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Cordless Telephone System basics

This page describes cordless telephone system basics and mention cordless phone frequency allocations. As we know almost all the modern apartments and homes usually houses telephone jacks in all the rooms. This makes it possible to move single telephone for use in all the rooms as required. Cordless telephone is much friendlier than this.

Cordless telephone system consists of two units viz. base unit and portable unit(referred as handset). Base unit is wired with standard telephone line. It is powered from AC mains and provide place to place and charge the handset unit on it.

cordless telephone system
Fig.1 Cordless Telephone System

The system is depicted in the fig-1. As shown in the figure, both base unit and handset will have two way radio transmission and reception. This is possible due to availability of RF transceiver on both the units to enable full duplex operation. To facilitate full duplex communication cordless telephone operates on two different frequencies one for transmit and another for receive. Base Unit transmits in the range from 43-46 MHz and receives in the 49 MHz range. Handset unit transmits in 49MHz range and receives in 43-46 MHz range.

FCC has allocated about 25 duplex channels for cordless telephone system. Here Frequency modulation technique is employed. Crystal control is provided to set these frequencies. This helps one set frequency channel such that interference can be avoided. FCC has limited transmit power to maximum value of 500 mW. Maximum usable range between base unit and portable unit is about 100 ft.

Cordless Telephone Frequency Allocations-New Channels

Base Unit Transmit frequency Handset Transmit frequency
43.72 48.76
43.74 48.84
43.82 48.86
43.84 48.92
43.92 49.02
43.96 49.08
44.12 49.10
44.16 49.16
44.18 49.20
44.20 49.24
44.32 49.28
44.36 49.36
44.40 49.40
44.46 49.46
44.48 49.50

Original Cordless frequency channels

Base Unit Transmit frequency Handset Transmit frequency
46.61 49.67
46.63 49.845
46.67 49.86
46.71 49.77
46.73 49.875
46.77 49.83
46.83 49.89
46.87 49.93
46.93 49.99
46.97 49.97

Advanced cordless systems also have beed developed which operates in PCS band from 902 to 928 MHz. There are 4 basic advanced cordless telephone systems exist.
•  The first category uses FM on channels in 900 MHz frequency range.
•  The second type operates in 900 MHz range and is digital. ADPCM is used.
•  The third type uses spread spectrum and operates at 902-928 MHz range.
•  The fourth type uses spread spectrum and operates at 2.4GHz range.

Telephony RELATED LINKS

Signalling Types
EPABX system basics
Circuit Switching vs Packet Switching

RF and Wireless Terminologies


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