OSI and TCP Models 1

The OSI Model

Open Standards Interconnection (OSI) was created by the International Organization for
Standardization (ISO). With the technology boom came the rise of several giants in the fields of
networking devices and software, including Cisco, Microsoft, Novell, IBM, HP, Apple, and others.
Each vendor had their own cable types and ports and ran their own communication protocols.
This caused major problems if you wanted to buy routers from one company, switches from
another, and servers from yet another.
There were workarounds for these problems, such as deploying gateways on the network that
could translate between protocols, but such solutions created bottlenecks (i.e., slow portions
of the network) and made troubleshooting very difficult and time-consuming. Eventually,
vendors had to agree on a common standard which worked for everyone, and the free suite of
protocols called Transmission Control Protocol/Internet Protocol (TCP/IP) was ultimately
adopted by most. In the end, those vendors who failed to adopt TCP/IP lost market share and went bust.
The ISO created the OSI model to help vendors agree on a set of common standards with which
they could all work. This involved dividing network functions into a set of logical levels or
layers. Each layer would perform a specific set of functions, so, for example, if your company
wanted to focus on network firewalls, they would work with other vendors’ equipment.
The advantage was that each device was designed to perform a specific role well, rather than
several roles inadequately. Customers could choose the best device for their solution without
being tied to one vendor. Troubleshooting became much easier because certain errors could be
traced to a certain OSI layer.
The OSI model divides all network functions into seven distinct layers. The layered model starts
at Layer 7 and goes all the way down to Layer 1. The more complex functions, which are closer
to the user, are at the top, moving down to network cable specifications at the bottom layer, as
illustrated

Table 1.2 – The OSI Model

Layer # Layer Name
7 Application
6 Presentation
5 Session
4 Transport
3 Network
2 Data Link
1 Physical

You can easily remember the names of the layers with the mnemonic “All People Seem To Need
Data Processing.” I would certainly get used to referring to each layer by its number because
this is how real-world network technicians use the OSI.
As data is passed down from the top layers to the bottom for transportation across the
physical network media, the data is placed into different types of logical data boxes. Although
we often call these data boxes “packets,” they have different names depending upon the OSI
layer. The process of data moving down the OSI model is referred to as encapsulation (see
Figure 1.13). Moving back up and having these boxes stripped of their data is called deencapsulation.