This section is a review of the OSI model, which was originally discussed in
CCNA: Cisco Certified
Network Associate Study Guide, 4th ed
., by Todd Lammle (Sybex, 2004). The
OSI model
(the Open Systems Interconnection reference model) is the template used to design applications
or protocols that allow nonhomogenous computers or networks to communicate with one
another. The ISO (International Organization for Standardization) developed the OSI model.
The OSI model consists of seven layers. Each layer communicates directly with its adjacent layers,
as well as with the corresponding layer of the destination system (depicted in Figure 36.1).
Communication between layers facilitates the transfer of data up and down the OSI model. Communication
between the corresponding layers of the source system and the destination system
enables two heterogeneous networks or computers to understand each other.
The OSI template defines the services and roles that each layer is to provide. Because each
layer provides different services and functions, the layers need to communicate so that the data
can be transmitted up and down the seven layers and onto the destination system. The following
list summarizes the responsibility of each of the seven layers, starting from the Physical layer
and working up to the Application layer:
Physical
This layer sends and receives bits with values of 1s and 0s. The Physical layer is
in charge of determining how it sends these values. If the physical connection between two
machines is fiber-optic, then the Physical layer has to use light to transmit the 1s and 0s. If
the connection is electrical, then electrical signals are sent to represent the 1s and 0s.
Data Link
This layer takes all the data that is accumulated as packets are handed from one layer
to the next and then packages it into frames. The Data Link layer equates the Network layer address
(IP address) to a data link address, or MAC address, of the next hop. Once the physical address is
known, the frame is sent to that address. The receiving interface uses the Data Link layer to extract
the packet from the frame, discards the frame, and then sends the packet up to the Network layer.
Network
This layer defines the topology of the network through the use of logical addressing.
Routing protocols use this information to route packets.
Transport
This layer takes care of end-to-end communications. It is responsible for the
connection to the destination system, as well as for packet segmentation and assembly. The
Transport layer includes both connection-oriented and connectionless protocols (for example,
TCP and UDP).
Session
This layer is responsible for coordinating communication among applications, which
it does through dialog-control methods.
Presentation
This layer negotiates syntax, so it is responsible for the proper method of presenting
the data to the Application layer. Some of the Presentation layer functions are compression/
decompression and encryption/decryption of data.
Application
This is the user and application interface. The Application layer is responsible for
data exchange and job management. It also handles file, print, message, database, and application
services.
You saw how the logical data flow of the OSI model works, but look at Figure 36.2, in
which you can see the actual data flow. This figure depicts data that is handed from the Application
layer all the way down to the Physical layer. At that point, the data is transmitted
across any variety of physical media to the next hop, or destination system. Once the 1s and
0s arrive at the Physical layer of the destination system, the information is sent to layer 2 (the
Data Link layer). This layer discards the frame, and then the extracted packet is handed up
to the Network layer. The network packet header is stripped off, and the resulting packet is
handed up to the Transport layer. This process is repeated for each layer until it arrives at the
Application layer.
Now that each layer of the OSI reference model has been explained briefly, you need to
focus on the functions of each layer in detail. This detail provides the necessary background
and information to effectively troubleshoot network problems that occur within specific layers
of the OSI model.