Redundancy

Redundancy is the use of extra elements than needed to keep the system performance in case one or more of the element fails.

Active Redundancy

Example: Truck
Photo source: Desa Ilmu 

Figure 8.2.1 shows a 18 wheeled truck.

     Active redundancy is the use of redundant elements all the time, means the redundant elements are always active.

     The truck in figure 8.2.1 shows the use of many tires to accommodate for its load. Similarly to using multiple independent pillars to support a roof, these tyres are the redundant elements and the burden of the load of the lorry is distributed to each of tyres. This will reduce system or element failure because the burden is not solely on one tyre at a time. 

  

Passive Redundancy

Example: Emergency lights

Photo source: FSS hallway

Figure 8.2.2 shows an emergency light.

     Passive redundancy is the use of redundant elements only when the active elements fails.

     The emergency lights in Figure 8.2.2 are used as redundant elements in this lighting system. In this case, the emergency lights acts as passive elements and the normal lights acts as active elements. In case of a blackout where the normal lights fail, then the emergency lights will light up. Therefore the emergency lights can be said as passive redundant elements because they are always idle until the normal lights fail.

Diverse Redundancy

Example: Phone alert system

Photo source: Friend's phone.

Figure 8.2.3 shows a mobile phone when it receives a call.

    Diverse redundancy is using different elements to resist a single cause of failure.

    Mobile phones (in Figure 8.2.3) uses a diverse redundancy principle in its alert system, for example when a phone receives a call. It uses sound, light and vibration all at the same time to alert the user about the incoming call.  For example, in case when the sound system of the phone fail to function, there are still vibration and lights to alert the user of the incoming call. Hence the diverse redundancy elements can be seen in terms of the various alert system of sound, light and vibration.

Homgeneous Redundancy

Example: Bicycle chain
Photo source: FSKPM

Figure 8.2.4 shows a bicycle chain.

     Homogenous redundancy is the use of multiple same elements to maintain the system's performance  in case of failure of one element.
     The bicycle seen in Figure 8.2.4 have a chain to operate its wheels. This chain consist of many  links (the small elements that make up the chain) that can be removed or added to adjust the length of the chain. In this example, the links are the homogenous redundant elements. This is because they can be attached back if one of the links is damaged. However, the chain is susceptible to one cause of failure, for example if the chain is cut, the chain will break and the bicycle will not function anymore.