|
Session Layer
The Session layer
is responsible for the creation, management, and termination of sessions
between systems. A session is best described as a type of managed connection
between systems for the purpose of a specific type of communication. For
example, a session might be created for the purpose of user authentication,
or to initiate a file transfer.
The Session layer
is also responsible for coordinating how the communication between systems
takes place, which is known as dialog control. In some sessions, only a
single system is allowed to communicate at any point in time, referred to as
half-duplex. The Session layer would be responsible for determining whose
turn it is in these situations, and for how long each system is allowed to
communicate. In other cases, both systems can communicate at once, which is
also known as full duplex. If the communication stream were somehow
interrupted, the Session layer would be responsible for recognizing this and
re-establishing the session.
Examples of Session
layer protocols:
-
Network
File System (NFS): Unix file system access
-
Structured Query Language (SQL): Local or remote database queries
-
Remote
Procedure Call (RPC): Client-server communication mechanism
-
AppleTalk
Session Protocol (ASP): AppleTalk client-server communication
mechanism
-
X Windows:
Remote desktop sessions
Protocol Stacks
A protocol stack is
a group of protocols that all work together to allow software or hardware to
perform a function. The TCP/IP protocol stack is a good example. It
uses four layers that map to the OSI model as follows:
-
Layer 1: Network Interface - This layer
combines the Physical and Data layers and routes the data between devices
on the same network. It also manages the exchange of data between the
network and other devices.
-
Layer 2: Internet - This layer
corresponds to the Network layer. The Internet Protocol (IP) uses
the IP address, consisting of a Network Identifier and a Host
Identifier, to determine the address of the device it is communicating
with.
-
Layer 3: Transport - Corresponding to the
OSI Transport layer, this is the part of the protocol stack where the
Transport Control Protocol (TCP) can be found. TCP works by asking
another device on the network if it is willing to accept information from
the local device.
-
Layer 4: Application - Layer 4 combines
the Session, Presentation and Application layers of the OSI model.
Protocols for specific functions such as e-mail (Simple Mail Transfer
Protocol, SMTP) and file transfer (File Transfer Protocol,
FTP) reside at this level.
As you can see, it
is not necessary to develop a separate layer for each and every function
outlined in the OSI Reference Model. But developers are able to ensure that
a certain level of compatibility is maintained by following the
general guidelines provided by the model.
The OSI model
plays an important role in standardizing communications between disparate
systems. Were it not for the standards and specifications outlined in the
OSI model, linking Apple, IBM, UNIX, and other systems would certainly
prove much more challenging.
In Conclusion Lets Recap The 7 OSI Layers
OSI model—Application layer
The Application layer lives at the top of the OSI model. It’s the king. It
talks to a system’s software applications; hence the name. Applications owe
this layer a debt of gratitude, because it provides them with access to
network services.
OSI model—Presentation layer
The Presentation layer determines the format used to share data between
systems. Redirectors operate at this level. On Microsoft systems, Server
Message Blocks (SMBs) are used. On Novell systems, the Novell Core Protocol
(NCP) is used.
OSI model—Session layer
In order to share data, two systems must create a communications link.
Called a session, this connection must perform identity and security duties.
At the Session layer, systems verify that they are talking to a specific
machine. OSI
model—Transport layer
The Transport layer has one of the toughest jobs of any layer. Essentially,
its role is the equivalent of being the OSI model’s FedEx courier. The
Transport layer is charged with ensuring the proper delivery of data
packets, as well as packing and unpacking the messages.
OSI model—Network layer
The Network layer handles the task of stamping messages with address
information. It also translates logical addresses and names into physical
addresses.
OSI model—Data-link layer
The Data-link layer is charged with ensuring the error-free transfer of data
frames from one system to another via the Physical layer. At the Data-link
layer, data bits received from the Physical layer are packaged into data
frames and sent to the Network layer. Or, when data frames are received from
the Network layer, the Data-link layer passes them to the Physical layer and
the network connection.
OSI model—Physical layer
The Physical layer was named after the physical wires that connect system
NICs. The Physical layer determines the type of cable (electrical, optical,
mechanical, etc.) in use.
-
Because the OSI model is 'The Standard', all
discussions of other communications models refer to it. You may be using
this model directly, but it will be used as a reference point to make sure
you haven't forgotten something in your new model. Know it well!!
|
Navigation
