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.
|– – – – –|
NOTE: Status PDUs are octet aligned i.e. their length must be multiple of 8 bits.
Status PDU Parameters
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
The PDU type field indicates the type of Control PDU.
SUFI: Super Field
Super-Field indicates which AMD PDUs are received correctly and which are missing.
SUFI has three sub-fields:
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:
|0000||No more data (NO_MORE)|
|0001||Window Size (WINDOW)|
|0101||Relative list (Rlist)|
|0110||Move Receiving Window (MRW)|
|0111||Move Receiving Window Acknowledgement (MRW_ACK)|
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.
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.
- Radio Link Control (RLC) protocol specification: 3GPP TS 25.322
- WCDMA Design Handbook
- UMTS Signaling: UMTS Interfaces, Protocols, Message Flows and Procedures Analyzed and Explained