The AMP (Asynchronous Mapping Procedure)

This post discusses and describes the AMP (Asynchronous Mapping Procedure) that one can use to map client signals into OTN signals and transport them through the Optical Transport Network (OTN).

What is the AMP (Asynchronous Mapping Procedure)?

This post describes the AMP (Asynchronous Mapping Procedure) for mapping a non-OTN CBR (Constant Bit Rate) client signal into an OPUk/ODUk signal.

ITU-T G.709 also states that the system designer can use AMP to map lower-speed ODUj tributary signals into a higher-speed OPUk server signal.  We will discuss that topic in another post.

NOTE:  Whenever ITU-T G.709 discusses procedures on how to map a CBR client signal into an OPUk/ODUk signal, it will often refer to the OPUk/ODUk signal as the Server signal.  We will use the terms OPUk/ODUk and Server interchangeably throughout the rest of this post.

ITU-T G.709 also defines two other mapping procedures, that one can use to map a non-OTN CBR client signal into a Server signal.

We discuss each of these other two mapping procedures in other posts.

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What is the Asynchronous Mapping Procedure?

The name Asynchronous Mapping Procedure means that the timing relationship between the client signal (being mapped into an OPUk payload) and the bit-rate of the OPUk payload are close in frequency but still asynchronous.

This timing relationship is different from that for the BMP (Bit-Synchronous Mapping Procedure).

In the case of BMP, the timing relationship between the Client Signal and the Server signal must be synchronized.  For AMP, we can say (in a “tongue-in-cheek manner”) that the timing relationship between the Client Signal and the Server signal is “close, but no cigar”!!  I’ll explain that comment below.

The System Designer must ensure that ALL the following is true before he/she can use the Asynchronous Mapping Procedure to map a non-OTN client signal into the OPUk payload.

  • The Client signal clock frequency must be within ±65pm of the OPUk Payload clock frequency.
  • The System-Designer must handle Rate differences (between the Client and the Server signal) via both fixed and variable stuffing.
  • This ±65ppm tolerance accounts for the maximum variable stuffing (justification) limits.

How AMP Works

We begin our discussion of AMP by taking a look at the OPUk Overhead bytes.

Figure 1 presents an illustration of the OPUk Frame (within the OTUk Frame).

OTUk Frame with OPUk Portion shown

Figure 1, Illustration of the OPUk Frame – within the OTUk Frame.

Figure 2 takes a closer look at the OPUk frame structure and shows a more detailed drawing of the OPUk Overhead Bytes within the OPUk Frame.

AMP Discussion - Basic Figure - Introduction of OPUk OH

Figure 2, Illustration of the OPUk Overhead Bytes

This figure shows five (5) Overhead Bytes that play a role in AMP.

  • JC1 – Justification Control Byte # 1
  • JC2 – Justification Control Byte # 2
  • JC3 – Justification Control Byte # 3
  • NJO – Negative Justification Opportunity Byte
  • PJO – Positive Justification Opportunity Byte

Figure 2 also breaks out and shows the bit format for each of the three Justification Control Bytes.  This figure shows that Bits 1 through 6 (within each of these 3 bytes), are labeled as RES (Reserved) and do not have a role in AMP.

But, this figure also shows Bits 7 and 8 (within these 3 bytes), which we have labeled JC7 and JC8, do have a role in AMP.

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Justification Events within an OPU Frame

We earlier stated that if the System Designer wishes to use AMP to map a non-OTN client signal into an OPUk frame, then he/she must make sure that the frequency differences between the client signal and that for the OPUk payload are within ±65ppm.

Since there is no requirement to phase-lock the OPUk Payload frequency to the client signal frequency (as ITU-T G.709 requires for BMP applications), it is unlikely that these two signals will be of the same rate.  

They will, most likely, be close (in frequency) to each other, but off in one direction or the other.

In this case, the Source (OTN) Path Terminating Equipment (PTE) will generate many of its OPUk frames with no justification events.  But, because these two signal frequencies will typically not be identical, the Source PTE will eventually have to generate OPUk frames with justification events.  We sometimes call these justification events, slip events in other forms of data communication.

NOTE:  These justification events are also similar to the pointer justification events that can occur in SONET/SDH Networks.

The Source PTE will use these Justification Events to compensate for the frequency differences between the client and the server signal.  

