The Frame Delay parameter can be used for on-demand OAM measurements of frame delay and frame-delay variation. When a MEP is enabled to generate frames with frame-delay measurement (ETH-DM) information, it periodically sends frames with ETH-DM information to its peer MEP in the same maintenance entity. Peer MEPs perform frame-delay and frame-delay variation measurements through this periodic exchange during the diagnostic interval.
A MEP requires the following specific configuration information to support ETH-DM:
- MEG level--MEG level at which the MEP exists
- Drop eligibility--marked drop ineligible
- Transmission rate
- Total interval of ETH-DM
- MEF10 frame-delay variation algorithm
A MEP transmits frames with ETH-DM information using the TxTimeStampf information element. TxTimeStampf is the time stamp for when the ETH-DM frame was sent. A receiving MEP can compare the TxTimeStampf value with the RxTimef value, which is the time the ETH-DM frame was received, and calculate one-way delay using the formula
frame delay = RxTimef – TxTimeStampf .
One-way frame-delay measurement (1DM) requires that clocks at both the transmitting MEP and the receiving MEPs are synchronized. Measuring frame-delay variation does not require clock synchronization and the variation can be measured using 1DM or a frame-delay measurement message (DMM) and a frame-delay measurement reply (DMR) frame combination.
If it is not practical to have clocks synchronized, only two-way frame-delay measurements can be made. In this case, the MEP transmits a frame containing ETH-DM request information and the TxTimeStampf element, and the receiving MEP responds with a frame containing ETH-DM reply information and the TxTimeStampf valued copied from the ETH-DM request information.
Two-way frame delay is calculated as
frame delay = RxTimeb – TxTimeStampf , where RxTimeb is the time that the frame with ETH-DM reply information was received. Two-way frame delay and variation can be measured using only DMM and DMR frames.
To allow more precise two-way frame-delay measurement, the MEP replying to a frame with ETH-DM request information can also include two additional time stamps in the ETH-DM reply information:
- RxTimeStampf--Timestamp of the time at which the frame with ETH-DM request information was received.
- TxTimeStampb--Time stamp of the time at which the transmitting frame with ETH-DM reply information was sent.
Discard frame-delay and frame-delay variation measurements for continuity and availability faults or when known network topology changes occur.
A MIP is transparent to the frames with ETH-DM information; therefore a MIP does not require information to support the ETH-DM function.
The figure below shows a functional overview of a typical network in which Y.1731 performance monitoring is used.