5.2 I-Header transmission reduction in static
5.2 I-Header transmission reduction in static wireless sensor networks 89
vi is the ”bottleneck” which bridges two sub-networks. Sensors in each of the two sub-networks have aggregated messages with the same I-Header. Therefore, before the wake-ups of the bottleneck sensor initiating the communications, only I-Headers are exchanged among sensors, and no messages are communicated.
vi
vl1
vl2
vl3
vr1
vr2
vr3
vr4
Figure 5.2. Demonstration of large I-Header communications with two sub-networks
In the two sub-networks, I-Headers are exchanged because sensors need to continuously check if there are new data in the messages of the neighbor sensors even though no new data has been aggregated.
Depending on how a sensor vi aggregates a new measurement data, two types of com-munications are categorized in order to control the behavior of the sensor vi differently in the considered algorithm.
• In a type-one communication, sensor vi wakes up and initiates the communi-cations with its neighbor sensors, i.e., sensor vi communicates with its neighbor sensors as a center. Sensorvi may aggregate new data by receiving messages from its neighbor sensors. A message broadcast is done later by vi. Therefore, any new data aggregated by the vi will eventually be aggregated by all its neighbor sensors.
Sensorvi performs twice I-Header communications during a wake-up in the type-one communication. In the first time, it broadcasts the I-Header to all its neighbor sensors to inform all its neighbor sensors the measurement data it has aggregated in the message. In the second time, vi broadcasts its I-Header after it aggregates data from the messages transmitted by its neighbor sensors.
• In a type-two communication, sensor vi is waked up as a neighbor sensor. In this type, vi may aggregate new data by receiving the broadcast message from
the center. Therefore, only the center sensor, which is one neighbor sensor of vi, knows the I-Header ofvi.
Sensor vi performs at most only once the I-Header transmission during a wake-up in a type-two communication. After vi received the I-header from the center sensor, it sends its I-Header to the center if it has aggregated new data to the center sensor.
Since the neighbor sensors of a sensor do not change in a static wireless sensor network, the sensor can use its buffer to store the I-Headers of its neighbor sensors which have been received from a previous I-Header communication. The stored I-Headers can be reused at the sensor when it communicates with its neighbor sensors, and its neighbor sensors have not aggregated any new data since their last communications.
With the categorization of the two types of communications and the capability of a sensor storing I-Headers of its neighbor sensors, I-Header communications can be reduced in static wireless sensor networks for both types of communications in three aspects.
• In the random gossiping, the center sensor (it is in a type-one communication) wakes up and initiates communications with its neighbor sensor. The first broad-casting of the I-Header of the center can be avoided in its type-one communica-tions if a sensor has not aggregated any new data since its last type-one commu-nications. In terms of avoiding an I-Header communication, neighbor sensors use the I-Header received in the last I-Header broadcasting of the center from their buffer.
• Neighbor sensors which are involved in type-two communications receive or re-cover the I-Header from the center. By comparison with the I-Header of their own messages, I-Header transmission from the neighbor sensors to the center can be avoided if their messages contain no new data to the center.
• After the center sensor aggregating the new data from the messages of the neigh-bor sensors, the center sensor can decide whether an I-Header broadcast is nec-essary by comparing its new I-Header and the I-Headers of its neighbor sensors.
If the new I-Header at the center sensor contains no new data to the neighbor sensors, the transmission is omitted by the center sensor.
In all three cases, communications of I-Headers can be avoided. However, a message indicating that I-Headers are to be transmitted or to be recovered still need to be exchanged. This message exchange can be realized mainly in two ways:
5.2 I-Header transmission reduction in static wireless sensor networks 91
• to transmit a 1-bit message telling the receiver that the I-Header shall be covered or will be transmitted, or
• to build an agreement between the transmitter and the receiver that at the par-ticular phase of the random gossiping, if the receiver does not receive any message in a given period of time a recover of I-Header is performed at the receiver.
The random gossiping algorithm with the reduction of I-Header communications in static wireless sensor networks is given in Algorithm 10.
Algorithm 10Random gossiping algorithm with I-Header reduction
1: vi initiates communications with its neighbor sensors in Ni;
2: vi informs sensors in Ni whether it has new data updated in the data set of Si of vi since last type-one communication;
3: if there is new data updated in the data set ofSi ofvi since last type one commu-nication then
4: vi broadcasts its I-HeaderIi to all its neighbor sensors inNi;
5: else
6: Sensors in Ni recover I-Header Ii that is received from the previous type-one communication initiated by vi;
7: end if
8: for sensor vj in neighbor sensors Ni do
9: if message ofvj contains data that is new to vi then
10: vj feed backs its Ij to vi;
11: else
12: if message of vi contains data that is new to vj then
13: vj informs vi that it requires message communication from vi;
14: else
15: vj transmits nothing;
16: end if
17: end if
18: end for
19: vi processes all feedbacks;
20: vi informs sensors inNi which sensors need to send their messages;
21: All informed neighbor sensors transmit their messages;
22: vi computes the new aggregated data;
23: if new aggregated data at vi contains new data for sensors in Ni then
24: vi broadcasts the new I-HeaderIi;
25: vi broadcasts the new message;
26: end if
When all the sensors in the static wireless sensor networks have the same I-Header, the network is converged.