Indicators

TWAMP Light collects statistics about the delay, jitter, and packet loss rate, as shown in Figure 1. The calculation process of each measurement indicator is as follows:

• Delay: is calculated based on the timestamps carried in probes. The session-sender sends a probe carrying the transmit timestamp t1, and the session-reflector replies with a response probe carrying the receive timestamp t1' and the respond timestamp t2'. Upon receipt of the response probe, the session-sender records the receive timestamp t2. The delay during a single round trip is then calculated as follows:

  Delay1 = t2 - t1 - ( t2' - t1' )

• Jitter: is the absolute deviation between two latencies in two consecutive round trips. According to the preceding formula, the delay in the next round trip is Delay2 = t4 – t3 – (t4' – t3'). The jitter is then calculated as follows:

  Jitter = | Delay2 - Delay1 |

• Packet loss rate: is calculated as follows based on the numbers of packets sent and received by the session-sender, which are assumed as P1 and P2, respectively:

  Loss = ( P1 - P2 ) / P1