What is the RDI (Remote Defect Indicator) Signal?
What does RDI Mean?
RDI is an acronym for Remote Defect Indicator.
Where is the RDI Signal Used?
RDI is a special type of alarm signal that a Network Element (within a Telecom/Datacom application) will generate and transmit (back towards upstream Network Equipment) anytime it detects some sort of servicing-affect defect within the signal from that same upstream Network Equipment.
Stated differently, the Network Element (NE) will transmit the RDI indicator (upstream) at the same time (and under the same conditions) that it will transmit the AIS signal downstream.
For example: If an NE were to declare the LOS (Loss of Signal) or the LOF (Loss of Frame) defect within its incoming telecom/datacom signal, then it will respond to this defect condition by transmitting the RDI signal (back) upstream towards the source of the defective signal.
Whenever the Network Element transmits this RDI signal upstream, it is notifying the upstream NE that there are problems with its data.
What EXACTLY is an RDI Signal?
The exact scheme that we use to transmit the RDI signal depends upon the telecom/datacom standard and network layer that we are operating in.
However, in most cases, the Network Element will typically transmit the RDI signal by setting a certain overhead bit-field (within the signal that it is transmitting back to the remote or upstream NE) to “1”.
The Network Element will consistently set this bit-field to “1”, within each frame of data (that it is transmitting back to the remote NE) for the duration that it is declaring the defect within its incoming data stream.
Likewise, the Network Element will end its transmission of the RDI signal by clearing that same overhead bit-field to “0”.
For OTN applications, we call the RDI signal the BDI (or Backward Defect Indicator) signal.
I have included posts that define and describe the BDI signal for both the OTUk and ODUk frames.
For example, SONET Line-Terminating Equipment will transmit the RDI-L indicator and SONET Path-Terminating Equipment will transmit the RDI-P indicator.
In the case of PDH signals (e.g., T1/E1 or T3/E3) will we typically call the RDI signal by other synonymous names such as FERF (Far-End Receive Failure) or the “Yellow Alarm”.
When do we transmit the RDI Signal?
We will go through a couple of examples to illustrate how and when we will transmit the RDI signal.
Example # 1 – The Unerred/Normal Condition
Figure 1 presents a very simple illustration of a portion of a 3R Repeater/Regenerator which consists of the following components:
- Two (2) Receive Line Interface blocks (one block is labeled W for West, and the other block is labeled E for East)
- Two (2) Receive Framer blocks (W – West, and E – East)
- Two (2) Transmit Line Interface blocks (W – West, and E – East)
- Two (2) Transmit Framer blocks (W – West, and E – East)
- CS (Clock Smoothing/Jitter Attenuation) PLL (Phase-Locked Loop)
- AIS OSC (Stand-Alone Oscillator)
- Two (2) Defect Decoder blocks (W – West and E – East)
In this figure, our 3R Repeater/Regenerator is receiving a good (error-free) signal from the West Terminal.
The 3R Repeater/Regenerator will first receive this signal through its Receive Line Interface (W) block.
Afterward, this signal passes through to the Receive Framer (W) block.
If the Receive Line Interface (W) and Receive Framer (W) blocks were to detect no problems within this signal, then the 3R Repeater/Regenerator would allow this signal to pass through both the Transmit Framer (E) and Transmit Line Interface (E) blocks as is.
The Receive Line Interface (W) and the Receive Framer (W) blocks would also do nothing to impact the data, that the Near-End Transmitter circuitry (e.g., the Transmit Line Interface (W) and Transmit Framer (W) blocks) is transmitting, back out to the West Terminal.
Figure 1 presents an illustration of this Normal (No Defect) Condition.
Figure 1, Illustration of the 3R Repeater/Regenerator – during Good/Normal Conditions.
Please note that I have grayed out the non-relevant portions of Figure 1, to focus our discussion on this Defect Declaration to RDI Generation mechanism, on the West-side of the 3R Repeater/Regenerator circuit.
Now, we will illustrate the case where we will transmit the RDI indicator.
Example # 2 – The dLOS/Abnormal Condition
Figure 2 presents another very simple illustration of a 3R Repeater/Regenerator.
However, in this figure, there is an impairment in the signal that originates from the West Terminal such that our Network Element is now declaring the dLOS (Loss of Signal) defect with this signal.
It is possible that this signal was severed by a backhoe or some other mishap.
Nonetheless, this means that our 3R Repeater/Regenerator is no longer receiving its signal from the West Terminal.
In this situation, the 3R Repeater/Regenerator will respond by doing all the following things.
- The Receive Line Interface (W) or the Receive Framer (W) blocks will declare the dLOS (Loss of Signal) defect with the signal that it is receiving from the West Terminal.
- The Transmit Framer (E) and the Transmit Line Interface (E) (which resides directly behind the Receiving Line Interface and Framer blocks – that are declaring the dLOS condition) will proceed to transmit the AIS indicator (to the East Terminal) as we discussed in the AIS post of this blog.
- Additionally, the Receive Line Interface (W) and/or the Receiver Framer (W) blocks will also send a control signal to its “near-end” transmitting circuitry [e.g., the Transmit Framer (W) and Transmit Line Interface (W) blocks] to command it to start sending the RDI signal, back out to the West Terminal.
Figure 2 presents an illustration of the dLOS/RDI Transmission Condition for our 3R Regenerator/Repeater.
Figure 2, Illustration of the 3R Repeater/Regenerator – during the dLOS/Abnormal Condition
The Transmit Framer (W) and Transmit Line Interface (W) blocks will continue to transmit the RDI indicator to the West Terminal for the duration that the Receive Line Interface (W) and the Receive Framer (W) blocks are declaring the dLOS defect with the signal that they are receiving from the West Terminal.
The Transmit Framer (W) and Transmit Line Interface (W) blocks will cease to transmit the RDI indicator once the Receive Line Interface (W) and the Receive Framer (W) blocks clear the dLOS defect and start to receive good/normal data from the West Terminal.
In addition to the dLOS defect, the Network Element will typically transmit the RDI Indicator (upstream) in response to any of the following other defects.
Why do we bother to transmit the RDI Signal?
The main reason that we send the RDI signal (back to upstream equipment) in response to service-affect defects is to alert that NE that there is a service-affecting defect, within its output signal between its location and that of the next downstream NE.
This notification aids in troubleshooting and system debugging of fault conditions in the network.