP+, SFP28, QSFP and QSFP28 are.

SFP
SFP (Small Form Factor Pluggable) – A transceiver or cable with a one or two lanes (channel) in each direction. All cables and transceivers commonly used in data-centers are bidirectional.
SFP transceivers are part of the Ethernet architecture, but not used in InfiniBand systems.

SFP+
SFP+ (small form-factor pluggable plus) is an enhanced version of the SFP. It supports 8 Gbit/s Fibre Channel, 10 Gigabit Ethernet and Optical Transport Network standard OTU2.
SFP+ also introduces direct attach for linking two SFP+ ports without additional fiber transceivers, including DAC (direct attach cable) and AOC (active optical cable), which are quite brilliant solutions for the short-distance direct connection between two adjacent network switches.

SFP-DD
SFP-DD, a double-density version of SFP, with 2 lanes in a form factor with same width as the SFP is defined.

SFP28
SFP28 (small form-factor pluggable 28) is an enhanced version of SFP+. SFP28 has the same common form factor as the SFP+, but supports 25Gb/s over a single lane. SFP28 provides a new way for networking upgrade: 10G-25G-40G-100G, which is an energy-efficient solution to meet the growing demands of next-generation data center networks.
SFP+ denotes the 10–14 Gb/s type of AOC/transceivers, while SFP28 is the notation for the 25-28 Gb/s products with an SFP form factor. The noted data rate is the data rate in each direction.

QSFP+
QSFP (Quad Small Form Factor Pluggable) – A bidirectional transceiver or cable with 4 lanes in each direction.
Standards: Electrical pinout, memory registers, and mechanical dimensions for both SFP and QSFP devices are defined in the public MSA (Multi-source Agreement) standards
QSFP+ is an evolution of QSFP (quad small form-factor pluggable). QSFP can carry 4 channels simultaneously and each channel can handle 1 Gbit/s data rate- hence the name Quad SFP. Unlike QSFP, QSFP+ supports 4x 10 Gbit/s channels. And the 4 channels can be combined into a single 40 Gigabit Ethernet link. The QSFP+ transceiver can replace 4 standard SFP+ transceivers, resulting in greater port density and overall system cost savings over traditional SFP+ products.

QSFP28
QSFP28 (quad small form-factor pluggable 28) is designed for 100G applications. It offers four channels of high-speed differential signals with data rates ranging from 25 Gbps up to potentially 40 Gbps, and finally, meet 100 Gbps Ethernet (4×25 Gbps) and 100 Gbps 4X InfiniBand Enhanced Data Rate (EDR) requirements. Note that, QSFP28 can do 4x25G and 2x50G breakout connection, or 1x100G depending on the transceiver type that is used.

QSFP+ denotes cables/transceivers for 4 x (10 – 14) Gb/s applications, while QSFP28 denotes the 4 x (24…28) = 100 Gb/s product range with QSFP form factor, used for InfiniBand EDR 100Gb/s ports and 100Gb/s Ethernet (100GbE) ports. The QSFP28 interface is specified in SFF-8679. QSFP56 denotes 4 x (50…56) Gb/s in a QSFP form factor. This form factor is used for InfiniBand HDR 200Gb/s and 200/400GbE Ethernet cables/transceivers.

QSFP-DD
QSFP-DD refers to a double-density version of the QSFP transceiver supporting 200 GbE and 400 GbE Ethernet. It employs 8 lanes operating at up to 25Gb/s NRZ modulation or 50Gb/s PAM4 modulation. QSFP-DD cables will in general not work in standard QSFP cages, but switches/NICs with QSFP-DD cages may support the older QSFP transceivers/cables.

OSFP
OSFP (Octal Small Form Factor Pluggable) is wider and longer than QSFP and accommodates 8 lanes side-by-side. This form factor is used for 200/400/800G transceivers.

