With the advantages of high transmission rate, high density and low equipment power consumption, 100G Ethernet has become the darling of the optical communication market, and the 100G optical module, 100G AOC and 100G DAC are important components for 100G Ethernet. The 100G AOC and 100G DAC are used as carriers for short-distance data transmission in enterprises or data centers, but there are some differences between them. This paper will introduce in detail what is 100G AOC/DAC and how to use these two types of cables.
What is 100G AOC?
100G AOC is a communication cable that requires external energy to convert electrical signals into optical signals or optical signals into electrical signals during communication. The optical transceivers at both ends of the cable provide photoelectric conversion as well as optical transmission functions, and the transmission rate reaches 100Gb/s. The AOC consists of two optical transceivers and a fiber optic patch cable, which has a long transmission distance, generally up to 100 meters. There are two types of 100G AOC: 100G QSFP28 AOC and 100G QSFP28 to 4X 25G SFP28 AOC. The former is an optical module receiver at both ends to achieve one-to-one transmission; the latter is a 100G QSFP28 interface at one end and four 25G SFP28 interfaces at the other end, which can provide customers with 100G data transmission.
What is 100G DAC?
DAC refers to the optical module installed on both ends of the high-speed patch cable, which is used to connect the switch to the router or server. DAC is widely used in storage area network, data center and high-performance computer connection. 100G DAC adopts silver plated conductor and foamed insulating core wire, which has excellent attenuation performance and low delay performance, so that the signal transmission is correct and can also improve the transmission speed. It is a short-distance connection solution instead of optical module. The price is much cheaper than the same type of optical module, and is widely welcomed in short-distance connection applications. The transmission distance of DAC is generally about 7m. The 100G DAC has 100G QSFP28 DAC direct cable and 100G QSFP28 to 25G SFP28 DAC two types. The former is an optical module receiver at both ends to achieve one-to-one transmission; the latter is a 100G QSFP28 interface at one end and four 25G SFP28 interfaces at the other end, which can provide customers with 100G data transmission.
100 AOC vs. 100G DAC
The biggest difference between AOC and DAC is that the transmission medium is different: AOC cable is made of fiber optic material (insulation), which belongs to fiber optic communication and is not subject to electromagnetic wave interference; while DAC cable is made of copper wire, which belongs to electrical communication and will be subject to electromagnetic interference. Moreover, the AOC contains a laser, while the DAC does not contain optical devices inside. As the price of fiber optic material is more expensive than copper cable, the cost price of the laser is also very high, so the price of the AOC is much higher than the DAC.
The advantages of DAC
·Higher data transmission rate: today’s DAC can support data transmission rates from 4Gbps to 10Gbps, which is higher than that supported by traditional copper cable.
·Strong interchangeability: with the development of copper technology, high-speed cables and optical fiber transceiver devices can be interchanged and hot swapped.
·Low cost: copper cable is cheaper than optical fiber, and the use of DAC will reduce the wiring cost.
·Good heat dissipation: DAC is made of copper core with good heat dissipation effect.
The advantages of AOC
·Greater bandwidth: no equipment upgrade is required, with a throughput of up to 40Gbps.
·Lightweight: much lighter than DAC.
·Low degree of electromagnetic interference: because optical fiber is a dielectric, it is not easy to be affected by electromagnetic interference.
How to apply AOC/DAC in 100G Network?
First, let’s understand what is QSFP28 optical module and SFP28 optical module.
QSFP28 optical module is a 4-channel 100G module, “28” means the maximum rate of each channel is 28G. The 100G optical module has undergone an upgrading process from CFP, CFP2, CFP4 to QSFP28, and the module volume is also shrinking during the upgrading. The QSFP28 optical module can be upgraded from 25G to 100G directly without transitioning through 40G network, so the cost is low. In addition, the power consumption of QSFP28 optical modules generally does not exceed 3.5W when operating, while the power consumption of other 100G optical modules usually ranges from 6W to 24W. QSFP28 optical modules are mainly used in data centers, where the transmission distance is shorter and the cost is lower compared to the CFP series.
The 25G SFP28 optical module is an optical module with a transmission rate of 25Gbps, mainly used for interconnecting servers and switches in data centers. 25G optical modules are low-speed optical modules in the optical module market, and the emergence of 25G optical modules has, to a certain extent, made up for the defect that the price of high-speed optical modules is too high and the rate of 10G optical modules cannot meet users’ needs. The biggest advantage of 25G Ethernet technology for the data center is that it can maximize the bandwidth and port density, and it can also use 25Gb/s single channel physical layer technology to realize 100GB data transmission, and it can provide the most energy-saving way to meet the growing needs of the next generation data center network.
Here are four 100G data center connectivity schemes using DAC and AOC respectively:
·100G QSFP28 to QSFP28 Direct Attach Cables: both ends are 100G interfaces, which are suitable for stacking and direct connection between 100G devices in short distance, and are used in the same rack.
·100G QSFP28 to 4x25G SFP28 Direct Attach Cables: for interconnection between 100G and 25G devices where one end is a 100G interface and the other end is four 25G interfaces, supporting a distance range of 1m – 5m.
QSFP28 to SFP28 DAC
·100G QSFP28 to QSFP28 Active Optical Cables: both ends are 100G interfaces, using OM3 fiber for a distance of 70m and OM4 fiber for 100m.
·100G QSFP28 to 4x25G SFP28 Active Optical Cables: Suitable for interconnection between 100G and 25G devices where one end is 100G interface and the other end is four 25G interfaces.
Generally speaking, when the distance between the access switch and the server NIC (NIC) is less than 5 meters for ultra-short distance scenarios, DAC is better than AOC in terms of cost, power consumption and thermal performance, and DAC is the most suitable choice. Under special scenarios, 100G QSFP28 to 4-way SFP28 DAC direct connection and other solutions can be used according to customer needs. The transmission distance from the top of rack switch (TOR) to the edge core switch is generally less than 100m. Here, the line integration begins to become higher. AOC has the advantages of light weight and small wire diameter. At this time, AOC is selected to simplify cable management. Moreover, the data center has very strict requirements for signal transmission. The strength of AOC compared with DAC lies in signal integrity and optical coupling design, which greatly ensures the reduction of bit errors in signal processing. In addition, because the high-frequency EMI signal is processed in the pluggable module, the EMI immunity of the AOC is higher than that of the DAC. There is no doubt that AOC is the first choice in the scenario of medium and short distance switch interconnection.
In short, when the requirements for the data center are not very high at this stage, DAC scheme is recommended for the interconnection between servers and switches within 5m/10m, and AOC scheme is recommended for the connection between switches. Of course, if you have sufficient funds and consider the performance more than the cost, you do not need to consider too much and can directly choose AOC.