Communicating Between Layers
Each layer of the OSI model uses its own protocol
to communicate with its peer layer in the destination
device. The OSI model specifies how each
layer communicates with the layers above and
below it, allowing vendors to focus on specific layers
that will work with any other vendor’s adjacent
layers.
Information is exchanged between layers using
protocol data units (PDU). PDUs include control
information (in the form of headers and trailers)
and user data. PDUs include different types of
information as they go up or down the layers
(called “the stack”). To clarify where the PDU is
on the stack, it is given a distinct name at each of
the lower levels.
At-a-Glance: OSI Model
In other words, a PDU that is a segment (Layer 4)
includes all the application layer’s information. A
packet (Layer 3) includes network layer control
information in addition to the data and control
information contained at the transport layer.
Similarly, a frame (Layer 2) is a PDU that includes
data link layer control information in addition to
the upper layer control information and data.
Finally, PDUs at the physical layer (Layer 1) are
called bits. 24
IT Certification CCIE,CCNP,CCIP,CCNA,CCSP,Cisco Network Optimization and Security Tips
What Problems Need to Be Solved?
An OSI layer can communicate only with the layers
immediately above and below it on the stack,
and with its peer layer on another device. A
process must be used so that information (including
data and stack instructions) can be passed
down the stack, across the network, and back up
the stack on the peer device.
immediately above and below it on the stack,
and with its peer layer on another device. A
process must be used so that information (including
data and stack instructions) can be passed
down the stack, across the network, and back up
the stack on the peer device.
OSI Layers and Definitions
OSI Layers and Definitions
The OSI layers are defined as follows:
Layer 1: Physical
Layer 2: Data link
Layer 3: Network
Layer 4: Transport
Layer 5: Session
Layer 6: Presentation
Layer 7: Application
The four lower layers (called the data flow layers)
define connection protocols and methods for
exchanging data.
The three upper layers (called the application layers)
define how the applications within the end stations
communicate with each other and with users.
Several mnemonics have been developed to help you
memorize the layers and their order. Here’s one:
Please Do Not Throw Sausage Pizza Away
The OSI layers are defined as follows:
Layer 1: Physical
Layer 2: Data link
Layer 3: Network
Layer 4: Transport
Layer 5: Session
Layer 6: Presentation
Layer 7: Application
The four lower layers (called the data flow layers)
define connection protocols and methods for
exchanging data.
The three upper layers (called the application layers)
define how the applications within the end stations
communicate with each other and with users.
Several mnemonics have been developed to help you
memorize the layers and their order. Here’s one:
Please Do Not Throw Sausage Pizza Away
Why Should I Care About the OSI Model?
The Open Systems Interconnection (OSI) model is
a conceptual framework that defines network functions
and schemes. The framework simplifies complex
network interactions by breaking them into
simple modular elements. This open-standards
approach allows many independent developers to
work on separate network functions, which can
then be combined in a “plug-and-play” manner.
The OSI model serves as a guideline for creating
and implementing network standards, devices, and
internetworking schemes. Advantages of using the
OSI model include the following:
• It breaks interrelated aspects of network operation
into less-complex elements.
• It enables companies and individual engineers to
specialize design and development efforts on
modular functions.
• It provides standard interfaces for plug-and-play
compatibility and multivendor integration.
• It abstracts different layers of the network from
each other to provide easier adoption of new
technologies within a layer.
a conceptual framework that defines network functions
and schemes. The framework simplifies complex
network interactions by breaking them into
simple modular elements. This open-standards
approach allows many independent developers to
work on separate network functions, which can
then be combined in a “plug-and-play” manner.
The OSI model serves as a guideline for creating
and implementing network standards, devices, and
internetworking schemes. Advantages of using the
OSI model include the following:
• It breaks interrelated aspects of network operation
into less-complex elements.
• It enables companies and individual engineers to
specialize design and development efforts on
modular functions.
