The BITMAP SUFI consists of the following fields:
- Type identifier field(BITMAP)
- LENGTH Filed
- First Sequence Number(FSN)
The BITMAP SUFI fields in the STATUS PDU are as follows:
|Type = BITMAP|
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 is the sequence number of the first bit in the bitmap. FSN is 12 bits long.
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.|
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|
The LENGTH = 0
LENGTH = 0000
FSN = 3.
So the twelve bits FSN field will be encoded as:
FSN = 3 (%0000 0000 0011)
LENGTH = 0
Bitmap = 1 octet
So the bitmap can be encoded as:
1 0 1 1 0 0 1 1
The encoding of the Bitmap SUFI as follows: