OSI MODEL
The Open Systems Interconnection reference model (OSI model) is a reference model used
to define how data communications occur on computer networks. It is divided into layers
that provide services to the layers above and below. These layers are associated with protocols and devices.
The OSI model was created and ratified by the International Organization for
Standardization (ISO), and is represented in the United States by the American National
Standards Institute (ANSI). This model was created to do the following:
■■ Explain network communications between hosts on the LAN or WAN.
■■ Present a categorization system for communication protocol suites.
■■ Show how different protocol suites can communicate with each other.
When we say “different protocol suites,” keep in mind that TCP/IP is not the only player
in town. However, it is by far the most common. If TCP/IP devices need to communicate
with other devices using other communication protocols, the OSI model can help to
describe how translation between the two will take place. In addition to being described by
the OSI model, TCP/IP has its own model—the TCP model, which is discussed toward the
end of this lesson. Know that network communications existed before the model was created. This model is an abstract way of categorizing the communications that already exist. The model was created to help engineers understand what is happening with communication protocols behind
the scenes. Let’s go ahead and break down the OSI into its distinct layers and functions
Defining the OSI Model Layers
The OSI model was created as a set of seven layers, or levels, each of which houses different protocols within one of several protocol suites, the most common of which is TCP/IP. The OSI model categorizes how TCP/IP transactions occur. It is invaluable when it comes to installing, configuring, maintaining, and, especially, troubleshooting networks.
Sometimes, a protocol suite such as TCP/IP is referred to as a protocol stack. The OSI
model shows how a protocol stack works on different levels of transmission (that is, how
it stacks up against the model). Because it is the most tangible, let’s start with the Physical
layer. Later, when you view the model, it will be from the seventh layer on top to the first
layer on the bottom. As mentioned previously, a LAN requires computers with network adapters. These must be connected in some way to facilitate the transfer of data. It is important to define how they are connected, as well as how they transmit the data. The OSI model layers do just that. The following list gives a brief description of each of the layers
Layer 1: Physical layer
This is the physical and electrical medium for data transfer. It
includes but is not limited to cables, jacks, patch panels, punch blocks, hubs, and multistation access units (MAUs). It is also known as the physical plant. Concepts related to
the Physical layer include topologies, analog versus digital/encoding, bit synchronization,
baseband versus broadband, multiplexing, and serial (5-volt logic) data transfer. If you can
touch it, it is part of the Physical layer, making this layer one of the easiest to understand.
The unit of measurement used on this layer is bits.
Layer 2: Data Link layer
This layer establishes, maintains, and decides how the transfer is
accomplished over the Physical layer. Devices that exist on the DLL are network interface
cards and bridges. This layer also ensures error-free transmission over the Physical layer
under LAN transmissions. It does this through the use of physical addresses (the hexadecimal address that is burned into the ROM of the NIC), otherwise known as the MAC address (to be discussed more later in this lesson). Just about any device that makes a physical connection to the network and can move data is on the Data Link layer.
The unit of measurement used on this layer frames.
Layer 3: Network layer
The Network layer is dedicated to routing and switching information
between different networks, LANs, or internetworks. This can be on the LAN
or WAN (wide area network). Devices that exist on the Network layer are routers and
IP switches. Now we are getting into the logical addressing of hosts. Instead of physical
addresses, the addressing system of the computer is stored in the operating system—for
example, IP addresses Now you can see that a typical computer really has two addresses: a physical or hardware-based address, such as a MAC address, and a logical or software-based address, such as an IP address. Part of the trick in networking is to make sure the two get along together! The unit of measurement used on this layer is packets
Layer 4: Transport layer
This layer ensures error-free transmission between hosts through
logical addressing. Therefore, it manages the transmission of messages through Layers 1
through 3. The protocols that are categorized by this layer break up messages, send them
through the subnet, and ensure correct reassembly at the receiving end, making sure there
are no duplicates or lost messages. This layer contains both connection-oriented and connectionless systems, which are covered later in this lesson, in the Defining the Transport
Layer section. Inbound and outbound ports are controlled by this layer. When you think
ports, think the Transport layer. The unit of measurement used on this layer is sometimes referred to as segments or messages. All layers above this use the terms data and messages.
Layer 5: Session layer
This layer governs the establishment, termination, and synchronization
of sessions within the OS over the network and between hosts—for example, when
you log on and log off. It is the layer that controls the name and address database for the
OS or NOS. NetBIOS (Network Basic Input/Output System) works on this layer
Layer 6: Presentation layer
This layer translates the data format from sender to receiver
in the various operating systems that may be used. Concepts include code conversion, data
compression, and file encryption. Redirectors work on this layer, for example, mapped network drives that enable a computer to access file shares on a remote computer.
Layer 7: Application layer
This is where message creation—and, therefore, packet creation—
begins. DB access is on this level. End-user protocols, such as FTP, SMTP, Telnet, and
RAS, work at this layer. For example, suppose you are using Outlook Express. You type
a message and click Send. This initiates SMTP (Simple Mail Transfer Protocol) and other
protocols, which send the mail message down through the other layers, breaking it down
into packets at the Network layer and so on. This layer is not the application itself, but the
protocols that are initiated by this layer
