Page 33 - Intelligent Communication Systems

CHAPTER 3 / COMMUNICATION NETWORK STRUCTURE 1 7

FIGURE 3.2 Layered structure of the OSI protocol.

A virtual circuit of the packet switching system is an example of connection-type
service.
On the other hand, in the case of the connectionless-type service, the Nth entity
is a functional module for the communication between a source and its destina-
tion. The entity has the functions for communication between peer nodes and the
functions for communication between the entity and the adjacent upper-level entity
or between the entity and the adjacent lower-level entity.
The Nth service provides the communication functions to the (N+ l)th entity.
Generally speaking, the Nth entity provides the Nth service to the (N+ l)th entity
by using the (N— l)th service provided by the (N- l)th entity in cooperation with
the peer Nth entity. The access point in which the (N + l)th entity receives the Nth
service is defined as the Nth service access point (SAP). The information exchanged
through the Nth SAP is defined as the Nth service primitive.
The Nth connection is a communication channel between the Nth entity and
the peer Nth entity. The channel is used for data transmission between the (N +1 )th
entity and the peer (N + l)th entity. The Nth connection is given a specific identi-
fier. The identifier is attached to the transmitting data. Therefore the Nth entity can
send the data to the (N+ l)th entity by recognizing the identifier.
The Nth protocol is defined as the protocol by which the Nth entity commu-
nicates with the peer Nth entity. In the protocols, there are the protocols for estab-
lishment of the connection, information control, and other necessary actions.
The unit of the data block in the Nth layer is defined as the Nth protocol data
unit (PDU). As shown in Figure 3.3, the (N + l)th PDU is manipulated as the Nth
service data unit (SDU). Basically, the (N+ l)th PDU is replaced by the Nth SDU.
According to the data length, more than one (N + l)th PDU are integrated into a
single Nth SDU. The Nth PDU is created by attaching the Nth protocol control iden-
tifier (PCI) to the Nth SDU.
Generally speaking, the current layer control information is attached to the
adjacent upper-level layer PDU to generate the current layer SDU. Figure 3.3
shows the relationship between PDU and SDU.