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.

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.

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

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

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:

1. UMTS RLC Status PDU: SUFI LIST
2. UMTS RLC Status PDU: SUFI RLIST
3. UMTS: Radio Link Protocol (RLC) Overview 25.322
4. UMTS: RLC Length Indicator (RLC LI)
5. UMTS Radio Link Protocol Decoder