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PROTOCOL BASICS 285
to be very ineffi cient by today ’ s FDX standards due to the turnaround times
involved.
6.3.8.2 Synchronous Data Link Control Synchronous Data Link Control
(SDLC), developed in the mid - 1970s, is at the heart of IBM ’ s System Network
Architecture (SNA). SDLC is a bit - oriented, point - to - point protocol that uses bit
strings to represent characters. SDLC uses CRC error correction techniques — specifi
cally known as Frame Check Sequence (FCS) here. SDLC supports high - speed
transmission and generally employs FDX, dedicated circuits. SDLC works either in
HDX or FDX, supports satellite transmission protocols, and works in point - to - point
or multipoint network confi gurations.
Up to 128 frames can be sent in a string, with each frame containing up to seven
blocks, each up to 512 characters. Each block within each frame is checked individually
for errors. Errored blocks must be identifi ed as such to the transmitting device
within a given time limit or they are assumed to have been received error free. As
a carefully timed, point - to - point protocol, SDLC depends on high - performance
circuits, usually in the form of dedicated leased lines.
The SDLC frame consists of synchronizing bits, data, and control characters sent
in a continuous data stream frame by frame. The specifi c elements of the SDLC
frame (Figure 6.17 ) are as follows and in sequence [19 – 21] . The same format applies
to High - level Data Link Control (HDLC) frames and X.25 packets:
• Flag (F): Flag bits, in a specifi c eight - bit pattern, alert the receiving device to
the transmission of the frame, thereby initiating the error - checking procedures.
The most commonly used fl ag character is 01111110 (7E in hexadecimal).
• Station Address (A): This address fi eld of eight bits identifi es the specifi c target
device for which the frame is intended, a group address for multiple target terminals,
or a broadcast address to all terminals. This fi eld also can be used to
distinguish commands from responses.
• Control (C): The eight - bit control fi eld identifi es the type of frame being trans-
mitted. An information frame is used for the transfer of messages, frame numbering
of contiguous frames in a message, and so on. A supervisory frame is
used for purposes such as to indicate a detected error in transmission, acknowledge
that frames have been received without error, request the transmission of
specifi ed frames, and order the transmitting device to stop sending.
F A C
Text
FCS
F
< 7 blocks of < 512 characters
Figure 6.17
Legend
F Framing bits
A Address
C Control
FCS Frame Check Sequence
SDLC frame.