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Chapter 2
Protocol Attributes
Frame Structure
SDLC uses three different frame structures: information, supervisory, and
unnumbered. The overall structure of the frames is similar, except for the
control frame. The control frame is varied to distinguish the type of SDLC
frame that is being used. Figure 2.32 gives the structure for the different
SDLC frames. Pay close attention to the bit values next to the send sequence
number within the control frame.
F I G U R E 2 . 3 2
SDLC frame structures
First, let's talk about the frame fields that are common among all three
frame types. As you can see, all three frames depicted in Figure 2.32 start
with a Flag field that is followed by an Address field. The Address field of
SDLC frames is different from other frame structures because only the
address of the secondary node is used instead of a destination and source
address. The secondary address is used because all communication is either
originated or received by the primary node; thus, it is not necessary to specify
its address within the frame.
The control frame follows the Address field. Information contained
within the control frame defines the SDLC frame type. The control frame
begins with a receive sequence number. This sequence number is used to tell
the protocol the number of the next frame to be received.
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