SUFI Fields in Control PDU

SUFI fields in Control PDU:

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.

Transmitter Receiver

How to inform peer about the missing PDUs, allowed transmission window size, move the receiving window?

Super-Field indicates which AMD PDUs are received correctly and which are missing.

STATUS PDU

SUFi has three sub-fields:

Type field is 4 bits long and may have one of the following values:

The length and value of each super field (SUFI) is dependent on its type and contents

1. The No More Data super-field

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.

2. Window sizeThe range of the WSN is [0, 2^12-1]. It changes the tx window size of the transmitter.

1 <= WSN <= Configured_Tx_Window_size

if WSN < Configured_Tx_Window_sizeVT(WS) = WSN

ElseVT(WS) = Configured_Tx_Window_size

.

3. The 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 SUFI 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.

0 0 0 0 0 0 1 0

0 0 0 0 0 0 0 0

0 1 0 0

4. List

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.

SNi

Length: 12 bits ”Sequence number” of AMD PDU, which was not correctly received.

Li 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.

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 pair0000 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.

Encoding of Status PDU

0 0 0 0 0 0 1 1

0 0 0 1 0 0 0 0

0 0 0 0 0 1 0 0

0 0 0 1

5. BITMAPThe BITMAP SUFI consists of the following fields:

Type identifier field(BITMAP) LENGTH Filed

First Sequence Number(FSN)

Bitmap

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

LENGTH

The LENGTH is a 4 bits long field. The LENGTH field signifies the size of bitmap in octets.The size of bitmap in octets = LENGTH + 1So if, LENGTH = 0000 = 0Size of bitmap = 0 + 1 = 1 octet

FSN

FSN is the sequence number of the first bit in the bitmap. FSN is 12 bits long.

Bitmap

In bitmap each bit position signifies whether FSN + bit_position is correctly received or not.bit_position can have two different values:

1 Sequence Number = (FSN + bit_position) has been correctly received.

0Sequence Number = (FSN + bit_position) has not been correctly received.

Example

Suppose the receiver needs to send the status of following PDUs to the transmitter.

Sequence Number of PDU Status

Sequence Number # 3 ACK

Sequence Number # 4 NACK







Step #1The LENGTH = 0LENGTH = 0000

Step #2FSN = 3.So the twelve bits FSN field will be encoded as:FSN = 3 (0000 0000 0011)

Step #3LENGTH = 0Bitmap = 1 octetSo the bitmap can be encoded as:

1 0 1 1 0 0 1 1

Step #4The encoding of the Bitmap SUFI as follows:

0 0 0 0 0 0 0 0

0 0 0 0 0 0 1 1

1 0 1 1 0 0 1 1

6. RLIST

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:

Bit 1 Bit 2 Bit 3 Bit 4

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:

X1X2X30When the last bit is 0, it signifies that the CW continues in next CW. The most significant bit is X1 within the codeword.

X1X2X31When the last bit is 1, the number is terminated in this codeword. This is the most significant CW in the number.

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 (rightmost bit of lastCW == 0 || lastCW == 0001){

Discard Status PDU

}

Example

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

3, 5, 6, 7, 8, 9, 10, 11, 12, 13

In this case there are a number of PDUs detected as erroneous starting from sequence number 3. So the encoding of the SUFI RLIST will be as follows:

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

Step #2The LENGTH = 4LENGTH = 0100

Step #3The 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 #4Now there is error burst continues up to sequence number 13. So the special burst indicator will be used.So now the distance will be 8. So the encoding of the codewords CW2, CW3 and CW4 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 = 1110 (The distance from sequence number 5 to 13 is 8 and the distance till max 7 can be represented in one CW so the error indication has to be carried in next CW also, so the last bit is 0

CW4 = 0011 7 error PDUs is already reported in previous CW only 8-7=1 is pending, and the error indication is terminated here so code word is 0011.

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

0 1 0 0 0 0 0 0

0 0 0 0 0 0 1 1

0 1 0 1 0 0 0 1

1 1 1 0 0 0 1 1

LENGTH

FSN

CW-1

CW-2

CW-3

CW-4

7. MRW

Transmitter uses MRW to request the Receiver to move its reception window and optionally to indicate the set of discarded RLC SDUs, as a result of an RLC SDU discard in the Sender

8. MRW_ACKReceiver acknowledges MRWSufi reception and N in MRW_ACK = Nlength in MRW

Note: SUFI selection is implementation dependent.

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SUFI Fields in Control PDU