Understanding the network.pdf - Back to Home

The OSI Model
The OSI model has the following seven layers:
• Application
• Presentation
• Session
• Transport
• Network
• Data link
• Physical
Each layer defines the function(s) needed to support end-to-end networked
communication. End-to-end means that it moves from the physical/hardware level
to the application/user interface level.
The OSI functions are subdivided into two layer groups: upper and lower. The
upper-layer services (7, 6, 5, and 4) relate to application services. Application
services, such as Web browsers, network mail clients, and Graphic User Interface
(GUI) client/server database applications, all use upper-layer services. The
lower-layer services (3, 2, and 1) are where host-to-host communication takes
place. Data created by the Upper-Layer Protocols (ULPs) is placed in datagrams.
Datagrams, as you recall, are logical groupings of data of a specific size that are
used to carry data created by the ULP. These datagrams are placed in transmission
protocol (Ethernet, Token Ring, and FDDI) frames.
If a service processes user data, it is an upper-level OSI function. If a service
transports data, it is a lower-level OSI function. Under the Internet-RM, the OSI
upper layers fall into the application and transport layers and the lower layers fall
into the Internet, network, and physical layers. Although the OSI-RM does provide
a better model for examining the entire communications process, the Internet-RM
still provides some value because it closely resembles a practical model of how the
network protocol communication process works.
The Seven Layers of the OSI-RM
The OSI-RM is known as the X.200 standard in the ISO universe. Because the
OSI-RM standard is the context for examining data communication protocols, this
book provides some examples from various protocol suites to establish perspective
for later chapters as to how the actual protocols function. The upper OSI reference
model standards perform the following functions, as noted in the following sections.