The traditional data center is mainly based on the 10G network architecture. In order to adapt to the scale deployment of AI, deep learning, big data computing and other services, the next-generation data center architecture is evolving to the 25G/100G network architecture.
Building a 25G/100G data center requires a large number of 100G optical modules, which account for a relatively high proportion of network construction costs. What are the 100G optical module standards? How do we choose? Today, we will briefly review the data center 100G optical module standards and package formats.
100G optical module standard organization
Before you begin to share the optical module standards, first understand the standardization organization of the lower optical module. The definition of optical modules is mainly two key organizations, namely IEEE and MSA (Multi Source Agreement), which complement each other and learn from each other.
Everyone knows that IEEE is the Institute of Electrical and Electronics Engineers, and 802.3 is a working group under IEEE. Many 10G, 40G, 100G, and 400G optical module standards are proposed by the IEEE 802.3 working group.
MSA is a multi-vendor specification. Compared with IEEE, it is a non-official organization. It will form different MSA protocols for different optical module standards. It can be understood as the behavior of enterprise alliances within the industry. In addition to defining the structural package of the optical module (including external dimensions, electrical connectors, pin assignments, etc.), the MSA also defines electrical and optical interfaces to form a complete optical module standard.
A long time ago, the optical module industry chain was very chaotic. Each manufacturer had its own structural package. The developed optical modules were large and small, and the interfaces were varied. In order to solve this problem, the MSA multi-source protocol came into being. Each manufacturer follows the standard package of the unified optical module proposed by MSA and the related interface, which is like the standardization of the charging port of the mobile phone. For 100G, MSA defined standards include 100G PSM4 MSA, 100G CWDM4 MSA, and 100G Lambda MSA.
100G optical module standard
In order to meet 100G interconnection scenarios of different distances, IEEE and MSA define more than ten 100G standards, but the mainstream is the following six standards.
▲100G optical module mainstream standard
The standard beginning with 100GBASE is proposed by IEEE 802.3.
As shown in FIG:
In the 100GBASE-LR4 name, LR means long reach, that is, 10Km, and 4 means four channels, that is, 4*25G, which are combined to be a 100G optical module capable of transmitting 10Km.
The naming rules for -R are as follows:
▲-R noun explanation
In addition to the IEEE 100GBASE series of standards, why does MSA also propose the PSM4 and CWDM4 standards?
100GBASE-SR4 and 100GBASE-LR4 are the most commonly used 100G interface specifications defined by the IEEE. However, for large data center internal interconnection scenarios, the distance supported by 100GBASE-SR4 is too short to meet all interconnection requirements, and the cost of 100GBASE-LR4 is too high. As a result, MSA brought a solution for mid-range connectivity to the market, and PSM4 and CWDM4 were the products of this revolution.
Of course, the capability of 100GBASE-LR4 completely covers CWDM4, but in the scenario of 2Km transmission, the CWDM4 solution is cheaper and more competitive.
The following figure is a schematic diagram of 100GBASE-LR4 and 100G CWDM4:
▲ 100GBASE-LR4 schematic
▲100G CWDM4 schematic
LR4 and CWDM4 are similar in principle, and the four parallel 25G channels are wavelength-multiplexed into a 100G fiber link through the optical device MUX and DEMUX. However, there are several differences between the two:
1. Optical MUX/DEMUX devices used by LR4 are more expensive
CWDM4 defines a CWDM interval of 20 nm, because the wavelength drift characteristic of the laser is about 0.08 nm/°C, and the wavelength variation in the 0~70 °C operating range is about 5.6 nm, and the channel itself also leaves some isolation bands.
Channel 1: 1264.5~1277.5nm
Channel 2: 1284.5~1297.5nm
Channel three: 1304.5~1317.5nm
Channel 4: 1324.5~1337.5nm
LR4 defines a LAN-WDM interval of 4.5 nm.
Channel 1: 1294.53~1296.59nm
Channel 2: 1129.02~1301.09nm
Channel three: 1303.54~1305.63nm
Channel four: 1308.09~1310.19nm
The larger the channel spacing, the lower the requirements for optical MUX/DEMUX devices and the cost savings.
2. The laser used in LR4 is more expensive and consumes more power.
CWDM4 uses DML (Direct Modulated Laser), while LR4 uses EML (Electro-absorption Modulated Laser).
DML is a single laser, and EML is two devices, one is DML and the other is EAM modulator, together called EML. The principle of DML is to realize the signal modulation by modulating the injection current of the laser. Since the magnitude of the injection current changes the refractive index of the active region of the laser, causing wavelength drift to generate dispersion, it is difficult to make high-speed signal modulation. Not far enough. 10KM is a little powerless for DML, only on EML.
Note: Chirp refers to a signal whose frequency changes (increases or decreases) over time. This signal sounds like a beeping sound of a bird.
