RFX144V24-S23 and RFX96V24-S23 Modem Designer’s Guide
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5-1
5. HDLC FRAMING
The modem supports HDLC framing.
High Level Data Link Control) protocol is a standard procedure used for data communications. SDLC (Synchronous Data
Link Control) is a bit-oriented protocol which is a subset of HDLC. The same format is used in both protocols although all
SDLC fields must be eight-bit octets. The modem uses the SDLC eight-bit octet format.
5.1 HDLC FRAMES
Data and control information on a HDLC link are transmitted via frames. These frames organize the information into a format
specified by an ISO standard that enables the transmitting and receiving station to synchronize with each other. This format
is shown in Figure 5-1.
FLAG
ADDR CONTROL
INFORMATION
FCS FLAG
FRAME STARTS
FRAME ENDS
1070F5-1 HDLC Frm
Figure 5-1. HDLC Frame
5.1.1 Frame
Fields
Flags
All frames start and end with a flag sequence. The beginning flag and the ending flag are defined by the bit pattern
01111110 (7Eh). The ending flag for one frame can also serve as the beginning flag for the following frame. If separate
ending and beginning flags are used, the final zero in the ending flag of one frame may also serve as the first zero of the
beginning flag in the following frame. This process is known as “zero-sharing”. The zero-sharing bit pattern is
011111101111110.
Address Field
The address field informs the receiver where the information is to go (if the primary station is transmitting) or where the
message originated (if a secondary station is transmitting). This field is eight bits in length for the “basic” format.
For the “extended” format, the length is N number of octets, each octet having the first bit a binary zero with the exception of
the last octet that begins with a binary one.
Broadcast Address = 11111111
Null Address = 00000000
Control Field
The control field defines the function of the frame. It may contain a command or response. The control field might also
contain send or/and receive sequence numbers. This field can be in one of the following formats:
1. Information Transfer Format
2. Supervisory
Format
3. Unnumbered
Format
This field is normally eight bits in length. However, certain protocols allow for an “extended” control field. For example, it is 16
bits in length for modulo 128 operation of the LAP and LAPB procedures.
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