T1 circuit testing can be performed with either of two methods: In‑service and Out‑of‑service testing.

In‑service testing is done during the routine maintenance and troubleshooting phase. This can be done on live data without removing the T1 circuit from service. Monitoring live data allows wireless service provider technicians to detect alarms, bipolar violations (BPV), and frame errors, but bit errors cannot be measured. Bit errors may, however, be estimated by measuring CRC errors or frame errors. For a step‑by‑step example of in‑service testing, refer to In‑service Measurements.

Out-of-service testing is done when the T1 circuit is initially installed and before final acceptance from the LEC. At that time, the circuit should be subjected to critical testing in order to guarantee the level of service per contract. Out-of-service testing may also be performed when the circuit performance is very poor. For a step‑by‑step example of out of service testing, refer to Out‑of‑service Measurements.