AC (Admission Control)

Admission control (AC) is a mechanism used in computer networks and telecommunications to ensure that new connections or traffic do not exceed the network's capacity or cause a degradation in the quality of service (QoS) provided to existing connections. Admission control can be performed at different levels of the network, such as at the application, transport, network, or link layer. In this article, we will focus on admission control at the network layer, which is also known as network admission control (NAC).

Network admission control is the process of determining whether a new connection request can be accepted by a network based on the network's current state and capacity. The admission control algorithm determines whether the network can handle the requested traffic while ensuring that the QoS requirements of existing connections are not compromised. The QoS requirements may include parameters such as bandwidth, delay, packet loss, jitter, and priority.

The admission control algorithm typically uses network measurements and parameters such as link utilization, available bandwidth, buffer occupancy, and traffic load to decide whether to admit or reject the new connection. The admission control algorithm may also consider other factors such as the connection type, the source and destination nodes, the service level agreement (SLA), and the network topology. The SLA specifies the QoS requirements and guarantees that are agreed upon between the network provider and the customer.

Admission control can be performed in a centralized or distributed manner. In a centralized approach, a dedicated admission control server or entity is responsible for accepting or rejecting new connections based on the network state and the SLA. The admission control server may communicate with other network elements to obtain network measurements and parameters. In a distributed approach, the admission control decision is made by the network nodes that are directly involved in the connection setup, such as the routers or switches. The nodes may exchange admission control messages to coordinate their decisions.

Admission control is an essential mechanism for ensuring that a network is utilized efficiently and fairly. Without admission control, new connections may cause the network to become congested, leading to delays, packet loss, and reduced QoS for existing connections. Admission control also enables the network provider to enforce the SLA and meet the customer's expectations. By rejecting connections that cannot be supported by the network, the admission control mechanism ensures that the network does not oversubscribe its resources and violate the SLA.

There are several admission control algorithms and techniques that can be used to determine whether a new connection can be admitted to the network. These algorithms can be classified based on their complexity, accuracy, and efficiency. Some of the common admission control algorithms are discussed below.

1. Threshold-based Admission Control: In this algorithm, a fixed threshold is used to determine whether a new connection can be admitted to the network. The threshold may be based on network parameters such as link utilization, buffer occupancy, or available bandwidth. If the network measurement exceeds the threshold, the connection is rejected. This algorithm is simple and efficient, but it may not be accurate enough to guarantee the QoS requirements of existing connections.
2. Measurement-based Admission Control: In this algorithm, the admission control decision is based on a more accurate measurement of the network state, such as link capacity or delay. The algorithm may use active or passive measurement techniques to obtain the network measurements. Active measurement involves injecting test packets into the network and measuring the response time, while passive measurement involves analyzing the network traffic to infer the network state. This algorithm provides better accuracy than threshold-based admission control, but it may require more resources and processing power.
3. Model-based Admission Control: In this algorithm, a mathematical model of the network is used to predict the behavior of the network under different traffic conditions. The model may take into account the network topology, the traffic patterns, the QoS requirements, and the network protocols. The admission control decision is based on the prediction of the model, which is compared to the SLA and the current state of the network. If the model predicts that the QoS requirements of the new connection cannot be guaranteed, the connection is rejected. This algorithm provides high accuracy and flexibility, but it may be more complex and require more resources to maintain the model.
4. Control-theoretic Admission Control: In this algorithm, control theory is used to optimize the network resource allocation and admission control decision. Control theory is a mathematical framework that enables the design of control algorithms to achieve a specific objective while taking into account the system's dynamics and uncertainties. The admission control decision is based on the feedback from the network measurements, which is used to adjust the control parameters of the system. This algorithm provides optimal resource allocation and adaptive admission control, but it may require significant expertise and computational resources.

Admission control can also be applied to different types of traffic, such as unicast, multicast, and real-time traffic. Unicast traffic refers to point-to-point communication between two nodes, while multicast traffic refers to one-to-many communication. Real-time traffic refers to traffic that has stringent delay and jitter requirements, such as voice and video. Admission control for real-time traffic is particularly challenging since it requires a low delay and jitter, which may conflict with other QoS requirements, such as bandwidth and packet loss.

In addition to the admission control algorithm, the admission control mechanism may also include other components such as policy enforcement, rate limiting, and traffic shaping. Policy enforcement ensures that the new connection complies with the network policies and security requirements. Rate limiting and traffic shaping limit the amount of traffic that can be sent by the new connection to avoid overloading the network.

Admission control is also used in wireless networks to manage the access of mobile devices to the network. In wireless networks, the admission control mechanism is used to determine whether a mobile device can join the network and how much bandwidth it can use. The admission control decision is based on the signal strength, the available bandwidth, and the number of active users in the cell. In cellular networks, admission control is also used to manage the handover of mobile devices between different cells.

In conclusion, admission control is a critical mechanism for ensuring that a network can provide the required QoS to existing connections while accepting new connections that comply with the SLA and network policies. Admission control algorithms can be simple or complex, accurate or approximate, and centralized or distributed. The choice of the admission control algorithm depends on the network requirements, the complexity of the network, and the resources available. Admission control can also be extended to different types of traffic and wireless networks.