100G QSFP28 module types and main differences

Nowadays, world information networks have become more interconnected than ever. This can be attributed to advancements in multiple telecommunication sectors, or as OSI calls it, Layers. Each layer has had an integral part in the rapid development of node-to-node connections and the technology behind it. As always, the driving part is the demand by the general population for exposure to data communication is higher than ever before and continues to rise as countries around the world continue their development.

At the time of writing this article in late 2019, the transceiver market had shifted heavily to support 100G speeds for 100 Gigabit Ethernet (100GbE) infrastructures, with continued development and upkeep. Modules that support this data rate are also integral in the recent 5G and New Radio (NR) wireless communication deployment across the globe, more on that in this article.

Nevertheless, 100G connection possibilities have emerged from the humble beginnings of GBIC (gigabit interface converter) through SFP, SFP+, XFP, and many other form factors that started to interconnect the world at large. Like many inventions or projects of grandeur, it has taken years to polish and refine 40GbE and 100GbE, which have been in research and development since the late 2000s. The first IEEE 802.3 communication standard to define speeds of 40 Gbit/s and 100 Gbit/s Ethernet is the 802.3ba release, which defined requirements and measures. Mainly defining that data transfer should be organized as separate streams of 4×25 Gbit/s or 10×10 Gbit/s lanes and physical link of MMF (Multi-mode fiber) reach up to 150 m, and SMF (Single-mode fiber) reach up to 40 km.

100G is not limited to one physical form factor, as many are available, for example, CFP, CFP2, CFP4, CXP, and QSFP28. However, QSFP28 is the most widespread and commonly used form factor for 100GbE or 100G connections in general, and companies such as CISCO, Juniper, and HP have implemented this in their latest hardware releases. Based on the prominence of the term 100G, it is important to understand the thought behind the QSFP28 form factor and the technical jargon surrounding physical links (connections) and signal transmission technologies that are available. A few examples of industry jargon, including many abbreviations, are SR4, LR4, and SWDM4, to name a few. Some of them can be easily understood, but others require additional reading up. This is what the next several paragraphs will be dedicated to.

Basics

100G QSFP28 form factor is defined by the IEEE 802.3bj standard and is used to define SFF-8665, which contains requirements and guidelines important for manufacturing and further use. More details on standards will be provided in a different article.

The first important physical characteristic that is defined is the transceiver connector, as there are two common variations of QSFP28 module connectors referred to as LC and MPO/MTP (image 1). MPO-type connector (right side of image 1) is used for the same λ, wavelengths simultaneous parallel transmission over fiber [over multiple fibers], and Double LC-type connector (left side of image 1) is used for xWDM (Wavelength Division Multiplex) technology-based transmission [over single or double fiber]. Single LC-type connector is rarely used as BIDI 100G QSFP28, popularity and demand are rather low.

Image 1.

The second important physical characteristic that is defined is the data transfer medium that connects modules, which mainly are SMF, MMF, and Twinaxial cabling, depending on the distance between nodes, and coincidentally, is the easiest approach to 100G QSFP28 categorization.

Twinaxial cabling (image 2) is used for PDAC (Passive Direct Attach Cables) for extra short port-to-port distances that commonly are less than 10 m or for ADAC (Active Direct Attach Cables) to support up to 100m long connections. Each Twinax end is attached to a transceiver electrical interface, as this data transfer medium is purely electrical. Additionally, higher data rates support breakout cables with, for example, one side is 100G QSFP28 and the other side is 4x25G SFP28 (image 2 100G-PDAC-QSFP-SFP).

Image 2

MMF (Multi-mode Fiber) is mainly used for Short Range to Intermediate Range, and is different from SMF in its core diameter. MMF has a larger core diameter that allows for shorter wavelengths (λ) to be used (e.g., 850 nm). Additionally, MMF has several categories from which only OM3, OM4, and OM5 support 100G speeds for distances up to 100 m, 150 m, and 150m, respectively. OM5 provides additional support for BIDI (Bidirectional) data transmission that allows for a new BIDI 100G implementation.

SMF (Single-mode Fiber), as previously mentioned, has a smaller core diameter and thus requires longer wavelengths of 1310 nm optimized in OS2, which provide less attenuation and dispersion, and thus data transmission can be achieved for greater distances (e.g., 40 km). When talking about 100G QSFP28 over SMF transmission distances are commonly from 500 m to 10 km, and Extended Range of 40 km, and with recent developments in ZR modules that can reach up to 80 km.

MMF-based transceivers

SR4 (Short Range 4 lane) transceivers are most used in industry as they allow connections over short distances up to 150 m. The transceiver has an MPO connector for transmitting data over 4 lanes and receiving data over an additional 4 lanes simultaneously, with a data rate of 25.78125 Gbps (100GbE speed) using 850 nm wavelength. That is why an 8-strand OM3 or higher MPO cable is required to link these types of transceivers.

