Page 32 - Intelligent Communication Systems

INTELLIGENT COMMUNICATION SYSTEMS
(1) Information must be transmitted properly. It is necessary to specify the
interface between a terminal and a line linked to the network. For example, the
physical conditions, such as electricity and the connection between a terminal and
a line, should be specified. It is also necessary to specify error detection and recov-
ery during data transmission and to specify the sequence control and flow con-
trol.
(2) Information must be processed properly. It is necessary to specify how to
transfer data between a terminal and a computer, the particular character set to be
processed between a terminal and a computer or between peer terminals or peer
computers, and the data format and commands to be processed in the network.

3.2.3 OS! Protocol
There are three logical components in OSI: application process, open system, and
transmission medium. The application process is the process conducted in a ter-
minal or in a computer. Open system is a platform that provides the information
processing and communication function between peer application processes. The
transmission medium is a line that transmits information and signals between open
systems. Open system provides the functions for interconnecting two or more sys-
tems and includes such equipment as a terminal or a workstation and a network in
which terminals and computers are interconnected.
In the OSI reference model, seven layer protocols are defined, from a physical-
level protocol to an application-level protocol. The lower-level protocols, such as
a physical-level protocol and a data link-level protocol, define the functions of the
communications hardware. The upper-level protocols, such as an application-level
protocol and a presentation-level protocol, define the functions of communication
processing. The protocols are a set of communication functions between peer nodes,
that is, the interface between them.
The protocols are well defined to ensure the transparency of the interconnec-
tion between peer entities and between neighboring layers. An upper-level proto-
col issues a request for the communication functions provided by the adjacent
lower-level layer. The adjacent lower-level layer provides its functions to the adja-
cent upper-level layer, although it does not control the adjacent upper-level layer.
Figure 3.2 shows the layered structure of the OSI protocol. In it, the (N+ l)th layer
is the adjacent upper-level layer of the Mh layer.
The OSI reference model is composed of seven layers. The functions of the
Nth layer are composed of the entity, service, and protocol of the Nth layer. The
Nth entity creates the Nth service by using the (N- l)th entity. The Nth service is
provided to the (N + l)th entity.
The Nth service is divided into connection-type service and connectionless-
type service. In case of the connection-type service, a connection is established
between a source node and its destination node before the data transmission begins.
After finishing the data transmission, the communication link is disconnected.