## 2 Categorization and Classification of Quality of Service Routing Protocols
### Probabilistic Routing Protocols (Hard Real Time)
The MCMP routing algorithm builds the detailed link information for memory, sustainable computation, and overhead for the resource restricted sensor nodes. MCMP uses the local link metrics and distance to estimate the path metric. Local link metrics can help to obtain the network scalability. The goal of MCMP is to employ multiple paths to improve the network performance using moderate energy consumption. However, the protocol always prefers to choose the path that consists of the minimum number of hops to fulfill the essential QoS parameters. As a result, the protocol leads to additional energy consumption. The Probabilistic routing protocol for Heterogeneous sensor networks (ProHet) was introduced in [24], as a probabilistic hard real-time approach that can handle asymmetric links in a dispersed fashion using local information. It uses low overhead with a guaranteed delivery rate. The working process of ProHet consists of two phases: the preparation phase that identifies the neighbor relationships and determines the reverse path for the asymmetric links, and the routing phase that selects the nodes in order to forward the message and send the acknowledgement. ProHet uses bidirectional routing abstraction to determine the reverse path for each asymmetric link. Then, it applies a probabilistic policy to select forwarding nodes based on chronological data using local information. The advantage of this protocol is to reduce energy consumption and to guarantee the delivery rate in wireless hybrid sensor networks. As with the previous QoS protocols, ProHet focuses only on hot-spot and energy consumption, as discussed in [24,25]. ProHet also lacks mobility support and decreases the throughput. The Potential-based Real-Time Routing (PRTR) protocol is a probabilistic hard real-time QoS routing protocol introduced for making basic routing decisions [26]. The PRTR involves multiple potential fields to combine the node and queue length fields using weighing parameters. The objective of this protocol is to reduce the congestion and handle non-delay-sensitive flows to bypass the hotspots that distribute these unnecessary data packets for multipath transmission. The PRTR also utilizes calculus theory to estimate the end-to-end delay bound for a single flow. PRTR provides scalability and is suitable for large-scale WSNs by just using local information. Furthermore, PRTR satisfies the real-time routing requirements and avoid the possible congestion that causes the packet loss. However, PRTR consumes additional energy by alleviating the congestion and packet loss. Cluster-based QoS Aware Routing Protocol (CQARP) is a probabilistic hard real-time QoS routing protocol introduced for cluster-based wireless sensor networks in [27]. This protocol employs a queuing model to tackle the non-real-time and real-time traffic. The protocol focuses on least end-to-end delay, improving the throughput and prolonging the network lifetime. The protocol involves the cost function with each link and applies the K-least cost path algorithm to determine the set of the efficient routing paths. Each path is verified against the end-to-end delay limitations. Once a path satisfies the limitations, it is selected as the path for sending the data to sink node. All the nodes are basically assigned a similar bandwidth ratio, which can create problems because some of the nodes require higher bandwidth. The strength of this protocol is to improve the throughput and prolong the network lifetime. Also, the issue of bandwidth assignment was resolved by using a different bandwidth ratio for each node. However, transmission delay was not considered, and the protocol also lacks mobility support. The working process of the protocol is depicted in Figure 3.