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An experimental Setup for measuring Power Consumption in WLANs

The TKN group investigates, develops and tests power save strategies for RF interfaces. In particular, the recent research deals with reduction of the energy needs of IEEE 802.11 as well as HIPERLAN WLANs.

For the sake of realistic power consumption parameters to be used in simulations, power consumption measurements of a WLAN network interface cards were necessary. Furthermore, we were in need of an experimental setup to verify our developed power save mechanisms.

Measurements

In our measurements we used Aironet's PC4800 PCMCIA air interfaces which implement a DSSS physical layer and a medium access control protocol according to the IEEE 802.11(b) specification. An IEEE 802.11 network interface card can basically be in 4 power relevant working modes when in action:

  • transmit
  • receive
  • idle
  • power save
We measured the power consumption in each of these modes while varying the following parameters:
  • transmission rate (1, 2, 5.5 and 11Mbit/s)
  • RF output power (1, 5, 10, 20 and 50 mW)
Furthermore, we measured the averaged power consumption for receiving and sending. For that purpose we assumed two mobile terminals whereby one of them sends continuously frames of a fixed size and the other one receives these frames. In addition to the above mentioned parameters we varied
  • distance and (5, 10 and 15 meters; there were walls between transmitter and receiver for 10 and 15m)
  • packet size (64 ... 2312 bytes)
and recorded throughput and packet error rate . By doing so, we were able to compute the average energy needs to successfully transmit/receive one bit of information. The results are used to verify the outcome of simulations which use the measured, mode-dependent power consumption parameters of the above described network setup.

Experimental Setup

Our experimental setup consists of two Laptops. Each of them are equipped with an Aironet PC4800 PCMCIA card. We chose Linux (kernel version 2.2.13) as operating system because of its public available source code. This was necessary to be able to modify and extend kernel and drivers for measurement purposes. The wireless cards were configured to work in ad hoc mode. The hardware setup for measuring power consumption is shown in Figure 1. Probes as well as the external power supply for the card are mounted via a PCMCIA extender card (see Figure 2).

Figure 1: Measurement Setup
Figure 1: Measurement Setup
Lupe
Figure 2: PCMCIA extender card
Figure 2: PCMCIA extender card
Lupe

Power consumption results were obtained by measuring the voltage which dropped across the Aironet PC4800 card. In order to obtain the current, a small resistor of 1 Ohm was placed in series with the PC4800 card. We measured the voltage drop across the resistor and computed the current by means of the resistance and voltage values. We used an external power supply in order to ensure the quality of our measurements (avoiding variances in voltage delivered by the notebook) and to keep the voltage across the PCMCIA card in the specified voltage range (4.75-5.25V).

Results

In Figure 3 and 4 we show exemplary results of the measurements. A technical report which provides more insights of the measurement setup and which contains all of the results will be published soon (check here).

Figure 3: Energy per successfully transmitted bit of information; RF output power 1mW
Figure 3: Energy per successfully transmitted bit of information; RF output power 1mW
Lupe
Figure 4: Energy per successfully transmitted bit of information; RF output power 50mW
Figure 4: Energy per successfully transmitted bit of information; RF output power 50mW
Lupe

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