What is the OTUk-AIS Indicator?

This post defines and describes the OTUk-AIS signal for OTN applications.

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What is the OTUk-AIS Maintenance Signal?

What exactly is an OTUk-AIS signal?

AIS is an acronym for Alarm Indication Signal.

Another post describes the role/function of AIS.

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.

Since this is an Unframed signal, then this means that all of the OTUk, ODUk and OPUk overhead fields (within the OTUk signal) will be overwritten with this PN-11 pattern.

Figure 1 presents a simple illustration of an OTUk-AIS signal.

OTUk-AIS Pattern

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.

OTUk Bit Rate

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.

Is there another type of AIS signal that OTN APPLICATIONS ARE ACTIVELY USING?

Yes, ITU-T G.709 also recommends the use of ODUk-AIS.  OTN Path Terminating Equipment is actively using the ODUk-AIS indicator.

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Author: Darrell Smith

Darrell Smith has more than 30 years of experience as an Electrical Engineer. He has about 20 years of experience as an Applications Engineer and the remainder of his time was spent in Hardware Design and Product Marketing. He will now be sharing his wealth of knowledge on this blog.

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