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

• 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