Two-way ETH-SLM and Two-way ETH-DM

Synthetic loss measurement (SLM) is part of the ITU-T Y.1731 standard. It can be used to periodically measure Frame Loss and Forward Loss Ratio (FLR) between a pair of point to point MEPs. Measurements are made between two MEPs belonging to the same domain and MA.

Synthetic loss measurement is a mechanism to measure frame loss using synthetic frames, rather than data traffic. A number of synthetic frames are sent and received, and the number of those that are lost is hence calculated to measure the loss.



A MIP is transparent to the frames with ETH-SLM information and therefore does not require any information to support the ETH-SLM functionality.


In a Two-Way ETH-SLM, Initiator (the source MEP) sends burst of Synthetic Loss Message (SLM) frames to Responder (the Remote MEP) and in turn receives Synthetic Loss Reply (SLR) frames to carry out synthetic loss measurements.

Click to expand in new window

A MEP transmits burst of SLM frames once for every Tx-interval time period. Whenever a valid SLM frame is received by a MEP, an SLR frame is generated and transmitted to the initiating MEP. With the information contained in SLR frames, a MEP determines frame loss for given measurement periods.

Loss measurement calculation

For each MA where Two-Way ETH-SLM is configured, an MEP maintains two local counters for each peer MEP for each CoS instance which plays a role in calculating Loss Measurement.

TxFC1: Counter for synthetic frames transmitted towards the peer MEP. A source MEP increments this counter with transmission of SLM frames while a responder MEP increments it with transmission of SLR frames.

RxFC1: Counter for synthetic frames received from the peer MEP. A source MEP increments this counter with reception of SLR frames while a responder MEP increments it with reception of SLM frames.

A MEP uses the following values to determine near-end and far-end frame loss in the measurement period:

Frame lossfar-end = | TxFCf[tc] – TxFCf[tp] | – | TxFCb[tc] – TxFCb[tp] |

Frame lossnear-end = | TxFCb[tc] – TxFCb[tp] | – | RxFCl[tc] – RxFCl[tp] |

On-demand SLM

The following points applies to On-demand SLM.

Delay Measurement

The ETH-DM is used for on-demand OAM to measure frame delay and frame delay variation.

Frame delay and frame delay variation measurements are performed by sending frames with ETH-DM information to the peer MEP and receiving frames with ETH-DM information during the diagnostic interval. Each MEP performs the frame delay and frame delay variation measurement.

For the MEP to support ETH-DM, you must configure the following.

Two-way ETH-DM

In a Two-way ETH-DM, an Initiator or a Source MEP sends frames with ETH-DM request information (DMM) to the Responder or a Remote MEP and in turn receives frames with ETH-DM reply information (DMR) to carry out two-way frame delay and two-way frame delay variation measurements.

Delay measurement calculation

When delay measurement is issued, a MEP transmits DMM frames with the 'TxTimeStampf' value.

When a valid DMM frame is received by a MEP, a DMR frame is generated and transmitted to the requesting MEP. A DMM frame with a valid domain level and a destination MAC address equal to the receiving MEP's MAC address is considered as a valid DMM frame. There are two additional timestamps which are used in the DMR frame to take into account the processing time at the remote MEP: 'RxTimeStampf' (Timestamp at the time of receiving the DMM frame) and 'TxTimeStampb' (Timestamp at the time of transmitting the DMR frame).

After receiving a DMR frame, a MEP tags the incoming frame with another timestamp 'RxTimeStampb' and uses the following values to calculate two-way frame delay.

Frame Delay = (RxTimeStampb - TxTimeStampf) - (TxTimeStampb - RxTimeStampf)

The MEP can also make two-way frame delay variation measurements based on its capability to calculate the difference between two subsequent two-way frame delay measurements.

Click to expand in new window
Two-Way ETH-DM

Test Profile

SLX-OS provides the default test profile, configurable test profile and configurable action profile that can be associated to a source and target MEP pair at the initiator or responder side which establishes a session. This facilitates the user to apply all the parameters which are configured within the profile at once for a measurement session instead of specifying each parameter through the command-line interface (CLI).

The following table provides information on the two important counters used for loss measurement.

Initiator Responder
TxFC1 SLM Tx count SLR Tx count
RxFC1 SLR Rx count SLM Rx count