Catch Up

You may need the following tutorial:

• UMTS RLC Status PDU: SUFI NO_MORE & SUFI ACK
• UMTS RLC Status PDU: SUFI LIST

RLIST (Relative- List) Super field

The SUFI RLIST consists of the following fields:

• Type identifier field(RLIST)
• LENGTH Filed
• First Sequence Number(FSN)<7li>
• Code Words (CW1, CW2, …, CWLENGTH )

The SUFI RLIST fields in the STATUS PDU are as follows:

LENGTH

The LENGTH is a 4bits long field. The LENGTH field signifies the number of codewords (CW) in the super field of type RLIST.

FSN

The length of the FSN super field is of 12 bits. The FSN is the “Sequence Number” of the first erroneous AMD PDU in the RLIST. When the LENGTH field is populated as “0000” at that time FSN is only present in the SUFI and it is the only erroneous AMD PDU.

CW

CW or codeword field is of 4 bits long. The encoding of the codeword field is as follows:

The first three bits of the CW are part of the number and the last one bit is a status indicator. The encoding of the last bit of the CW field as 0 or 1 can be as following:

By default, the number given by the CWs represents a distance between the previous indicated erroneous AMD PDU up to and including the next erroneous AMD PDU.

One special value of CW is defined:

000 1: ‘Error burst indicator’

The error burst indicator means that the next CWs will represent the number of subsequent erroneous AMD PDUs (not counting the already indicated error position).

After the number of errors in a burst is terminated with XXX 1, the next codeword will again by default be the least significant bits (LSB) of the distance to the next error.

Special case while the STATUS PDU will be discarded:

If the last CW, as indicated by the value of the LENGTH field, does not contain a “1″ in its rightmost position, or the last CW, as indicated by the value of the LENGTH field does contain a “1″ in its rightmost position, but is a special “error burst indicator” CW, the encoding of the RLIST SUFI is invalid, and the STATUS PDU is discarded.

Example

Suppose following erroneous PDU Sequence Numbers need to be reported in Rlist SUFI:

3, 5, 6, 7, 8, 9, 10

In this case there are a number of PDUs detected as erroneous starting from sequence number 5. So the encoding of the SUFI RLIST will be as follows:

Step #1
FSN = 3.
So the four bit FSN field will be encoded as:
FSN = 3 (%0000 0000 0011)

Step #2
The LENGTH = 3
LENGTH = 0011

Step #3
The next SN = 5.
Distance = 5 – 3 = 2

CW1 = 0101 (As the distance is 2 and again the codeword is terminated here so the last bit is 1).

Step #4
Now there is error burst continues up to sequence number 10. So the special burst indicator will be used.
So now the distance will be 5. So the encoding of the codewords CW2 and CW3 will be as follows:

CW2 = 0001 (Special error bust indicator is encoded here while the next CW will signify the length of the error burst)

CW3 = 1011 (The distance from sequence number 5 to 10 is 5 and also the distance is terminated in this CW. So the last bit is 1)

Step #5
The encoding of the SUFI RLIST as follows:

Reference

1. Radio Link Control (RLC) protocol specification: 3GPP TS 25.322
2. WCDMA Design Handbook

The List Super-Field consists of a type identifier field (LIST), a list length field (LENGTH) and a list of LENGTH number of pairs.

LENGTH

• Length: 4 bits
• The number of (SNi, Li) pairs in the super-field of type LIST.
• The value 0000 is INVALID and STATUS PDU is discarded.

• Length: 4 bits
• Number of consecutive AMD PDUs not correctly received following AMD PDU with”Sequence number” SNi.

Example

In this example:

• RLC AMD PDUs with Sequence Number 0, 1, 2, 3, 6, 7 are received correctly
• PDUs with sequence number 4 and 5 are missing.

