Admission control

Admission control is a network management function that decides whether a new connection or traffic flow can be accepted based on available bandwidth, accepting or rejecting requests to protect quality of service for existing connections.

What Is Admission Control?

Admission control is a network management function that decides whether a new connection or traffic flow should be accepted into a network based on whether the required resources, particularly bandwidth, can be allocated without degrading the quality of service already committed to existing connections. When a request for a new flow arrives, the admission control algorithm evaluates available capacity against the flow's stated requirements and either accepts the connection and reserves the necessary resources or rejects it to protect existing traffic. The function is a foundational component of quality-of-service (QoS) architectures in telephony, IP networking, wireless systems, and real-time multimedia delivery.

Admission control originated in circuit-switched telephony, where accepting a call required confirming that a complete transmission path with adequate channel capacity existed between endpoints. The concept carried over into packet networking as differentiated services and real-time applications imposed demands that best-effort packet forwarding could not guarantee.

Parameter-Based and Measurement-Based Methods

Two broad classes of admission control methods have been developed for packet networks. Parameter-based admission control (PBAC) makes the admission decision by comparing a flow's declared traffic descriptor, including peak rate, average rate, and burst size, against the resources available on each link along the path. The approach is deterministic and predictable but depends on accurate traffic characterization by the source; if the declared parameters overestimate actual usage, network capacity is wasted.

Measurement-based admission control (MBAC) takes a different approach: rather than relying on declared parameters, it measures actual traffic on the link in real time and infers available capacity from observation. When a new flow requests admission, the algorithm estimates whether the measured aggregate plus the new flow's expected contribution will exceed the link's capacity target. MBAC typically achieves higher network utilization than PBAC because it uses empirical evidence of what existing flows are actually consuming, though it introduces a small risk of accepting connections during traffic bursts. Research published in the journal Applied Mathematics analyzed both families of methods under simulated IP network conditions, demonstrating that MBAC delivers higher throughput in variable-load scenarios.

Resource Reservation and Signaling

Admission control in the IntServ model of IP QoS is implemented in conjunction with the Resource Reservation Protocol (RSVP), defined in IETF RFC 2205. RSVP allows a receiver to request resource reservations along the path from source to destination by sending a reservation message that each router processes. Each router on the path runs an admission control function: it checks whether it can support the requested reservation given its current allocations, and if so, installs state and forwards the reservation message upstream. If any router cannot satisfy the request, it returns a rejection message and the reservation fails.

The DiffServ model avoids per-flow signaling by aggregating traffic into classes and applying admission control at the domain boundary based on traffic conditioning agreements. Flows entering a DiffServ domain are policed against a contracted service profile, and excess traffic is dropped or re-marked rather than negotiated hop-by-hop.