National Physical Laboratory (NPL) situated at New Delhi, India maintains the IST using five commercial cesium clocks. They have started the service of digital transmission of highly accurate Indian Standard Time over the telephone lines. This service is available to Indian users.
Tele clocks based Digital Clocks are connected to a DSL Modem via the RS 232 interface. The modem in turn is connected to the telephone network via the phone line. The Digital Clock Unit (DCU) dials to the NPL to get the time data over the phone line. The hour, minutes and seconds data is preceded by a particular signature to identify the incoming data confidentially.
An illustration is given below for better understanding.
Tele clock side(New Delhi)
The master clock as shown maintains a highly accurate IST time. It is connected to the tele clock format generator in digital format. This data is then routed using an electronic switch over the phone line to the user.
DCU side (anywhere in india)
On this side, the DCU is connected to the telephone network via DSL modem. It dials to get connected to the modem at New Delhi to get accurate IST time.
Kindly refer to digital clocks for specifications.