GuidesThe OSI Model - Part Two

The OSI Model – Part Two

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by Ryan Smith

In Part One of this article, I covered the basics of the OSI model and gave a few tips for
memorizing the seven layers of the stack. In this part, I’m going to cover each of the
seven layers of the OSI model individually and present more detail to how the OSI model
is utilized by networking.

The OSI Model:

A continuation of a previous article outlining an introduction to the OSI Model, focusing on each of the seven layers of the OSI model individually.

To refresh our memory, the OSI model takes the task of internetworking – that is, host
to host networking – and divides that up into what is referred to as a vertical stack.
This vertical stack consists of seven different layers:

  1. Physical
  2. Data Link
  3. Network
  4. Transport
  5. Session
  6. Presentation
  7. Application

Now let’s take a closer look a the individual layers of the OSI model. We’re going to
look at the layers in what is referred to as bottom-up, meaning we’re looking at them
from a physical standpoint up to an application standpoint as opposed to top-down where
you look at the application first down to the physical level.

Layer 1 – Physical

The Physical layer handles the transfer of bits (0’s and
1’s, remember?) from one computer to another. This is where the bits are actually converted
into the electrical signals that travel across the physical circuit.  Connectors, pins,
electrical currents, encoding and light modulation are all part of different physical layer specifications.

Examples include –
V.35, V.24, RJ45, Ethernet, FDDI, B8ZS

Layer 2 – Data Link

The Data Link layer deals with getting data from one
particular medium to another.  It provides delivery across an
individual link. The Data Link layer ensures that messages are delivered to the proper device
and translates messages from the Network layer into bits for the Physical
layer to transmit. It formats the message into data frames and adds a customized
header containing the hardware destination and source address.

Examples include – IEEE 802.3/802.2,
HDLC, Frame Relay,
802.5/ 802.2

Layer 3 – Network

The Network layer is responsible for routing through an internetwork and
for network addressing. This means that the Network layer is responsible for
transporting traffic between devices that are not locally attached. Routers, or
other layer-3 devices, are specified at the Network layer and provide the
routing services in an internetwork.
When a packet is received on a router interface, the destination IP address
is checked. If the packet is not destined for the router, then the router will
look up the destination network address in the routing table. Once an exit
interface is chosen, the packet will be sent to the interface to be framed and
sent out on the local network. If the entry for the destination network is not
found in the routing table, the router drops the packet.

Examples include – IP, IPX, AppleTalk DDP

Layer 4 – Transport

Services located in the Transport layer both segment and reassemble data
from upper-layer applications and unite it onto the same data stream. They
provide end-to-end data transport services and can establish a logical connection
between the sending host and destination host on an internetwork.

Examples include – TCP, UDP, SPX

Layer 5 – Session

The Session layer is responsible for setting up, managing, and then tearing
down sessions between Presentation layer entities. The Session layer also
provides dialog control between devices, or nodes. It coordinates communication
between systems and serves to organize their communication by offering
three different modes: simplex, half-duplex, and full-duplex. The Session
layer basically keeps different applications’ data separate from other applications

Examples include – RPC, SQL, NFS, NetBios names

Layer 6 – Presentation

The Presentation layer gets its name from its purpose: It presents data to the
Application layer. It’s essentially a translator and provides coding and con-version
functions. A successful data transfer technique is to adapt the data
into a standard format before transmission. Computers are configured to
receive this generically formatted data and then convert the data back into its
native format for actual reading (for example, EBCDIC to ASCII). By providing
translation services, the Presentation layer ensures that data transferred
from the Application layer of one system can be read by the
Application layer of another host.

Examples include – JPEG, ASCII, EBCDIC, TIFF, GIF, PICT, encryption, MPEG, MIDI

Layer 7 – Application

The Application layer of the OSI model is where users communicate to the computer.
The Application layer is responsible for identifying and establishing the availability
of the intended communication partner and determining if sufficient resources for the
intended communication exist.

Examples include – Telnet, HTTP, FTP, WWW browsers, NFS, SNMP

Ryan Smith

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