When these measurements are performed, they provide a baseline for service providers to define SLAs with their customers. They enable service providers to validate the quality of the service delivered and can provide them with a tool to create value-added services that can be measured and demonstrated to customers. For example, these tests provide performance statistics and commissioning verification for virtual LAN (VLAN), virtual private networks (VPNs) and transparent LAN services (TLS), all of which use Ethernet as an access technology. The SLA criteria defined in RFC 2544 can be precisely measured using specialized test instruments. The performance verification is usually done once the installation is complete. The measurements are taken when the network is out of service to make sure that all parameters are controlled.