Page 35 - Intelligent Communication Systems

" CHAPTER 3 / COMMUNICATION NETWORK STRUCTURE 19
The second layer, the data link layer, defines the procedures of connection or
disconnection and the transmission control between adjacent nodes. It contributes
to the precise, efficient, and prompt transmission between adjacent nodes. It has
the functions of error correction control, a sequence control, and a flow control.
Examples of this layer are the basic transmission control, high-level data link con-
trol (HDLC), and LAPB.LAPD of ISDN or logical link control (LLC) of a local
area network. Media access control (MAC) protocols of the local area network,
such as CSMA/CD, token bus, and token ring, are included in the data link layer
and the physical layer.
The third layer is a network layer. Using the functions of this layer, a transpar-
ent transmission path is established between a source and its destination. The func-
tions include flow control, routing, and sequence control, providing for precise and
speedy data transmission control throughout the network. Examples of these func-
tions include the X.25 protocol, which is the user network interface of the packet
switching network, and the X.75 protocol, which is a network-to-network interface.
The fourth layer is a transport layer. Using the functions of this layer, a source
process and its destination process are linked and a transmission path established
between the source process and its destination process to communicate together.
This layer is also called the end-to-end transmission layer. It provides the functions
of flow control, sequence control, the composition and decomposition of data, and
the detection of data loss during transmission.
There are five classes in this fourth layer. Class 0 provides functions such as
connection establishment between peer processes, data composition or decompo-
sition, or transmission of the transport protocol data unit (TPDU). The higher the
class, the more advanced the functions that are accommodated. For example, class 4
provides not only the basic functions but also the advanced functions, such as flow
control, sequence control, multiplication, and error check and control, in order to
support high-quality transmission even over low-quality transmission lines.
The fifth layer is a session layer. It provides the conversational functions
between the adjacent entities of a presentation layer. Namely, this layer provides
the functions by which the connection called session is established, maintained,
and released. Additionally it provides some kinds of conversation styles and check-
ing functions for error recovery.
The sixth layer is a presentation layer. It provides the data conversion facili-
ties to the application programs or terminals in an application layer. The services
include code or character conversion, data form or layout conversion, data com-
pression of images, and encryption/decryption for security.
The seventh layer is an application layer. It provides a client with application
programs for accessing the OSI environment. In the application layer are functions
for file transfer, job transfer, virtual terminal, database access, transaction pro-
cessing, or the mail handling system (MHS).
The OSI reference model has been used for other standardization activities of
computer communication or data communication since it was established. An exam-
ple of a protocol structure on the packet switching network is shown in Figure 3,4,