400G what are the differences

• OSFP = Octal Small Form Factor Pluggable
• QSFP-DD = Quad Small Form Factor Pluggable Double Density

OSFP has a max power consumption of < 15W where QSFP-DD is designed for < 12W. The thermal management for OSFP is in included in the transceiver where as for QSFP-DD the design is up to the host device. QSFP-DD, as the name already states, is basically a QSFP - therefore the ports are backwards compatible for QSFP56, QSFP28 or QSFP+ which has a lot of advantages for the system vendors. OSFP could be adapted to QSFP but who wants to run additional hardware between device and transceiver?
With QSFP-DD, OSFP & PAM4 a new chipset is used in these high speed transceivers. It's commonly called DSP (digital-signal-processor). It combines electrical signals to an amount of optical signals as well as handling the modulation, like PAM4. These are the features which are already in use. But the DSP is even more powerful than this and put us in a position to use different combinations of speed as well as protocols and data-rates in the future. Just one example is 400G ZR4 which is QAM16, not PAM4 modulated. In addition this DSP could be used finding the appropriate data-rate on any given link.
The major advantage for OSFP is the bigger size. Usually it's 'the smaller, the better' but in this case OSFP are more robust in terms of heat dissipation, and more spacious in terms of electrical and optical components; this could come in handy for future use cases. Also, the size allows OSFPs to be 800G ready.
Whether you want to use QSFP or QSFP-DD will be up to your network setup and scalability, and the choice which devices you'd like to use. QSFP-DD devices can hold up to 36x QSFP-DD in 1U, while OSFP can only hold 32x - on the other hand heat dissipation is a challenge for QSFP-DD devices.

Formfactor Overview

Formfactor	max Datarate	Connector Types	Modulation scheme	Applications
SFP56	50G	LC-Duplex	1x53.125Gb/s(PAM4) electrical interface 1x53.125Gb/s(PAM4) optical interface	5G Data-centers
SFP56-DD	100G	LC-Duplex	2x53.125Gb/s(PAM4) electrical interface 1x106.25Gb/s(PAM4) optical interface	100G Ethernet Data Center High-speed interconnects within and between switches, routers and transport equipment Server-Server Clusters, Super-computing interconnections Proprietary backplanes Interconnects rack-to-rack, shelf-to-shelf, board-to-board, board-to-optical backplane
QSFP-56	200G	LC-Duplex MTP/MPO-12 (APC)	4x53.125Gb/s (PAM4) electrical and optical interface 4x53.125Gb/s (PAM4) electrical and optical interface	200G Ethernet Data Centers
QSFP-DD (QSFP56-DD)	400G	LC-Duplex MTP/MPO-12 MTP/MPO-16 CS-Duplex Quad-SN	8x25 Gb/s (NRZ) electrical interface 8x53.12 Gbps (PAM4) electrical interface 4x 106.25Gb/s (PAM4) optical interface	Data Center Interconnect 400G Ethernet Infiniband interconnects Enterprise networking
OSFP	400G 800G	LC-Duplex MTP/MPO-12 MTP/MPO-16 CS-Duplex Quad-SN Quad-MDC	8x53.125 Gbps (PAM4) electrical interface 4x 106.25Gb/s (PAM4) optical interface 8x 106.25 Gb/s (PAM4) electrical interface 8x 106.25 Gb/s (PAM4) optical interface	Data Center Interconnect 400G Ethernet Infiniband interconnects Enterprise networking

Available 400G QSFP-DD & OSFP Products

Description/ Connector	Distance	Wavelength	Connector	Details	Interoperability	Types
QSFP-DD
QSFP-DD SR8	70m (OM3) 100m (OM4)	8 x 850nm	MTP/MPO-16	IEEE 802.3cm (400GBASE-SR8)
QSFP-DD DR4	500m (G.652)	4 x 1310nm	MTP/MPO SN	IEEE 802.3cm (400GBASE-DR4) 4 x 100GG operations on SMF	SFP56-DD DR	(with SN-Connector)
QSFP-DD DR4+	2km (G.652)	4x 1310nm	MTP/MPO SN	4 x 100GG operations on SMF (100G Lambda MSA 100G-FR compliant)	SFP56-DD FR	(with SN-Connector)
QSFP-DD FR4	2km (G.652)	1271nm, 1291nm, 1311nm, 1331nm	LC	IEEE 802.3cu (400GBASE-FR4)
QSFP-DD LR4	10km (G.652)	1271nm, 1291nm, 1311nm, 1331nm	LC	IEEE 802.3cu (400GBASE-LR4)
OSFP
OSFP SR8	70m (OM3) 100m (OM4)	8 x 850nm	MTP/MPO-16	IEEE 802.3cm (400GBASE-SR8)		O.854HG.01.M
OSFP DR4	500m (G.652)	4 x 1310nm	MTP/MPO	IEEE 802.3cm (400GBASE-DR4) 4 x 100GG operations on SMF	SFP56-DD DR
OSFP DR4+	2km (G.652)	4x 1310nm	MTP/MPO	4 x 100GG operations on SMF (100G Lambda MSA 100G-FR compliant)	SFP56-DD FR	O.134HG.2
OSFP FR4	2km (G.652)	1271nm, 1291nm, 1311nm, 1331nm	LC	IEEE 802.3cu (400GBASE-FR4)
OSFP LR4	10km (G.652)	1271nm, 1291nm, 1311nm, 1331nm	LC	IEEE 802.3cu (400GBASE-LR4)

 
Note: a QSFP-DD LR4 is interoperable with a QSFP-DD LR4. I didn't mention this in the column "interoperability" for a better overview. This applies to all other types as well.
From a technical point of view a "split operation" is possible with some QSFP-DD mentioned above. QSFP-DD are transmitting mainly 4x 100G on the optical side. On the optical path you have to check how to split up the signal to 4 dedicated 100G lines. For the ones with MTP/MPO receptacle this is not a problem with Breakout-cables.
For connecting to SFP56-DD we will add the appropriate SFP56-DD in near future.