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Unformatted text preview: hone company's switching system. The modem attached to
the user's computer then sends and receives data over the telephone line. Just as
in the case of a telephone call, the charge for data transmission service in this case
depends on the duration of communication (circuit establishment) and the distance
between the two computers.
2. Leased line. Leased line (also known as private line or dedicated line) is a
special conditioned telephone line that directly and permanently connects two
computers. A leased line can be used for both voice and data transmissions.
Hence, if an organization has two offices in two different cities, it is often cheaper
for the organization to acquire its own leased line between the two offices which
can be used by the organization for all its voice (telephone calls) and data
transmissions between the two offices. The charges for a leased line are often
based on channel capacity (bps) and distance (air miles).
3. Integrated Services Digital Network (ISDN). The ISDN is a telephonic system
that provides digital (not analog) telephone and data services. As it supports digital
services (including digitized voice), the ISDN telephone users enjoy noise-free,
CD-quality sound. Moreover, with the ISDN, no modem is necessary because it
supports digital transmission of all types of data (including voice). This also
results in very short call set-up time between two ISDN subscribers. The main factor that led to the development of the ISDN was the adoption of
digital transmission by many public telephone systems. Voice traffic in such
networks is encoded using pulse-code modulation at 64 Kbps. Naturally, these
networks could also carry data, resulting in integrated digital networks (IDN).
Based on the transmission and switching capabilities, the ISDNs are currently of
two types - narrowband ISDN and broad-band ISDN. Narrow-band ISDN is based
on 64 Kbps bit-streams that are combined into higher-capacity "trunks" using
time-division multiplexing. For example, 32 64-Kbps channels...
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This document was uploaded on 04/07/2014.
- Spring '14