QoS Admission Control Service (QoS ACS)

Definition of QoS Admission Control Service (QoS ACS) in The Network Encyclopedia.

What is QoS Admission Control Service (QoS ACS)?

A Microsoft Windows 2000 service that can be used to centrally designate when, how, and by whom shared network segment resources will be used.

How It Works

QoS Admission Control Service (QoS ACS) is based on the Subnet Bandwidth Management (SBM) specification defined by the Internet Engineering Task Force (IETF). QoS ACS operates at the network layer and can service all transport protocols in the TCP/IP protocol suite, including Remote Display Protocol (RDP), User Datagram Protocol (UDP), and Transmission Control Protocol (TCP). A QoS ACS host (a server running Windows 2000 with the QoS ACS service installed and configured) uses the Resource Reservation Protocol (RSVP) as a message service for sending and receiving priority bandwidth requests.

A QoS ACS host controls the bandwidth for the subnet to which it is connected. The QoS ACS host uses multicasting to send out messages called beacons to inform clients on the subnet that it is ready to receive bandwidth allocation requests. Clients on the subnet that desire access to shared network resources (such as multimedia servers) first submit their bandwidth request to the QoS ACS server so that it can determine whether sufficient bandwidth is available to allocate to the clients. Bandwidth is then allocated based on the current state of resource and bandwidth availability on the subnet and the QoS ACS policy rights of the requesting user. These policy rights are defined in Active Directory.

A client’s request for bandwidth will be rejected if the QoS ACS host determines that the user does not have the right to reserve bandwidth on the subnetwork or the subnetwork does not have sufficient resources to support the request at that time. If the client’s request is rejected, the client must decide whether to try accessing the resource using a best-effort service level or wait until later when priority bandwidth becomes available and can be allocated to the client. If the request is approved, the QoS ACS host logically allocates the requested bandwidth and forwards the client’s resource request to the appropriate server on the network.

No configuration is required for Windows 2000 or Windows 98 clients, and non-Windows clients can request bandwidth, provided they are running suitable SBM client software.