Page 171 - DCAP406_DCAP_207_Computer Networks

Computer Networks/Networks

Notes administers end-to-end control and error checking to ensure complete data transfer. The functions
provided at this layer are transport address to network address mapping, makes multiplexing
and splitting of transport connections, flow control, virtual circuit management and error checking
and recovery. The transport layer’s job also includes breaking the messages from the session
layer into segments. The transport-layer protocols including Transmission Control Protocol
(TCP), User Datagram Protocol (UDP), Name Binding Protocol and OSI transport protocols
provide either connection-oriented or connectionless transmission. Quality of service is also
one of the functions of the transport layer.

11.1 Transport Service

The basic function of the transport layer is to respond to service requests from the session layer
and issue service requests to the network layer. To accomplish this task, it accepts data from the
session layer, splits it up into smaller units if required, pass these smaller units to the network
layer and ensure that the packets of data reassemble correctly at the destination machine. The
transport layer indents to perform all these function efficiently and keep the session layer
isolated from the necessary changes in the hardware technology. The transport layer provides
the following services:
11.1.1 Services Provided to the Upper Layers

Transport layer in conjunction with network layer intends to provide efficient, reliable and cot
effective services to its users through processes in application layer. The software and hardware
that are used in the transport layer is called transport entity. The transport entity is located
either in the kernel of the operating system or network interface card or remote user process or
the library package meant for network applications.

Did u know? Like, network layer, the transport also provides connection oriented and
connectionless services. Under normal conditions, in both cases, connection is established
to transfer data and after successful transfer of data the connection is released.
The transport layer establishes a distinct network connection for a transport connection required
by the session layer. When the transport connection needs a high throughput, the transport
layer establishes multiple network connections where it divides the data among the multiple
network connections to improve throughput. It also manages the bandwidth and thus reduces
the cost of establishing a connection. It does so by multiplexing several transport connections
onto the same network connection. The multiplexing always remains transparent to the session
layer. Thus transparent layer isolates the upper layers from the technology, design and
imperfections of the subnet.

11.1.2 Quality of Service

Transport layer bridges the gap of the services provided by the network and therefore enhances
the quality of service provided to the users. The possible parameters for the quality of service as
offered by the transport layer are connection establishment delay, connection establishment
failure probability, throughput, transit delay, residual error ratio, protection priority, resilience,
etc.

Connection establishment delay: It is the amount of time when an acknowledgement is received
from the destination machine to which a connection is requested. Obviously, lesser is the delay,
better is the service.

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