Quality of Service (QoS) mechanisms in packet-switched networks prioritise different types of traffic by categorising data packets and assigning them different levels of priority or service quality. This prioritisation ensures that more critical or time-sensitive data, such as voice or video traffic, is transmitted with higher priority, reducing latency and packet loss for these types of data.

QoS mechanisms use various techniques to achieve this prioritisation:

• Classification and Marking: Packets are identified and marked according to their type of service, such as voice, video, or data.
• Queue Management: Routers and switches use different queues for packets of different priorities. Higher-priority packets are processed more quickly, while lower-priority traffic may experience longer delays.
• Traffic Shaping and Policing: These techniques regulate the flow of packets to prevent network congestion. Traffic shaping delays packets to comply with a specified rate, while policing drops or marks packets that exceed the rate limit.
• Congestion Avoidance: Protocols like TCP adjust their transmission rate based on network congestion signals, giving priority to high-priority traffic during periods of congestion.

By implementing these QoS mechanisms, packet-switched networks can effectively manage and allocate bandwidth, ensuring that critical services maintain high performance and reliability even in congested network environments.

Packet switching presents unique challenges for network security, primarily due to the way data is fragmented and transmitted over various paths, potentially exposing it to various vulnerabilities across the network. The implications for security in a packet-switched network include the risk of interception, tampering, and unauthorised access to data packets.

To address these concerns, several security measures are implemented:

• Encryption: Encrypting data within packets is crucial for protecting the contents from interception and tampering. Protocols like SSL/TLS and IPSec encrypt data at different layers, ensuring secure transmission across the network.
• Authentication and Integrity Checks: These mechanisms verify the authenticity of data packets and ensure that they have not been altered during transit. Techniques such as digital signatures and HMACs (Hash-based Message Authentication Codes) are commonly used.
• Secure Routing Protocols: Enhanced routing protocols that include security features help in preventing attacks such as route spoofing and man-in-the-middle attacks.
• Firewalls and Intrusion Detection Systems (IDS): These security systems monitor network traffic for suspicious activities and block potentially harmful packets.
• Virtual Private Networks (VPNs): VPNs create a secure tunnel for data packets over a public network, providing an additional layer of security for sensitive data.

By incorporating these security measures, packet-switched networks can mitigate the inherent security risks, ensuring the confidentiality, integrity, and availability of data as it traverses the network.

The Maximum Transmission Unit (MTU) is a critical concept in packet-switched networks, representing the largest size of a packet or frame that can be sent in a network transaction. The significance of the MTU lies in its impact on network performance and efficiency. If a packet exceeds the MTU, it must be fragmented into smaller packets, which can lead to increased overhead and decreased throughput. Properly setting the MTU ensures that packets are as large as possible without the need for fragmentation, optimising network efficiency. However, if the MTU is set too high, packets may suffer from delays or even loss due to the inability of the network or receiving end to handle such large packets. Therefore, configuring the MTU appropriately is essential for balancing packet size with network capabilities, ensuring smooth data transmission, and reducing unnecessary network load. The MTU varies depending on the network type and protocol; for instance, Ethernet typically has an MTU of 1500 bytes, while PPPoE networks have a smaller MTU due to additional encapsulation.

Packet switching significantly enhances network scalability and facilitates future expansion. In a packet-switched network, data is broken down into small, manageable packets, which are then routed independently based on the current network conditions. This decentralised and dynamic approach allows for greater flexibility compared to circuit-switching, where a dedicated channel is required for each communication session. As the network grows and the number of users increases, packet switching efficiently manages the additional traffic without necessitating a proportional increase in infrastructure. The ability to route packets through multiple paths also means that as the network expands, it can easily incorporate new nodes and pathways. Moreover, packet switching is inherently designed to handle a wide range of data types and communication protocols, making it adaptable to future technological advancements and increasing data demands. This scalability is crucial in today's ever-evolving digital landscape, where networks must continuously adapt to accommodate growing numbers of devices and data-intensive applications.

Packet-switching techniques in wired and wireless networks differ primarily in how they handle data transmission and adapt to the inherent characteristics of their mediums. In wired networks, packet switching benefits from a stable and consistent physical connection, leading to generally lower packet loss and higher data transmission rates. Wired networks can leverage advanced routing algorithms and sophisticated error-correction methods due to their relative stability and predictability.

In contrast, wireless networks must contend with more variable conditions, such as interference, signal attenuation, and mobility-induced challenges. As a result, packet-switching techniques in wireless environments are designed to be more resilient to packet loss and to adapt dynamically to changing signal strengths and connection quality. This includes the use of more robust error-correction codes, packet retransmission strategies, and protocols that can adjust transmission rates or switch channels in response to fluctuating signal conditions. Additionally, wireless packet switching often employs mechanisms to efficiently manage the shared and limited wireless spectrum, such as contention-based access methods and power control techniques. The fundamental principles of packet switching remain the same across both mediums, but the implementation details are tailored to suit the specific requirements and limitations of wired and wireless environments.