RCVDL56DPFL/SP, RCV56DPFL/SP, and RCV336DPFL/SP Designer’s Guide
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5-1
5. HDLC OPERATION
The HDLC (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 MDP uses the SDLC protocol but it is referred to as HDLC to
avoid confusion.
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. Flags and the frame check sequence are distinguished from the other fields by status bits in the MDP
interface memory.
Flags
All frames start and end with a flag sequence. The beginning flag and the ending flag are defined by the bit pattern
01111110 (7E). 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.
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. Certain protocols allow for an extended control field of 16 bits in length.
Information Field
The MDP does not distinguish between the address field, the control field, or the information field. The information field does
not have a set length; however, this field is in the SDLC protocol format of 8-bit bytes.
Zero Insertion
Since flags mark the beginning and ending of a frame, some method must be implemented to inhibit or alter the transmission
of data that appear as flags. The method used is called “zero insertion”. HDLC procedures require that a zero be transmitted
following any succession of five continuous ones. This includes all data in the address, control, information and Frame Check
Sequence (FCS) fields. Use of zero insertion denies any pattern of 01111110 to ever be transmitted between beginning and
ending flags.
The MDP transmitter always performs zero insertion when in HDLC mode.
Zero Deletion
When transmitting flags, zero insertion is disabled. During reception of data, after testing for flag recognition, the receiver
removes a zero that immediately follows five continuous ones. This is termed “zero deletion”. A one that follows five
continuous ones signifies either a frame abort (i.e., at least seven ones with no zero insertion) or a flag (i.e., 01111110). The
sixth one is, therefore, not removed.
The MDP receiver always performs zero deletion when in HDLC mode.
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