DECADENCIA DEL PRIORATO DE MAGACELA

The proactive routing component of C2HR, is similar to OLSR [70], in that each node builds a routing table based on a topological view of the network. Each node maintains a next hop entry to each desti- nation as well as the associated total cost of the path, generated using Dijkstra’s shortest path algorithm [52]. Since the algorithm is designed to suit low power personal devices, links are selected in a way that minimises energy consumption while also minimising the number of hops. In wireless communi- cations, however, sending data through a least-hop path, implies an increase in the transmission range, which in turn, means an increase in power consumption [53]. On the other hand, relying on longer paths may deteriorate the quality of service. There is, therefore, a trade-off between energy conservation and acceptable path lengths that a packet should follow. The length of a path considered to be “acceptable” differs from one QoS class to another; this information can be extracted from the application and used to set a threshold for the maximum number of hops that a packet is allowed to traverse from source to destination. The idea being to use suboptimal routes in order to save energy and prolong the lifetime of the network, while ensuring the delay introduced by multi-hop routing remains at a tolerable level.

7.3.1.1

Link Costs

The cost of the link between node i and node j (denoted as Cij) indicates the energy required to send

the data via that link. This metric depends on a number of parameters, namely transmission energy (Etxi), residual energy (Eri) and channel bit error rate (berij). Etxi denotes the energy required by node i to transmit a single data unit, which is a function of the distance between i and j; Eri indicates

the battery power remaining at i, and berijgives an indication of the number of times a packet needs to

be retransmitted in order to ensure it is successfully received. Sending the same packet r times requires

required to forward data on the link ij.

As noted in [33], the energy metric varies with time and as nodes become more involved in network activities, their residual energy decreases. When all nodes have a high level of residual energy, it is preferable to route via the paths which incur the least transmission energy. However, when the battery power of some nodes falls below a critical level, it is advisable not to include them in the routing path. This achieved by increasing the link cost associated with sending data through those nodes. A link cost function that satisfies this paradigm is expressed as:

Cij = a.Etxαib.E

θ

ri+ φ.ber (7.1)

where α and θ are binary factors, and a, b and φ are normalising factors. If α is 0 and the channel’s bit error rate is negligible, the deciding factor would be the residual energy. Similarly, if θ is 0, then routes are chosen so that transmission energy is minimised.

All of the required parameters for computing the link cost are readily available to devices from

local measurements. Etxis extracted from the transmit power level set through LMP, Eris obtained by

monitoring battery level, and the bit error rates can be measured as described in Chapter 4

7.3.1.2

Topology View Generation

When nodes have no knowledge of the network topology, the leader builds a tree rooted at itself by send- ing a topology query message, to each node in the connected component, which is relayed from parent to child until it reaches the leaves in a tree expansion process. If a node receives a duplicate query message from another parent, it signals that it is already connected to a parent so that it is removed from the list of children, thus ensuring a node only has a single parent. When the message reaches a leaf node, the latter generates a query reply packet containing, in addition to its address and residual energy, its adjacency information table, which includes the node’s one-hop neighbours, together with their associated transmit powers and bit error rates. Table 7.1 gives an example of the adjacency information table stored at node 4 in Figure 5.1(a).

Neighbour Count Address Etx ber

3

8 E_tx4,8 ber4,8

9 E_tx4,9 ber4,9

10 E_tx4,10 ber4,10

Table 7.1: Adjacency Information Table

Each non leaf node receiving the topology reply message forwards it up the tree, after appending its information, until the message returns to the leader. In order to ensure that the topology query message reaches every node in the connected component, a node does not send the topology reply packet unless it is either a leaf node or it receives a reply from all of its children within a pre-defined timeout. The

format of the topology query and topology reply packets are shown in Figure 7.2 and 7.3 respectively.

Figure 7.2: Topology Query Packet Format

The different fields making up the topology query message are described below:

• T: This field identifies the type of topology view generation message. In the case of topology query, it is set to 0.

• Leader Address: This is the 48-bit BD ADDR of the leader.

• Sequence Number: The sequence number is a monotonically increasing variable that is incre- mented by 1 each time the leader initiates a topology query message.

The pair <Leader Address, Sequence Number> uniquely identifies a topology query message, such that if the leader’s identity changes, the new leader can initiate its own topology view tree if required.

Figure 7.3: Topology Response Message Format

Figure 7.3 indicates that the topology reply message carries the entire topology information of the network, which is relayed from child to parent until it reaches the root, i.e., the leader. As the number of nodes increases, so does the size of the reply packet, which means that for large networks, the topology reply message needs to be transported in several baseband packets. This method incurs the same overhead as relying on individual replies; however, nodes do not need to continuously relay the

information, therefore nodes can enter one of the power saving modes while waiting for the message to reach them, thereby saving energy.

Once the leader gathers the necessary information from all the nodes, it builds an N × N connec- tivity matrix, where N is the number of nodes, whose (i, j) entry represents the link cost between i and j as described by Equation 7.1, then distributes this matrix along the tree so that nodes can generate their routing tables using Dijkstra’s shortest path algorithm.

7.3.1.3

Topology Maintenance

In an ad hoc network, energy and channel conditions vary with time, meaning that link costs also vary with time. Nodes need, therefore, to update the leader with these variations so that all routes are up-to-

date. For this purpose, the leader generates a topology query message every Tutime slots and repeats the

tree expansion/shrinking process as described above. Furthermore, in order to maintain a proximity view, each node exchanges hello beacons with its immediate neighbours at regular intervals. In addition to signalling their presence, nodes use the hello messages to convey other information such as a change in residual energy or the estimated channel error rates. These packets are transported as DM1 packets and include the address of the originator as well as parameter updates. In Bluetooth PANs, the beacons are exchanged when the master polls its slaves or the updates can be obtained directly from the low power mode beaconing mechanism, both of which can be easily implemented in LMP. When a node fails to hear the beacon from one of its adjacent nodes after a predefined timeout, it assumes that the link is lost and informs the leader so that the connectivity matrix can be updated and the updates redistributed. If topological changes occur while data is being routed, the detecting node informs the leader and the source node, so that the latter can look for an alternative route by initiating a route search procedure.