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Content for  TS 38.323  Word version:  18.0.0

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1…   4…   5…   5.4…   5.8…   5.13…   6…   6.2.3…   7…   A…   B… 
5.13 Uplink data switching  5.14 Uplink Data compression and decompression  5.14.1 UDC protocol  5.14.2 Configuration of UDC  5.14.3 UDC header  5.14.4 Uplink data compression  5.14.5 PDCP Control PDU for UDC feedback  5.14.6 Pre-defined dictionary  5.14.7 UDC buffer reset procedure  5.14.8 UDC checksum error handling  5.15 Data volume calculation for delay status reporting 
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5.13  Uplink data switching |R16|  p. 31

For DAPS bearers, when upper layers request uplink data switching, the transmitting PDCP entity shall:
  • for AM DRBs, from the first PDCP SDU for which the successful delivery of the corresponding PDCP Data PDU has not been confirmed by the RLC entity associated with the source cell, perform retransmission or transmission of all the PDCP SDUs already associated with PDCP SNs in ascending order of the COUNT values associated to the PDCP SDU prior to uplink data switching to the RLC entity associated with the target cell as specified below:
    • perform header compression of the PDCP SDU using ROHC as specified in the clause 5.7.4;
    • perform integrity protection and ciphering of the PDCP SDU using the COUNT value associated with this PDCP SDU as specified in the clause 5.9 and 5.8, respectively;
    • submit the resulting PDCP Data PDU to lower layer, as specified in clause 5.2.1.
  • for UM DRBs, for all PDCP SDUs which have been processed by PDCP but which have not yet been submitted to lower layers, perform transmission of the PDCP SDUs in ascending order of the COUNT values to the RLC entity associated with the target cell as specified below:
    • perform header compression of the PDCP SDU using ROHC as specified in the clause 5.7.4;
    • perform integrity protection and ciphering of the PDCP SDU using the COUNT value associated with this PDCP SDU as specified in the clause 5.9 and 5.8, respectively;
    • submit the resulting PDCP Data PDU to lower layer, as specified in clause 5.2.1.
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5.14  Uplink Data compression and decompression |R17|  p. 32

5.14.1  UDC protocol  p. 32

The UDC protocol is based on RFC 1951 (DEFLATE Compressed Data Format Specification).
Static Huffman coding tree defined in RFC 1951 is used as the DEFLATE compression strategy.
UDC Data Block should be byte-alignment. Z_SYNC_FLUSH is used as the DEFLATE byte-alignment with corresponding reference RFC 1979, wherein the fixed last four bytes, 0x00 0x00 0xFF 0xFF, are removed before transmission.
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5.14.2  Configuration of UDC  p. 32

The PDCP entities associated with DRBs can be configured by upper layers, see TS 38.331, to use UDC. If UDC is configured, the UE shall apply UDC compression function (details see Annex B) to process the received PDCP SDU from upper layers corresponding to the configured DRB. The size of compression buffer is configured by upper layers via bufferSize. If pre-defined dictionary is configured by upper layers, the UE shall first set the compression buffer to all zeros and then prefill the configured pre-defined dictionary in the compression buffer upon configuration of UDC. If pre-defined dictionary is not configured by upper layers, UE shall set the compression buffer to all zeros.
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5.14.3  UDC header  p. 32

UDC header (1 byte) is added in UDC compression function followed by UDC data block. The UDC header contains the information about whether the current PDCP SDU is compressed by UDC protocol or not. Only the compressed packets are stored in the buffer. The UDC header also contains a reset bit to inform the decompressor that the compression buffer has been reset. The validation bits (checksum) of the compression buffer are also contained in UDC header. Checksum mechanism could be used to resolve miss-match (if any) between the compression and de-compression buffers. If both SDAP and UDC are configured for a DRB, the UDC header shall be located after the SDAP header. Figure 5.14.3-1 shows the location of the UDC header in a PDCP data PDU.
Reproduction of 3GPP TS 38.323, Fig. 5.14.3-1: Location of UDC header in a PDCP Data PDU
Figure 5.14.3-1: Location of UDC header in a PDCP Data PDU
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5.14.4  Uplink data compression  p. 33

The UDC protocol generates UDC packets, each associated with one PDCP SDU.
A UDC packet is associated with the same PDCP SN and COUNT values as the related PDCP SDU. The uplink data compression is not applicable to the SDAP header and the SDAP Control PDU if included in the PDCP Data PDU.

5.14.5  PDCP Control PDU for UDC feedback  p. 33

At reception of a PDCP Control PDU for UDC feedback from lower layers, the receiving PDCP entity shall:
  • deliver the corresponding UDC feedback to the UDC protocol without performing deciphering/integrity verification.

5.14.6  Pre-defined dictionary  p. 33

One standard dictionary for SIP and SDP and one operator defined dictionary can be used as pre-defined dictionaries in UDC. The standard dictionary for SIP and SDP consists of the first 3468 bytes of the dictionary for SigComp defined in RFC 3485. When UDC is configured, at most one dictionary, configured by upper layers, is put into the tail of the compression buffer. Also, the compression buffer acts as a FIFO and hence the content of the dictionary is to be totally pushed out of the compression buffer after the size of transmitted uncompressed packets compressed by UDC exceeds the compression buffer size. If the size of dictionary is larger than the compression buffer size, only the tail of the dictionary is inserted in the compression buffer.
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5.14.7  UDC buffer reset procedure  p. 33

UDC works on the condition that compression buffer and de-compression buffer are synchronized. UDC buffer reset mechanism is to resynchronize buffer when error is detected. For resynchronization, UE shall reset the compression buffer to all zeros.

5.14.8  UDC checksum error handling  p. 33

UDC checksum error notification PDCP control PDU indicates the compression buffer and de-compression buffer are out of synchronization. When receiving the notification, the UE shall trigger UDC buffer reset procedure to resynchonize the compression buffer.

5.15  Data volume calculation for delay status reporting |R18|  p. 33

For the purpose of MAC delay status reporting, the transmitting PDCP entity shall consider the following as delay-critical PDCP data volume:
  • the delay-critical PDCP SDUs for which no PDCP Data PDUs have been constructed;
  • the PDCP Data PDUs that contain the delay-critical PDCP SDUs and have not been submitted to lower layers;
  • the PDCP Control PDUs;
  • for AM DRBs, the PDCP SDUs to be retransmitted according to clause 5.1.2 and clause 5.13;
  • for AM DRBs, the PDCP Data PDUs to be retransmitted according to clause 5.5.
If a PDCP SDU becomes a delay-critical PDCP SDU, and if the corresponding PDCP Data PDU has already been submitted to lower layers, the delay-critical indication for the PDCP Data PDU is provided to lower layers.
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