OTUkV – Functionally Compliant OTU Frames

This post describes the differences between a Fully-Compliant OTUk frame and a Functionally-Compliant OTUkV frame.

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What are OTUkV/Functionally-Compliant OTUk Frames?

There are two broad categories of OTUk Frames.

  • There are the Fully-Compliant OTUk Frames (which we will simply refer to as OTUk frames).
  • And, there are the Functionally-Compliant OTUk Frames (which we will refer to as OTUkV frames).

To help show how the OTUkV frames are different from that of the OTUk frames, I will discuss the Fully-Compliant OTUk frames first.

Please note that you can find a much more in-depth discussion of the Fully-Compliant OTUk Frame here in the OTUk Post.

The Fully-Compliant OTUk Frame

There is only one type of Fully-Compliant OTUk frame.

The Fully-Compliant OTUk frame is a frame that entirely complies with all the ITU-T G.709 recommendations for an OTUk frame.

It has the exact field formats, field sizes/types that are specified within ITU-T G.709.  Figure 1 presents an illustration of the Fully-Compliant OTUk frame.

ITU-T G.709 Fully Compliant OTUk Frame

Figure 1, Illustration of the Fully-Compliant OTUk Frame

The OTUk Post already discusses and defines each of these OTUk Overhead fields.  Thus, we will not repeat that information in this post.

ITU-T G.709 states that the Fully-Compliant OTUk Frame should have all the following features/attributes.

  • The overall frame size should be a 4 Row by 4080 Byte Column Structure.
  • It contains an OTUk Overhead of 14 bytes that match the Overhead fields that we describe in the OTUk Post.
  • The frame contains the three rows by 14-byte column ODUk Overhead field that matches the Overhead fields that we describe in the ODUk Post.
  • The OTUk frame contains a 4 row by 3808-byte OPUk field, along with a 4 row by 2-byte column OPUk overhead, that matches the Overhead fields that we describe in the OPUk Post.
  • It includes a 4 row by 256-byte column structure for FEC (Forward Error Correction) that was computed using the Reed Solomon, RS(255,239) scheme.

Some industry people simply refer to this type of FEC as “GFEC” (because it complies with the ITU-T G.709 requirements for FEC).

NOTE:  We will be referring to the Reed-Solomon FEC (as called out for in ITU-T G.709) as “GFEC” throughout the rest of this post.

If the OTUk frame differs from these characteristics by even one item, then we cannot refer to this type of frame as being a Fully-Compliant OTUk frame.

We must then refer to this type of frame as being a Functionally Compliant OTUkV frame.

The Functionally-Compliant OTUk (OTUkV) Frame

In contrast to the Fully-Compliant OTUk frame, ITU-T G.709 identified six (6) different variations of OTUkV frames that can exist.
These six variations are:

  • OTUkV frame with Alternative 7% FEC (also referred to as the OTUk-v frame)
  • OTUkV frame with Larger/Stronger FEC
  • OTUkV frame with Smaller FEC
  • OTUkV frame without FEC
  • OTUkV frame with Different Frame Structure and FEC Area
  • OTUkV frame with Different Frame Structure and No FEC Area

We will describe each of these variations of OTUkV frames below.

NOTE:  The reader should not consider this list of types of OTUkV frames to be an exhaustive list.

Other variations within these frames are possible (and still qualify at OTUkV frames).

OTUkV Frame with Alternative 7% FEC

This type of OTUkV frame is ALMOST the same as the Fully-Compliant OTUk frame.  It has the same set of fields (payload and overhead bytes).  It also has the same frame size (e.g., 4080 byte-columns by four rows).

The only difference between this particular Functionally-Compliant OTUkV frame and the Fully-Compliant OTUk frame is in how the FEC is calculated.

Figure 2 presents an illustration of the field format for this particular OTUkV frame.

Functionally Compliant OTUkV Frame with Alternative 7 Percent FEC

Figure 2, Illustration of the OTUkV Frame with Alternative 7% FEC

There are other “7% FECs” out there other than the Reed-Solomon FEC (or GFEC).

If an OTUk frame uses one of these alternative types of FECs, rather than the “GFEC,” then we need to refer to this frame as an OTUkV Functionally Compliant frame.

NOTE:  Some OTN-related documentation refers to these type functionally-compliant OTUk frames as OTUk-v frames.

OTUkV frame with Larger/Stronger FEC

Some applications need the use of a Stronger FEC.

These are applications in which the system design requires a much larger NCG (Net Coding Gain).

Long-Haul applications (where there are long fiber spans between 3R regenerators) are examples of such applications.

