Open Loop and Closed Loop Congestion Control

Congestion control refers to methods and systems that either eliminate congestion once it has occurred or stop it before it starts. Two types of congestion control systems are distinguished, first those that stop congestion before it starts and second those relieve it once it has started.

These two groups consist of:

1. Unclosed loop

2. Enclosed circuit

• The protocols that should be followed to avoid congestion are called the "open loop." That is, there shouldn't have been traffic jams in the first place. This is predicated on the idea that effective design execution can stop congestion before it starts.
• The Close loop enables the system to identify and eliminate congestion by allowing it to enter the condition of congestion when it arises. This is predicated on the received feedback mechanism. One can identify and clear network congestion with the assistance of feedback.

Open Loop Congestion Control

Open-loop congestion control policies aim to stop traffic before it starts. Either the source or the destination manages the congestion. Stated differently, an open loop control system is one whose operation is contingent upon time. The feedback is not present in the open-loop system. 

Adopted Policies for Open Loop Congestion Control

• Retransmission Policy: This is the policy that handles packet retransmission. A packet must be retransmitted if the sender believes it was lost or corrupted. The network may become more congested as a result of this broadcast. Retransmission times must be established with efficiency optimisation and congestion prevention in mind.
• Window Policy: Congestion may also be impacted by the sender's window type. While certain packets may be successfully received at the recipient end, many packets in the Go-back-n timeframe are resent. This duplication can worsen and intensify the network's congestion. Selective repeat window should, therefore, be used since it transmits the particular packet that might have been dropped.
• Discarding Policy: An excellent discarding policy is one in which the routers can preserve message quality while partially rejecting corrupted or less sensitive packages and preventing congestion. Routers might reject less important packets while transmitting audio files to avoid congestion and preserve the audio file's quality.
• Acknowledgment Policy: The acknowledgement policy that the receiver imposes may impact congestion because acknowledgements are a component of the network load. There are various ways to avoid acknowledgement-related congestion. The recipient should transmit acknowledgement for N packets as opposed to just one. The recipient should send an acknowledgement only when a packet has to be sent or when the timer expires.
• Admission Policy: An admission policy should have a method to avoid congestion. Switches in a flow should transmit a network flow only after verifying that it meets its resource requirements. The router should refuse to make a virtual network connection if there is a possibility of congestion or if there is already congestion in the network to stop it from getting worse.

Closed Loop Congestion Control

Closed-loop control techniques aim to alleviate congestion as soon as it occurs. Because the system is closed-loop controlled, its input determines its output. One or more feedback loops connect the system's input and output. The closed-loop system is designed to automatically give the desired result by comparing the desired output with the actual input. The closed-loop system produces the error signal, the difference between the input and output.

The following are some of the techniques for closed-loop congestion control:

• Backpressure: Back pressure is a congestion control mechanism that operates from one node to another, propagating against the direction of data flow. It is only possible to use the backpressure technique on virtual circuit networks. Every node in such a virtual circuit knows the upstream node from which a data flow is coming. In this congestion control strategy, a crowded node stops receiving data directly upstream from the node or nodes.As a result, there may be congestion on the upstream node nodes, which causes them to reject data from their upstream node or nodes.
• Choke Packet: In this congestion control technique, a crowded router or node notifies the source of the congestion by sending a specific kind of packet known as a choke packet. Unlike the backpressure approach, the crowded node does not notify its upstream node of the congestion. When using the choke packet method, a congested node alerts the source station immediately; the intermediate nodes the packet passed through are not alerted.
• Implicit Signaling: The congested node or nodes and the source do not communicate while implicit signaling is present. When it receives no acknowledgement, the source surmises network congestion someplace. Therefore, it is assumed that network congestion is the cause of the acknowledgement delay.The source slows down upon detecting this congestion.TCP employs this kind of congestion control policy.
• Explicit Signaling: Unlike the choke packet method, explicit Signalling involves the congested nodes sending a signal to the source or destination to tell them of the congestion. Explicit Signalling involves including the signal in the data packets, while choke-packed methods utilize a separate packet. Explicit communication can take place in both forward and backward directions.

A bit is set in a packet travelling against congestion when backward Signalling is used. This section alerts the source of the congestion and tells it to slow down.A bit is set in a packet travelling towards congestion as part of forward Signalling. This section alerts the destination of the traffic. In this instance, the receiver reduces the congestion by using measures like delaying down acknowledgements.

Differences between Open-Loop Control System and Closed-Loop Control System

The Open-Loop Control System and Closed-Loop Control System are distinguished in the following table: 

Similarities between Open-Loop Control System and Closed-Loop Control System

• They seek to accomplish a particular output or objective.
• Both control systems use sensors and actuators to measure and regulate the output.
• Both control systems use a feedback mechanism to track and modify the output.
• To determine the output, mathematical models or algorithms are used by both kinds of control systems.
• A variety of applications, including automation, robotics, and industrial control, can use both control systems.

Advantages of Open-Loop Control System

• Low cost and maintenance
• Easy to design and execute.
• Rapid operation speed.
• Applicable to systems that don't need exact control.

Disadvantages of Open-Loop Control System

• One of the drawbacks of the open-loop control system is its impreciseness.
• Vulnerable to changes and disruptions from outside sources.
• Unsuitable for systems that need a high level of precision and accuracy.

Advantages of Closed-Loop Control System

• Excellent precision and accuracy
• Sturdy against changes and disruptions from the outside world.
• Fit for systems that need a high level of precision and accuracy.

Disadvantages of Closed-Loop Control System

• Its slower operating speed compared to open-loop control systems
• Its complexity in design and implementation.
• Its high cost and maintenance.

Conclusion

In summary, two primary control systems are utilised to govern and manage the behaviour of physical systems: closed-loop and open-loop control systems. Although each type of system has benefits and drawbacks, closed-loop control systems have feedback, which makes them more reliable and consistent overall. We anticipate an increase in the development of closed-loop control systems to monitor and govern various physical systems as technology advances.