## 2 Categorization and Classification of Quality of Service Routing Protocols
### Probabilistic Routing Protocols (Soft Real Time)  
The Multi-Path and Multi-SPEED (MMSPEED) Protocol is introduced to guarantee the probabilistic QoS in [28]. The QoS provisioning is done in two domains: reliability and timeliness. The reliability domain supports many reliability requirements using probabilistic multipath forwarding, and the timeliness domain can be achieved by ensuring multiple packet delivery. These mechanisms are realized in a localized manner without using global network information. The local geographic packet forwarding is improved with a dynamic benefit that compensates for local conclusion imprecision as a packet travels to its destination. The key goal of MMSPEED is to guarantee end-to-end requirements with a localized manner that supports the adaptability and scalability for large scale dynamic WSNs. It also ensures QoS differentiation in both timeliness and reliability domains. MMSPEED greatly improves both timeliness and reliability. However, MMSPEED uses greedy forwarding and geographic routing, which may not improve the performance of the network. Also, it is not a good option for long life applications because in such applications the data transmission exceeds the required energy. Figure 3: Queuing model in the cluster-based wireless sensor networks [27]. The Stateless Protocol for real-time communication (SPEED) is introduced to maintain QoS for WSNs, based on soft real-time end-to-end guarantees. SPEED controls the congestion during heavy traffic load [29]. The Stateless Geographic Non-Deterministic forwarding (SNFG) routing module is used in SPEED, which works with a combination of four other modules at the network layer. SNFG maintains traffic delivery speed across a WSN using a two-tier adaptation, including traffic delivery at the networking layer and packet regulation at the MAC layer. SPEED consists of several components including Neighborhood Feedback Loop (NFL), application Programming Interface, backpressure rerouting, delay-estimation scheme, last mile processing, Nondeterministic Geographic Forwarding (NGF) algorithm, and last mile processing. SPEED consumes slightly more energy than other QoS protocols because it delivers more packets under heavy congestion. The strength of SPEED is to reduce end-to-end delay. The energy efficient and QoS aware multi-hop routing protocol (EQSR) maximizes the network lifetime [30]. In EQSR, delayed sensitive traffic is handled and forwarded effectively to the sink node using a service differentiation concept. The goal of EQSR is to improve throughput and to reduce the end-to-end delay by using multiple paths. The protocol uses residual energy, node buffer size, and signal-to-noise-ratio for determining the best next hop. In addition, EQSR uses a data aggregation model to handle the real-time and non-real-time traffic. EQSR uses a path discovery phase that consists of initialization phase, a primary path discovery phase, and an alternative paths discovery phase. The path discovery phase is based on directed diffusion [31]. The sink node uses multiple path discovery in order to determine the set of neighboring nodes, which are able to send the data towards the sink from the source node. Furthermore, EQSR applies a procedure of path refreshment, path selection, traffic allocation, and data transmission for maintaining the QoS and handling the different types of traffic. Message-initiated Constrained-Based Routing (MCBR) maintains the QoS requirements [32]. MCBR is composed of explicit specifications for route constraints, QoS provisioning, and constraint-based destinations for handling the messages, and s set of QoS-based meta-schemes. The routes are set up through network flooding from the source to the destination. The data message is transmitted from the source to the destination through the route that fulfills the QoS provision for a given data message. The general purpose of a meta-data routing scheme in MCBR is to improve the end-to-end delay and throughput.