Needless to say, routing in wireless sensor networks (WSNs) is different from routing in conventional wired networks. It is also different from routing in WLANs and MANETs due to several unique characteristics of WSNs. This post focuses on the unique characteristics of WSN that affects the design of routing protocols.
Sensor nodes are inherently constrained in terms of processing power, power supply, memory and storage capacity, all of which require careful planning and management.
It is impossible to use a global addressing scheme for mobile nodes because WSN may involve the deployment of a large number of nodes — the overhead cost of maintaining the ID would be very high. Therefore, the traditional IP-based protocols do not apply to WSNs.
Unlike typical communication networks, most sensor applications require the forwarding and aggregation of sensed data from multiple sources to a base station or a central coordinator.
This work analyzes the effect of rate-based pacing over TCP (i.e. Reno and Vegas) with delayed acknowledgement strategy.
TCP acknowledgement (ACK) packets must travel upstream to the TCP source against the downstream flow of TCP data packets. Correlated arrivals of TCP data and ACK packets lead to contention for the wireless channel, which can cause collisions and even packet losses. These problems arise even for a single TCP flow on a multi-hop wireless network. As a result, throughput degrades as the length of the multi-hop path increases.
The TCP data sender transmits data to the TCP data receivers and the receiver transmits ACKs back to the sender as segments arrive. The sender would retransmit any segments that are not acknowledged by the receiver and this provides reliability. Initially, a TCP receiver generates an ACK for every incoming segment. These ACKs are cumulative and acknowledge all in-order segments that have arrived at the receiver. If a receiver has received an out-of-order segment, an ACK is transmitted. However, it will not acknowledge the incoming segment, but rather a duplicate ACK for the last in-order segment is generated.
In an optional delayed acknowledgement strategy, delayed ACKs allow a receiver to refrain from transmitting an ACK for every incoming segment. In the case of rate-based pacing, pacing is a hybrid between pure rate control and TCP’s use of acknowledgments to trigger new data to be sent into the network. This pace or rate should be based on a fraction of prior estimates of data transfer rate, since that is the closest estimate of available bandwidth, which is called the rate-based pacing. Using pacing can be avoided starting TCP slow start after a packet loss or when an idle connection resumes.
For wireless ad hoc networks, the issue of routing packets between any pair of nodes becomes a challenging task because the nodes can move randomly within the network. A path that is considered optimal at a given point in time might not work at all a few moments later. Traditional routing protocols, such as DSDV, are proactive in that they maintain routes to all nodes. They react to any change in the topology even if no traffic is affected by the change and they require periodic control messages to maintain routes to every node in the network. As mobility increases, more of scarce resources, such as bandwidth and power, is used. Alternative reactive routing protocols, such as DSR and AODV, determine when routes are needed to route packets to to prevent nodes from updating every possible route in the network.
On the other hand, the role of TCP is important to transport data packets. The TCP protocol provides reliability, flow control, congestion avoidance, fairness, and in-order delivery. Originally, the protocol did not have congestion avoidance, causing the networks to become overloaded. TCP Tahoe introduced congestion avoidance, where dropped packets are used as an indication of congestion, and slow start, where the initial window size keeps doubling until congestion is detected. Later enhancements, such as TCP-Reno, New Reno, Vegas and Westwood, perform differently in wireless environments.
The nature of how mobile devices work gives rise to privacy concerns because it can be used as a tracking device to identify your location (see this post). RFID tags, the size of a grain of sand, have been used to tag items that we buy when we go shopping, e.g. for groceries or clothing. The use of this technology will become more widespread.
While the use of RFID tags make it easier for suppliers and retailers to track their inventories, it raises privacy concerns because if it is not disabled, the tag can be used to track the user’s movements. The video below explains this issue.
Among possible application for sensor networks are emergency response, medical monitoring, logistics and inventory management, and battlefield management. For certain applications, broadcasting unprotected data over the wireless medium might pose serious security concerns, especially for life-critical systems.
Implementing a security protocol in wireless sensor networks poses a set of unique challenges not found in wired networks. Compared to desktop computers, sensor nodes have limited processing power, storage, bandwidth and energy. The requirements for WSN security are:
