Information Technology Reference
In-Depth Information
SABM
1111P110
DM
1111F000
DISC
1100P010
UA
1100F110
FRMR
1110F001
I.2.7 Frame check sequence field
The frame check sequence (FCS) field contains an error detection code based on cyclic re-
dundancy check (CRC) polynomials. It is used to check the address, control and information
fields, as previously illustrated in Figure I.2. HDLC uses a polynomial specified by CCITT
V.41, which is
16
12
5
1
. This is also known as CRC-16 or CRC-CCITT.
G
(
x
)
=
x
+
x
+
x
+
x
I.3 Transparency
The flag sequence
01111110
can occur anywhere in the frame. To prevent this a transpar-
ency mechanism called zero-bit insertion or zero stuffing is used. There are two main rules
that are applied, these are:
•
In the transmitter, a 0 is automatically inserted after five consecutive 1's, except when the
flag occurs.
•
At the receiver, when five consecutive 1's are received and the next bit is a 0 then the 0 is
deleted and removed. If it is a 1 then it must be a valid flag.
In the following example a flag sequence appears in the data stream where it is not supposed
to (spaces have been inserted around it). Notice that the transmitter detects five 1's in a row
and inserts a 0 to break them up.
Message:
00111000101000 01111110 01011111 1111010101
Sent:
00111000101000 011111
0
10 01011111
0
1111010101
I.4 Flow control
Supervisory frames (
S[]
) send flow control information to acknowledge the reception of
data frames or to reject frames. Unnumbered frames (
U[]
) set up the link between a primary
and a secondary, by the primary sending commands and the secondary replying with re-
sponses. Information frames (
I[]
) contain data.
I.4.1 Link connection
Figure I.6 shows how a primary station (node A) sets up a connection with a secondary sta-
tion (node B) in NRM (normal response mode). In this mode one or many secondary stations
can exist. First the primary station requests a link by sending an unnumbered frame with:
node B's address (
ADDR_B
), the set normal response mode (
SNRM
) command and with poll
flag set (
P=1
), that is,
U[SNRM,ABBR_B,P=1]
. If the addressed secondary wishes to make a
connection then it replies back with an unnumbered frame containing: its own address
