An Overview of Wireless Sensor Networks

Sensor networks are basically distributed networks composed of tiny sensing devices which come equipped with a microprocessor chip, memory and enabled with a low-cost/short-range WIFI communication. These are ideally different from standard networks in that wireless sensor networks have major energy constraints, low data transmission rates and information flows that range widely [21-23]. In normal circumstances, sensor networks involve a large number of distributed nodes with self organizing capabilities which translate into a multi-hop WIFI network. This multi-hop ability enables enhanced energy efficiency within the network. It is possible to deploy WSN with multiple unattended nodes such that the core network architecture poses a significant challenge in WSN research studies [2, 24]. At present, the novel field of WSN research studies incorporates a number of diverse disciplines posing complex challenges to modern computer science, wireless communication and mobile computing [25-28].

It is a commonly accepted fact that wireless mediums suffer higher bit error rate (BER) in contrast to wired mediums [29]. It is important not to assume the topology between nodes in wireless networks is fully connected but as a network with a logical topology which constantly changes depending on node or user movement from one point to another. Majority of the studies carried out on WSNs relate to finding solutions to problems consistent with wireless communication. Present research on WSNs is dedicated towards finding ways with which to improve on the Quality of Service (QoS) for multimedia applications [30, 31]. It is therefore important for one to stay well informed on current WSN research as well as other issues unique to WSNs and similarly improvised networks. The principal performance objectives for WSNs are energy efficiency and throughput neutralization with temporary delay and fairness being considered as secondary objectives [32, 21]. Standard wireless mediums share sensor nodes which necessitates the need for efficient medium access control (MAC) operations without isolating changes in topology, multi-hop communication, power saving, throughput upgrading and network density.

The IEEE 802.11 standards for wireless communication are widely used though a number of research studies have shown dilapidated performance when this standard is applied in distributed network set-ups [33, 34]. Studies have revealed IEEE 802.11 MAC protocols fail to perform as expected in these environments with regard to energy efficiency, as well as other performance parameters such as high density topology and throughput. Current research findings have inspired future research to focus on MAC schematics suitable for WSNs operating in multi-hop modes with no excesses in performance dilapidations. One cannot however, underscore the phenomenal achievements of wireless local area networks (WLAN) based on the IEEE 802.11 standard in the recent past. This robustly established standard has dominated the wireless network devices market more so due to its comparatively low costs. It is envisaged that future standards will coexist with 802.11 networks as well as other technologies [33]. Newer standards like the IEEE 802.11 (Bluetooth) and IEEE 802.15.4 (ZigBee) are at present gaining increased attention in wireless sensor applications development [14, 35].

The prevalence of energy resource inefficiencies in WSN communication systems are projected to pose significant challenges in the development effective WSN communication system in years to come. It is expected that emergent wireless sensor networks will have improved energy efficiency capabilities and reliable connectivity given that contemporary devices are becoming more compact. The ever-increasing density of wireless devices will tend to pose further constraints on connectivity and battery performance. To limit nodal power losses, system components have to be optimized and compatible with efficient protocols without limiting overall network throughput [36, 37]. Research studies continue to focus on networking abilities and the maximization of network lifetime powered by irreplaceable batteries. Though energy efficiency has dominated WSN research endeavors, there is considerable interest in real-time applications relative to imaging and video sensors. This poses further challenges relative to explicit QoS requirements which include network density, end to end delay and throughput [38].

Literature on Transmit Power Control (TPC) protocol  

TPC is a commonly used energy conservation method meant to optimize the performance of wireless networks. For multi-hop WSNs, TPC operability has been challenging attracting a lot of research in search for answers to this effect. A transmitter applies TPC in an attempt to utilize the least possible power necessary to transmit a signal to its destination and unintentionally leads to reductions in interference. TPC therefore not only conserves power but also reduces nodal interference prolonging network lifespan and throughput. Previous research proposes that in densely deployed wireless sensor networks TPC is expected to manage power consumption [39, 41]. Therefore, it is possible to utilize the least possible power while maintaining throughput thus reducing energy loss and prolonging the lifespan of wireless sensor nodes [42-77].

Prior to embarking on an in depth study of TPC protocols it is necessary to briefly describe contemporary MAN protocols and similarly relevant terminologies:

• The Lightweight medium access protocol (L_MAC) [42]: This is a TDMA schematic for wireless sensor networks and is a modification of Eyes MAC (E-MAC). Each sensor node in L-MAC selects only one time slot through sequential slot reserving from neighboring single-hops. Each node chooses a timeslots plied in the transmission of control frames. L_MAC utilizes the TDMA protocol and notifies neighboring nodes control frame transmissions. The L-MAC is fundamentally applied towards the reduction of multi-hop latencies and for improved energy efficiency.
• Berkely-MAC protocol (B-MAC) [43]: this contention based routing protocol manages network functions which include synchronization, routing and organization. It incorporates control frame channel reservations request to send or clear to send (RTS/CTS). This protocol limits collisions through low power listening (LPL), clear channel assessments (CCA), link layer ACK. B-MAC is presently a sensor node default dedicated MAC protocol.
• Sensor-MAC protocol (S-MAC) [42, 44]: utilizes time-slot occupancy slitting a time interval twice, one for the listening time-slot the other for the sleeping time-slot. Periodic sleeping consistent with S-MAC improves power efficiency.
• Time Division Multiple Access (TDMA) [18]: This is a time splitting schematic applied in mediums with shared channels. It serves to allocate splitting capabilities to a transmitter by dividing available frequencies in a number of time slots.
• Carrier Sense Multiple Access (CSMA) [45]: It’s a MAC protocol applying node verification techniques to check medium channel activity prior to transmitting data on a shared frequency channel.
• Low Energy Adaptive Clustering Hierarchy (LEACH) [46]: It is a highly regarded clustering protocol for wireless sensor networks. It utilizes cluster heads (CH) selected at random to periodically change its role based on the level of power drain such that energy loads are evenly shared among all sensor nodes in a network.

TPC in IEEE 802.11 wireless networks

The IEEE 802.11 standard serves to identify two operating types: active/ sleep. When a sensor node has the ability to swap data streams it is in active mode. Energy use in this mode is not so much dependent on the state of operability though it is of a higher value. Conversely, the sleeping mode for sensors allows no relay of data streams and energy use is therefore considerably low [47]. This segment illustrates TPC operability with specific regard to IEEE 802.11 wireless networks.