UMTS RLC Status PDU: SUFI RLIST

Catch Up

You may need the following tutorial:

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:

Type = RLIST
LENGTH
FSN
CW-1
CW-2
—–
CW-1
CW-LENGTH

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:

X1X2X30 When the last bit is 0, it signifies that the CW continues in next CW. The most significant bit is X1 within the codeword.
X1X2X31 When 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 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:

0 0 1 1 0 0 0 0
0 0 0 0 0 0 1 1
0 1 0 1 0 0 0 1
1 0 1 1
LENGTH
FSN
CW-1
CW-2
CW-3

Reference

  1. Radio Link Control (RLC) protocol specification: 3GPP TS 25.322
  2. WCDMA Design Handbook UMTS RLC Status PDU: SUFI RLIST
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About Prashant Panigrahi

I am Prashant Panigrahi, live in Sweden. Now working as wireless professional and this website is just a reflection of what I learned. You can connect me on LinkedIn, Twitter
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