Routing protocols in WSN 1.1 WSN Routing Scheme Data collected by sensor nodes in a WSN is typically propagated toward a base station (gateway) that links the WSN with other networks where the data can be visualized and analyzed. In small sensor networks where sensor nodes and a gateway are in close proximity, direct (single-hop) communication between all sensor nodes and the gateway may be feasible. However, most WSN applications require large numbers of sensor nodes that cover large areas, necessitating an indirect (multi-hop) communication approach. That is, sensor nodes must not only generate and disseminate their own information, but also serve as relays or forwarding nodes for other sensor nodes. The process of establishing paths from a source to a sink (e.g., a gateway device) across one or more relays is called routing and is a key responsibility of the network layer of the communication protocol stack. A. When the nodes of a WSN are deployed in a deterministic manner (i.e., they are placed at certain predetermined locations), communication between them and the gateway can occur using predetermined routes. B. When the nodes are deployed in a randomized fashion (i.e., they are scattered into an environment randomly), the resulting topologies are nonuniform and unpredictable. In this case, it is essential for these nodes to do the following:- - self-organize, that is, they must cooperate to determine their positions - Identify their neighbors, and discover paths to the gateway device. In the following figure (1): it shows the single and multi-hop routing model Page 1
Figure (1): Single-hop routing model (left) versus multi-hop routing model (right). This design of a routing protocol is challenging due to the unique characteristics of WSNs, including resource scarcity or the unreliability of the wireless medium. For example, the limited processing, storage, bandwidth, and energy capacities require routing solutions that are lightweight, while the frequent dynamic changes in a WSN (e.g., topology changes due to node failures) require routing solutions that are adaptive and flexible. 1.2 WSN Routing Classification There are various ways to classify routing protocols. Figure (2) presents three different classifications based on the network structure or organization, the route discovery process, and the protocol operation Page 2
A. Network Organization Protocol Most routing protocols fit into one of three classes. Flat-based routing protocols consider all nodes of equal functionality or role. hierarchical-based routing protocols, different nodes may assume different roles in the routing process, that is, some nodes may forward data on behalf of others, while other nodes only generate and propagate their own sensor data. Location-based routing protocols rely on the location information from nodes to make routing decisions. B. Route Discovery Protocol It can used for route discovery and to distinguish between different types of routing protocols. Reactive protocols discover routes on-demand that is, whenever a source wants to send data to a receiver and does not already have a route established. A reactive route discovery incurs ( تعاني ) delays before actual data transmission can occur, proactive routing protocols establish routes before they are actually needed Proactive Protocols (table driven). This category of protocols is also often described as table-driven, because local forwarding decisions are based on the a) Contents of a routing table that contains a list of destinations, Combined with one or more next-hop neighbors that lead toward these destinations and b) Costs associated with each next hop option. In this type the disadvantages are: routes are established that may never be needed. Further, the time interval between route discovery and actual use of the route can be very large, potentially leading to outdated routes (e.g., a link along the route may have broken in the meantime). Hybrid protocol: it exhibits characteristics of both reactive and proactive protocols. Page 3
c) Protocol Operation Negotiation-based protocols aim to reduce redundant data transmissions by relying on the exchange of negotiation messages between neighboring sensor nodes before actual data transfers occur. Multipath-based protocols use multiple routes simultaneously to achieve higher performance or fault tolerance. Query-based routing protocols are receiver-initiated, that is, sensor nodes send data in response to queries issued by the destination node. QoS-based routing protocols is to satisfy a certain Quality of- Service (QoS) metric (or a combination of multiple metrics), such as low latency, low energy consumption, or low packet loss. Coherent-based protocols perform only a minimum amount of processing (e.g., eliminating duplicates, time-stamping) before sensor data is sent to receivers and data aggregators. However, in noncoherent-based protocols, nodes may perform significant local processing of the raw data before it is sent to other nodes for further processing. 