SR4 lane characteristics

SWDM4 (Short Wavelength Division Multiplex) transceivers use xWDM technological solution to transmit data over a single fiber with each Tx/Rx having different wavelengths (L0λ = 850 nm, L1λ = 880 nm, L2λ = 910 nm, and L3λ = 940 nm as visible in table 2). Transceiver has connectors that can differ by manufacturer, but mostly are available with a Double-LC connector. When compared to SR4, a major difference is that SWDM4 is scarcely available and is more expensive to produce, but requires less fiber to transmit and receive the same amount of data as SR4.

SWDM4 lane characteristics

SMF-based transceivers

PSM4 (Parallel Single Mode 4 lane) transceivers are the SMF equivalent of the SR4 transceivers type, as they use an MPO connector for transmitting and receiving data simultaneously using 1310 nm wavelength over 4 independent Tx lanes and the same number of different Rx lanes. Initially module was standardized for connection over OS2 for distances up to 500 m. Since then base transmission range has increased to 2 km and recently extended to 10 km. This can be attributed to advances in electrical components and a reduction in optical losses.

PSM4 lane characteristics

CWDM4 (Coarse Wavelength Division Multiplex 4 lane) is equivalent to the SWDM4 transceiver type as it implements four different wavelengths for data Tx and Rx. The module has an LC connector as it is similar to a conventional 1G/10G CWDM MUX/DEMUX system, as λ (Lambdas) in use are the same as for lower channels of ITU-T G.694.2 CWDM (L0λ = 1271 nm, L1λ = 1291 nm, L2λ = 1311 nm and L3λ = 1331 nm as visible in table 4). That is why connecting two transceivers takes up fewer fibers and is more efficient for use in Intermediate Distances (IR) of up to 2 km.

CWDM4 lane characteristics

LAN WDM and also commonly referred to as LR4 (Long Range 4 lane) transceivers, are commonly used in distances of 10 km and above. Compared to other xWDM solutions, LAN WDM has denser channel spacing (L0λ = 1295.56 nm, L1λ = 1300.05 nm, L2λ = 1304.58 nm, and L3λ = 1309.14 nm as visible in Table 5) as the maximum distance between channels L0 and L3 wavelengths is 15.66 nm. This is possible by using the OS2 media type that is optimized 1310 nm wavelength. Most of the previously mentioned transceiver types that have a 10 km range do require additional FEC (Forward Error Correction) feature to transmit without losses, but LAN WDM can work without this feature, which reduces additional costs, and this serves as one of the main reasons why LAN WDM is adopted quickly for distances of equal or larger than 10 km.

When talking of the maximum distance for LAN WDM, it is hard to say, as it changes year by year. Recently market had modules with a maximum range of 30km, but now, as demand for distance has come up, modules can reach up to 40 km, a ER distance more cost-efficiently, with the latest development in ZR range of up to 80 km with relatively low BER (Bit Error Rate) and signal jitter.

LAN WDM lane characteristics

Round up

Below is a comprehensive Table 6 that depicts the module data transfer medium, data transfer technology, and the distance it can reach. Table 7 is a quick overview with specific parameters and their approximate values for each module type. Additional module that are not included in this table is the transceiver, which has ONLY TX or RX function, with RX being used in data centers for data deep inspection or creation of asymmetric link groups.

Possible reach distance over SMF or MMF with respective data transfer technology

Transceiver parameters by type at 100GbE data transmission

• *a Distance is based on OM4/OM5.
• *b PSM4 500m, 2 km versions have less optical budget (3.1 dBm) while 10 km has around 6.1 dBm.
• *c Data rate based on 100GbE transceivers
• *d Optical budget can change transceiver by transceiver, but should never be less than the lower value.
• *e SR4 modules have been optimized for ≤2.5 W but ≤3.5 W are more widespread.

QSFP28 transceivers can also support InfiniBand, OTU4, and 128GFC connections, and values would be similar, with optical budgets slightly better, and for this, different hardware may be required (some hardware support multi-rate speeds).

Summary

In conclusion, 100G modules can differ in form factor. QSFP28 is more common and is used to support 100GbE data communication infrastructure and help establish a starting basis for 5G. It can be said that 100G is the way to go as new transmission technologies are becoming more widespread and diverse (e.g., PAM-4), but as with everything, there is a replacement on the horizon. Next generation of 200G and 400G modules, the next utilize form factors such as QSFP-DD, will be used in top-of-the-rack switches or other technological solutions that require near instant data exchange between two points and are expected to be more widely available by mid-to-latenear-instantaneoustop-of-rack 2020.

Data transmission technologies such as xWDM (e.g. SWDM4, CWDM4, LAN WDM) or split lane solutions (e.g., SR4, PSM4) and mediums such as SMF, MMF or Twinaxial cabling may differ but basic principles of separate lane data streams (e.g., 4, 10) to achieve greater combined data throughput will stay and be adopted for next generation of transceiver that will support even greater speeds.

Thank you for reading!