How to Choose Optical Modules for Switch Stacking?

NADDOD Jason Dec 28, 2022

Switch stacking refers to the combination of multiple switch devices that support the stacking feature, logically combined into one switching device. The main switch is responsible for the operation, management and maintenance of the system, and other switches can be used as the backup of the main switch while processing services. Once the master switch cannot work, the system will quickly and automatically select a new master switch to ensure uninterrupted business. Through switch stacking, high network reliability and large data volume forwarding can be realized, and at the same time network management can be simplified.

At present, there are three main types of switch stacking (there must be a switch that supports the stacking function):

  1. DACDirect Connection Stacking
  2. AOC Direct Connection Stacking
  3. Optical Module + Optical Fiber Jumper Stacking

Since DAC, AOC, and optical modules can all realize the stacking of switches, do you know when to use DAC or AOC? When to use optical module + optical fiber jumper?

Firstly, let’s talk about DAC, which is a cable assembly with fixed length and fixed modules at both ends. Modular connectors and copper cables cannot be separated. The modules at both ends of the DAC do not have expensive optical lasers and electronic components, and can only transmit electrical signals, not optical signals. Compared with AOC and optical modules, the price of DAC is much lower. It is a low-cost short-distance connection solution, which can save cost and power consumption in short-distance applications.

However, the transmission medium used by DAC is copper, which is more susceptible to electromagnetic interference. Therefore, optical fibers are generally used for long-distance signal transmission. Optical fibers are far superior to copper cables in terms of insulation and isolation characteristics, and the transmission distance is longer. Generally speaking, in common applications, switch stacks below 7 meters (in one rack) use DACs, which can greatly reduce costs. AOC or optical modules + fiber optic jumpers will be used to expand the capacity between devices with a distance of more than 7 meters (in one computer room).

Secondly, let’s talk about AOC. It is composed of a fixed-length optical cable and two modules at both ends. The module and the cable cannot be separated. It looks similar to DAC, but the transmission method and usage environment are different. The modules at both ends of the AOC can provide photoelectric conversion and have the function of optical transmission. Theoretically, the combination of AOC and 2 optical modules + 1 fiber jumper can achieve the same transmission effect. And under the same transmission distance, the cost of AOC is slightly lower than the cost of 2 optical modules + 1 fiber jumper.

However, because the length of the optical cable of the AOC is fixed, and the two modules are connected together, if one module cannot work, the entire AOC needs to be replaced.

But if it is a stack of 2 optical modules and 1 jumper, if one module cannot work, only one module needs to be replaced. Therefore, AOC is generally used for short-distance transmission in data centers. When the transmission distance is within 30 meters, AOC will be a better choice because it can save costs.

To sum up, from the perspective of practicality and economy, DAC should be used for data transmission or switch stacking below 7 meters. AOC is used for data transmission or stacking of switches between 7-30 meters, and optical modules + optical fiber jumpers are used for more than 30 meters.