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