System information
SDLC
Book Title
10-212
• Multipoint—Involves one primary and multiple secondaries.
• Loop—Involves a loop topology, with the primary connected to the first and last secondaries.
Intermediate secondaries pass messages through one another as they respond to the requests of
the primary.
• Hub go-ahead—Involves an inbound and an outbound channel. The primary uses the outbound
channel to communicate with the secondaries. The secondaries use the inbound channel to
communicate with the primary. The inbound channel is daisy-chained back to the primary
through each secondary.
Frame Format
The SDLC frame format is shown in Figure 10-1.
Figure 10-1 The SDLC Frame Format
As Figure 10-1 shows, SDLC frames are bounded by a unique flag pattern. The address field always
contains the address of the secondary involved in the current communication. Because the primary
is either the communication source or destination, there is no need to include the address of the
primary—it is already known by all secondaries.
The control field uses three different formats, depending on the type of SDLC frame used. The three
SDLC frames are described as follows:
• Information (I) frames—These frames carry upper-layer information and some control
information. Send and receive sequence numbers and the poll final (P/F) bit perform flow and
error control. The send sequence number refers to the number of the frame to be sent next. The
receive sequence number provides the number of the frame to be received next. Both the sender
and the receiver maintain send and receive sequence numbers. The primary uses the P/F bit to tell
the secondary whether it requires an immediate response. The secondary uses this bit to tell the
primary whether the current frame is the last in its current response.
Flag Address FlagData FCSControl
111 or 2 1 or 2 2Variable
Field length,
in bytes
Receive
sequence
number
Send
sequence
number
0
Poll
final
Information
frame format
Receive
sequence
number
1
Poll
final
Supervisory frame format
Function
code
1
0
1
Poll
final
Function
code
Function
code
Unnumbered frame format