Synchronous Data Link Control (SDLC) is based on a synchronous, more-efficient, faster, and
flexible bit-oriented format. SDLC has several derivatives that perform similar functions with
some enhancements: HDLC, LAPB (Link Access Procedure, Balanced), and IEEE 802.2, just to
name a few. HDLC is the default encapsulation type on most Cisco router serial interfaces.
SDLC is used for many link types. Two node types exist within SDLC: primary nodes and
secondary nodes. Primary nodes are responsible for the control of secondary stations and for
link management operations such as link setup and teardown. Secondary nodes talk only to the
primary node when fulfilling two requirements. First, they have permission from the primary
node; second, they have data to transmit. Even if a secondary node has data to send, it cannot
send the data if it does not have permission from the primary node.
Primary and secondary stations can be configured together in four different topologies:
Point-to-point This topology requires only two nodes—a primary and a secondary.
Multipoint This configuration uses one primary station and multiple secondary stations.
Loop This configuration uses one primary and multiple secondary stations. The difference
between the loop and multipoint setups is that in a loop, the primary station is connected between
two secondary stations, which makes two directly connected secondary stations. When more
secondary stations are added, they must connect to the other secondary stations that are currently
in the loop. When one of these stations wants to send information to the primary node, it must
transit to the other secondary stations before it reaches the primary.
Hub go-ahead This configuration also uses one primary and multiple secondary stations, but
it uses a different communication topology. The primary station has an outbound channel. This
channel is used to communicate with each of the secondary stations. An inbound channel is
shared among the secondary stations and has a single connection into the primary station.
Frame Structure
SDLC uses three different frame structures: information, supervisory, and unnumbered. Overall,
the structure of the frames is similar among all three, except for the Control frame. The Control
frame is varied to distinguish the type of SDLC frame that is being used. Figure 36.10 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.
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 36.10 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, rather than 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.
The P/F or Poll Final number following the receive sequence number is used differently
by primary and secondary nodes. Primary nodes use the information to communicate to the secondary
node that an immediate response is required. The secondary node uses the information
to tell the primary node that the frame is the last one in the current dialog.
The Data field follows the Control frame. As with other frame types, the FCS field comes
next and is used to calculate the CRC. SDLC frames differ again with the last field, which is
another Flag field like one at the beginning of the frame.
Now that we have discussed the frame structure, let’s examine the three different frame
types. Information frames carry exactly that—information destined for the upper layer protocols.
Supervisory frames control SDLC communications; they are responsible for flow control
and error control for I-frame (information). Unnumbered frames provide the initialization of
secondary nodes, as well as other managerial functions. 1109