Power-Efficient Routing in Wireless Sensor Networks - Networked Embedded Systems - page 146

it that it has joined the cluster. Setup and data transmission phases are analogous to LEACH, i.e.,
cluster heads calculate and communicate a TDMA schedule to all the nodes in the same cluster, so
data transmission by each node can only occur in the assigned time slot. he only difference is that,
besides data, nodes also transmit their current energy values to the cluster head. In this way, each
cluster head knows which node has the highest energy inside its cluster and when the round ends
this node can be directly elected as the cluster head for the next round. After the Data Transmission
phase MECH introduces a Forwarding phase, in which a hierarchical relationship among clusters is
defined. Thanks to this hierarchical relationship multi-hop data forwarding from a cluster head to
the sink is now possible. LEACH, on the contrary, always uses direct transmission from cluster heads
to the sink. he Forwarding phase uses two parameters to create the hierarchy: hop count from the
sink node and energy. he phase is started by the sink node, which periodically broadcasts a message
to all the cluster heads. Each cluster head updates the hop count and the minimum energy of the
path, both contained in the message, and rebroadcasts the packet. At the same time, each cluster
head maintains the ID of the best node for forwarding data towards the sink, that is, the node with
the lowest hop count or, if several nodes have the lowest hop count, the one with the highest energy.
Simulations show that MECH improves on LEACH in terms of network lifetime thanks to the
more balanced cluster setup, although it introduces more overhead. hanks to the multi-hop forward-
ing, scalability for large-area WSNs is also improved. However, no QoS is addressed for multi-hop
transmissions.

7.5.2.4 Threshold-Sensitive Energy-Efficient Sensor Network (TEEN) Protocol

The TEEN [Man] is a hierarchical protocol based on LEACH. TEEN is also the first routing pro-
tocol explicitly targeted at reactive WSNs, where nodes sense their environment continuously, but
data is not periodically transmitted, as only significant variations in the values of sensed parame-
ters are reported. TEEN defines two thresholds, the hard threshold ( H T .

These values determine when sensor data needs to be transmitted. More specifically, when the sensed
value exceeds its H T , the node switches on its transmitter and sends the sensed data and stores
thevalue.hen,aslongasthesensedvalueremainsgreaterthan H T ,thenodewillretransmitthe
sensed data and store its value every time the sensed value differs from the stored one by at least
S T . In this way, the number of data transmissions is greatly reduced, further improving both energy
consumption and responsiveness.
TEEN also extends LEACH in terms of network architecture, providing hierarchical clustering. In
LEACH and MECH nodes are grouped into two categories, cluster-head nodes and non-cluster-head
nodes. In LEACH cluster heads communicate directly with the BS, while in MECH they can operate
multi-hop transmissions based on hop count and energy. However, in both protocols only one-level
cluster heads are defined. TEEN, on the other hand, uses a hierarchical clustering scheme, as depicted
in Figure .. Cluster nodes transmit data to their first-level cluster heads. hese in turn forward data
to the next-level cluster heads (e.g., second-level cluster heads), and so on, until the uppermost-level
cluster heads, which transmit directly to the sink node, are reached.
As cluster formation and transmission is based on LEACH, but there are fewer data transmissions,
TEEN provides more energy saving than LEACH. However, it only works for reactive networks, so it
is not applicable when data has to be continuously updated. he problem of long distances between
cluster heads and the sink is only partially resolved with hierarchical clustering. In fact, multi-hop
forwarding is always toward higher-level cluster heads. By increasing the levels in the hierarchy, nodes
have to cover a larger area and no guarantee is given that the distance from the sink is reduced. So
the problem is not solved but only moved to the highest-level cluster heads. Hence, the scalability of
TEEN in terms of geographical dimensions is low, as it is limited by the transmission range of the
(highest-level cluster-head) nodes.

) and the soft threshold ( S T )