Category Archives

3 Articles

Data Center/Data Center Interconnection/Gigalight Wiki

What is Data Center Interconnect/Interconnection?

Posted by gigalight on

Data Center Interconnection means the implements of Data center Interconnect (DCI) technology. With the DCI technology advances, better and cheaper options have become available and this has created a lot of confusion. This is compounded by the fact that a lot of companies are trying to enter this market because there is a lot of money to be made. This article is written to straighten out some of the confusion.

According to the different applications, there are two parts of data center interconnections. The first is intra-Data Center Interconnect (intra-DCI) which means connections within the data center. It can be within one building or between data center buildings on a campus. Connections can be a few meters up to 10km. The second is inter-Data Center Interconnect (inter-DCI) which means connections between data centers from 10km up to 80km. Of course, connections can be much longer but most of the market activity for inter-DCI is focused on 10km to 80km. Longer connections are considered Metro or Long-haul. For reference, please see the table below.

DCI Distance Fiber Type Optics Technology Optical Transceivers
intra-DCI 300m MMF NRZ/PAM4 QSFP28 SR4
500m SMF QSFP28 PSM4
2km QSFP28 CWDM4
10km QSFP28 LR4
inter-DCI 10km SMF Cohernet QSFP28 4WDM-10
20km QSFP28 4WDM-20
30km to 40km QSFP28 4WDM-40
80km to 2000km CFP2-ACO


The big bottlenecks are in the intra-DCI and therefore, the highest volume of optical transceivers are sold here generating the most revenue, however, it is low margin revenue because there is so much competition. In this space, may of the connections are less than 300m and Multi-Mode Fiber (MMF) is frequently used. MMF is thicker, and components are cheaper because the tolerances are not as tight, but the light disperses as it bounces around in the thick cable. Therefore, 300m is the limit for many types of high speed transmission that use MMF. There is a data center transceiver with a transmission distance up to 100m over OM4 MMF for example.

Gigalight 100GBASE-SR4 100m QSFP28 Optical Transceiver

100G QSFP28 SR4 for MMF up to 100m

In a data center, everything is connected to servers by routers and switches. Sometimes a data center can be one large building bigger than a football field and other times data centers are built on a campus of many buildings spanning many blocks. In the case of a campus, the fiber is brought to one hub and the connections are made there. Even if the building you want to connect to might be 200m away, the fiber runs to a hub, which can be more than 1km away, so this type of routing increases the fiber distance. Some of the distances between buildings can be 4km, requiring Single Mode Fiber (SMF), which has a much narrower core, making it more efficient, but also increasing the cost of all related components because the tolerances are tighter. Therefore, with data centers growing, so has the need for SMF as the connections get longer within the data center. With SMF you have the option to drive high bandwidth with coherent technology, and we’ll see more of this in the future. Previously coherent was only used for longer distances, but with cost reductions and greater efficiency versus other solutions, coherent is now being used for shorter reaches in the data center.

Gigalight 100GBASE-LR4 Lite 4km QSFP28 Optical Transceiver

100G QSFP28 LR4L for SMF up to 4km

500m is a new emerging market and because the distance is shorter, a new technology is emerging, and that is silicon photonics modulators. EMLs (Externally Modulated Lasers) perform modulation within the laser, but with silicon photonics, the modulator is outside the laser and it’s a good solutions for distances of 500m. In an EML, the modulator is integrated into the same chip, but is outside the laser cavity, and hence is “external”. For silicon photonics, the laser and modulator are on different chips and usually in different packages. Silicon photonics modulators are based on the CMOS manufacturing process that is high scale and low cost. A continuous wave laser with silicon photonic modulation is very good for 500m applications. EMLs are more suitable for longer reaches, such as 2-10km. Therefore, with data centers growing, so has the need for single mode fiber as the connections get longer within the data center. With SMF you have the option to drive high bandwidth with coherent technology, and we’ll see more of this in the future. Previously coherent was only used for longer distances, but with cost reductions and greater efficiency versus other solutions, coherent is now being used for shorter reaches in the data center.