AOC
AOC (Active Optical Cable) – An optical fiber cable with an optical transceiver with the fibers bonded inside and not removable. The optical transceiver converts the host electrical signals into light pulses and back. Bonding the fiber inside means the AOC only needs to be tested electrically and eliminates the costly optical testing.
Transceiver (transmitter and receiver) is a converter with an electrical connector in one end and optical connector in the other end. It can have one or more parallel lanes in each direction (transmit and receive).
Transceiver or AOC? – You can argue that two transceivers connected with a patch cable replace an AOC. However, if you don’t have cleaning tools and experience with optical connectors, it is safer to use an AOC where the optical cable is fixed inside the ‘connector’. The AOC’s ‘connectors’ are actually similar to detachable transceivers, but they work as a kit with a well-known transceiver at the other end. AOCs don’t have any issue with multi-vendor interoperability. Nevertheless, it is easier to replace a pair of transceivers than an AOC since you don’t have to install a new cable as the cable is already in place.
Traditionally, AOCs are more common in InfiniBand installations, while transceivers with patch cables are more common in Ethernet systems with structured cabling.
DAC (Direct Attached Copper) cable or PCC (Passive Copper Cable) – A high-speed electrical cable with an SFP or QSFP connector in each end, but no active components in the RF connections. The term ‘passive’ means that there is no active processing of the electrical signal. The DACs still have an EEPROM, a memory chip in each end, so the host system can read which type of cable is plugged in, and how much attenuation it should expect.

Comparisons in SFP vs SFP+ vs SFP28 vs QSFP+ vs QSFP28 vs QSFP-DD vs OSFP
After figuring out what SFP/SFP+/SFP28/QSFP+/QSFP28/QSFP-DD are, the following part will give detailed comparisons of SFP vs SFP+, SFP28 vs SFP+, QSFP vs QSFP28 and SFP28 vs QSFP28 vs QSFP-DD vs OSFP

SFP vs SFP+: Same Size with Different Speed and Compatibility
SFP vs SFP+ transceivers, both are virtually identical in size and appearance. This allows the equipment producer to reuse existing SFP physical designs for network switches with SFP+ ports. As for the difference, the clear one is that they support different transmission speeds, SFP is up to 4 Gbit/s while SFP+ is 10 Gbit/s. Besides, they comply with different specifications. SFP is based on SFF-8472 protocol, and SFP+ conforms to SFF-8431 and SFF-8432. And in terms of SFP vs SFP+ compatibility, SFP+ ports often accept SFP optics but at a reduced speed of 1 Gbit/s. And SFP+ transceiver can not be plugged into an SFP port, otherwise, the product or port may be damaged.

SFP28 vs SFP+: Can I Use SFP28 Transceiver in SFP+ Ports?
The answer is definitely yes. From the above, it’s clear that SFP28 is the upgrade version of SFP+ that SFP28 has been upgraded to handle 25 Gbit/s per lane. They use the same form factor, and the pinouts of SFP28 and SFP+ connectors are mating compatible. So SFP28 will work with SFP+ optics but at a reduced speed of 10 Gbit/s. And SFP+ module will work well with SFP28 port on a network switch if the port can be set up for 10G transmission, otherwise the SFP+ modules can not work. When it comes to copper cable, SFP28 copper cable possesses significantly greater bandwidth and lower loss compared with SFP+ version.

SFP28 vs QSFP28: Work on Different Principles
Though there is a number “28” in their names, SFP28 and QSFP28 transceiver actually adopt different sizes and working principles. SFP28 supports only one channel with 25 Gbit/s, while QSFP28 supports 4 separate lanes, and each is 25 Gbit/s. Both of them can be used in 100G networks, but the SFP28 is applied in the form of QSFP28 to SFP28 breakout cables. The following shows a direct connection for 100G QSFP28 to 4x SFP28 DAC.

QSFP+ vs QSFP28: Different Speeds for Different Uses
QSFP+ and QSFP28 transceivers integrate 4 transmitting and 4 receiving channels and share the same size. Besides, the product family of QSFP+ and QSFP28 both includes transceiver module and DAC/AOC cable, but with different speeds. QSFP+ module supports 1x 40 Gbit/s and QSFP+ DAC/AOC cable supports 4x 10 Gbit/s. QSFP28 module is capable of transmitting data at 100 Gbit/s, and QSFP28 DAC/AOC cable can run at 4x 25 Gbit/s or 2x 50 Gbit/s. Note that, usually, QSFP28 modules can’t break out into 10G links. But it’s another case to insert a QSFP+ module into a QSFP28 port if switches support (how to realize 4x 10G mode on QSFP28 100G port, please visit QSFP28 100G Port Play with 40G, 25G and 10G). In this situation, a QSFP28 can break out into 4x10G like a QSFP+ transceiver module.

