Synchronous Data Link Control (SDLC) Sessions 459
SDLC Frames
There are three types of frames to discuss with SDLC: the U-frame, the I-frame, and the
S-frame. These operate in the control field of the SDLC frame, as shown in Figure 12-26.
Figure 12-26
SDLC Frame Format
·
Unnumbered frames (U-frames)--These frames are primarily used for controlling the
data link, diagnostics, initializing procedures, and transferring data.
·
Supervisory frames (S-frames)--A 1 in bit position 1 and a 0 in bit position 2 designates
S-frames. S-frames are used for control functions such as acknowledging the receipt of
information frames, requesting polls, and reporting status.
·
Information frames (I-frames)--Information frames send and receive counts to ensure
proper ordering. Information frames are designated by a 0 in bit position 1 of the control
field. I-frames are primarily used for sending user data, although some control information
is allowed.
Flag 8
bits
Address 8 bits
Control field
8 or 16 bits
I-frames - 00000000
U-frames - 11000000
S-frames - 10000000
Info
length varies
FCS 16 bits
Flag 8 bits
87200333.book Page 459 Wednesday, August 22, 2001 2:53 PM