100GE PSM4 2km QSFP28 Optical Transceiver

100G QSFP28 PSM4 for SMF up to 500m/2km

100GE CWDM4 2km QSFP28 Optical Transceiver

100G QSFP28 CWDM4 for SMF up to 2km

100GBASE-LR4 10km QSFP28 Optical Transceiver

100G QSFP28 LR4 for SMF up to 10km


Inter-DCI is typically between 10km and 80km, including 20km and 40km. Before we talk about data center connectivity, let’s talk about why data centers are set up the way they are and why 80km is such an important connection distance. While it is true that a data center in New York might backup to tape in a data center in Oregon, this is considered regular long-haul traffic. Some data centers are geographically situated to serve an entire continent and others are focused on a specific metro area. Currently, the throughput bottleneck is in the metro and this is where data centers and connectivity are most needed.

100GE 4WDM-20 20km QSFP28 Optical Transceiver

100G QSFP28 4WDM-20 for SMF up to 20km

100GE 4WDM-40 40km QSFP28 Optical Transceiver

100G QSFP28 4WDM-40 for SMF up to 40km

Say you have a Fortune 100 retailer and they are running thousands of transactions per second. The farther away a data center is, the more the data is secure because the data center is so far away and separate from natural disasters, but with the increased distance there are more “in flight” transactions are at risk of being lost due to latency. Therefore, for online transactions there might be a primary data center that is central to retail locations and a secondary data center that is around 80km away. It’s far enough away not to be affected by local power outages, tornadoes, etc, but close enough that there is only a few hundred milli-seconds of latency; therefore, in the worst case a small number of transactions would be at risk.

In another example of inter-DCI, as if a certain video is getting a lot of views, the video is not only kept in its central location, but copies of the video are pushed to metro data centers where access is quicker because it’s stored closer to the user, and the traffic doesn’t tie up long haul networks. Metro data centers can grow to a certain size until their sheer size becomes a liability with no additional scale advantage and thus they are broken up into clusters. Once again, to guard against natural disasters and power outages, data centers should be far away. Counterbalancing this, data centers need to have low latency communication between them, so they shouldn’t be too far away. There is a compromise and the magic distance is 80km for a secondary data center, so you’ll hear about 80km data center interconnect a lot.

It used to be that on-off keying could provide sufficient bandwidth between data centers, but now with 4K video and metro bottlenecks, coherent transmission is being used for shorter and shorter distances. Coherent is likely to take over the 10km DCI market. It has already taken over the 80km market but it might take time before coherent comes to 2km. The typical data center bottlenecks are 500m, 2km, and 80km. As coherent moves to shorter distances, this is where the confusion comes.

The optical transceiver modules that were only used within the data center are gaining reach, and they’re running up against coherent solutions that were formerly only used for long distances. Due to the increasing bandwidth and decreasing cost, coherent is being pulled closer into the data center.

The other thing to think about is installing fiber between data centers. Hopefully this is already done, because once you dig, it’s a fixed cost, so you put down as many fibers as you can. Digging just for installing fiber is extremely expensive. In France when they lay fiber, they use another economic driver. Whenever you put in train tracks, you put in fiber at the same time, even if it is not needed. It’s almost for free because they are digging anyway. Fibers are leased to data centers one at a time; therefore, data centers try to get as much bandwidth as possible onto each fiber (this is also a major theme in the industry). You might ask, why not own your own fiber? You need to have a lot of content to own your own fiber. The cost is prohibitive. In order to make the fiber network function, all the nodes need to use the same specification and this is hard. Therefore, carriers are usually the ones to install the full infrastructure.

Article Source: John Houghton, a Silicon Valley entrepreneur, technology innovator, and head of MobileCast Media.

Data Center/GIGAC™ Cabling/Gigalight News/Industry News/MTP/MPO Cabling

Gigalight’s First Successful Project for the Russian ISP Market Based on GIGAC™ Cabling

Posted by gigalight on

Shenzhen, China, May 9, 2018 − The Gigalight's GIGAC™ MTP/MPO Cabling Portfolio has won the first big order in the Russian ISP market. In the next three years, Gigalight will provide the largest Russian Internet service provider with the GIGAC™ high-density cabling products for the data centers in major Russian cities.

