Public Switched Data Network(PSDN)
In public switched data network all data to be transmitted is first assembled into one or more message units.called packets.These packets include both the source and the destination network address.The destination is determined by the destination address within the packet,where the packet will be finally passed to the destination.
Integrated Service Digital Network(ISDN)
ISDN(Integrated Service Digital Network)is a fully digital transmission over ordinary telephone copper wire as well as over other media.Home and business users who install an ISDN adapter(in place of a modem)can see highly graphic Web pages arriving very quickly(up to 128Kbps).ISDN requires adapter at both ends of the transmission so your access provider also needs an ISDN adapter.
Modem
There are two levels of service:the Basic Rate Interface(BRI)intended for the home and small enterprise,and the Primary Rate Interface(PRI),for larger organizations.
USB modem
ADSL(Asymmetric Digital Subscriber Line)
Unlike regular dial up phone service,ADSL provides continuously available,"always on" connection.ADSL simultaneously accommodates analog(voice) information on the same line.ADSL is generally offered at downstream data rates from 512Kbps to about 6 Mbps.