Wireless Sensor Networks : Technology, Protocols, and Applications

TRANSPORT PROTOCOL DESIGN ISSUES 235
their energy sooner). This may also result in a disconnect between more
distant nodes and the sink.
As a connectionless transport control protocol, UDP is also not suitable for
WSNs. Here are some reasons:
Because of the lack of flow and congestion control mechanisms in UDP,
datagram loss can result in congestion. From this point of view, UDP is also
not energy efficient for WSNs.
UDP contains no ACK mechanism; therefore, the lost datagrams can be
recovered only by lower or upper layers, including the application layer.
7.2 TRANSPORT PROTOCOL DESIGN ISSUES
WSNs should be designed with an eye to energy conservation, congestion control,
reliability in data dissemination, security, and management. These issues often
involve one or several layers of the hierarchical protocol, and can be studied either
separately in each layer or collaboratively in cross layers. For example, congestion
control may involve only the transport layer, but energy conservation may be
related to the physical, data link, network, and perhaps all other high layers. Generally,
transport control protocols’ design include two main functions: congestion
control and loss recovery. For congestion control, one needs to detect the onset
of congestion and to determine when and where it has occurred. Congestion can
be detected, for example, by monitoring node buffer occupancy or link load
(such as wireless channel). In the traditional Internet, methods to control congestion
include selective packet dropping at a congestion point, such as is used in active
queue management (AQM) schemes, rate adjustment at the source node, such as
the technique of additive increase multiplicative decrease (AIMD) in TCP, and
the use of routing techniques. For WSNs, one should consider carefully how to
detect congestion and how to overcome it, because sensors have limited resources.
These protocols must consider simplicity and scalability, to save energy, and ways
to prolong the life of sensor batteries. For example, one may use an end-to-end
mechanism such as that utilized in TCP or hop-by-hop backpressure such as that
implemented in the asynchronous transfer mode (ATM) or frame relay networks.
End-to-end approaches are very simple and robust, but they can result in additional
traffic in the networks. However, hop-by-hop approaches usually detect congestion
quickly, and as a result, introduce less additional network traffic. Due to energy
constraint at the sensors, there is a clear trade-off between end-to-end and hopby-hop
mechanisms which should be considered carefully when designing congestion
control algorithms for WSNs.
Packet loss in wireless sensor networks is usually due to the quality of the
wireless channel, sensor failure, and/or congestion. WSNs must guarantee certain
reliability at the packet or application level through loss recovery, in order to relay