Latency Reduction

Correct Answer: Implement Quality of Service (QoS)

The best solution to improve performance on multiple Voice over IP (VoIP) calls would be to implement Quality of Service (QoS). QoS prioritizes specific types of data traffic to ensure that important data (such as voice or video streaming) receives priority over less critical data. It can significantly reduce latency for time-sensitive applications like VoIP. Decreasing the bandwidth limit won't help improving the VoIP call quality. In fact, it might worsen the situation as VoIP calls need a certain bandwidth to maintain a minimum level of quality. Disabling all firewalls is not a solution either. While it may reduce some overhead and latency, it opens up the network to a slew of security risks and threats, which is not desirable. Implementing a Virtual Private Network (VPN) doesn't inherently improve VoIP call quality. VPN can indeed lead to better security, but in terms of performance, it might introduce additional latency due to data encryption and decryption, hence not the best solution for this problem.

Correct Answer: Change the Wi-Fi channel

Changing the Wi-Fi channel can help to reduce latency in a wireless network. This is often the case when there are other Wi-Fi networks nearby on the same channel, which is causing interference. By switching to a different channel, the user may be able to avoid this interference and reduce latency. Disabling the 802.11ac protocol, on the other hand, would likely increase latency, as this protocol is designed to provide high data rates and low latency. Similarly, disabling Wi-Fi security would not reduce latency and would also make the network vulnerable to unauthorized access. Lowering the MTU size could potentially reduce latency, but it would also reduce the maximum size of data packets that can be sent, which could negatively impact network performance.

Correct Answer: Implement WAN optimization

The best way to reduce latency for the company's WAN connections between branch offices would be by implementing WAN optimization. WAN optimization is a collection of techniques for increasing data-transfer efficiencies across Wide Area Networks (WANs). This could include techniques such as data deduplication, compression, latency optimization, and caching. It is specifically designed to address the issues the question is concerned with. Switching to public IP addresses is not going to affect latency in a meaningful way. Upgrading routers to Layer 3 switches will not specifically target latency issues, and may in fact increase latency under certain circumstances. Disabling the branch firewall is a security risk and while it may reduce some latency due to the fact that the traffic does not need to be inspected, it is far from a best practice. WAN optimization was specifically designed to address the problem in question and is the best choice.