1.3 Routing Metrics Routing is considered node-centric when sensor data is explicitly sent to one or more receivers. Most routing protocols focus on unicast routing, that is, forwarding of sensor data to exactly one receiver. Multicast and broadcast routing approaches, on the other hand, disseminate data to multiple or all nodes, respectively. Data-centric routing is used when nodes are not explicitly addressed, but instead receivers are implicitly described by certain attributes. For example, a query issued by the gateway device may request temperature readings and only sensors that can collect such information respond to the query. Sensor networks can vary widely in scale, the geographic areas they cover, and their position-awareness. Global addressing schemes (such as IP addresses used on the Internet) may be unavailable and even nonfeasible, particularly in networks with heterogeneous nodes and node mobility. Routing metrics are used to express a variety of objectives of a routing protocol with respect to the consumption of these resources or the Page 4
performance an application perceives. This section provides a brief overview of commonly used routing metrics in WSNs. 1.3.1 Minimum Hop The most common metric used in routing protocols is minimum hop (or shortest hop), that is, the routing protocol attempts to find the path from the sender to the destination that requires the smallest number of relay nodes (hops) in figure (3). In this simple technique, every link has the same cost and a minimum-hop routing protocol selects the path that minimizes the total cost of data propagation from source to destination to reduce end-to-end delay. Figure 3: Choosing of optimal path For example, The format of the setup packet is defined as Setup.ID.hop.energylevel, where the Setup indicates that it is a setup packet and should be flooded throughout the entire network, ID is the ID number of the sending node, hop represents the hop count between the sending node and the sink node, and energylevel is the battery energy level of the sending node. For the setup packet broadcasted by sink node, the hop count value is set to 0 and the energylevel value is set to a very large value in figure (4). Page 5
Figure 4: Single and Multi-hop in WSN s 1.3.2 Minimum energy consumed per packet: The goal is to minimize the total amount of energy expended for the propagation of a single packet from the source to the destination. The total energy is then the sum of the energy consumed by each node along a route for receiving and transmitting the packet. Figure (5) shows an example of a small sensor network, where a source node wishes to transmit a packet to a destination node using a route that minimizes the packet s energy overheads. The number on each link indicates the cost of propagating the packet over this link. As a consequence, the packet will travel via nodes A D G (with a total cost of 5). Note that this is different from the minimum-hop route (B G). Figure (5): Comparison of routing choices using different energy metrics. 1.3.3 Minimum variance in node power levels: All nodes within the network are considered equally important and the challenge is to distribute the energy consumption across all nodes in Page 6
the network as equally as possible. The goal of such an approach could be to maximize the lifetime of the entire network. 1.3.4 Maximum (average) energy capacity: The focus in this manner is the energy capacity (i.e., the current battery charge level) of the nodes. A routing protocol that uses this metric would then favor routes that have the largest total energy capacity from source to destination. In Figure 5, the numbers in parentheses below the nodes indicate the nodes remaining energy capacity. In this example, a routing protocol could select path C E G, which has the largest total capacity (i.e., 8). A routing protocol that uses this metric must be carefully designed to avoid the pitfall of choosing unnecessarily long routes in order to maximize the total energy capacity. 1.3.5 Maximum minimum energy capacity: The primary routing goal could be to select the path with the largest minimum energy capacity. This technique also favors routes with larger energy reserves, but also protects low-capacity nodes from premature expiration. In Figure 5, a protocol using this metric would choose )مبكرا( B G, since the minimum capacity along this route is 2, which is larger than the minimum capacities of all other possible routes. Question#1 For the network topology shown in Figure 6, identify the optimal routes for source A to sink M according to the following criteria (describe how you compute the cost for the optimal route). The numbers X/Y along each link indicate the latency (X) and energy cost (Y) for transmitting a single packet over the link. The number Z under each node indicates the node s remaining energy capacity. (a) Minimum number of hops (b) Minimum energy consumed per packet (c) Maximum average energy capacity (eliminate hops that would result in a higher average but unnecessarily add to the route length!) (d) Maximum minimum energy capacity Page 7
Figure 6: Topology of Question 1 Page 8