Introduction to Telephone systems By Abhijeet Gole Telephones • The sending of phonic information from a particular point to distant point is achieved using telephone. • ‘Tele’ means distance and phone means phonic(sound) Working of Telephone Subscriber loop • Speech transmitter • Speech receiver • Bell system to receive the ringing tone • Anti-side tone coil • Cradle switches • Dialing mechanism Transmitter and Receiver Transmitter Receiver Subscriber’s local loop • Coils 1,2,3 and 4 are marked with mutual inductance, act as Anti side-tone induction coil (ASIC) Transmission Bridges Control Functions • Signaling • Hunting • Preparing • Holding • Releasing • Supervising • Metering Working of the Call Telephone Traffic • Amount to traffic at office= α • Number of calls Average Duration of Calls • The traffic carried= N x T Erlang or TU • Where N is the number of calls carried out during a particular duration T is the duration of the call expressed as fraction of the duration in which the number of calls have been noticed. Example 1 • If there are 100 calls carried out during busy hour each having an average holding duration of 3 minutes, the amount of traffic carried out can be calculated as follows: • The number of unit calls carried out=100. The duration of the call expressed as fraction of the duration in which the number of calls have been noticed 3/60 =1/20 The amount of traffic = N x T = 100 x 1/20 = 5 Erlang or TU Example 2 • If there are 200 calls are carried out in duration of 15 minutes with an average holding duration of 3 minutes, the traffic can be calculated as follows • Number of calls carried out= 200 • Duration of calls as fraction of the duration of calls have been noticed 3/5= 1/5 The amount of traffic carried out= 200x 1/5 = 40 TU or Erlang Switching • Switches select a particular outlet to be connected to a particular inlet. • Switching stage is called as node. • Notes are connected by links Two parts of switch 1) Switch hardware through which the information flows 2) The switch controller which receives signal from the user and actuates the connection between two points Control overlay switches Nodal Configurations Process of development in Switching technologies Thought of the day • Do what you can with all you have, wherever you are. • What you lack in talent can be made up with desire, hustle and giving 110% all the time Wireless information networks Applications of WL Information Systems • The wireless information networks cover wide range of communication applications • • • • • Broadcast Multicast services Cordless Cellular telephony Paging and data communications. Evolution of Telephone Systems • Early telephone exchange system. • Automatically switching • Automatic switching in several local exchange. Broadband Communication Systems • High-capacity data transferring • Video conferencing • Video on demand • Always on Internet services • Interactive multimedia communications • Web serving • Local Area Networks (LAN) • Similar applications Terrestrial Fixed Wireless Access (TFWA) • The digital subscriber loop available in public telephone network has got several drawbacks like laying and maintenance of the cable system. • FWA overcomes these drawbacks and at the same time it can be used in the areas that cannot be reached by digital subscriber loops (DSL). • Important requirements for implementation of wireless broadband by fixed wireless access is the availability of radio frequency spectrum. Mobile Communication • Wide Area Networks (range > 10 km) • Local Area Networks (range 1-10 km) • Personal Area Networks (range 20 m) The important features of the IMT 2000 • High degree of worldwide compatible design. • Compatibility between IMT 2000 and the public telephone network already in use. • Better quality and reliability. • Compact terminal for worldwide use. • Global roaming compatibility. • Wide range of services coverage—multimedia operation. INFORMATION TRANSMISSION • The information assumes any one of the following forms: (i) Speech (ii) Picture (ii) Picture (iii) Text (iv) Data State diagram for telephone network. TRANSMISSION LINKS • Twisted pair • Coaxial cables • Terrestrial microwave • Satellites links • Fibre optics Fibre optic links - Advantages • Large number of information can be carried by a single fibre. • Due to its light frequency of operation, it can provide large bandwidth through which a large number of information channelscan be accommodated. • Signal to noise ratio will be quite high when fibres are used as transmission links. • This is because most of the noise enters into the channel through electromagnetic interference. • But in fibre communication, electromagnetic interference cannot take place. • The electromagnetic waves have got no effect in the operation of fibres. Fibre optic links - Advantages • The losses encountered with fibre optic links are negligible and a greater length of the links can be realized. • This avoids repetition of information which are required in conventional links up to a distance of even 100 km. • Fibre optic links can be used without repeaters. Beyond such distances only fibre optic links will use optical repeaters. • The recurrence is available for the simple design of optical repeaters. However conventional transmission links like the coaxial cable requires repeaters at frequent intervals. Fibre optic links - Advantages • The availability of the fibre in modern telecommunication environment is quite good. • Fibres can provide support for multiple cost/performance levels, from small systems to supercomputers. • Fibres have got the ability to carry multiple existinginterference sets for existing channel and network protocol. MODEMS • Schematic Structure of Modem. • Modem standards Data compression • Data compression attempts to save bits by compressing the recurring character in the information chain. Data compression standards • MNP5- MNP5 standard which makes use of Huffman encoding. • Lempel-Ziv encoding- used by V.42 bits, the ISO standard. Drawbacks in using data compression • The compressing of already compressed data can result in less amount of gain. • In practical applications, the data that are transmitted through modems are compressed before that process. • Invariably, many of the voice digitizers of modern time provide data compression also. Drawbacks in using data compression • The data rates higher than 100 Kbps place considerable pressure on the traditional microcomputer serial port controller that controls the communication between the serial port and the modem (Universal Asynchronous Receiver/Transmitter –UART Chip) • Most modems rely on microcomputer for the transfer of data. • At high transmission rates the CPU of the microcomputer gets overloaded. • This effect is quite common in Windows bases multitasking environments. This will results in loss of data.
