™ Multiplexing Application-Cont

™   Digital Services o    Digital services are largely offered nowadays o    Digital services are less sensitive than analog ones o    Telephone line acts as an antenna and pick up noise both in case of analog and digital TX o    In analog, both noise and signal are analog o    In digital, signal can easily be separated Switched / 56 Service It is digital version of Analog switched line Allows data rates of up to 56 Kbps Both parties must subscribe to the service A caller with normal telephone service cannot connect to a telephone or computer with this service even with a Modem Because this service is already digital, subscribers do not need modems They need DSU (Digital service unit) DSU changes the rate of digital data created by the subscriber's device to 56 Kbps and encodes it in the format used by service provider DSU is often included in dialing process DSU is more expensive than Modem So why would a subscriber pay for switched/56 service and DSU? Because digital line has better speed, better quality and less susceptibility to noise Bandwidth on Demand Digital Data Service (DDS) Digital version of analog leased line Max. data rate of 64 Kbps Like switched/56, DDS requires a DSU DSU for this service is cheaper than switched /56 DSU because it does not need a dial pad Digital Signal (DS) Service Telephone companies saw the need to develop a hierarchy of digital services much like those used for Analog services The next step was DS service DS is a hierarchy of digital signals