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 Cordless telephones primer 
団陳陳陳陳陳陳by Blue Coyote調
                            
 Covers: CT1, CT2, and DECT 
                            
塒様(Version 1.00 13/06/96)余
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Cordless telephones (Version 1.00 13/06/96) by Blue Coyote

Standards covered in this document

CT1 - Analogue
CT2 - Analogue / digital
DECT - Digital

CT1

CT1 uses two radio frequencies and analogue technology to provide a full duplex speech path between the handset and the base station. The two frequencies are spaced well apart; in the direction base to handset the frequency is 1.7MHz (at the top end of the medium waveband on a domestic radio), and in the direction handset to base the frequency is 47MHz.

Only eight RF channels are allocated, and the phone has no ability to hunt for a free channel, hence it can be blocked off by another cordless telephone which is set to use the same channel. This channel allocation setting is done at the factory.

CT2

CT2, the second generation of cordless phones, uses a digital speech path in any one of the forty 100KHz wide RF channels in the frequency range 864-868MHz. The channels are not allocated in pairs. Instead full duplex operation is obtained by the use of a digital technique known as Time Division Duplex (TDD). With TDD the two halves of a telephone conversation are first converted into digital form and then they are divided into a number of small data packets. Each packet is then compressed to one half of its original size before the two sets of data are interleaved on the same carrier frequency.

Each handset has up to 11 different security codes programmed into it at the time of manufacture. This enables the base station to be programmed to recognize up to eight separate handset identities to deal with simultaneously, and this allows base stations to facilitate a PABX function.

The CT2 specification defines a Common Air Interface (CAI), which means that all CT2 handsets and base stations can communicate with each other, regardless of manufacturer. The modulation methid enployed is two-level FSK with frequency deviations of

(a) 14.4 to 25.2 kHz above the carrier frequency representing binary 1 (b) 14.4 to 25.2 kHz below the carrier frequency representing binary 0

A single RF channel is used for both directions of transmission using the 'ping-pong' version of TDD. Speech signals in either direction of transmission are sampled and coded into digital form at 32kbits/s. The 2ms duration samples are transmitted at 72kbits/s in 1ms bursts to allow the bits to be compressed into packets of data of 1ms duration. Forty RF channels are available do that CT2 is a combined FDMA/TDD system.

When a call is intiated, there is initally no synchronisation between the handset and the base station. For a link to be set up so that the two items are able to communicate, they must first tune to the same RF channel and then synchronise with one another.

Incoming call to base station

When a incoming call is detected by the base station, it scans the 40 RF channels to find a free one with and adequate signal-to-noise ratio. The base station then transmits a call signal over the selected channel. Periodically the handset moves out of its sleep state into its scan state, in which it scans the RF channels. When the call signal is detected on one of the the RF channels, the handset remains on that frequency and achieves bit syncronisation with the base station. The handset then checks that the call is for it (not for some other handset); if so, burst synchronisation is obtained to establish a link to the base station. The handset then rings until answered, when speech can commence.

Handset originating call

When the 'Call' button on the handset is pressed, te handset scans the 40 RF channels to frind a free one with adequate signal-to-noise ratio. The handset then signals the base station over the selected channel. The base station is continously monitoring all the 40 RF channels and so it rapidly detects the call from the handset. Synchronisation occurs as above and base station returns dialling tone to caller.

The transmission power level of CT2 is about 10mW, with a range of approximately 100m.

DECT

The Digital European Cordless Telephone system uses a cellular radio-like technology The DECT system uses a three dimension cellular layout in which there may be cells above and below one another as well as side by side layout and is designed for high density use.

The DECT system uses the frequency band 1.88-19GHz and this band is divided up into ten separate carrier frequencies. In turn, each carrier frequency is divided into 23 time slots, any two of which are used for a conversation The system provides 32kbit/s voice channels using TDD.

During a a conversation a handset continuously compares the signal-to-noise ratio obtained in its time slot with the signal-to-noise raio in the corresponding time slot in another RF channel. If the other time slot is better, the handset will change over to use the better time slot.

DECT uses FDMA/TDMA/TDD techniques to provide 120 duplex channels using 10 separate carrier frequncies and multiplexing 12 send channels and 12 receive channels onto each carrier. The bit rate per channel is 1152kbits/s and the modulation is GMSK with a frequency deviation of +/- 228kHz and a carrier spacing of 1728kHz.

Glossary

TDD - Time Division Duplex
TDMA - Time Division Multiple Access
RF - Radio Frequency
FDMA - Frequency Division Multiple Access GMSK - Gaussian Minimum Shift Keying

Greetings

In general, anyone cool enough for 1066, Beyond Phear, or DockMaster especially the SysOps: AtR0CitY, Kry0, and Eck. Also Darkcyde, Fugitive, and Harl.
And Zo.


Blue Coyote
13/06/96