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16 Nov 2006 18:06:56 Digital switch A ‘switch’ is the facility at the telephone company that connects subscribers to each other both locally and over long distance. A digital switch is a device that manages digital signals produced at or passed through the central office of a telephone company and forwards them over the company’s backbone network. The users’ analog signals are transformed into digital signals after being collected from the telecom office’s channel banks by the device and are switched with other incoming signals to the Wide Area Network (WAN). The number of lines and the features determine the class of digital switches. For example, a Private Branch Exchange is a digital switch owned by a private company. A Centrex is a digital switch that handles switching for a private company at the central office of the telephone company. Digital switches function by linking two or more digital virtual circuits together, based on a dialled telephone number. Calls are established between switches using the Signalling System 7 (SS7) or a related protocol. A digital switch performs Time-Division Multiplexing (TDM) switching of digitised signals. When a person speaks over a phone, the voice is normally ‘encrypted’ and then restored for the person at the receiving end. In the process, the voice is delayed by a tiny fraction of a second. The voice is not ‘live’, it is reconstructed. This is because, speech is coded by digital switches at the rate of 8,000 time slices per second. For each time slice - the amount of processing time for each programme - a digital Pulse Code Modulation (PCM) representation of the tone is done. The digits are then directed to the receiving end, where the reverse process takes place, generating sound for the receiver. To some telephone switches, concentrators are not connected directly but instead are used to connect calls between other telephone switches. These are normally intricate machines located in a central exchange and are referred to as ‘carrier-level’ switches. In small places, a few telephone exchanges consist only of remote or satellite switches and the ‘parent’ switch would be located far off. The remote switch relies on the parent switch for routing and number plan information. A remote switch is capable of routing calls between local phones by itself, without using trunks to the parent switch. In this fast changing telecom scenario in a highly competitive economic environment, telecom service providers have to be cautious with their capital expenditure. They have to make investments in technology with enhancements to digital switching systems that result in quick return on investments. The thrust should be in identifying applications involving digital switches that would improve profitability. The growth in voice traffic is estimated to be around 10 per cent per annum, while that of data is around 125 per cent per annum. This provides an overwhelming opportunity for Local Exchange Carriers (LECs) to shift towards a packet network to accommodate the growth in data. It is estimated that by 2010, the LECs will have transformed 90 per cent of the switch capacity to Asynchronous Transfer Mode (ATM). Links: Digital Switch Content Team -- BSNL Portal -- Intelligroup Asia Pvt. Ltd. |