Please see the post on “slip events” to understand the mechanics of slip events (e.g., why they occur and how we can elegantly handle these events).

In AMP, a Source PTE can generate three (3) different types of OPUk frames.

  • OPUk frames with NO Justification Events (e.g., The Nominal Condition)
  • OPUk frames with Negative-Justification events (occurs periodically, if the Client Bit Rate > OPUk Bit Rate)
  • OPUk frames with Positive-Justification events (occurs regularly, if the Client Bit Rate < OPUk Bit Rate)

We will discuss each of these types of OPUk frames below.

The Normal Situation – The Source PTE creates an OPUk Frame with No Justification Activities

For AMP applications, many of the OPUk frames, that a Source PTE generates, will belong to this category.

Figure 3 shows a drawing of an OPUk Frame, with NO Justification events occurring.

OPUk Frame - AMP Discussion - No Justification Event

 

Figure 3, An Illustration of an OPUk Frame with NO Justification events occurring

This figure shows that this OPUk frame does NOT have a Justification Event by:

  • Setting at least two (out of the three) sets of JC7 and JC8 bit-fields to [0, 0] as we show above.
  • The PJO byte field is currently carrying a client data byte (as it usually does) – in this OPUk frame.
  • The NJO byte field is NOT transporting a client data byte but is (instead) carrying a stuff byte (which is also a nominal condition).

The settings within the JC7 and JC8 bit-fields alerts the Sink PTE (at the other end of the network) that there is no justification event occurring within this OPUk frame and:

  • That the PJO byte-field is currently carrying client data, and
  • That the NJO byte field is transporting a stuff-byte (and is NOT transporting client data).

NOTE:  If (by design or accident) the OPUk payload rate is equivalent to the Client bit-rate, then the Source PTE will ALWAYS generate these types of OPUk frames (e.g., with NO Justification Events). 

If the System Designer has achieved this synchronous relationship (between the OPUk and the Client signal) by design, then this becomes Bit-Synchronous Mapping (BMP).

Negative Stuff Event, (Occurs if the Client Bit Rate > OPUk Bit Rate)

If the Client-data bit-rate is slightly faster than that for the OPUk payload rate, then (with no intervention) the Mapper Buffer will eventually fill-up.

AMP uses Negative-Stuffing (or Negative Justification) to prevent the Mapper buffer from Overflowing.  

In this case, the Source PTE will (temporarily) increase our Server (OPUk) signal’s bandwidth (to pull additional data out of the Mapper Buffer), by forcing both the PJO and the NJO bytes to carry client-data momentarily.

Hence, for systems in which the Client data bit rate is faster than the OPUk payload bit rate, the Source PTE will periodically need to generate OPUk frames with Negative Justification, to keep the Mapper Buffer from filling up.

Figure 4 shows a drawing of such an OPUk frame.

OPUk AMP Discussion - Negative Justification

 

Figure 4, Illustration of an OPUk Frame with Negative Justification occurring

This figure shows that this OPUk frame has a Negative Justification Event by setting at least two (of the three) sets of JC7 and JC8 bits to [0, 1], as illustrated above.  

The Source PTE will also (for this particular OPUk) use both the NJO and the PJO byte fields to carry client data bytes.

When the Source PTE sets the JC7 and JC8 bit-fields to these values, it is notifying the Sink PTE (at the remote end of the network) of the roles of the NJO and PJO bytes, within this OPUk frame.

NOTES:

  1. For the NO Justification OPUk frames, the NJO byte is considered to be a part of the OPUk Overhead and usually does not transport any client data.  But, for Negative Justification OPUk frames, the NJO byte will (for this OPUk frame) be carrying client data.
  2. If the Client-data bit-rate is faster than the OPUk Payload rate, then the PJO byte will ALWAYS carry client data (in every OPUk frame). In this case, we will sometimes use the NJO byte to transport client data (within OPUk frames with justification events).

Positive Stuff Event, (Occurs if the Client Bit Rate < OPUk Bit Rate)

If the Client-data bit-rate is slightly slower than that for the OPUk Payload rate, then (with no intervention) the Mapper Buffer will eventually be depleted.

AMP uses Positive-Stuffing (or Positive Justification) to avoid depleting the Mapper buffer.  

