Classroom Companion: Business

3.2 
 Early Attempts of Service Integration
Attempts to design one integrated network supporting services carried by the 
PSTN and dedicated data networks started in the late 1970s resulting in the 
Integrated Services Digital Network (ISDN) standard in 1988. The failure of 
ISDN was that it did not amalgamate the telephone network and the packet data 
network into a common network except at the subscriber line. The transport net-
work for telephony and data still existed as two separate networks. The next attempt 
was to merge the telephone network and packet data network into a single trans-
port network using the Asynchronous Transfer Mode (ATM) technology. This 
attempt failed because the technology was ready for implementation at the same 
time as the Internet expanded into a worldwide platform open for access for every-
one. At the same time, cellular mobile communications grew rapidly offering 
location- independent services on a global scale.
To appreciate the challenges associated with service integration, some basic 
technological concepts must be understood: circuit switching, packet switching, 
connection- oriented transfer of data, and connectionless transfer of data.
Definition 3.1
Circuit switching implies that a physical two-way (duplex) connection between the 
users exists so long as the communication session lasts.
3.2 · Early Attempts of Service Integration

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The telephone network is a circuit switched network where a two-way physical 
connection is established over several telephone exchanges for the duration of the 
call. A circuit switched connection can be used for only one communication ses-
sion at a time.
Definition 3.2
Packet switching implies that chunks of data (e.g., speech or video samples, emails, 
web pages, and so on) are sent as independent packets of data.
At the switching devices in the network (the routers), the packets are queued before 
they are forwarded to the next router or to the recipient. Moreover, packets belong-
ing to different communication sessions are arbitrarily mixed when they are sent 
over the communication links between routers and between routers and terminals. 
For this reason, packet switched networks offer better utilization of the communi-
cation infrastructure than circuit switched networks: for voice communication, 
each direction of a telephone circuit is on average busy only 40% of the time so that 
60% of the transmission capacity is wasted. For the transfer of pictures and docu-
ments over the telephone network (telephoto or facsimile), both directions of com-
munication are used, but only one is occupied by transfer of information.