RLC Bitmap Super Field (BITMAP-SUFI)


The 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:

Type = BITMAP
LENGTH
FSN
Bitmap

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 + 1
So if, LENGTH = 0000 = 0
Size 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.
0 Sequence 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
Sequence Number # 5 ACK
Sequence Number # 6 ACK
Sequence Number # 7 NACK
Sequence Number # 8 NACK
Sequence Number # 9 ACK
Sequence Number # 10 ACK

Step #1
The LENGTH = 0
LENGTH = 0000

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

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

1 0 1 1 0 0 1 1

Step #4
The 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
LENGTH
FSN
Bitmap

Reference


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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