Page 726 - 5G Basics - Core Network Aspects

2 Transport aspects

Table 8-10 – PTM-TC frame type and coding

Valid length
Coding
DTU frame type Header extension (bytes)
[t3t2t1t0]
(Note)
Idle 0000 No N/A
Complete data packet 1110 Yes 1-4 039
Complete eoc packet 1111 Yes 1-4 039
Start of data packet 1100 Yes 1-4 039
Start of eoc packet 1101 Yes 1-4 039

Continuation of the packet (data or eoc) 1000 Yes 1-4 039
End of the packet (data or eoc) 1010 Yes 1-4 039
Reserved by ITU-T All other N/A N/A
values
NOTE – The maximum DTU size is (255-2) × 16 = 4 048 bytes, and there is an overhead of nine bytes (three bytes DTU header,
four bytes ECS and two bytes DTU frame header). The component values in the maximum DTU size are as follows: 255 =
maximum length in bytes of the FEC codeword, 2 = the minimum number of redundancy bytes in the FEC codeword and 16 is
the maximum number of FEC codewords in one DTU as defined in Table 6-1.

An idle DTU frame shall only be used as the last frame of the DTU payload, with the length equal to the
number of remaining bytes of the DTU payload. The payload of an idle DTU frame is vendor discretionary.
An idle DTU frame may follow any other type of DTU frame in the DTU payload. If no other DTU frame type
is available for the DTU payload, an idle DTU frame shall be the only frame of the DTU payload.
A DTU generated in response to a dummy DTU request (see Table 8-7) shall contain only an idle DTU frame
and shall always be marked as a dummy DTU in the auxiliary field of the DTU header.
A DTU generated in response to a TX DTU Req (see Table 8-7) and containing only an idle DTU frame shall
be marked as either a dummy DTU or a normal DTU depending on the control parameter TPS_TESTMODE
(see Table 8-8).
If the TPS-TC is configured with TPS_TESTMODE disabled, a DTU generated in response to a TX DTU Req
primitive and containing only an idle DTU frame shall be marked as a dummy DTU in the auxiliary field of
the DTU header (see clause 8.2.1.3) except if only dummy DTUs have been transmitted over the α
reference point during a time interval of one superframe. In this last case, the DTU shall be marked as a
normal DTU (using the SID of normal DTU) and the count of time shall be reset.
NOTE – This mechanism guarantees a minimum background rate of normal DTUs for performance monitoring
purposes.
If the TPS-TC is configured with TPS_TESTMODE enabled, a DTU generated in response to a TX DTU Req
primitive and containing only an idle DTU frame shall be marked as a normal DTU in the auxiliary field of
the DTU header (see clause 8.2.1.3).

All DTUs marked in the DTU header as dummy DTUs shall be discarded by the receiver. All idle DTU frames
shall be identified at the receiver by decoding the DTU frame type (in the DTU frame header) and shall be
discarded by the receiver.

The payload of a complete data packet DTU frame shall consist of the original user data packet. The
payload of a complete eoc packet DTU frame shall consist of an eoc packet. If the original user data packet
or eoc packet is bigger than the remaining space available in the given DTU payload or bigger than a
payload of the entire DTU, it shall be spread over several DTUs using the following DTU frame types:

• start of data/eoc packet, followed by
• one or more continuation of this packet, followed by
• end of this packet.

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