Cable/Transceiver Form Factors and Connector

Definition	Image
DAC (Direct Attach Copper) cable with QSFP connector
DAC with SFP connector
AOC (Active Optical Cable) with QSFP connector
QSA (QSFP to SFP Adapter)
QSFP transceiver
QSFP28 Transceiver for 100G transmission
QSFP56 Transceiver for 200G transmission
QSFP-DD transceiver 8 lane 200/400G transceiver
OSFP transceiver 8 lane 400G transceiver
SFP transceivers 25G SFP28 Transceiver (~1 W)

NOTE:
QSFP56/SFP56 has same 4-channels & 1-channel as QSFP28/SFP28 generation
Same Duplex LC and MPO-12 optical connector as QSFP28/SFP28 generation
QSFP56 offers more space and thermal dissipation capacity
50G PAM4 doubles the data rate
SFP56 ports accept SFP28 devices; QSFP56 ports accept QSFP28 devices
QSFP28/SFP28 ports will NOT accept newer QSFP56/SFP56 devices
SFP-DD ports will accept SFP+, SFP28, and SFP56 devices
SFP-DD is a 2-channel device, and hence requires a new optical connector scheme. Two types are currently (2019) supported by the SFP-DD MSA: Corning/US Conec MDC, and Senko SN.

Reach of Transceivers
Transceivers are classified with data- rate and reach, governed by the IEEE Ethernet standards.
For 100 - 400 Gb/s transceivers the most common definitions are:
100GBASE-CR4 - 100 Gb/s, standard for DAC cables (twisted pair) for short reaches, up to about 7 m.
100GBASE-SR4 - 100 Gb/s, SR4=Short Reach (100 meters on OM4 multimode fiber), 4 fibers
100GBASE-LR4 - 100 Gb/s, LR=Long Reach (10 km using WDM on SMF), 2 fibers
100GBASE-ER4 - 100 Gb/s, ER=Extended Reach (30-40 km using WDM on SMF), 2 fibers
100GBASE-ZR - 100 Gb/s, ZR is not an IEEE standard, 80+ km reach.
200GBASE-CR4 - 200 Gb/s on DAC (passive copper) twisted pair cable, up to 3 m
200GBASE-SR4 - 200 Gb/s, SR4=Short Reach (100 meters on OM4 multimode fiber), 4 fibers
200GBASE-DR4 - 200 Gb/s, DR4 = 500 meters on single mode fibers, 4 fibers per direction
200GBASE-FR4 - 200 Gb/s, FR4 = 2 km, single mode fibers using WDM, 1 fiber per direction
200GBASE-LR4 - 200 Gb/s, LR4 = long reach, 10 km, single mode fibers using WDM, 1 fiber per direction
400GBASE-DR4 - 400 Gb/s, 500 meters on single mode fiber, 4 fibers each direction
400GBASE- FR4 - 400 Gb/s, WDM, 2 km on 1 single mode fiber/direction, 4 electrical lanes
400GBASE-FR8 - 400 Gb/s, WDM, 2 km on 1 single mode fiber/direction, 8 electrical lanes
All 200/400 Gb links use PAM4 signaling which implies that Forward Error Correction (FEC) is required.
The interface types listed above are examples for 100, 200, and 400 GbE links. The IEEE 802 standards define a wide range of standards for different Physical Media Devices (PMDs) and PMD Naming Conventions figure below. Some of the transceiver types are not IEEE standards but separate industry MSAs (Multi-Source Agreements) usually formed by a leading transceiver company.  PSM4, SWDM4, CWDM4 and 400G FR4, are examples.

Transceivers and Cable Connectors