In this case, the Source PTE will temporarily reduce the Server signal’s (e.g., the OPUk signal) bandwidth (for this OPUk frame), by forcing both the PJO and the NJO bytes to transport stuff (and NOT client) data.

Hence, for designs in which the Client data bit rate is slower than the OPUk payload, the Source PTE will periodically generate OPUk frames with Positive Justification, to avoid depleting the Mapper Buffer.

NOTE:  For the NO Justification OPUk Frames, the PJO byte field will be carrying client data.  But, during Positive Justification frames, the PJO byte will instead, transport a stuff (or dummy) byte.  In other words, the Positive Justification OPUk frame will not carry client data within the PJO byte-field. 

This action momentarily slows the rate at which the Server is pulling data out of the Mapper Buffer, and keeps the buffer from being depleted.

Figure 5 shows a drawing of an OPUk frame with Positive Justification occurring.

OPUK Frame - AMP Discussion - Positive Justification Frame

 

Figure 5, Illustration of an OPUk Frame with Positive Justification occurring

This figure shows that this OPUk frame has a Positive Justification event by setting at least two (of the three) sets of JC7 and JC8 bits to “[1, 1].

When the Source PTE sets the JC7 and JC8 bit-fields to these values, it is notifying the Sink PTE (at the remote end of the network) of the roles of the NJO and PJO bytes, within this OPUk frame.

NOTE:  If the Client data rate is slower than the OPUk Payload rate, then the NJO byte will ALWAYS function as a stuff byte (e.g., never carrying client data) and the PJO will sometimes function as a stuff byte (during OPUk frames with justification events).

Interpreting the JC7 and JC8 Bits

Table 1 summarizes how to understand the settings of the JC7 and JC8 bits (within two out of the three Justification Bytes) and the corresponding roles for the NJO and PJO bytes within the OPUk Frame.

Table 1, How to Interpret the Settings of the JC7 and JC8 bits within Two (of the Three) Justification Bytes, and the corresponding roles for the NJO and PJO bytes within the OPU Frame – When Transporting a Non-OTN Client Signal

Relationship between JC7 and JC8 bits and the Justification of OPUk

AMP and De-Mapping Jitter

AMP poses some challenges for the System Designer’s efforts to control de-mapping jitter to meet system requirements.  BMP offers the best de-mapping jitter, of any of the three, recommended Mapping Procedures.  

However, AMP imposes all of the following contributions (and challenges) to meeting de-mapping jitter requirements.

  • The presence of OTUk/ODUk/OPUk overhead bytes within the OTN signal (this challenge exists for BMP as well)
  • Fixed-byte stuffing – while mapping the client signal into the OPUk frame (this challenge exists for BMP as well)
  • Justification Events, which are also known as variable-stuffing, is unique to AMP.

Each Justification Event imposes 8UI-pp of mapping-related jitter within the client signal.

The System Designer will need to implement some clock-smoothing or jitter attenuation scheme to comply with de-mapping jitter requirements.

If the System Designer is transporting SONET/SDH data (which has very stringent jitter requirements) through the OTN, then we recommend that they use BMP instead of AMP.  Otherwise, the designer will have to implement some very robust jitter attenuation solutions (at the Sink PTE) when de-mapping this client signal from the OPUk frame.

ITU-T G.709 Recommendations on Using AMP

Table 2 lists out the Non-OTN Client signals that ITU-T G.709 recommends using AMP when mapping these signals into each of the OPUk/ODUk Structures.

Table 2, List of Client Signals that ITU-T G.709 Recommends using AMP when mapping into an OPUk Structure

ITU-T G.709 Recommendations for AMP - Asynchronous Mapping Procedure

Can we use AMP for all OPUk rates?

We can use AMP for the OPU1, OPU2, and OPU3 server signals.  However, we cannot use AMP for mapping client signals into an OPU0 or OPU4 server signal.

ITU-T G.709 recommends that we use GMP for mapping client signals into OPU0 and OPU4 signals.

Summary

Table 3 summarizes the timing requirements (between the Client Clock Signal and that for the OPUk/ODUk clock) that the System Designer must follow before using any of the ITU-T G.709 Recommended Mapping Procedures.  Please note that I have highlighted the AMP items (within Table 3) with a “Red Rectangular” outline.

Table 3, Mapping Procedure Timing Requirements

ITU-T G.709 Requirements to use AMP - Asynchronous Mapping Procedure

 

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