What is the OTUk-AIS Maintenance Signal?
What exactly is an OTUk-AIS signal?
AIS is an acronym for Alarm Indication Signal.
Whenever an OTN Network Equipment (NE) transmits an OTUk-AIS signal, it will do so by generating and transmitting an Unframed PN-11 (e.g., PRBS11) Pattern.
More specifically, ITU-T G.709 defines this PN-11 sequence by the generating polynomial: 1 + x9 + x11.
Figure 1 presents a simple illustration of an OTUk-AIS signal.
Figure 1, Simple Illustration of an OTUk-AIS signal.
What is the timing/frequency requirements for an OTUk-AIS signal?
The OTN NE will need to transmit this signal at the same nominal bit-rate for an ordinary OTUk signal.
Just like for any ordinary OTUk signal, the OTN STE will need to transmit this data at the nominal bit-rate ± 20ppm.
Table 1 presents the nominal bit-rates for the OTUk signals (and in-turn for the OTUk-AIS signal) for each value of k.
Table 1, Required Bit Rates for the OTUk-AIS signal.
Does the OTN NE need to align the PN-11 pattern sequence with the OTUk frame?
In short, the answer is No.
The length of the PN-11 sequence is 2047 bits (e.g., 211 – 1). And the capacity of a given OTUk frame is 130,560 bits.
Since the numeral 130,560 is NOT an integral multiple of 2047, then the PN-11 sequence will certainly cross OTUk frame boundaries.
When would OTN Network Equipment transmit/generate an OTUk-AIS signal?
For the time-being, ITU-T G.709 has not defined a set of conditions (or defects), upon which the OTN STE would transmit the OTUk-AIS signal.
ITU-T G.709 has reserved this type of signal as a place-holder for future use.
The standards committee may (at a later time) specify a set of conditions upon which an OTN STE would transmit the OTUk-AIS signal.
Additionally, ITU-T G.709 is NOT requiring that OTN Equipment or Chip Vendors design their products to be capable of generating/transmitting the OTUk-AIS signal.
However, ITU-T G.709 is mandating that OTN Equipment and Chip Vendors be capable of receiving, detecting and flagging the OTUk-AIS pattern.
Click HERE for Information on How an STE does declare and clear the dAIS (OTUk-AIS) defect condition.
Why are even talking about an OTUk-AIS Signal, if we are NOT currently required to generate it?
Once again, the Standards Committee has reserved this signal for FUTURE USE.
They do envision defining a set of conditions for which an OTN Section Terminating Equipment (STE) would generate and transmit the OTUk-AIS signal in a system application.