The data center is the core support platform for cloud computing. The development of cloud computing poses severe challenges to the data center network architecture. The traditional electrical interconnection network architecture is difficult to meet the needs of cloud applications in terms of bandwidth, equipment overhead, energy consumption, and management complexity. Therefore, the optical interconnection network architecture characterized by low energy consumption, low overhead, and high bandwidth has emerged and received widespread attention.
Technical Challenges of Electrical Interconnection Networks
The development of cloud computing has resulted in the explosive growth of traffic in the data center. Therefore, the use of 10GigE switching architecture at the access layer and 40G/100GbE switching architecture at the core layer has basically become the future development trend of data center networks. In this case, the data center’s electrical interconnection architecture will face the following technical challenges.
For copper cables, there is a certain trade-off between the communication bandwidth and transmission distance under the rated power, that is, as the communication bandwidth increases, the transmission distance of copper cables will decrease. For the communication bandwidth of 10Gbps, the transmission distance of 10G copper will be less than 10m. To obtain a longer transmission distance, the power of the transmitter needs to be greater than 6W/port. Therefore, copper cable is no longer an ideal transmission medium as the demand for link bandwidth increases.
Since the transmission rate of electrical signals is limited by signal loss and intersymbol interference on a single line, the signal transmission rate is often increased by increasing the bit width of the line in chip design. However, this method will eventually be limited by the chip packaging area. Therefore, as the signal rate increases, the number of ports supported by the switch chip will gradually decrease. For example, under the current technology level and packaging constraints, if the port rate is 80Gbps, the maximum number of ports supported by the switch chip based on electrical integration technology is only 64.
With the improvement of link bandwidth, the transmission loss of electrical signals in copper cables or backplane lines increases, so the signals need to be processed with complex pre-emphasis and error control. In addition, due to the limitation of port density and switching capacity, the electrical switching network needs to use more equipment and more complex interconnection methods to meet the needs of large-capacity switching, which increases the equipment and wiring costs of the network.
Energy consumption has become one of the bottleneck factors restricting the development of data centers. According to forecasts, from 2012 to 2020, the peak computing capacity and bandwidth of high-performance computing systems will increase at a rate of 10 times/4 years and 20 times/4 years respectively, but its energy consumption is only allowed to increase at a rate of 2 times/4 years. speed growth.
This poses a serious challenge to the design of data centers. The network energy consumption of the data center accounts for about 23% of the total energy consumption. With the improvement of the link rate and switch capacity, the proportion of network energy consumption will continue to increase.
Since the current energy consumption of electrical switches does not decrease proportionally with load reduction, the commonly used energy-saving strategies for electrical interconnection networks mainly include:
① Integrate network traffic into fewer links and switching devices and turn off idle network devices; ② Reduce the link rate to save transmitter energy consumption. The above two strategies will bring performance loss to a certain extent.
Advantages of Optical Interconnect Technology
Optical interconnection technology has great potential to solve the above problems. Under optimal conditions, the photoelectric hybrid network Helios can save 2 times the cost, 5 times the equipment overhead, and 8 times the network energy consumption compared with the electric network of the same scale. The main reasons why the optical interconnect architecture can achieve the above optimization include the following aspects.
In terms of transmission bandwidth, coherent receivers based on 100 Gbps PM-QPSK modulation have been commercially available. Combined with DWDM technology, the transmission bandwidth of a pair of single-mode optical fibers can reach 12Tbps. Further, in terms of transmission distance, the current unrepeated transmission distance of multi-mode fiber and single-mode fiber can easily meet the interconnection needs in the data center;
In terms of switching capacity, the optical switching architecture can achieve higher switching capacity.
This is because:
① The loss and crosstalk of high-speed optical signals are much smaller than those of electrical signals;
② Through wavelength division multiplexing technology, the number of channels carried in a single optical waveguide can be increased by dozens of times;
In terms of network overhead, optical fiber has higher bandwidth, smaller cross-sectional area, and lighter weight, so it can bring better heat dissipation and reduce network wiring overhead. At the same time, because most optical switching units have the characteristics of transparent transmission for signal rate, modulation mode, protocol, etc., the network can upgrade the link bandwidth without replacing the optical switching equipment.
In addition, because optical switches can achieve greater switching capacity and higher port density, many all-optical interconnection networks or optical-electrical hybrid networks can achieve flat architecture design. This greatly reduces the equipment and management overhead of the network;
In terms of energy consumption, optical interconnection technology can well solve the trade-off between energy consumption and performance.
This is mainly because:
① The optical signal has lower loss and longer transmission distance, so the optical link can use lower transmission power;
② For the all-optical switching architecture, the signal does not need to experience O/E (optical- Electrical) and E/O (electrical-optical) conversion process;
③ The optical switching architecture can be constructed using passive or low-energy optical devices, so it can further reduce network energy consumption.
It can be seen that due to the limitations of communication bandwidth, switching capacity, equipment overhead, and energy consumption, it is difficult for electrical interconnection networks to meet the communication needs of cloud computing data centers. Therefore, optical interconnection technologies with high bandwidth and low energy consumption have been favored by researchers’ focus. Especially with the development of silicon photonics technology and optical integration technology, the cost of optical switching equipment is decreasing, which promotes the application and deployment of optical interconnection technology in data centers.