Specifications

Sierra Wireless, Inc. CDPD Primer
2130006 Rev 1.0 Page 20
Figure 4-1: ISO layered communications architecture
4.3.1. The Application Layer (Layer 7)
The top layer of the stack includes interaction with the end user. It allows for protocols and
services required by a particular user-designed or commercially written application. Particular
user requirements and application services that can be used by more than one application are
contained in this layer.
4.3.2. The Presentation Layer (Layer 6)
Layer 6 defines the method of representing information for exchange by applications. It is
concerned only with the syntax of the transferred data and not with the meaning of the data itself.
It provides the representation of:
• Data transferred between applications
• The data structures that the applications use
• Operations on the data structures
4.3.3. The Session Layer (Layer 5)
The session layer allows cooperating application processes to organize and synchronize the
conversation between them and to manage the data conversation. During a session, these services
are used by applications to regulate dialogue by ensuring an orderly message exchange on the
session connection.
4.3.4. The Transport Layer (Layer 4)
This layer provides reliable, transparent transfers of data between cooperating session entities. It
optimizes the available network services to provide the performance required by each session.
Connection-oriented transport protocols regulate the flow of data, detect and correct errors, and
multiplex data end to end.
4.3.5. The Network Layer (Layer 3)
The network layer provides packet routing and relaying between end systems on the same network
or on interconnected networks, independent of the transport protocol used. It can also provide
service enhancements, flow control, and load leveling.