In open-loop control, policies are applied to prevent congestion before it occurs. Let's understand this in detail.

Explanation:
Open-loop control is a type of control system in which the control actions are pre-determined and do not depend on the system's current state or feedback. In the context of network congestion control, open-loop control is used to prevent congestion before it happens.

- Open-loop Control:
Open-loop control is a one-way control mechanism where the control actions are determined in advance based on anticipated conditions. There is no feedback loop to adjust the control actions based on the system's current state.

- Congestion:
Congestion occurs when the demand for network resources exceeds the available capacity, leading to a degradation in network performance and potential packet loss. Congestion can significantly impact the efficiency and reliability of network communication.

- Policies to Prevent Congestion:
To prevent congestion before it occurs, various policies can be applied:

1. Traffic Shaping:
Traffic shaping involves controlling the rate of data transmission to manage the flow of network traffic. By regulating the rate at which packets are sent, traffic shaping helps prevent congestion by ensuring that the network resources are not overwhelmed.

2. Quality of Service (QoS):
QoS policies prioritize certain types of network traffic over others based on predefined rules. By assigning different levels of priority to different types of traffic, QoS ensures that critical traffic (such as real-time video or voice) receives preferential treatment, reducing the likelihood of congestion.

3. Admission Control:
Admission control policies determine whether a new flow or connection can be admitted into the network based on available resources. By carefully managing the allocation of network resources, admission control prevents congestion by limiting the number of concurrent flows or connections.

4. Load Balancing:
Load balancing involves distributing network traffic across multiple paths or resources to avoid overloading a single point. By evenly distributing the traffic, load balancing helps prevent congestion by utilizing available resources efficiently.

- Conclusion:
In open-loop control, policies such as traffic shaping, QoS, admission control, and load balancing are applied to prevent congestion before it occurs. These policies help regulate the flow of network traffic, prioritize critical traffic, control the number of concurrent connections, and distribute traffic across available resources, ensuring efficient and reliable network communication.