Abstract: A data transmission system includes probes connected between end user sites and a data switching network. Each probe is connected to the switching network via an access channel wherein transmission circuits establish paths between the sites through the access channel and switching network. The probes capture and retransmit data traveling between the sites over respective transmission circuits, and can thereby insert service level analysis (SLA) messages into the data traffic in order to actively communicate network performance information to other probes. For each transmission circuit, the probes periodically collect measurements related to one or more network performance metrics, including: round-trip delay (RTD), data delivery ratio (DDR) and network availability. During each SLA measurement cycle, a sequence of SLA messages is exchanged over each transmission circuit, which messages contain data used to determine RTD and DDR.