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IT Essentials — Assessing Infrastructure and Networks

Function: The session layer provides services for management of remote connections at very basic
levels of interaction. Layer 5 is responsible for enabling the interaction of local and remote
processes.

Layer 6 — Presentation
The presentation layer is primarily concerned with data conversion. Numerous standardization
protocols are used to ensure interoperability between systems and applications like ASCII and
UNICODE. If conversion is possible between two such standards, the presentation layer performs this
function, but it also performs compression, decompression, encryption, and decryption, although all
such tasks are not exclusively part of this layer.
Function: The presentation layer is concerned with taking data form a wide variety of application
layer sources and making the data available to other applications and network standard protocols.
The presentation layer represents a departure from the layers associated with data in motion.
Presentation applies to data at rest as well as data in motion. The presentation layer also
coordinates the encapsulation of data at rest, in compressed files, encrypted files, and compound
files (i.e., files containing other files like email attachments).

Layer 7 — Application
The application layer and the presentation layer function together in most cases. Applications that
organize data into standard formats for interoperation use presentation layer file formats. Those
formats are opened to the user by applications with awareness of that file type. For example, most
users automatically associate the application, MS Word, with the file type “.docx.” These two layers
have distinct functions, but differ from the strictly data-in-motion layers: 2, 3, 4, and 5.
Function: Various other applications generate and consume data at this level. This layer is the most
diverse, but also the most familiar to users. The applications that generate and modify user data
implement the application layer of the stack. It is a subtle difference, but this layer is not the
applications themselves; rather, it is the formatted data product of those applications.

Network Protocols

I have previously heard that protocols are the languages computers talk to each other. Is that
correct?

Yes, that is a simple interpretation. 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.

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