Encoding of Status PDU

Explanation

Octet #1:

0: D/C: Data or control PDU. As it is a control PDU this bit is set to 0.
000: Control PDU Type: 000 indicates it is a Status PDU.
0011: SUFI Type. This is SUFI LIST

Octet #2 and #3

0001: LENGTH field. This indicates the number of (SNi, Ni) pairs. Here there is only one pair
0000 0000 0100: (SNi): The start sequence number. This is the Sequence number of the missing PDU. Here the missing PDUs start from Sequence Number 4 (100).
0001: (Li): The number of consecutive missing PDUs following sequence number 4 (SNi). Here there is only 1 (Sequence number 5)

Rest of the Status PDU may be encoded for SUFI ACK and Padding.

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

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

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

The Length Indicator is used to indicate, each time, the end of an SDU occurs in the PDU. The Length Indicator points out the number of octets between the end of the last Length Indicator field and up to and including the octet at the end of an SDU segment. Length Indicators are included in the PDUs that they refer to. The size of the Length Indicator may be either 7 bits or 15 bits. A Length Indicator group is a set of Length Indicators that refer to a PDU. If there can be more than one Length Indicator, each specifying the end of an SDU in a PDU, the order of these Length Indicators must be in the same order as the SDUs that they refer to. In the case where the end of the last segment of an SDU exactly ends at the end of a PDU and there is no LI that indicates the end of the SDU, the next Length Indicator, shall be placed as the first Length Indicator in the following PDU and have value LI=0. In case this SDU was the last one to be transmitted, a PDU consisting of an RLC Header with LI=0 followed by a padding Length Indicator and padding may be transmitted. If a 7-bit Length Indicator is used for the following PDU then the LI with value LI=0000000 shall be placed as the first Length indicator and its SN shall be incremented by 2 before it is transmitted (this can only occur in UM). The LI may be represented in either 7-bits or 15 bits.

For AM Mode:

If AMD PDU Size is <= 126 octets then 7-bits LI be used. If AMD PDU Size is > 126 octets then 15-bits LI be used.

For UM Mode:

If UMD PDU Size is <= 125 octets then 7-bits LI be used. If UMD PDU Size is > 125 octets then 15-bits LI be used

Length Indicator(7 bits) Decimal Description

RLC LI Encoding In UM Mode

Assumptions

• RLC PDU Size = 20 Octets.
• Sequence No (7 Bits)
• Extension (1 Bit)
• LI (7 Bits)

SDU Data (in Hex): 01 02 03 04 05 Length Of SDU = 5 Octets

Encoded RLC PDU

Explanation

RLC Header: The first 4 octets (indicated by blue) in the above PDU are RLC headers.

01: 0000 0001 :Sequence No – 0, Extension –1 (i.e. Next Octet contains a LI)

F9: 1111 1001 : LI = 124, Extension –1 (i.e. Next Octet contains a LI)

0B: 0000 1011 : LI = 5 (Length of the SDU), Extension – 1(i.e. Next Octet contains a LI)

FE: 1111 1110 : LI = 127 Extension –0 (i.e. Next Octet contains Data)

Data: Octet Numbers from 5 to 9 are the data octets containing SDU data.

Filler/Padding: Octet Numbers from 10 to 20 are the filler octets.

RLC LI Encoding In AM Mode

Assumptions

• RLC PDU Size = 20 Octets.
• D/C (1 bit) (1: Data, 0: Control)
• Sequence No (12 Bits)
• Polling bit (1 Bit) (1: Status Required)
• Header Extension (2 Bits) (00: Data Follows, 01 LI follows)
• LI (7 Bits)
• Extension (1 Bit)

SDU Data (in Hex): 01 02 03 04 05 Length of SDU = 5 Octets

RLC Header: The first 4 octets (indicated by blue) in the above PDU are RLC headers.

80: 1000 0000 01: 0000 0001: 01: D/C –1 Sequence No – 0, Polling – 0,HE–01 (i.e. Next Octet contains a LI)

0B: 0000 1011 : LI = 5 (Length Of A SDU), Extension – 1(i.e. Next Octet contains a LI)

FE: 1111 1110 : LI = 127, Extension –0 (i.e. Next Octet contains Data)

Data: Octet No 5 – 9 are the data octets containing SDU data.

Filler: Octet No 10 – 20 are the filler octets.

Reference

Radio Link Control (RLC) protocol specification: 3GPP TS 25.322