El análisis de la representación

mx and bx are the coefficients for various operational set up such as point to point,

broadcast and discard traffic. Figure 5-2 presents the result from [70] on the energy consumption during transmission, reception and the idle period of the tested IEEE 802.11 interface device. It is worth noting that the result indicates that the energy consumed during reception is marginally more than during an idle period, this finding is supported by another measurement conducted by [71].

Figure 5-2: Experimental snapshot on the Energy consumption of IEEE 802.11 (directly reproduced from [70])

In wireless sensor networks (WSN) where there is a greater emphasis on energy conservation as they are operated with limited battery power and computing capability. The discussion of whether it is more energy efficient to transmit over many short hops or few long hops has been a topic debated by various researchers [20], [73], [74], [75] and [76]. One of the drawbacks of the multi hop approach is that using other nodes as relays can cost more energy due to the additional effort for multiple transmission and reception as well as the increase in delay and bottleneck. In contrast, a single hop needs to go over a longer distance with more transmit energy. Over the last few years, some researchers such as [77] claimed that multi-hop network implementations can reduce energy consumption by 40% less compared to an equivalent single-hop network. However, some researchers [74], [75] and [76] argue that long-hop implementations consume less energy in relaying data compared to equivalent multi-hop networks due to simpler routing protocols, lower communication overhead, and higher overall efficiency.

In [20] and [21], the authors examine the energy consumption of a direct communication to the base station versus minimum energy multi-hop routing protocols in a wireless sensor network as shown Figure 5-1. Using the energy modelled summarised in Table 5-1, their analysis and simulation results indicate that when the transmission distance is small, transmission and receive energy are identical and a direct transmission is therefore more energy efficient than minimum energy routing protocols.

Operation Energy Consumption

Transmission ETx(kb,r) = Eelec×kb + eamp ×kb×r2

Reception ETx(kb,r) = Eelec×kb

Table 5-1: Leach energy consumption model

Where kb length of a message in bits, Eelec is the transmitter and receiver circuitry and

eamp is the transmitter amplifier.

The claim made by LEACH in regards to the energy consumption of short hops and long hops was supported by M. Haenggi in [73], [74], [75] and [76]. From their

analysis in [76], M. Haenggi concluded that the total energy reduction in transmitting over short hop is negligible as the circuitry of low power transceivers will dominate the energy consumption of sensor network nodes and that relatively high transmit peak power is required to maintain network connectivity, therefore it is more beneficial to transmit via a long hop if a reliable connection can be made [76]. M.Haenggi also claimed that long-hop transmission does not necessarily produce greater interference than multiple transmissions at lower power as Signal to interference ratio (SIR) does not depend on absolute power levels. Assuming that all nodes increase their power by the same factor, the increase in transmitted energy will not reduce the probability of packet reception as SIR would actually remain constant.

In [72], they explore the conditions in which multi-hop routing is more energy efficient than a direct transmission and when two hop strategy is optimal in wireless sensor networks (WSN). They argued that the energy consumption model used in [76] were unrealistic as it does not reflect the practical performance of commercially available sensor network nodes during transmission and reception. Figure 5-3 illustrates the current consumption of commercially available wireless sensor network transceiver model used in [72]. Their experimental tests using actual sensor networking hardware showed that it is more energy efficient using multi-hop schemes and that its efficiency is dependent upon source to sink distance and energy consumed in reception.

Figure 5-3: An example of Current consumption of transceivers model in Wireless Sensor Nodes (Directly reproduced from [72])

Wireless sensor network (WSN) energy efficiency has been widely based on a sensor node power consumption modelled by [20], [21]where the major impact of the energy consumption of a node is largely dependent upon the transmission range and transmitter circuitry. However, the quality of service (QOS) of a transmission also plays a vital role in the energy consumption as poor QOS will result in a longer delay thus increase energy consumption. In [51], the author analyses the energy consumption of wireless sensor devices by separating out the power consumption of each hardware component and incorporating the characteristics with RF transceivers to understand the impact on the QOS of the network. The power consumption in [51] is derived from the structure of a communication module found in a typical WSN node, in which the total power consumption for transmission and reception is:

5.3

5.4

Where PTB and PRB is the power consumption in baseband DSP circuitry, PTCF and

PRCF is the front end switching power consumption and PRA is the reception power

amplifier. PA(r) is the power consumption of the power amplifier and is dependent

upon the transmission range r. Since PTB together with PTCF and PRB together with

PRCF and PRA are not dependent upon transmission range r, therefore these

components can be modelled respectively as a constant and . They demonstrated that the upper bound limit of power efficiency of the network is proportional to the power consumption of the transmission and reception circuitry as well as the power amplifier of the tested devices. They also concluded that multi-hop schemes are more energy efficient than single hop only if the source to sink transmission length cannot be reached via single hop.

The above discussions clearly demonstrate that whether to use single hop or multi-hop to achieve optimum energy efficient solution is still continuously analysed and debated amongst various researchers. Therefore the network will be limited to two

hops, i.e. data from cluster members will be transmitted to the cluster head which in turn relay the data directly to a HBS. The limitation on the number of hops is primarily to reduce the relaying burden on the cluster heads and decrease the end-to- end bottleneck that can exist along transmission links from source to sink which therefore severely limits the network capacity.