LTE RLC PDU Structure

In this tutorial the basic of LTE RLC PDU structure is discussed. The detail encoding of RLC PDUs will be discussed in the upcoming tutorials.

In LTE RLC PDUs can be categorized as:
Data PDU

Control PDU

  • Status PDU

RLC Data PDU

Note: RLC SDUs are bit strings which are bye aligned (Multiple of 8 bits)

TM Data PDU (TMD PDU)

TMD PDU consists only of Data field and does not consist of any RLC header.

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UM Data PDU (UMD PDU)

UMD PDU consists of UMD PDU header and the data part.

UMD PDU Header

UMD PDU Header consists of:

Fixed Part

  • Framing Info (FI)
  • Extension bit (E)
  • Sequence Number (SN)

Extension Part

  • Extension bit (E)
  • Length Indicator (LI)

NOTE: RRC can configure UM RLC to use 5 bit or 10 bit SN

  • When 5 bit SN is used the length of the fixed part of the UMD header is 1 byte.
  • When 10 bit SN is used the length of the fixed part of the UMD header is 2 bytes with 3 bits of R1 fields.

There are different types of encoding schemes of UMD PDUs can be possible.

NOTE: The different encoding schemes will be described with examples in upcoming tutorials.

UMD PDU with 5 bit SN

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UMD PDU with 10 bit SN (No LI)

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UMD PDU with 5 bit SN (Odd number of LIs)

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UMD PDU with 5 bit SN (Even number of LIs)

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UMD PDU with 10 bit SN (Odd number of LIs)

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UMD PDU with 10 bit SN (Even number of LIs)

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AM Data PDU (AMD PDU)

AMD PDU is byte aligned. The AMD PDU header part consists of a fixed part and an extension part.

AMD PDU Header

Fixed Part

  • Data/Control bit (D/C)
  • Re-segmentation Flag (RF)
  • Polling bit (P)
  • Framing Info (FI)
  • Extension bit (E)
  • Sequence Number (SN)

Extension Part

  • Extension bit (E)
  • Length Indicator (LI)

AMD PDU (No LI)

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AMD PDU (Odd number of LIs)

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AMD PDU (Even number of LIs)

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AMD PDU Segment

AMD PDU segment header consists of a fixed part and an extension part.

Fixed part

  • Data/Control (D/C)
  • Re-segmentation Flag (RF)
  • Polling bit (P)
  • Framing Info (FI)
  • Extension bit (E)
  • Sequence Number (SN)
  • Last segment Flag (LSF)
  • Segment Offset (SO)

Extension part

  • Extension (E)
  • Length Indicator (LI)

AMD PDU segment (No LI)

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AMD PDU segment (Odd number of LIs)

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AMD PDU segment (Even number of LIs)

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RLC Control PDU

Status PDU

STATUS PDU consists of a STATUS PDU payload and a RLC control PDU header.

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NOTE: All the RLC PDU structure will be described with examples in separate tutorials.

Reference