Wireless Sensor Networks : Technology, Protocols, and Applications

234 TRANSPORT CONTROL PROTOCOLS FOR WIRELESS SENSOR NETWORKS
Mobile IP works as follows: When a terminal moves into a new network, it registers
with the FA of the new network and subsequently receives a COA. At this
time, either the terminal or the FA informs the terminal’s HA of the COA. When
a corresponding terminal sends packets to the mobile terminal, those packets are
forwarded to the HA, which will, in turn, forward them to the mobile terminal’s
COA. Packets from the mobile terminal to the corresponding terminal are sent
directly to the corresponding terminal. Therefore, there is an asymmetrical routing
process between the corresponding terminal and the mobile terminal called the triangular
routing, which leads to a longer path from the corresponding terminal to
the mobile and therefore to low efficiency. In the process of mobility, since handoff
results from movement and may cause packet loss and TCP timeout, the TCP sender
is forced to reduce its rate, which may lead to low throughput even though physical
link may offer sufficient bandwidth.
7.1.4 Feasibility of Using TCP or UDP for WSNs
Although TCP and UDP are popular transport protocols and deployed widely in the
Internet, neither may be a good choice for WSNs. For the most part, there is no
interaction between TCP or UDP and the lower-layer protocols. In wireless sensor
networks, the lower layers can provide rich and helpful information to the transport
layer and enhance the badly needed system performance.
Following are other problems that make either TCP or UDP unsuitable for
implementation in WSNs:
TCP is a connection-oriented protocol. However, in WSNs, the number of
sensed data for event-based applications is usually very small. The three-way
handshake process required for TCP is a large overhead for such a small
volume of data.
In TCP, segment loss can potentially trigger window-based flow and congestion
control. This will reduce the transmission rate unnecessarily when, in
fact, packet loss may have occurred as a result of link error and there may be
no congestion. This behavior will lead to low throughput, especially under
multiple wireless hops, which are prevalent in WSNs.
TCP uses an end-to-end process for congestion control. Generally, this results
in longer response to congestion, and in turn, will result in a large amount of
segment loss. The segment loss, in turn, results in energy waste in the
retransmission. Furthermore, a long response time to congestion results in
low throughput and utilization of wireless channels.
TCP uses end-to-end ACK and retransmission when necessary. This will
result in much lower throughput and longer transmission time when RTT is
long, as is the case in most WSNs.
Sensor nodes may be within a different hop count and RTT from the sink. The
TCP operates unfairly in such environments. The sensor nodes near the sink
may receive more opportunities to transmit (which results in them depleting