These applications will need a more robust FEC, which (in turn) will need a larger FEC area within the OTUkV frame.

Figure 3 presents an illustration of this type of OTUkV frame.

Functionally Compliant OTUkV Frame with Larger FEC

Figure 3, Illustration of the OTUkV Frame with Larger/Stronger FEC

Since we are using a larger FEC (and larger FEC area), this type of OTUkV frame will have a larger size than that for the Fully-Compliant OTUk frame.

This type of OTUkV frame will have 4080 + X byte columns and 4-byte rows.

NOTE:  In most cases, for a given value of k (in OTUk/kV), the Frame Repetition rate will be the same for all OTUk/kV-type frames.

For example, if you look at the OTUk post, you will see that the Frame Repetition Rate for an OTU2 signal is 82,028 frames/second.

We can state that this also means that the Frame Repetition rate for an OTU2V frame (that is of a different size than that for the OTU2 frame) will also have this same frame repetition rate.

This means that the OTUkV frame, which is larger than its OTUk counterparts, will need to operate at a higher bit rate to transmit these frames than that to transmit the OTUk frames.

Similarly, OTUkV frames that are smaller in size than their OTUk counterparts will need to operate at a lower bit rate to transmit these frames than that to transmit the OTUk frames.

OTUkV frame with Smaller FEC

There will be applications where we will not need an FEC as large as the 7% GFEC.  In these applications, we can get away with using smaller FECs.  Figure 4 presents an illustration of an OTUkV frame with this kind of FEC.

Functionally Compliant OTUkV Frame with Smaller FEC

Figure 4, Illustration of the OTUkV Frame with Smaller FEC

Figure 4 shows that this type of OTUkV frame is of the same frame size as that for the Fully-Compliant OTUk frame.

In this case, the actual FEC takes up less “real-estate” than that for the GFEC.  However, the unused portions of the FEC field are filled with an All-Zeros pattern, to “pad out” the remaining FEC byte-fields.

OTUkV frame without FEC

There will be applications that will require the use of OTUkV frames without FEC.  Some of these applications will typically be very low-latency applications (e.g., for Enterprise Applications such as Real-Time Stock Quotes, etc.).

FEC coding and decoding all require some number crunching that does consume a finite amount of time and increases latency.

Figure 5 presents an illustration of this type of OTUkV frame.

Functionally Compliant OTUkV Frame with NO FEC

Figure 5, Illustration of the OTUkV frame without FEC

NOTE:  Some applications will implement the “No-FEC” OTUkV frame by filling in the entire FEC field (as drawn in Figure 1) with an all-zero pattern.

In this case, the “No-FEC OTUkV frame” would be of the same size as that of the Fully-Compliant OTUk frame.

Figure 6 presents an illustration of this example OTUkV frame.

Fully Compliant OTUk Frame with All Zeros FEC

 

Figure 6, Illustration of the No-FEC OTUkV frame with the entire FEC field set to an All-Zeros pattern

OTUkV frame with Different Frame Structure and FEC Area

OTUkV frames that are of completely different Frame Structure (from that of the Fully-Compliant OTUk frame) can be (and are) sent out onto the OTN network.

Before the days of OTUCn, some people used these type of frames to (for example) support “200Gbps or More” Operation.

In this case, an entire OTU4/4V frame (within a given OTU4 signal) could be mapped into one of these structures.  Afterward, we could bit-interleaved this structure with other such structures (from another OTU4 signal) to achieve “200Gbps” transmission.

I will elaborate on the actual mechanics behind this scheme in another post.

Figure 7 presents an illustration of the With-FEC version of this “Different Structure” OTUkV frame.

OTUkV Structure with Different Frame Structure and FEC

Figure 7, Illustration of the OTUkV Frame with Different Frame Structure and FEC Area

OTUkV frame with Different Frame Structure and No FEC Area

This type of frame would have a similar use, like that for in the previous section.

The only difference between this frame and that of the previous frame is that this particular frame does not contain an FEC.

Again, a possible application (for this type of frame) would be to support 200Gbps (or higher rate) applications.

In this case, we would map an OTU4/4V frame into this structure.

Afterward, we would then combine this signal with another by bit-wise multiplexing this data will another such signal (from another OTU4/4V signal) when transmitting this data to the line.

In this case, an FEC might not be needed, because the OTU4/4V frames (that are being carried within this structure) might already have their own FEC.

Figure 8 presents an illustration of this type of OTUkV frame.

OTUkV Structure with Different Frame Structure and No FEC

Figure 8, Illustration of the OTUkV Frame with Different Frame Structure and No FEC Area

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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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