The control field has a length of 8 bits.
The information covered by this field is:
- D7 = telegram type
- D5 = telegram repetition status
- D3 + D2 = telegram priority
Telegram type
0 = Extended frame: length of the telegram = 9..263 octets
1 = Standard frame: length of the telegram = 8..23 octets
Importance: see here.
Telegram repetition status
0 = Repeated
1 = Not repeated (original)
Telegram priority
00 = System
10 = Urgent
01 = Normal
11 = Low
Fixed bits
D6 = 0
D4 = 1
D1 = 0
D0 = 0
D1 and D0 are called the preamble bits