Page 27 - DCAP406_DCAP_207_Computer Networks

Computer Networks/Networks

Notes communicating for a long time between senders and receivers. However, they are notable for
wastage of bandwidth.
In connection-oriented service, each packet is associated with a source/destination connection.
These packets are routed along the same path, known as a virtual circuit.
Connectionless service adopts the mechanism of the postal system. Each message is broken into
packets and enclosed in an envelope. The envelope contains the full address. The envelope is
then routed independently. The order of the packets is not guaranteed. They are suitable for
sending short messages and notable to provide bandwidths in short time intervals.
In connectionless service, a router treats each packet individually. The packets are routed through
different paths through the network according to the decisions made by routers.

Quality of Service

Reliable services guarantee the delivery of data, which is implemented by acknowledgements.
However, this introduces overheads and thus reducing the efficiency. In case of file transfer, we
need a reliable connection-oriented service while an unreliable connection-oriented service is
appropriate for digitized voice traffic.

Example: Registered e-mails are example of reliable connectionless service with
acknowledgement while an unreliable connectionless service without acknowledgement is
appropriate for junk e-mail. They have a high probability of arrival but no guarantee. In the
client-server model, the request-reply command is another example of connectionless service.

Service Primitives

The Connection-oriented or connectionless service is specified by a set of primitives available
to a service user to interact with the service provider. These primitives enable the service
provider to perform some action or report on an action taken by a peer entity. Primitives have
parameters to define conditions. For example, request and response primitives, the sender and
the receiver may negotiate some conditions like message size. They are part of the protocol. A
confirmed service is defined with a request, an indication, a response, and a confirm primitives.
An unconfirmed service has a request and an indication primitive only.

Relationship of Services to Protocols

Services and protocols are distinct concepts and are important to establish and release connections
between sender and receiver.
A service is defined as a set of primitives that are nothing but actions or operations. They are
provided to the upper layer by an immediate lower layer. It defines only nature of actions to
perform by the layer upper to the service-initiating layer.
A protocol defines set of rules to describe the format and meaning of the frames, packets or
messages that are exchanged by the peer entities within the same layer. Entities use protocols to
implement their service definitions.

Task What do you understand by layered protocol? Explain, why protocols are layered?

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