Application Layer protocols contribute to the security of data transmission by implementing various security measures tailored to specific applications. For example, the Hypertext Transfer Protocol Secure (HTTPS) is an extension of HTTP that adds a layer of security for web communications. HTTPS incorporates Transport Layer Security (TLS) to provide encrypted communication between web browsers and servers, protecting the data from eavesdropping, tampering, and forgery. Similarly, secure versions of email protocols, like Secure POP3 (POPS) and Secure IMAP (IMAPS), use encryption to safeguard email transmissions. These protocols ensure that sensitive data, such as personal information, login credentials, and financial transactions, are securely transmitted over the network, thus maintaining confidentiality and integrity. The Application Layer, therefore, plays a vital role in providing end-to-end security in various internet services, making it a crucial component in the overall network security architecture.

The Internet Layer contributes significantly to the scalability of a network through its routing and addressing mechanisms. The Internet Protocol (IP), the core protocol of this layer, allows for the assignment of unique IP addresses to each device on a network. This addressing system is hierarchical, which makes it scalable to accommodate a growing number of devices. Furthermore, the Internet Layer's routing functionality, which involves determining the best path for data packets to travel across networks, is essential for handling increased network traffic efficiently. As networks expand and more devices and routes are added, the Internet Layer's routing protocols adapt to these changes, ensuring that data continues to be transmitted efficiently regardless of the network's size. This adaptability and efficiency in handling increased network complexity are key to the scalability of modern networks.

The Transport Layer is capable of handling multimedia transmissions, and this is primarily achieved through the use of different protocols suited for various types of media. For instance, the User Datagram Protocol (UDP) is often used for streaming audio and video data. UDP is preferred for multimedia because it offers faster data transmission by forgoing the error-checking and recovery processes that are characteristic of TCP (Transmission Control Protocol). This results in reduced latency and smoother streaming, which is essential for real-time multimedia applications. However, it should be noted that UDP does not guarantee data integrity, so some level of quality compromise is accepted in return for speed and fluidity. For non-real-time multimedia, where data integrity is more crucial, TCP might be used instead. Thus, the Transport Layer, by providing both TCP and UDP, caters to the diverse needs of multimedia transmissions, balancing between quality and performance as required by the specific application.

The Link Layer plays a crucial role in determining the quality of service in a network by managing the direct data transmission between network nodes. This layer is responsible for the actual physical connection between devices, as well as the data framing, physical addressing, and error detection and handling. The quality of these functions directly impacts the overall network performance. For instance, efficient data framing and error detection mechanisms ensure data integrity and minimize the need for retransmissions, which can slow down the network. Additionally, the Link Layer's role in handling the physical and logical connections impacts the network's ability to handle high volumes of traffic and its resilience to interruptions or interference. The efficiency and reliability of the Link Layer directly influence the network's throughput, latency, and overall stability, which are critical components of quality of service.

Protocol stacks ensure compatibility between different hardware and software systems through the implementation of standardized protocols at each layer of the stack. Each protocol defines specific rules and formats for communication, which are universally recognized and followed by network devices and software. This standardization means that any device or software that adheres to these protocols can communicate effectively with others in the network. For example, the TCP/IP protocol suite, widely used in internet communication, has universally accepted standards for data transmission. By following these standards, different devices and software from various manufacturers can communicate seamlessly, regardless of their underlying hardware differences. This interoperability is crucial for the global connectivity and functionality of the internet, allowing diverse systems to exchange data reliably and efficiently.