3. LR4 requires an additional TEC (Thermo Electric Cooler)
Because there is only a 4.5 nm spacing between adjacent channels of LR4, the laser needs to be placed on the TEC for temperature control. The TEC Driver chip needs to be placed on the circuit, and the Laser is also integrated into the TEC material. As a result, the cost of the LR4 is increased compared to the CWDM4.
Based on the above three points, the optical module of the 100GBASE-LR4 standard is more expensive, so the 100G CWDM4 standard proposed by MSA is a good complement to the gap caused by the excessive cost of 100GBASE-LR4 within 2Km.
In addition to CWDM4, PSM4 is also a medium-distance transmission scheme. What are the advantages and disadvantages of PSM4 compared to CWDM4?
The 100G PSM4 specification defines eight single-mode fiber (4 transmit and 4 receive) point-to-point 100 Gbps links, each transmitted at 25 Gbps. Four channels of the same wavelength and independent are used for each signal direction. Therefore, the two transceivers typically communicate via an 8-fiber MTP / MPO single mode jumper. The PSM4 has a transmission distance of up to 500 meters.
▲PSM4 schematic
▲CWDM4 vs PSM4
After talking about the 100G long-distance optical module standard, look at the 100G short-range optical module.
The 100G short-range optical module standards mainly include 100GBASE-SR10 and 100GBASE-SR4. In order to meet the 100G demand appearing on the market, the 100GBASE-SR10 standard was first proposed and applied to the short-distance interconnection of 100G.
The 100GBASE-SR10 standard uses 10 x 10Gbps parallel channels for 100Gbps point-to-point transmission. The rate of electrical signals is 10G, and the optical signal rate is also 10G. It uses NRZ modulation and 64B/66B encoding. Because IEEE 802.3 proposed the 100GBASE-SR10 standard as early as 2010, the application specific integrated circuit (IC) can only support 10G, which is CAUI-10 (10 channels x 10Gbps).
▲100GBASE-SR10 schematic
As the switch ASIC chip electrical interface rate is increased from 10Gpbs to 25G bps, the electrical interface standard is upgraded from CAUI-10 (10 channels x 10Gbps) to CAUI-4 (4 channels x 25Gbps), and the channel is reduced from parallel 10 channels of SR10 to parallel With 4 channels, the number of devices in the optical module is reduced, the cost is reduced, the module size is reduced, and power consumption is reduced.
The reduction of the size of the optical module makes the density of the 100G interface provided by the switch per 1U space larger. Based on the above advantages, 100GBASE-SR4 has replaced the 100GBASE-SR10 as the mainstream standard of 100G short-range optical modules.
▲100GBASE-SR4 schematic
100G optical module package
Only the optical interface and electrical interface specifications of the optical module are not enough, and the required structural package is required to be a complete optical module solution. The package formats of 100G optical modules mainly include CFP, CFP2, CFP4, and QSFP28.
CFP was first proposed, short-distance transmission application 100GBASE-SR10 standard, long-distance transmission application 100GBASE-LR4. The first-generation CFP long-distance transmission scheme is as follows. Since the electrical interface capability is only CAUI-10, the built-in Gearbox (10:4 Serializer in the figure below) is required to convert 10 x 10 Gbps and 4 x 25 Gbps electrical signals. Later, as the electrical signal was upgraded to CAUI-4, the second generation CFP (CFP2/CFP4) long-distance transmission scheme did not require the built-in Gearbox.
▲ first generation CFP optical module long distance solution
However, the CFP size is too large. As the integration of optical modules is getting higher and higher, the later development direction is to make the size smaller and the power consumption lower. CFP can be evolved to CFP2, CFP4, and then QSFP28 appears later. . Compared to the CFP4, the QSFP28 is smaller in size and lower in power consumption. The smaller size of the QSFP28 allows the switch to have a higher port density (typically 36 Gigabit interfaces can be deployed per board). Currently, QSFP28 is the mainstream package format for 100G optical modules in the data center.
▲CFP/CFP2/CFP4/QSFP28 optical module size comparison
Finally, to summarize, how to choose the internal interconnect optical module of 25G/100G data center, I suggest you refer to the following standards:
100G short-range interconnection scene (TOR-LEAF) not exceeding 100 meters, using 100GBASE-SR4 QSFP28 optical module;
100G mid-range interconnection scene (LEAF-SPINE) from 100 meters to 500 meters, using 100G PSM4 QSFP28 optical module;
100G medium long distance interconnection scene (LEAF-SPINE, SPINE-CORE) of 500 meters to 2Km, using 100G CWDM4 QSFP28 optical module;
Long-distance interconnection scenario (CORE-MAN) over 2Km, using 100GBASE-LR4 QSFP28 optical module.
Finally, the explanation of the terminology is attached.
If you would like more information of 100G Transceiver, please contact Ivy by email or skype: sales6@htfuture.com
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