Power Saving Scheme

Oh and Han [32] observed a demand-based slot assignment (DSA) algorithm to allocate time slots based on the bandwidth demand of each node in a tree based network topology during each data collection round. They proposed DSA to allocate a sequence of receiving slots and a sequence of sending slots to each sensor node, which significantly reduced the power consumption of nodes at lower depths of the network tree topology. Based on that approach, a cross-layer energy efficient scheme which results in a loosely coupled style without creating any extra interfaces could be more likely to collaborate between routing demands and frame slots to achieve energy conservation in sink-oriented wireless sensor networks. In this research, we are interested in deploying an adapted proactive routing protocol (INTRA-CEEP) in the intensity region and an adapted reactive routing protocol (INTER-CEEP) cross-layered with cooperative pattern MAC in the far-site region of the sink-oriented WSN. Considering either the static duty cycle or traditional CSMA schemes (Carrier Sense Multiple Access) may result in energy wastage under light loads, it seems so natural that a dynamic duty cycle scheme is desperately needed to avoid such situation in the power-short networking environment. The essential of power savings for CEEP is to implement a source routing protocol (e.g., AODV, DSR), which route table cache or query packets may record the IDs of the intermediate nodes it has traversed during the route discovery phase [18], at the sensor nodes in the far-site region of the network. Once the destination is reached, the reply packet (RREP) then copies in its header the entire route sequence retrieved from the request packet (RREQ) to respond

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to the source node (i.e., DSR), as illustrated in Figure 4-2(a) (b).

Figure 4-2. INTER-CERP implements a source routing scheme to discover the route path

1 6 3 7 2 5 4

(b) Creation of the routing reply RREP Source Destination 1-2-5-7 1-2-5-7 1-2-5-7 1 6 3 7 2 5 4

(a) Creation of the routing request RREQ Source Destination 1 1-2 1-2 1-2-5 1-2-5 1-2-3 1-2-5-6

Among the sensor MACs, pattern sensor MAC (e.g., SMAC, PMAC) has been investigated as a cooperative scheme which tries to save more power without yielding on the throughput. The sleep-wakeup pattern cyclically repeated during the main time frame is a reserved plan for a sensor node to turn off its transceiver in a time slot when the pattern bit is 0, while bit 1 indicates the node intends to stay awake during the time slot [49]. The new patterns generated at the end of each main time frame on neighbouring nodes are exchanged to each other during an amount of time, called pattern exchange time frame. Based on comparing with the receiving node’s pattern status, a sensor node can simultaneously adjust its tentative sleep-wakeup pattern onto a transmitting schedule. The observers of dynamic sensor MACs tends to increase the

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number of 0 bits in a pattern during light traffic, whereas the pattern goes back to 1 if it has data to send at any time. In this research, we’d like to adapt this kind of cooperative pattern scheme cross-layered with the source routing information, which is retrieved from route cache or query packets in different topological cases at the network layer during route discovery. In Figure 4-3(a), node i forwards a RREP packet to node h via node j, while node k forwards another RREP packet to node g via node j. In this case, node j has to create its wakeup pattern to receive any data from node h or node g at the most appropriate time. In Figure 4-3(b), node i forwards a RREP packet to node h via node j, while node h forwards another RREP packet to node i via node j. In this case, node j also has to create its wakeup pattern to receive any data from node h or node i at the most appropriate time. In Figure 4-3(c), node i forwards multiple RREP packets from different sources at different time to node h via node j. In this case, node j also has to create its wakeup pattern to receive any data from node h at the earliest time.

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Figure 4-3. Illustrations of combining different wakeup patterns initiated by RREP packets on node j

j

i

k

g

h

(a) A cross case

i

j

h

j

i

h

By perceiving the least distance (hops_to_source * 2) to receive a data packet, an INTER_CEEP node only needs to wakeup after that time period based on a demanded slot assignment in a sink-oriented WSN. In other words, a sensor node can turn off its transceiver during the least distance of time slots without compromising on the throughput. However, by considering the transmission interferences among the adjacent nodes, a combination operation of the crossed wakeup patterns is illustrated as the pseudocoded algorithms in Table 4-2.

Table 4-2. Pseudocoded algorithms to overlap different wakeup patterns on node j --- I. In process of receiving an incoming RREP packet at network layer of node j

1. receiving PACKET from MAC layer ; 2. if (node j is not destination) then

34 4. retrieve DISTANCE to the source node ;

5. send DISTANCE down to MAC ;

6. end if

7. forward PACKET ; 8. end if

--- II. In process of forwarding an outgoing RREP packet at MAC layer of node j

1. receiving PACKET from network layer ;

2. calculating the least index of slots needed to receive data packet ; 3. if (it appears within current pattern repeat time frame) then 4. wakeup all slots after it ;

5. else if (it appears within next pattern repeat time frame) then 6. wakeup all slots after it ;

7. end if

8. forward PACKET ;

--- Thereafter, it has been a significant objective to evaluate the impact of this networking performance with cross-layer schemes implemented at sensor nodes in the following section of simulations.