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OSI Layer 2: Data-Link Layer Protocols and Applications

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 it requires an imme-
diate response. The secondary node uses the information to tell the primary
node that the frame is the last one in the current dialogue.

After the P/F bit, the send sequence number is used to identify the current

frame's sequence number. Following the send sequence number, one or two
bits are used to define the frame type. Table 2.18 specifies the bit values and
the corresponding frame type.

The Data field follows the control field. As with other frame types, the

FCS

field follows and is used to calculate the CRC. SDLC frames differ again

with the last field. As with the beginning of the frame, the frame ends with
another Flag field.

Now that we have discussed the frame structure, we need to discuss the

three different frame types. We will begin with Information frames. Infor-
mation frames carry exactly that--information destined for the upper-layer
protocols. Supervisory frames control SDLC communications; they are
responsible for flow-and-error control for I-frame (information). Unnum-
bered frames provide the initialization of secondary nodes, as well as other
managerial functions.

SDLC Derivatives

Other protocols, such as HDLC, Link Access Procedure, Balance (LAPB),
IEEE 802.2, and Qualified Logical Link Control (QLLC), are derivatives
of SDLC.

T A B L E 2 . 1 8

SDLC Frame Types

Bit Value

Frame Type

Information

0 1

Supervisory

1 1

Unnumbered

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