CFD functions

CFD functions, which are implemented through the MPs, include:

• Continuity check (CC).

• Loopback (LB).

• Linktrace (LT).

• Alarm indication signal (AIS).

• Loss measurement (LM).

• Delay measurement (DM).

• Test (TST).

Connectivity faults are usually caused by device faults or configuration errors. Continuity check examines the connectivity between MEPs. This function is implemented through periodic sending of CCMs by the MEPs. A CCM sent by one MEP is intended to be received by all the other MEPs in the same MA. If a MEP fails to receive the CCMs within 3.5 times the sending interval, the link is considered as faulty and a log is generated. When multiple MEPs send CCMs at the same time, the multipoint-to-multipoint link check is achieved. CCM frames are multicast frames.

Similar to ping at the IP layer, loopback verifies the connectivity between a source device and a target device. To implement this function, the source MEP sends loopback messages (LBMs) to the target MEP. Depending on whether the source MEP can receive a loopback reply message (LBR) from the target MEP, the link state between the two can be verified.

LBM frames are multicast and unicast frames. The device can send and receive unicast LBM frames, and can receive multicast LBM frames but cannot send multicast LBM frames. LBR frames are unicast frames.

Linktrace is similar to traceroute. It identifies the path between the source MEP and the target MP. The source MEP sends the linktrace messages (LTMs) to the target MP. After receiving the messages, the target MP and the MIPs that the LTM frames pass send back linktrace reply messages (LTRs) to the source MEP. Based on the reply messages, the source MEP can identify the path to the target MP. LTM frames are multicast frames and LTRs are unicast frames.

The AIS function suppresses the number of error alarms reported by MEPs. If a local MEP does not receive any CCM frames from its peer MEP within 3.5 times the CCM transmission interval, it immediately starts sending AIS frames. The AIS frames are sent periodically in the opposite direction of CCM frames. When the peer MEP receives the AIS frames, it suppresses the error alarms locally, and continues to send the AIS frames. If the local MEP receives CCM frames within 3.5 times the CCM transmission interval, it stops sending AIS frames and restores the error alarm function. AIS frames are multicast frames.

The LM function measures the frame loss in a certain direction between a pair of MEPs. The source MEP sends loss measurement messages (LMMs) to the target MEP. The target MEP responds with loss measurement replies (LMRs). The source MEP calculates the number of lost frames according to the counter values of the two consecutive LMRs (the current LMR and the previous LMR). LMMs and LMRs are unicast frames.

The DM function measures frame delays between two MEPs, including the following types:

• One-way frame delay measurement

  The source MEP sends a one-way delay measurement (1DM) frame, which carries the transmission time, to the target MEP. When the target MEP receives the 1DM frame, it does the following:

  • Records the reception time.

  • Calculates and records the link transmission delay and jitter (delay variation) according to the transmission time and reception time.

  1DM frames are unicast frames.

• Two-way frame delay measurement

  The source MEP sends a delay measurement message (DMM), which carries the transmission time, to the target MEP. When the target MEP receives the DMM, it responds with a delay measurement reply (DMR). The DMR carries the reception time and transmission time of the DMM and the transmission time of the DMR. When the source MEP receives the DMR, it does the following:

  • Records the DMR reception time.

  • Calculates the link transmission delay and jitter according to the DMR reception time and DMM transmission time.

  DMM frames and DMR frames are unicast frames.

The TST function tests the bit errors between two MEPs. The source MEP sends a TST frame, which carries the test pattern, such as pseudo random bit sequence (PRBS) or all-zero, to the target MEP. When the target MEP receives the TST frame, it determines the bit errors by calculating and comparing the content of the TST frame. TST frames are unicast frames.