|
After developing SDLC, IBM submitted it to
various standards committees. The International Organization for
Standardization (ISO) modified SDLC to create the High-Level Data
Link Control (HDLC) protocol. The International Telecommunication
Union-Telecommunication Standardization Sector (ITU-T), formerly CCITT,
subsequently modified HDLC to create Link Access Procedure
(LAP), and then Link Access Procedure, Balanced (LAPB). The
Institute of Electrical and Electronics Engineers (IEEE) modified High-level
Data Link Control (HDLC) to create IEEE 802.2. Each of
these protocols has become important in its own right, but SDLC
remains the primary SNA link-layer protocol for WAN links.
A sample SDLC frame is shown in the
figure to the left with each field described below:
- Flag—This field initiates
and terminates error checking.
- Address—This field contains
the SDLC address of the secondary station, which indicates whether
the frame comes from the primary or secondary. This address can
contain a specific address, a group address, or a broadcast
address. A primary is either a communication source or a
destination, eliminating the need to include the address of the
primary.
- Control—This field employs
three different formats, depending on the type of SDLC frame
used:
- The Information (I) frame
carries upper-layer information and some control information.
This frame sends and receives sequence numbers, and the poll
final (P/F) bit performs 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 sender and receiver maintain
send- and receive-sequence numbers.
A primary station uses the P/F bit
to tell the secondary whether it requires an immediate response. A
secondary station uses the P/F bit to tell the primary whether the
current frame is the last in its current response.
- The Supervisory (S) frame
provides control information. An S frame can request and suspend
transmission, report on status, and acknowledge receipt of I
frames. S frames do not have an Information field.
- The Unnumbered (U) frame
is used for control purposes and is not sequenced. A U frame can
be used to initialize secondaries. Depending on the function of
the U unnumbered frame, its control field is 1 or 2 bytes. Some
U unnumbered frames have an Information field.
- Data—This field contains a
path information unit (PIU) or exchange identification (XID)
information.
- FCS—This field precedes the
ending flag delimiter and usually requires a CRC calculation. The
CRC is recalculated by the receiver. If the result differs from
the value in the original frame, an error is detected.
As you can see, SDLC framing is fairly
complex, but not much more so than its successors. SDLC and its
derivative protocols have had a long history and it's still going
strong, as evidenced by the continued widespread use of SNA (commonly
used by banks). SDLC is the first WAN of five data link layer
protocols to be discussed in this chapter. Next you'll learn about
X.25, Frame Relay, ISDN, and ATM.
|