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Layered Service Network Model
When referencing networks, it helps
to conceptualize the different network Figure 11: The Open Systems Interconnection Model
“layers” using a model. Sometimes
collectively referred to as the network
stack, the most commonly used
network-layered model is the Open
Systems Interconnection (OSI)
Seven-Layer model, as shown in
Figure 11.

Like many IT concepts, this model is
not universal, but it may be helpful
when thinking about services
provided by a network stack. Most
operating systems provide a network
stack that contains a series of
applications allowing for remote
connections and sending/receiving of
data to remote devices. Each layer For more details on the specifics of the OSI
has a responsibility and operates Seven-layer Model, see Appendix D and
independently of other layers. Appendix E.
Additionally, each layer accepts data
from the higher level and performs its
required functions before passing it to a lower level. This is referred to as passing information
down the “network stack” and allows developers to assume that necessary services will have
been provided by lower layers. It also requires that the services they develop provide stable
interoperation “up the stack.”

Information passed down from a higher layer is most often intact. It can be divided or combined
as needed at the new layer because all data from the higher level is simply a field of data. Control
information called metadata (data about data) is added; this metadata is usually called a header.
Portions of this guide will reference the different layers.

Network Protocols
The protocol of a network is an agreed-upon format for exchanging or transmitting data between
systems (or up and down the network stack). Protocols define a number of agreed-upon
parameters, such as the method to compress data, the type of error checking to use, and
mechanisms for systems to signal when they have finished either receiving or transmitting data. A
simple analogy is a telephone conversation in which the recipient of the call says “hello” when
answering the call, and the caller responds, “hello,” establishing a voice protocol (speaking in an
agreed-upon language).
Some common network protocols include Ethernet, Transmission Control Protocol/Internet
Protocol (TCP/IP), File Transfer Protocol (FTP), Hypertext Transfer Protocol (HTTP), and Secure
Sockets Layer (SSL) Protocol. Simple descriptions of each are available in Appendix F.

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