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Chapter 2
Protocol Attributes
HDLC
High-level Data Link Control (HDLC) is used in the same manner as
SDLC--for serial connections and for synchronous operations. This proto-
col is an enhancement over SDLC because it provides a 32-bit checksum and
three different transfer modes: normal, asynchronous response, and asyn-
chronous balanced.
These modes allow secondary nodes to communicate with primary nodes
by using different methods. The normal modes are the same as SDLC uses;
the secondary nodes can talk only to the primary node when they have per-
mission. The other two methods are unique to HDLC.
Asynchronous response allows the secondary node to communicate
with the primary node without first having to obtain permission from the
primary node.
The final method, asynchronous balanced, creates a third type of node,
known as a combination node. As you can imagine, this node combines the
roles of a primary and secondary node.
IEEE 802.2
This protocol uses three different frame types to provide different services to
IEEE 802.3, IEEE 802.5, and IEEE 802.4. The nomenclature for the differ-
ent types is simply Type 1, Type 2, and Type 3:
Type 1 Unacknowledged connectionless service, which does not pro-
vide for any flow or error-control information.
Type 2 Connection-oriented service. Connections are established and
flow-and-error control is inherent.
Type 3 Acknowledged connectionless service. Connections are not
established, yet flow-and-error control is inherent.
LAPB
This protocol uses the ABM (asynchronous balanced method) transfer
mode. All nodes are combination modes. Hierarchy is established by the
node initiating the communication. The frame for LAPB is similar to that of
SDLC and HDLC.
This ends our discussion of SDLC and its derivative protocols. As you
have seen, they all have many similarities to SDLC. Enhancements were
made to create more effective and efficient protocols.
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