4. Power Terminology
Constraints on power and energy usage define what type of network the node can be connected to and the duration and frequency of such a connection. [RFC6606] distinguishes a number of classes of energy limitation. The present document extends and refines these terms with more information on how power usage is managed.
4.1. Scaling Properties
The power and energy available to a node may vastly change over time. Several key characteristics need to be described:
Event energy: The energy used in a single event, such as a packet transmission. Often, one of the larger contributions to the event energy is the radio energy used for the actual transmission; pre-transmission and post-transmission energy contributions, such as radio calibration and local computations/cache lookup necessary to prepare the parameters for transmission, may significantly add to that. Depending on the node architecture, turning on a device may cost some fixed amount of energy, independent of the energy expended while the device is on. (Sometimes, there is a fixed energy budget available for the event, regardless of whether it is actually expended, such as with duty cycling that uses the same time period for sleep and transmission.)
Time spent in different power states: Here, power states are characterized by the current draw in this state. For constrained nodes, there are typically various sleep states that trade off some power consumption for a more immediate availability of the node when needed. A much higher power consumption state is the power state where the node is fully turned on; there might be multiple different levels of this state. Such levels might correspond to working states like radio listening, radio transmitting, or processing; they might also correspond to different power states, such as when different CPU speeds are used.
4.2. Classes of Energy Limitation
Devices can be categorized based on the type and level of energy limitation they are subject to. The term "mains-powered" is often used to imply unlimited energy. The terms used in the present document and in [RFC6606] are as follows:
| Name | Type of energy limitation |
|---|---|
| E0 | Event energy-limited |
| E1 | Period energy-limited |
| E2 | Lifetime energy-limited |
| E9 | No direct quantitative limitations to available energy |
Table 2: Classes of Energy Limitation
E0: Event energy-limited. Nodes in this category have to limit the energy for a single event (such as a packet transmission) to make it possible to handle a number of such events on the net energy available from a period of energy harvesting.
E1: Period energy-limited. Nodes in this category can use a significant amount of energy in a single event (e.g., transmit a packet), but are limited in the sum of the energy available for a number of such events in a period of time.
E2: Lifetime energy-limited. Nodes in this category use their energy budget for the entire lifetime of the node. Typically, it is not practical to replace or recharge the energy for such a node.
E9: No direct quantitative limitations to available energy. Nodes in this category have no direct quantitative limitations to available energy. Energy is still limited, but at a higher level, such as by the limited budget of the owner of the node or by the availability of suitable electrical power infrastructure.
4.3. Strategies for Using Power for Communication
Especially when wireless transmission is used, the radio often consumes a big portion of the total energy consumed by the device. Design parameters, such as the available spectrum, the desired range, and the bitrate aimed for, influence the power consumed during transmission and reception; the duration of transmission and reception (including potential reception) influence the total energy consumption.
Different strategies for power usage and network attachment may be used, based on the type of the energy source (e.g., battery or mains-powered) and the frequency with which a device needs to communicate.
The general strategies for power usage can be described as follows:
Always-on: This strategy is most applicable if there is no reason to be concerned about power consumption at all. The device can stay on in the usual manner all the time. It may be useful to employ power-friendly hardware or limit the number of wireless transmissions, CPU speeds, and other aspects for general power-saving and cooling needs, but the device can be connected to the network all the time.
Normally-off: Under this strategy, the device sleeps such long periods at a time that once it wakes up, it makes sense for it to not pretend that it has been connected to the network during sleep: the device reattaches to the network as it is woken up. The main optimization goal is to minimize the effort during the reattachment process and any resulting application communications.
If the device sleeps for long periods of time and needs to communicate infrequently, the relative increase in energy expenditure during reattachment may be acceptable.
Low-power: This strategy is most applicable to devices that need to operate on a very small amount of power but still need to be able to communicate on a relatively frequent basis. This implies that extremely low-power solutions need to be used for the hardware, chosen link-layer mechanisms, and so on. Typically, given the small amount of time between transmissions, despite their sleep state, these devices retain some form of attachment to the network. Techniques used for minimizing power usage for the network communications include minimizing any work from re-establishing communications after waking up and tuning the frequency of communications (including "duty cycling", where components are switched on and off in a regular cycle) and other parameters appropriately.
Table 3 provides a summary of the strategies described above.
| Name | Strategy | Ability to communicate |
|---|---|---|
| P0 | Normally-off | Reattach when required |
| P1 | Low-power | Appears connected, perhaps with high latency |
| P9 | Always-on | Always connected |
Table 3: Strategies of Using Power for Communication
Note that the discussion above is at the device level; similar considerations can apply at the communications-interface level. This document does not define terminology for the latter.
A term often used to describe power-saving approaches is "duty-cycling". This describes all forms of periodically switching off some function, leaving it on only for a certain percentage of time (the "duty cycle").
[RFC7102] only distinguishes two levels, defining a Non-Sleepy Node as a node that always remains in a fully powered-on state (always awake) where it has the capability to perform communication (P9) and a Sleepy Node as a node that may sometimes go into a sleep mode (a low-power state to conserve power) and temporarily suspend protocol communication (P0); there is no explicit mention of P1.