1. How Circuit-Switches
Digital Networks Work
Circuit-Switched
Central Office Switches
ISDN
TCP/IP

2. Circuit-Switched
Circuit-switched is a type of network in which a physical path is obtained
for and dedicated to a single connection between two end-points in the
network for the duration of the connection.

3. Central Office Switch
A central office switch is a large rack of equipment located in a telephone
company's central office that switches calls locally or to long-distance carriers over
dedicated circuits.

4. ISDN
ISDN is a telephone system network. Prior to the ISDN, the phone system was
viewed as a way to transport voice, with some special services available for data.
The key feature of the ISDN is that it integrates speech and data on the same lines,
adding features that were not available in the classic telephone system. There are
several kinds of access interfaces to the ISDN defined: BRI, PRI and B-ISDN.

5. TCP/IP
TCP/IP is the set of communications protocols used for the Internet and other similar
networks. It is named from two of the most important protocols in it: the TCP and the IP,
which were the first two networking protocols defined in this standard. Today's IP
networking represents a synthesis of several developments that began to evolve in the
1960s and 1970s, namely the Internet and LANs , which emerged in the mid- to late-1980s,
together with the invention of the World Wide Web by Tim Berners-Lee in 1989.

6. References
http://searchnetworking.techtarget.com/sDefinition/0,,sid7_gci211787,00.html
http://www.mindspeed.com/web/products/applications/index.jsp?catalog_id=620
&cookietrail=0
http://en.wikipedia.org/wiki/ISDN
http://en.wikipedia.org/wiki/TCP/IP