The Digital services are more advantageous than the analog services in the sense that they are less sensitive to noise and other forms of interference. The cost of installation is also very low as compared to analog services as it needs only to differentiate between two or three levels of voltage instead of a continuous range of values.
Digital systems are further subdivided into-
a) Switched/56 service,
b) DS and c) DDS.
a) Switched/56 service: It is a digitalised version of analog switched line that allows data rate up to 56Kbps. To communicate through this service, both parties must subscribe. A caller with normal telephone service cannot connect to a telephone or computer with switched/56 even by using a modem. Subscribers don’t need modems to transmit digital data, but instead they need another device called the DSU (Digital service unit) which changes the rate of digital data created by subscriber’s device to 56Kbps and encodes it into a format used by service provider.
DSU is more expensive than modems, but still preferred as it has advantages like better speed, better quality, less susceptibility to noise and can allow bandwidth on demand service (which facilitates video conferencing, multimedia etc.).
b) Digital Data Service (DDS): It is the digitalised version of analog leased line with a maximum data rate of 64Kbps. Like Switched/56, DDS also requires the use of a DSU. The DSU in this service is however cheaper than the one used in Switched/56. It does not use a dial-pad like the Switched/56.
c) Digital Signal (DS) Service: DS is an advanced version of the Switched/56 and the DDS. It incorporates a hierarchy of digital services like that of an analog service. Some of its hierarchical services are:
· DS0: Resembles DDS, Single digital channel of 64Kbps.
· DS1: 1.54Mbps service. It can carry other desired multiplexed channels also.
· DS2: 6.31Mbps service. It can be used as a single or multiplexed channel.
· DS3: 44.37Mbps service.
· DS4: 274.17Mbps service.