# IoT-Based Solid Waste Management Solutions: A Survey
## 6 Standard Protocols Used in Waste Management Systems
### Application Layer Protocols  
At the application layer, the protocols are used for end-user communication and are usually integrated in middleware solutions for IoT [92]. End-user applications detect systems, which meansthey can communicate directly with lower layers of the protocol stack, such as web servers widely used in system integration and communication between different applications.
* **Constrained Application Protocol (COAP).** The COAP is an application layer protocol developed to support applications within IoT systems [93; 94]. Based on the Representational State Transfer (REST) functionalities over HTTP [95], REST is a transport protocol used in networks with low power nodes, mobile applications, and social networks, being able to transfer data between client and server in a more direct way, in addition to being a cached connection protocol. Unlike REST, COAP is linked to User Datagram Protocol (UDP), which makes it a lighter and more appropriate protocol for IoT applications, containing adaptations of HTTP functionalities for low power consumption when operating on links in the presence of noise and packet loss.
* **Message Queue Telemetry Transport (MQTT).** MQTT [96] is a publishing and signing transport protocol based on a TCP/IP server-client structure developed for the connection between embedded applications and middleware. It uses one-to-one, one-to-many, and many-to-many routing mechanisms, ideal for IoT systems, providing flexibility and simplified deployment. MQTT has a fixed 2-byte header suitable for devices with limited resources, such as connections with low bandwidth, battery leaks or untrusted links, and IoT requirements.
* **Extensible Messaging Presence Protocol (XMPP).** The XMPP is an instant messaging protocol over the Internet independent of operating system, designed for chat, voice and video calls, and telepresence [97]. It supports authentication, access control, privacy metering, encryption, and interoperates with other protocols. XMPP communication, based on text using XML, establishes an overload to the system that is solved with XML streams compression using EXI [98] discussed and based on Reference [99].
* **Advanced Message Queuing Protocol (AMQP).** AMPQ [100] is an open standard IoT connection layer protocol applied to a message-oriented environment with a publishing and signing structure. It supports reliable communication through primitives that guarantee delivery but requires a reliable transport protocol, such as TCP. It is facially interoperable with other protocols with communication-based on message transfers and queues using a SWAP to route messages to the appropriate queues.
* **Data Distribution Service (DDS).** DDS [101] is a subscription and publishing protocol developed for real time communications of machine to machine (M2M). In contrast to AMPQ and MQTT, DDS has a decentralized structure and does not require the presence of a broker. It uses multicast as a form of guaranteed traffic delivery and excellent QoS that supports 23 queues with a variety of communication parameters, such as security, urgency, priority, durability, and reliability.