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
|
||
|
_ _ _ _ _ _ _
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||
|
SUFI k
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|
PADDING |
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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 |
|
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
|
7 |
6 |
5 |
4 |
3 |
2 |
1 |
0 |
|
0 |
0 |
0 |
0 |
0 |
0 |
1 |
0 |
|
0 |
0 |
0 |
0 |
0 |
0 |
0 |
0 |
|
0 |
1 |
0 |
0 |
|
|||
|
Padding |
|||||||
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
000000000100: LSN, Last Sequence Number: 4 (100)
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
Radio Link Control (RLC) protocol specification: 3GPP TS 25.322
Related Posts:
- UMTS RLC Status PDU: SUFI LIST
- UMTS RLC Status PDU: SUFI RLIST
- UMTS: Radio Link Protocol (RLC) Overview 25.322
- UMTS: RLC Length Indicator (RLC LI)
- UMTS Radio Link Protocol Decoder
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