*
l
l
Controls congestion if too many packets are on
the network
Translates logical addresses or names into
physical addresses
Has accounting functions to count packets orbits
sent by users to produce billing information
This layer acts in our postal service analogy, like the
regional mail distribution centers throughout the
country. The trucks are directed to the centers and are
routed along the best path to their final destinations.
Layer 4—The transport layer ensures data units
are delivered error-free, in sequence, with no losses or
duplications. It relieves higher layer protocols from any
concern with the transportation of data between them,
as follows:
l
l
l
l
l
Message segmentation—accepts data from the
session layer, splits it up into smaller units, and
passes the units down to the network layer
Establishes and deletes host-to-host connections
across the network
Multiplexes several message streams onto one
channel and keeps track of which message
belongs to which connection
Provides reliable end-to-end delivery with
acknowledgment
Provides end-to-end flow control and window
management
The transport layer functions are provided by the mail
truck dispatcher, who takes over if there is a wreck out
in the system. If the network goes down, the transport
layer software will look for alternate routes or perhaps
save the transmitted data until the network connection is
reestablished.
Layer 5—The session layer allows users on
different machines to establish sessions between them.
It performs the functions that enable two applications to
communicate across the network, performing security,
name recognition, logging, administration, and other
similar functions. Unlike the network layer, this layer is
dealing with the programs in each machine to establish
conversations between them, as follows:
Allows two applications processes on different
machines to establish, use, and terminate a
connection (or session)
Performs synchronization between end-user
tasks by placing checkpoints in the data stream
so if the network fails, only the data after the last
checkpoint has to be retransmitted
Provides dialogue control (who speaks, when,
how long, and so on)
The session layer in our postal agency recognizes
different zip codes and reroutes letters.
Layer 6—The presentation layer formats data to
be presented to the application layer. It can be viewed as
the translator for the network. This layer provides a
common representation for data that can be used
between the application processes. The presentation
layer relieves the applications from being concerned
with data representation, providing syntax
independence, as follows:
l
l
l
Encodes data in a standard way (integers,
floating point, ASCII, and so on)
Provides data compression to reduce the number
of bits that have to be transmitted
Provides data encryption for privacy and
authentication
This layer functions like a translator who translates a
letter from French into English.
Layer 7—The application layer serves as the
window for the application process to access the OSI
environment. This layer represents the services that
directly support users and application tasks. It contains
a variety of commonly needed protocols for the
following:
Network virtual terminals
File transfers
Remote file access
Electronic mail
Network management
In our analogy, the application layer is the person who
writes or reads the letter.
CABLING
A data communications network must have cabling
to allow individual computers and other peripherals to
talk to one another and share resources. And wouldn’t it
be easier if there were only one type available? There
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