Data centers are very important for modern large IT business units. As the largest Internet service provider in Russia, this client has its own data centers with the major target to optimize the network and improve the quality of the business. On the picture below, the right is the previous organization of the racks and on the left is the current installation.

Data Center Cabling Racks

The Gigalight company together with the expertise partner in Russia have solved this challenge and ensured the reliability for the current network.

Data Center Cabling Racks

Almost the full range of Gigalight optical transceivers and GIGAC™ cabling products, including GIGAC™ MTP/MPO patch cables, trunk cables, harness cables, hydra cables, GIGAC™ racks and cassettes are used in this project. They are particularly reliable and safe, and can withstand temperatures up to 70 °C. Typical uses for the cables include delivering optimal performance and data transmission for information systems.


This order follows after the years of the hard work with the Russian market. Well-known companies, public and private operators of data and communications networks are placing their trust in Gigalight’s expertise for years. Their confidence is based on our powerful cabling solutions, optical transceivers manufacturing capacity, and the tireless support we provide to our customers.

Gigalight is the world's design innovator in the optical interconnect field and this order sees it continue to build on this strong position. The company has rich experience in the development and production of optical transceivers, GIGAC™ MTP/MPO cables and passive optical components. In addition to connectivity solutions for the interconnect field, Gigalight also develops checkers and programming boards for the production lines, data centers and our global partners.

About Gigalight:

Gigalight is global optical interconnection design innovator. A series of optical interconnect products include: optical transceivers, passive optical components, active optical cables, GIGAC™ MTP/MPO cablings, and cloud programmers & checkers, etc. Three applications are mainly covered: Data Center & Cloud Computing, MAN & Broadcast Video, and Mobile Network & 5G Optical Transmission. Gigalight takes advantage of its exclusive design to provide clients with one-stop optical network devices and cost-effective products.

Article Source:

Data Center

The Trend of DSP’s Application in Data Center

Posted by gigalight on

The data center 100G has begun to be used on a scale, and the next-generation 400G is expected to begin commercial use by 2020. For 400G applications, the biggest difference is the introduction of a new modulation format, PAM-4, to achieve a doubled transmission rate at the same baud rate (device bandwidth). For example, the single-lane baud rate of DR4 used for transmissions up to 500m need to reach 100Gb/s. In order to realize the application for such rate, the data center optical transceiver modules began to introduce Digital Signal Processor (DSP) chips based on digital signal processing to replace the clock recovery chips of the past to solve the sensitivity problem caused by insufficient bandwidth of the optical devices. Can DSP become a broad solution for future data center applications as expected in the industry? To answer this question, it is necessary to understand what problems the DSP can solve, what its architecture is, and how the development of its costs and power consumption trends in the future.


The Problems that DSP Can Solve

In the field of physical layer transmission, DSP was first applied in wireless communications for three reasons. First, the wireless spectrum is a scarce resource, and the transmission rate demand has been increasing. Increasing the spectrum efficiency is a fundamental requirement for wireless communications, so DSP is required to support a variety of complex and efficient modulation methods. Second, the transmission equation of the wireless channel is very complicated. The multipath effect, and the Doppler effect in the high-speed motion, can’t satisfy the wireless channel’s compensation demand with the traditional analog compensation. DSP can use various mathematical models to compensate the channel well Transmission equation. Third, the Signal-to-Noise Ratio (SNR) of the wireless channel is generally low, and the Forward Error Correction (FEC) should be used to improve the sensitivity of the receiver.


In the field of optical communications, DSP was first commercially used in long-distance coherent transmission systems over 100G. The reason is similar to that of wireless communications. In long-distance transmission, since the laying cost of optical fiber resources is very high, the improvement of spectral efficiency to achieve higher transmission rates on a single optical fiber is an inevitable requirement for operators. Therefore, after the use of WDM technology, the use of coherent technology based-on DSP has become an inevitable choice. Secondly, in long-distance coherent transmission systems, by using of a DSP chip, the dispersion effects, non-linear effects caused by transmitter (Tx) and receiver (Rx) devices and the optical fiber itself, and phase noise introduced by the Tx and Rx devices, can be easily compensated without the need for Dispersion Compensation Fiber (DCF) that placed in the optical link in the past. Finally, in long-distance transmission, due to the attenuation effect of optical fibers, an optical amplifier (EDFA) is generally used to amplify the signal every 80km to reach a transmission distance up to 1000km. Each amplification will introduce noise to the signal, reducing the SNR of the signal, therefore, the FEC should be introduced to improve the receiver’s receiving ability during long-distance transmission.


