Information Technology Reference
In-Depth Information
HDLC
I
I.1 Introduction
The data link layer is the second layer in the OSI seven-layer model and its protocols define
rules for the orderly exchange of data information between two adjacent nodes connected by
a data link. Final framing, flow control between nodes, and error detection and correction are
added at this layer. In previous chapters the data link layer was discussed in a practical man-
ner. It is a use protocol as it provides a model for interfacing to a serial bus.
The two types of protocol are:
Asynchronous protocol.
Synchronous protocol.
Asynchronous communications uses start-stop method of communication where characters
are sent between nodes, as illustrated in Figure I.1. Special characters are used to control the
data flow. Typical flow control characters are End of Transmission (EOT), Acknowledge-
ment (ACK), Start of Transmission (STX) and Negative Acknowledgement (NACK).
Synchronous communications involves the transmission of frames of bits with start and
end bit characters to delimit the frame. The two of the most popular are IBM's synchronous
data link communication (SDLC) and high-level data link control (HDLC). Many network
data link layers are based upon these standards, examples include the LLC layer in IEE 802. x
LAN standards and LAPB in the X.25 packet switching standard.
Synchronous communications normally uses a bit-oriented protocol (BOP), where data is
sent one bit at a time. The data link control information is interpreted on a bit-by-bit basis
rather than with unique data link control characters.
10101101
11100101
00101100
00000001
10101101
10001101
Asynchronous
communications
END
START
10101011101001......01011101010
Synchronous
communications
Figure I.1
Asynchronous and synchronous communications
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