Data Transfer Techniques
Connection oriented and Connectionless are two distinct techniques used in data
communications to transfer data. Each has its own advantages and
disadvantages.

Connection-oriented
Requires a session connection (analogous to a
phone call) be established before any data can be sent. This method is
often called a "reliable" network service. It can guarantee that data will
arrive in the same order. Connection-oriented services set up virtual links
between end systems through a network, as shown in Figure 1. Note that
the packet on the left is assigned the virtual circuit number 01. As it
moves through the network, routers quickly send it through virtual circuit
01.
Figure 1: Data Transfer Services

Connectionless Does not require a session connection between sender
and receiver. The sender simply starts sending packets (called datagrams)
to the destination. This service does not have the reliability of the
connection-oriented method, but it is useful for periodic burst transfers.
Neither system must maintain state information for the systems that they
send transmission to or receive transmission from. A connectionless
network provides minimal services.
Connection-oriented methods may be implemented in the data link layers of the
protocol stack and/or in the transport layers of the protocol stack, depending on
the physical connections in place and the services required by the systems that
are communicating. TCP (Transmission Control Protocol) is a connection-oriented
transport protocol, while UDP (User Datagram Protocol) is a connectionless
network protocol. Both operate over IP.
The physical, data link, and network layer protocols have been used to implement
guaranteed data delivery. For example, X.25 packet-switching networks perform
extensive error checking and packet acknowledgment because the services were
originally implemented on poor-quality telephone connections. Today, networks
are more reliable. It is generally believed that the underlying network should do
what it does best, which is deliver data bits as quickly as possible. Therefore,
connection-oriented services are now primarily handled in the transport layer by
end systems, not the network. This allows lower-layer networks to be optimized
for speed.
LANs operate as connectionless systems. A computer attached to a network can
start transmitting frames as soon as it has access to the network. It does not
need to set up a connection with the destination system ahead of time. However,
a transport-level protocol such as TCP may set up a connection-oriented session
when necessary.
The Internet is one big connectionless packet network in which all packet
deliveries are handled by IP. However, TCP adds connection-oriented services on
top of IP. TCP provides all the upper-level connection-oriented session
requirements to ensure that data is delivered properly. MPLS is a relatively new
connection-oriented networking scheme for IP networks that sets up fast labelswitched paths across routed or layer 2 networks.
A WAN service that uses the connection-oriented model is frame relay. The
service provider sets up PVCs (permanent virtual circuits) through the network as
required or requested by the customer. ATM is another networking technology
that uses the connection-oriented virtual circuit approach.
Copyright (c) 2001 Tom Sheldon and Big Sur Multimedia.
All rights reserved under Pan American and International copyright
conventions.
Taken from: Tom Sheldon’s Linktionary.com
Available at :
http://www.linktionary.com/c/connections.html