To sum up, DSP can solve three problems. First, it supports high-order modulation formats and can improve the spectral efficiency. Second, it can solve the effects caused by components and signal-channel transmission. Third, it can solve the SNR problem.


Then, whether there are similar requirements in data center has become an important basis for us to judge whether we should introduce DSP.


First of all, let’s take a look at the spectrum efficiency. Does data center need to improve spectrum efficiency? The answer is yes. But unlike the lack of wireless spectrum resources and insufficient optical fiber resources in the transmission network, the reason for improving spectrum efficiency in data center is that the insufficient bandwidth of the electrical/optical devices and the insufficient number of wavelength division/parallel paths (limited by the size of optical transceiver modules). Therefore, to meet the needs of future 400G applications, we must rely on increasing the single-lane baud rate.


The second point is that for single-lane 100G and above applications, current Tx electrical driven chips and optical devices can not reach bandwidths above 50GHz. Therefore, it is equivalent to that a low-pass filter is introduced at the transmitter. The performance on the code is inter-symbol interference in the time domain. Taking the application of 100G PAM-4 as an example, the bandwidth-limited modulation device will make the width of the optical eye diagram of the signal very small, then the clock recovery based on the analog PLL in the past could not find the best sampling point, making the receiver unable to recover the signal (this is also why the TDECQ needs to introduce an adaptive filter for equalization in the standards). After introducing the DSP, the signal can be directly spectrally compressed at the Tx end. For example, the extreme approach is to artificially introduce intersymbol interference between two symbols to reduce the signal bandwidth of the Tx end. At this time, the eye diagram of PAM-4 on the oscilloscope will become PAM-7 form. The Rx end recovers the signal through an adaptive FIR filter. In this way, the uncontrollable analog bandwidth effect in the modulating/receiving device becomes a known digital spectrum compression, reducing the bandwidth requirement for the optical device. Fujitsu’s DMT (Discrect-Multi-Tone) modulation technology, which has been promoted in conjunction with DSP, can even use a 10G optical device to transmit 100G signals.


Third, does FEC technology really need to be introduced at the module end? Inside the data center, the maximum transmission distance is not more than 10km. The link budget is about 4dB with the loss of the joint. Such SNR effects caused by the link is basically negligible. Therefore, the FEC in the data center is not intended to solve the link SNR, but to solve the performance shortage of the optical devices. At the same time, we need to consider that the electrical interface signal at the optical module end is upgraded from 25G NRZ to 50G PAM-4 (net rate) in the 400G era, so it is often necessary to turn on the electrical FEC to meet the requirements for transmission from the optical transceivers to the switches. In this case, reopening the FEC on the module side is not necessary and has no effect. Because for FEC, we mostly discuss the error correction threshold. For example, 7% FEC error correction threshold is at 1E-3 Bit Error Rate (BER), that is to say that FEC is able to correct all errors below this BER, and FEC above this BER is essentially useless (regardless of the Burst Error which is usually solved with Inter-leaver). Therefore, there is no difference between the effect of using multiple FECs and using only the best FEC. Considering the power consumption and delay caused by FEC on the module side, it may be better to open FEC on the switch side in the future.


The Architecture of DSP

In the optical communication field, DSP generally consists of several parts: the front-end analog digital mixing section, including ADC (Digital-to-Analog Converter, required), DAC (Analog to Digital Converter, optional) and SerDes, digital signal processing section (including FEC) ) and the PHY section. The PHY section is similar to the CDR chip with the PHY function, and will not be described here.