• It provides standard interfaces for plug-and-play
compatibility and multivendor integration.
• It abstracts different layers of the network from
each other to provide easier adoption of new
technologies within a layer.
Layer 7, application:
Layer 7, application: The application layer provides networking services to a
user or application. For example, when an e-mail is sent, the application
layer begins the process of taking the data from the e-mail program and
preparing it to be put onto a network, progressing through Layers 6
through 1.
The combination of the seven layers is often called a stack. A transmitting
workstation traverses the stack from Layer 7 through Layer 1, converting the
application data into network signals. The receiving workstation traverses the
stack in the opposite direction: from Layer 1 to Layer 7. It converts the
received transmission back into a chunk of data for the running application.
When the OSI model was created, there was an industry initiative that tried to
implement a universal set of OSI network protocols, but it was not adopted.
Most popular protocols today generally use design principles that are similar
to and compatible with the OSI model, but they deviate from it in some areas
for various technical reasons. That said, the OSI model is still considered the
basis of all network communication.
user or application. For example, when an e-mail is sent, the application
layer begins the process of taking the data from the e-mail program and
preparing it to be put onto a network, progressing through Layers 6
through 1.
The combination of the seven layers is often called a stack. A transmitting
workstation traverses the stack from Layer 7 through Layer 1, converting the
application data into network signals. The receiving workstation traverses the
stack in the opposite direction: from Layer 1 to Layer 7. It converts the
received transmission back into a chunk of data for the running application.
When the OSI model was created, there was an industry initiative that tried to
implement a universal set of OSI network protocols, but it was not adopted.
Most popular protocols today generally use design principles that are similar
to and compatible with the OSI model, but they deviate from it in some areas
for various technical reasons. That said, the OSI model is still considered the
basis of all network communication.
Layer 6, presentation:
Layer 6, presentation: The presentation layer provides formatting services
for the application layer. For example, file encryption happens at this layer,
as does format conversion.
for the application layer. For example, file encryption happens at this layer,
as does format conversion.
Layer 5, session:
Layer 5, session: The session layer manages connections between hosts. If
the application on one host needs to talk to the application on another, the
session layer sets up the connection and ensures that resources are available
to facilitate the connection. Networking folks tend to refer to Layers 5 to 7
collectively as the application layers.
the application on one host needs to talk to the application on another, the
session layer sets up the connection and ensures that resources are available
to facilitate the connection. Networking folks tend to refer to Layers 5 to 7
collectively as the application layers.
Layer 4, transport:
Layer 4, transport: The transport layer is responsible for taking the chunk
of data from the application and preparing it for shipment onto the network.
Prepping data for transport involves chopping the chunk into smaller
pieces and adding a header that identifies the sending and receiving application
(otherwise known as port numbers). For example, Hypertext Transfer
Protocol (HTTP) web traffic uses port 80, and FTP traffic uses port 21.
Each piece of data and its associated headers is called a packet.
of data from the application and preparing it for shipment onto the network.
Prepping data for transport involves chopping the chunk into smaller
pieces and adding a header that identifies the sending and receiving application
(otherwise known as port numbers). For example, Hypertext Transfer
Protocol (HTTP) web traffic uses port 80, and FTP traffic uses port 21.
Each piece of data and its associated headers is called a packet.
Layer 3, network:
Layer 3, network: The network layer is where the majority of communications
protocols do their work, relying on Layers 2 and 1 to send and receive
messages to other computers or network devices. The network layer adds
another header to the front of the packet, which identifies the unique source
and destination IP addresses of the sender and receiver. The process of routing
IP packets occurs at this level.
protocols do their work, relying on Layers 2 and 1 to send and receive
messages to other computers or network devices. The network layer adds
another header to the front of the packet, which identifies the unique source
and destination IP addresses of the sender and receiver. The process of routing
IP packets occurs at this level.
Subscribe to:
Posts (Atom)