- Facts:
- Each communication
protocol use
it's own (fixed)
frame format (= structure)
- The frame format is
always
as follows:
+--------------+------------+-----+ | header info | user data | CRC | +--------------+------------+-----+
- The header info contains
information about:
- The identity of the sender
- The identity of the receiver
- The "identity" (= sequence number) of the user data
- etc, etc
- Each communication
protocol use
it's own (fixed)
frame format (= structure)
- The structure of
an HDLC frame:
- The HDLC frame is
transmitted
between
2 consecutive FLAG:
- The HDLC frame is
transmitted
between
2 consecutive FLAG:
- The meaning of the
fields:
- Address field:
- The address field contains the
address of
the destination station
- The address field contains the
address of
the destination station
- Control field
contains:
- The frame type
- Identifies the type of the HDLC frame
- Send sequence numbers
- Identifies the position of the frame in the sequence of sent messages
- Receive sequence number
- Used to acknowledge the receiption of frames
We will discuss the send and receive sequence numbers for error control soon...
- Poll indication
- Set this bit to give permission to a secondary station to transmit
- The frame type
- Data field:
- Contains the user message
- Frame check sequence (FCS):
- Contains a CRC code for the entire HDLC frame
- Address field:
- There are 2 version of
HDLC frames:
- Normal format
- Extended format (uses more bits for larger values)
- The HDLC frame in normal format:
The extended format uses:
- Same length for address field
- The control field is 16 bits
- The FCS field is 32 bits
- Question:
- How do we know which version will be used ???
Answer:
- When a communication device
starts, it
runs an initialization protocol
- It will send probes
on all
it communication interfaces
- The device will respond with a protocol setting message that includes a frame type
- I.e., the start up protocol will determine a frame type between the communicating devices