Comparing DAC/AOC Cables vs. DSP/LPO Optical Modules for Data Center Network Interconnects

NADDOD Brandon InfiniBand Technical Support Engineer Mar 15, 2024

In the modern three-layer CLOS network architecture of data centers, the interconnection links between the Spine and Leaf layers, as well as between the Leaf and ToR (Top of Rack) layers, are generally limited to within 2 kilometers in length. Especially, the physical links between Leaf and ToR often do not exceed a mere 10 meters. These links, which involve both short-distance and long-distance optical modules, account for one-third of all physical links within the cluster. Meanwhile, the direct connection links between server network cards and TOR switches, although also within 10 meters, constitute two-thirds of the total number of links, primarily relying on DAC (Direct Attach Cable) or AOC (Active Optical Cable) for connectivity.




Interconnection Solution between Network Cards and TOR Switches


AOC has attracted attention due to its lightweight design, long transmission distance of up to 300 meters, flexible cabling, and immunity to electromagnetic interference. However, due to its integration of precision optical transceiver components, it comes with relatively higher costs. The core structure of AOC includes a pair of optical transceivers and an intermediate section of optical cable. The optical transceiver chips at both ends are responsible for converting electrical signals into optical signals and efficiently transmitting them through the optical fiber. The use of multimode optical fibers and VCSEL light sources in AOC ensures low signal attenuation, significantly extending the transmission distance, and allowing users to customize the length as needed.


In comparison, DAC, with its construction of silver-plated copper conductors and foamed insulation cores, does not require optical-electrical conversion chips, resulting in a significant cost advantage. Its total cost of ownership (TCO) is only about one-fifth of the AOC solution, while also providing high reliability and better latency performance. However, limited by the physical properties of the copper medium itself, DAC has a shorter effective transmission distance, especially during the transition from 400G to 800G, where the transmission distance will be further reduced to around 2 meters.



With the continuous increase in network speed and bandwidth, DAC (Direct Attach Cable) appears inadequate in addressing the challenges of cable length and density within the cabinet. The loss of copper cables increases with the rate, resulting in increasingly limited transmission distances. In particular, the increase in channel count leads to a rapid expansion of cable volume, posing challenges to cable management and heat dissipation within the cabinet.


To overcome the traditional DAC bottleneck in high-bandwidth and long-distance transmission, the industry is gradually shifting towards "electrical relay" solutions such as ACC (Active Copper Cable) or AEC (Active Electrical Cable), or continuing to adopt the "electrical-to-optical" AOC (Active Optical Cable) scheme. ACC and AEC offer a cost and power consumption compromise between passive DAC and active AOC, providing an ideal choice within specific speed ranges. However, when the rate increases to the 112G-PAM4 level, re-timers with CDR functionality or even DSP-based solutions are required, undoubtedly increasing transmission latency and power consumption.


To keep pace with the rapid growth of data center east-west traffic, meet the demands of high-bandwidth, high-density interconnect applications, and strive for lower energy consumption and costs, major Internet giants are increasing their investment in new-generation DAC technology. Their aim is to develop direct copper cable solutions suitable for a wider range of scenarios.


Interconnection Solutions between TOR and LEAF, and between LEAF and SPINE


The interconnection between TOR and LEAF, and between LEAF and SPINE typically employs optical modules, which serve as the heart of the optical communication network and play a critical role in optoelectronic conversion. At the transmitting end, the optical module processes electrical signals through driving chips and converts them into stable modulated optical signals using lasers, enabling high-speed transmission of information in optical fibers. At the receiving end, it restores the optical signals into electrical signals and outputs them through pre-amplifiers.


Optical module work principal


DSP (Digital Signal Processing) chips are powerful engines in modern communication technology, capable of high-speed processing of digital signals. Particularly in the field of optical communication, as the rate exceeds 50Gb/s, the polarization mode dispersion of optical fibers significantly increases, posing a serious threat to signal quality and transmission distance. At this point, DSP chips with integrated CDR functionality become crucial for combating and compensating for such distortions, especially in optical modules above 200G, where they are almost indispensable.


Optical module

DSP chips can be divided into two categories: PAM4 DSP and coherent DSP, depending on the modulation method. PAM4 DSP uses four different levels of amplitude for signal transmission, with each symbol carrying 2 bits of information, suitable for short-distance interconnection within data centers. Currently, mainstream specifications cover the range from 100G to 800G. Coherent DSP chips utilize coherent modulation and heterodyne detection techniques, particularly suitable for long-distance transmission, often used in interconnection between data centers and telecommunications markets. The current maximum bandwidth is 400G, although the research and development progress is slightly lagging behind PAM4, its potential is enormous.


Another noteworthy optical module technology, Linear-drive Pluggable Optics (LPO), discards CDR or DSP designs and adopts linear analog components, along with TIA and DRIVER chips with EQ functions, significantly reducing power consumption and latency. However, there are compromises in terms of bit error rate and transmission distance. LPO is suitable for short-distance connections within data centers, such as between servers and switches, or even high-speed interconnections between GPUs in High-Performance Computing (HPC) centers. With the maturation and large-scale production of LPO technology, it is expected that in the era of 800G, its low-cost characteristics may significantly impact the DSP chip market and lead future trends in optical communication development in specific scenarios.


Where to Buy High Speed 400G/ 800G Optical Module and DAC/AOC Cables?


As a leading provider of comprehensive optical network solutions, NADDOD possesses top-notch capabilities in research and development, manufacturing, and technical services in the industry. It has deep technical expertise and rich project experience in the fields of data centers, high-performance computing, and artificial intelligence. NADDOD continuously provides users with innovative, efficient, and reliable computing and network products, solutions, and services.

NDR Transceivers

NADDOD has its latest lineup of cutting-edge optical module products, including the OSFP-800G-2xSR4H, OSFP-800G-2DR4LH, OSFP-800G-2xFR4H, OSFP-400G-SR4H, OSFP-400G-DR4H, OSFP-800G-CU1H, O2Q56-400G-CU1H, O2O112-800G-CU1H, and O4O112-800G-CUTH. These new offerings represent the pinnacle of optical connectivity technology and showcase NADDOD's commitment to innovation and excellence.


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