- The PSI Message is a 256-byte Message, that a given Source PTE (Path Terminating Equipment) will repeatedly transmit to the Sink PTE.
- The Source PTE will repeatedly transmit this PSI Message via the PSI byte (within each ODUk/OPUk frame).
- The purpose of this PSI Message is to permit the Source PTE to inform the Sink PTE of the type of traffic that this particular ODUk signal is transporting.
- The first byte (Byte 0 – within the PSI Message) will be the PT (or Payload Type) byte.
- This means that there are still 255 other bytes, that are available to transport information, within each PSI Message.
I show an illustration of the PSI byte-field and the PSI Message below in Figure 1.
Figure 1, Illustration of the PSI byte-field and PSI Message
A Multiplex Structure ODUk
If the Source PTE (that is transmitting an ODUk signal to the remote Sink PTE) has set the PT byte value (within each PSI Message) to either 0x20 or 0x21, then this means that this ODUk signal is a Multiplex Structure ODUk signal.
If a given ODUk signal is a Multiplex Structure ODUk signal then this means that it is transporting at least lower-speed ODUj signal within its payload (where k > j).
n this case, the Source PTE (or upstream circuitry) has mapped and multiplexed lower-speed ODUj signals into this particular higher-speed ODUk signals.
For the OTN to work properly, the Source PTE needs to send sufficient information to Sink PTE, about the type of traffic/data that a given ODUk signal is carrying.
The Sink PTE needs more information than the PT byte value
So if the Source PTE sets the PT Byte value (within each outbound PSI Message) to 0x20 or 0x21, then it is “telling the Sink PTE” that this ODUk signal is a Multiplex Structure signal.
However, the Sink PTE needs more information for it to be able to properly handle this ODUk data-stream.
In particular, the Sink PTE needs to “know” how many, and what type of lower-speed signals that this ODUk is signal transporting.
Let’s take a Look at an ODU4/OPU4 Signal
For example, if we are dealing with an ODU4 signal, and if the PT byte is set to 0x21, then the PSI Message (that this ODU4/OPU4 signal transports) could have the format that we show below in Figure.
Figure 2, Illustration of the PSI Message for an ODU4/OPU4 that is transporting 80 ODU0 signals
Figure 2 shows the PSI Message, that a Source PTE would transport (within an ODU4 signal) if that ODU4 signal was transporting 80 ODU0 signals (that have been mapped and multiplexed into this ODU4).
Please note that ODU4/OPU4 signals can transport other types of multiplexed traffic. For example, it can transport any of the following types of multiplexed traffic.
- 80 ODU0 signals.
- 40 ODU1 signals
- 10 ODU2 or ODU2e signals
- 2 ODU3 signal
- Some number of ODUflex signals (provided that the total bandwidth of all of these signals does not exceed 80 time-slots or the OPU4 payload carrying capacity of 104.35597533 Gbps).
- Various combinations of each of the above signals (again, provided that the total bandwidth of all of these signals does not exceed 80 time-slots).
How to Read/Decipher these Multiplex Structure Identifier fields
Figure 2 shows that each PSI Message (starting at Byte 2) has 80 consecutive bytes of data, in which bit 1 (the left-most bit) is set to the value of “1”. When bit 1 (within these fields) is set to “1”, then it means that some lower-speed ODUj signal is using this particular time-slot (or bandwidth).
These 80 bytes (of data) are the Multiplex Structure Identifier for this ODU4 signal. When the Sink PTE reads in these PSI Messages, it will “know” that this particular ODU4/OPU4 signal is transporting 80 lower-speeds ODUj (ODU0) signals. In this case, the Sink PTE has sufficient information about this ODU4/OPU4 signal, to be able to handle it properly.
NOTE: we mention that each time-slot (for PT = 0x21 applications) represents approximately 1.25Gbps of bandwidth.
I have also included below Figure, which will help you better understand these Multiplex Structure Identifier fields.
Figure 3, Multiplex Structure Identifier – Bit Definitions for ODU4/OPU4 Applications.