Page 30 - DCAP406_DCAP_207_Computer Networks

Unit 2: Network Software

maintaining the connection. In broadcast networks, the routing problem is simple, so the network Notes
layer is often thin or even nonexistent.
The next three layers are task oriented and have to do with the operations performed by the user
rather than with the network.

Transport Layer (Layer 4)

This layer guarantees the orderly and reliable delivery of data between end systems after
accepting data from the session layer. Data is accepted from the Session layer and split up into
smaller units, if needed. Session layer passes the data to the Network layer and ensures that the
packets arrive correctly at the receiving side.

Basically, it performs connection management based upon the throughput conditions. In normal
condition, one network connection corresponds to multiple transport connections. In high
throughput condition, one transport connections correspond to multiple network connection.
The most popular protocol suite TCP/IP uses this layer. Transport layer also performs additional
functions such as data multiplexing and de-multiplexing. This layer divides up a transmitting
message into packets and reassembles it at the receiving end. Service offered at this layer
includes an error-free point-to-point channel to deliver messages in the order in which they
were sent. The transport layer is a true source-to-destination or end-to-end layer. Flow control
between hosts is also needed but different from between routers (similar principles will apply
to both).

Session Layer (Layer 5)

The session layer is responsible for establishing, maintaining, and arbitrating the dialogs between
communicating applications. It also provides enhanced services useful in some applications, for
example, remote login, remote file transfer, etc. It is also responsible for the orderly recovery
from failures by implementing appropriate check pointing mechanisms.

Presentation Layer (Layer 6)

The presentation layer performs functions related to the syntax and semantics of the information
transmitted that include formatting and displaying of received data by terminals and printers.
It is concerned with differences in the data syntax used by communicating applications. This
layer is responsible for remedying those differences by resorting to mechanisms that transform
the local syntax (specific to the platform in question) to a common one for the purpose of data
exchange.
For example, it performs encoding of data in a standard agreed upon way to facilitate information
exchange among heterogeneous systems using different codes for strings, for example, conversion
between ASCII and EBCDIC character codes. It facilitates data compression for reducing the
number of bits to be transmitted and encrypts data for privacy and authentication, if necessary.

Application Layer (Layer 7)

The application layer provides support services for user and application tasks. It determines
how the user will use the data network. It allows the user to use the network. For example, it
provides network-based services to the end user.
Examples of network services are distributed databases, electronic mail, resource sharing, file
transfers, remote file access and network management. This layer defines the nature of the task
to be performed.

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