Classroom Companion: Business

Definition 3.3
Connection-oriented transfer means that an association is established between the 
users before information is transferred between them. The association is retained 
until all information has been transferred.
Definition 3.4
Connectionless transfer means that no such association is established, and data is 
transferred in independent packages.
A circuit switched connection is always connection oriented, while a data connec-
tion may either be connection oriented or connectionless. Connection-oriented 
packet switched connections are also referred to as virtual circuits. Note that the 
Internet protocol (IP) itself is connectionless. Connection-oriented data transfer is 
achieved by imbedding a connection-oriented protocol in the IP packets. The most 
used connection-oriented protocol on the Internet is the Transmission Control 
Protocol (TCP) (the TCP/IP protocol suit).
The idea behind the Integrated Services Digital Network (ISDN) was to provide 
both telephone services and packet switched data transmission over the infrastruc-
ture of the telephone network. It turns out that it is impossible to incorporate 
packet switching in a circuit switched environment. The ISDN, therefore, consists 
of two parallel transport networks: the telephone network and the—now obso-
lete—X.25 packet switched data network. Therefore, the ISDN never became a 
 
Chapter 3 ·
Convergence of Technologies and Services

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common network for circuit switched and packet switched services. On the other 
hand, data services and telephone services can easily be integrated on the copper 
lines connecting the user to the network by simple multiplexing methods. The 
ISDN signaling protocol and the multiplexing on the subscriber line also allows 
the ISDN user to connect to several other users at the same time.
The two most important advantages of ISDN are that new subscriber lines 
need not be deployed for data transmission and that several simultaneous commu-
nication sessions can be established on each subscriber line. However, ADSL and 
related subscriber line technologies provide similar and simpler solutions to pro-
vide broadband data combined with telephony over subscriber lines. ISDN are also 
nicknamed as “Innovations Subscribers Don’t Need” or “It Still Does Nothing” 
since the technology does not offer much more than was already available on the 
PSTN and dedicated data networks.
Asynchronous Transfer Mode (ATM) was developed for supporting both tele-
phony and broadband data over a common packet switched high-speed optical 
network (140 megabits per second). The development of ATM took place during 
the 1990s coinciding with the commercialization of the Internet. Data and speech 
samples are sent in small data packets containing only 48 bytes of information. 
Short packets (or cells as they are called) sent at high speed makes it easy to avoid 
that the voice and video signals are destroyed by timing fluctuations. Because of 
the particular routing algorithm chosen for packet transfer, all data is sent in a 
connection-oriented mode; that is, a complete connection must be established 
between the users before data is sent. This applies independently of the amount of 
data being sent—even the transfer of less than 48 bytes fitting into a single cell 
requires the establishment and release of a connection.
At the time when the ATM specification had been finalized, the Internet had 
been commercialized and had already replaced most of the X.25 networks. Internet 
is a simple and cheap network, and, initially, users were not urging for integrating 
voice and data communication. Because of the popularity of the Internet and the 
rapid increase in mobile communication, ATM was never realized in large scale, 
and the ATM Forum promoting the technology became the Broadband Forum in 
2005 concerned with broadband network technologies in general.