The main function of ADC and DAC is to convert analog signal and digital signal, which is a bridge between the modulation device and digital signal processing section. The ADC/DAC mainly has four key indicators which are sampling rate, sampling effective bit width, analog bandwidth and power consumption. For the 100G PAM-4 application, the sampling rate of ADC in the Rx end needs to reach 100Gs/s. Otherwise, Alias will be generated during sampling, which will cause distortion to the signal. The effective sampling width is also very important. For PAM-4 applications, it does not mean that 2 effective bits can satisfy the requirement of digital signal processing, but at least 4. Analog bandwidth is currently the main technical challenge for ADC/DAC. This index is limited by both effective bit widths and power consumption. Generally, there are two ways to implement high bandwidth ADC/DAC which are GeSi and CMOS. The former has a high cutoff frequency and can easily realize the high bandwidth. The disadvantage is very high power consumption, so it is generally used in instrumentation. The cutoff frequency of CMOS is very low, so to achieve high bandwidth, multiple sub-ADCs/DACs must be sampled using an interleaving method. The advantage is low power consumption. For example, in a coherent 100G communication system, a 65Gs/s ADC with 6 effective bits is composed of 256 sub-ADCs with a sampling rate of 254Ms/s. It must be noted that although the ADC has a sampling rate of 65Gs/s, its analog bandwidth is only 18GHz. With a clock jitter of 100fs, the theoretical maximum analog bandwidth of 4 effective bits width is only up to 30GHz. Therefore, an important conclusion is that under the condition of using DSP, the bandwidth limitation of the general system is no longer the optical device, but the ADC and DAC.



In the data center applications, and the digital signal processing unit is still relatively simple. For example, for 100G PAM-4 applications, it performs spectral compression of the transmitted signal, nonlinear compensation, and FEC encoding (optional) in the Tx end, then the ADC uses an adaptive filter to compensate the signal and digital domain CDR in the Rx end (separate external crystal support is required). In the digital signal processing unit, the FIR filter is generally used to compensate the signal. The Tap number and the decision function design of the FIR filter directly determines the performance of the compensation DSP and power consumption. It should be particularly pointed out that the DSP application in the field of optical communications is facing with a large number of parallel computing problems. The main reason is the huge difference between the ADC sampling frequency (tens or even 100Gs/s) and the digital circuit operating frequency (up to several hundred MHz), in order to support the ADC of 100Gs/s sampling rate, digital circuits need to convert the serial 100Gs/s signals into hundreds of parallel digital signals for processing. It can be imagined that when the FIR filter only adds one Tap, the actual situation is that hundreds of Taps needs to be added. Therefore, how to deal with the balance of performance and power consumption in the digital signal processing unit is the key factor to determine the quality of the DSP design. In addition, inside the data center, optical transceiver modules must meet the interoperability prerequisites. In practical applications, the transmission performance of a link depends on the overall performance of the DSP and analog optical devices in the Tx and Rx ends. It is also a difficulty to design a reasonable standard to correctly evaluate the performance of the Tx and Rx ends. When the DSP supports that FEC function is opened in the the physical layer, how to synchronously transmit and receive the FEC function of the optical transceivers also increases the difficulty of data center testing. Therefore, so far, coherent transmission systems are interoperable among manufacturers’ devices, and do not require interoperability among different manufacturers. (The TDECQ performance evaluation method is proposed for PAM-4 in 802.3.)


Power Consumption and Cost

Because DSP introduces DAC/ADC and algorithm, its power consumption must be higher than the traditional CDR chip based on simulation technology. And the method that DSP lowers the power consumption is relatively limited, mainly depending on the promotion of the process of tape, for instance, upgrading from the current 16nm to 7nm process can achieve a 65% reduction in power consumption. The current design power consumption of the 400G OSFP/QSFP-DD based on the 16nm DSP solution is around 12W, which is a huge challenge for the thermal design of the module itself or the future front panel of the switch. Therefore, it may be based on the 7nm process to solve the 400G DSP problem.


Price is always a topic of concern to data center. Unlike traditional optical devices, DSP chips are based on mature semiconductor technology. Therefore, larger chip costs can be expected to fall under the support of massive applications. Another advantage of DSP’s future application in data centers is flexibility, which can meet the application requirements of different data rates and scenarios by adjusting the DSP configuration in the same optical device configuration.


Article Source:


Related Gigalight 100G QSFP28 Optical Transceivers: