HDLC - Computer Busses - page 572

In asynchronous balanced mode (ABM) all stations have the same priority and can per-
form the functions of a primary and secondary station.

I.2.2 HDLC frame format

HDLC frames are delimited by the bit sequence 01111110 . Figure I.3 shows the standard
format of the HDLC frame, the 5 fields are the:

• Flag field.

• Address field.

• Control field.

• Information field.

• Frame check sequence (FCS) field.

Flag

Flag FCS

Address Control Information

01111110

01111110

16 bits
CRC-16

8 bits 8/16/24

bits

8 or 16
bits

Multiple of
8 bits

8 bits

Span of CRC calculation

Span of zero insertion

Figure I.3 HDLC frame structure

I.2.3 Information field
The information fields contain data, such as OSI level 3, and above, information. It contains
an integer number of bytes and thus the number of bits contained is always a multiple of
eight. The receiver determines the number of bytes in the data because it can detect the start
and end flag. By this method, it also finds the FCS field. Note that the number of characters
in the information can be zero as not all frames contain data.

I.2.4 Flag field

A unique flag sequence, 01111110 (or 7Eh ), delimits the start and end of the frame. As this
sequence could occur anywhere within the frame a technique called bit-insertion is used to
stop this happening except at the start and end of the frame.
I.2.5 Address field
The address field is used to address connected stations an, in basic addressing, it contains an
8-bit address. It can also be extended, using extended addressing, to give any multiple of 8
bits.
When it is 8 bits wide it can address up to 254 different nodes, as illustrated in Figure I.4.
Two special addresses are 00000000 and 11111111 . The 00000000 address defines the
null or void address and the 11111111 broadcasts a message to all secondaries. The other
254 addresses are used to address secondary nodes individually.