Page 1261 - 5G Basics - Core Network Aspects

Transport aspects 2

As the client data has to fit into the payload area of the server signal, the maximum value of Cm and as such
the maximum client bit rate is limited by the size of the server payload area.

C m   Pt  m, server (D-17)
Pm,server: maximum number of (m bits) data entities in the server payload area

In order to extract the correct number of client information entities at the de-mapper, Cm(t) has to be
transported in the overhead area of the server frame or multiframe from the mapper to the de-mapper.

At the mapper, Cn(t) is determined based on the client and server clocks. The client data is constantly
written into the buffer memory. The read out is controlled by the value of Cm(t).

At the de-mapper, Cm(t) and CnD(t) are extracted from the overhead and used to compute Cn(t). Cm(t)
controls the write enable signal for the buffer. The client clock is generated based on the server clock and
the value of Cn(t).

Cn(t) has to be determined first, then it has to be inserted into the overhead as Cm(t) and CnD(t) and
afterwards Cm(t) client data entities have to be inserted into the payload area of the server as shown in
Figure D.5.

The Cn(t) value determines the Cm(t) and CnD(t) values; Cm(t) = floor (n/m  Cn(t)) and CnD(t) = Cn(t) – (m/n 
Cm(t)). The values of CnD(t) are accumulated and if CnD(t) ≥ m/n then m/n is subtracted from CnD(t) and
Cm(t) is incremented with +1. These latter two values are then encoded in the overhead bytes. This Cm(t)
value is applied as input to the sigma-delta process.

Payload Payload
OH OH
area area
Determine C n
Payload Payload
OH OH
area area
Insert C and C into OH Extract C and C from OH
m nD m nD
Payload Insert C Payload Extract C
m
OH area client data OH area client data m
Mapper Demapper
G.709-Y.1331(12)_FD.5

Figure D.5 – Processing flow for GMP in OTN

During start-up or during a step in the client bit rate, the value of Cn(t) will not match the actual number of
n-bit client data entities arriving at the mapper buffer and the Cn(t) determination process has to adjust its
value to the actual number of n-bit client data entities arriving. This adjustment method is implementation
specific. During the mismatch period, the mapper buffer fill level may increase if more n-bit client data
entities arrive per multiframe than there are transmitted, or decrease if less n-bit client data entities arrive
per multiframe than there are transmitted.
To prevent overflow or underflow of the mapper buffer and thus data loss, the fill level of the mapper
buffer has to be monitored. For the case where too many m-bit client data entities are in the buffer, it is
necessary to insert temporarily more m-bit client data entities in the server (multi)frame(s) than required
by Cn(t). For the case too few m-bit client data entities are in the buffer, it is necessary to insert temporarily
fewer m-bit client data entities in the server (multi)frame(s) than required by Cn(t). This behaviour is similar
to the behaviour of AMP under these conditions.
The OTN supports a number of client signal types for which transfer delay (latency) and transfer delay
variation are critical parameters. Those client signal types require that the transfer delay introduced by the
mapper plus de-mapper buffers is minimized and that the delay variation introduced by the mapper plus
de-mapper buffers is minimized.

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