Digital To Analogue Signals Modulation As Pstn example essay topic

Introduction to the PSTN The Public Switched Telephone Network (PSTN) was designed to enable analogue (sound) communication between telephones. It is a well-established network with extensive infrastructure providing universal coverage. PSTN is a circuit switched network. When a customer requests a service path, an electronic connection is established between their telephone connection and the local switching exchange (the local loop). The local exchange is interconnected to a network of main circuits and exchanges. When making a transmission each link in the network is dedicated to the connection for the duration of the transmission.

Once the communication has been completed, the switching device releases the channel making it available for other communication sessions. The PSTN is constantly being upgraded to increase the suitability to handle digital data more efficiently by introducing fibre optics and digital technology to work on packet switched basis. Connections from the local exchange to users premises are generally still analogue. Analogue to digital conversion is carried out at the source exchange. Digital to analogue conversion is carried out at the destination exchange. Communication Channels A communication channel (the link between two computers) is characterised by its transmission medium and transmission mode.

The transmission medium refers to wired (twisted-pair cable, coaxial cable, fibre optic cable) or wireless connection (microwave or satellite). The transmission mode refers to the modem. Converting Digital to Analogue Signals - Modulation As PSTN is not designed to carry binary electrical signals digital data must be converted into analogue form to be transmitted. This is achieved through modulation by attaching a modem which interfaces between the computer and PSTN. Modems convert digital data into a series of tones, sends the tones across the telephone network where they are demodulated at the receiver end back into digital data. The modem produces a high frequency signal that acts as a basis for the information signal, otherwise known as a carrier signal: A carrier signal carries no information.

The amplitude, frequency or phase of the signal can be manipulated to represent the encoding of the data. Frequency modulation is most commonly used as other methods of amplitude and phase modulation create errors. This works by the digital data being encoded onto the carrier by alternating the frequency: A binary code of 1 could be represented by a 1000 Hz signal, and a binary code of 0 could be represented by a 2000 Hz signal. Modulation protocols exist which manufacturers must adhere to ensure that modems can communicate with each other. Limitations of PSTN in Supporting Data Transmissions The universal access provided by the PSTN has lead to the network becoming the medium over which most users access the Internet. The sudden increase in customers using the 'dial up' method of accessing the Internet flooded the system as it introduced an unprecedented amount of data traffic onto the network.

This highlighted the following limitations of using the PSTN to support data communications: . It is the slowest form of electronic transmission. Data speeds are limited by the low bandwidth of the PSTN (30000 Hz), although modems can increase the speed of transmission... The PSTN is an inefficient system for carrying data as the maximum bandwidth is used for every transmission giving full dedication of the bandwidth without full utilisation, where as data networks vary the band with according to the required amount needing to be utilised... The PSTN is an expensive medium in comparison to data networks, which operate at a fraction of the cost of circuit switched networks... Data carried through the PSTN is more easily corrupted than data transmission through specialised data networks...

Users are charged for the duration of the connection rather than the volume of quality of the data transmission... Scalability - the network was designed for short duration calls, whereas the average connection for Internet usage is over an hour creating higher usage of the system. On occasion system capacity has been exceeded, overwhelming the system as traffic consumed all available circuit switched resources.