UMTS RLC Status PDU: SUFI NO_MORE & SUFI ACK

The STATUS PDU is used to exchange status information between two RLC AM entities.

Why STATUS PDU is required

  • by the receiving entity to inform the transmitting entity about missing PDUs at the receiving entity;
  • by the receiving entity to inform the transmitting entity about the size of the allowed transmission window;
  • by the transmitting entity to request the receiving entity to move the receiving window.
D/C PDU Type SUFI
SUFI 1
– – – – –
SUFI k
Padding

NOTE: Status PDUs are octet aligned i.e. their length must be multiple of 8 bits.

Status PDU Parameters

D/C

This one bit field tells if the PDU is a Status PDU or AM Data PDU.

  • If D/C = 1, then the PDU is AM data PDU
  • If D/C = 0, then PDU is a Status PDU

PDU Type

The PDU type field indicates the type of Control PDU.

Bit PDU Type
000 Status PDU
001 Reset PDU
010 Reset-Ack PDU
011-111 Reserved

SUFI: Super Field

Super-Field indicates which AMD PDUs are received correctly and which are missing.
SUFI has three sub-fields:

  • Type
  • Length
  • Value

Type field is 4 bits long and may have one of the following values:

  • Super-Field indicates which AMD PDUs are received correctly and which are missing.
  • SUFi has three sub-fields:
    • Type
    • Length
    • Value
Bits Description
0000 No more data (NO_MORE)
0001 Window Size (WINDOW)
0010 Acknowledgement (ACK)
0011 List (LIST)
0100 Bitmap (BITMAP)
0101 Relative list (Rlist)
0110 Move Receiving Window (MRW)
0111 Move Receiving Window Acknowledgement (MRW_ACK)
1000 Poll (POLL)
1001-1111 Reserved

No more data SUFI

No more data SUFI indicates the end of a Status PDU. Everything after this SUFI will be regarded as padding and will be discarded.
NO_MORE SUFI is always the last SUFI if it is included in a STATUS PDU.

Acknowledgement Super Field

  • The ‘Acknowledgement’ super-field consists of a type identifier field (ACK) and a sequence number (LSN).
  • Acknowledges the reception of all AMD PDUs with “Sequence Number” < LSN (Last Sequence Number) that are not indicated to be erroneous in earlier parts of the STATUS PDU.
  • LSN is of 12 bits.
  • SUFI ACK is always the last SUFI if it is included in a Status PDU. So SIFI NO_MORE and SUFI ACK can not be included in the same Status PDU.

Example

0 0 0 0 0 0 1 0
0 0 0 0 0 0 0 0
0 1 0 0

Explanation

Octet 1:

0: D/C: Data or Control. As it is a Control PDU, this bit is set to 0.
000: Control PDU Type: It is a STATUS PDU, so 000.
0010: SUFI Type. 0010 means SUFI ACK

Octet 2 & 3:
000000000100: LSN, Last Sequence Number: 4

LSN is encoded as 4, this means all PDUs with Sequence Number less than 4 are correctly received.
In this example PDUs with Sequence Number 0, 1, 2, 3 are correctly received.

Padding: The rest part of the Control PDU is padding.

NOTE: Other RLC SUFIs will be described in separate tutorials.

Reference

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9 Comments




  1. Hi Prashant,

    This is excellent information on RLC.

    Can you please take a example scenario and explain the operation of the AM RLC ????


  2. I will try to add some RLC Tutorials in future. Is there any specific AM operation you want to learn?


  3. Hi Prashant,

    I am basically interested in learning the whole procedure of AM RLC transmit side and receiving side operation.

    Also, a additional information on the MAC layer procedures ( For e.g:- Process of flow for MAC – d and MAC – hs ) would be helpfull.

    If you are having time constraints, then please point me to the right set of documents to read.


  4. Hi,

    Thanks for your feedback. I will add some more Layer 2 tutorials.


  5. Hello Prashant,

    Can you please tell me how these value are decoded as

    02 98 80 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

    Decoded

    D/C FIELD [1 Bit ] :: 0 [Acknowledged mode Control PDU]

    PDU TYPE [3 Bits] :: 0 [ STATUS_PDU ]

    SUFI TYPE [4 Bits] :: 2 [ SUFI_ACK ]

    LSN [12 Bits] :: 2440 [ Last Sequence Number ]

    Rest is Padding

    000000000000000000000000000000000000000000000000000000000000000000000000000000

    My doubt is regarding to the LSN 2440, as we can see the 12 bits are 98 8 how we are decoding it to 2440.


  6. uttam ,.. 988 is in HEX ,, u hav to convert it to DECIMAL.. 988 Hex is 2440 in decimal.

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