Cordless Telephones
A cordless  telephone uses  a  radio link  instead of a physical  cord to connect the handset to the
base station.  Both  the  mobile  handset and the  base  station  are  radio  transceivers.  The  first
generation  of  cordless  telephones  uses  analogue  techniques and it is known as CT1. CT1 has
been  followed by a digital system known as CT2.  Both CT1 and CT2 rely upon the base station
to connect  the mobile handset to the PSTN.  An even  more recent system is  DECT; this system
employs digital cellular technology to provide a mobile telephone service to large PABXs.
 

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 1.7 MHz,  and in the  direction handset to base the
frequency is 47 MHz. The CT1 system has a number of disadvantages:

  The quality of the received speech is not very good.

  Transmissions can be received by a sound broadcast radio receiver.

  Only eight r.f. channels are allocated.

  A telephone has no ability to search for a free channel and hence it can easily be blocked off by
      another cordless telephone that has been set to use the same channel.

  The range is limited to about 50 m.
 
 

CT2

   The second generation of cordless telephones, known as CT2, uses a digital speech path in any
one of  forty 100 kHz  wide r.f.  channels in the frequency band  864-868 MHz. 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 packets of data.
Each  packet of  data is then compressed to one-half  its natural length before the two sets of data
are interleaved on the same carrier frequency.

   Each handset has up to 11 unique identity codes loaded in at manufacture.  This  enables each
base station to be programmed to recognize up to eight separate handset  identities that it is able
to deal  with  simultaneously,  and this allows base stations to provide a  PABX  function.  Since
every  handset is uniquely  identified,  there is  little  risk of  privacy invasion.  A handset can also
send  recall and  other  tone  signals  to the base station  which can  relay them  on to the  PSTN.
This  feature  allows a  CT2  user at  home and  at  the  office to  employ  the  same  handset as a
PABX extension.

  The CT2 specification defines a common air interface (CAI), which means that all CT2 handsets
and  base stations  can  communicate  with one  another,  regardless of  their  manufacturer.  The
modulation method that is employed 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 deviation below the carrier frequency indicating binary 0.

  This is shown by Figure. 44 . A single r.f. channel is used for both directions of transmission
using the 'ping-pong' version of TDD shown in Figure 45  . Speech signals in either direction of
transmission  are sampled and coded into digital form at 32 kbit/s. The 2 ms duration samples
are transmitted at 72 kbit/s in 1 ms bursts to allow the bits to be compressed into packets of
data of 1 ms duration. Forty r.f. channels are available  so that CT2 is a combined FDMA/TDD
system.
 

 
Fig. 44  Two-level FSK
Fig. 45  CT2 uses a form of TDD
 
  When a call is initiated, there is initially no synchronization between the handset and the base
station. For a link to be set up so that the two items are able to communicate with one another,
they must first be tuned to the same r.f. channel and then they must synchronize with one another.

  Incoming call to base station: When an incoming call is detected by the base station, it scans
the 40 r.f. channels to find a free one that has an 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 sate into its SCAN state, in which it scans the r.f. channels. When the call signal is
detected on one of the r.f. channels, the handset remains on that frequency and achieves bit
synchronization with the base station. The handset then checks that the call is for it (not for some
other handset) ; if so , burst synchronization is obtained to establish a link to the base station. The
ringer of the handset then rings until the call is answered, when speech can commence.

   Handset originating a call : When a handset wishes to make a call, the CALL button is pressed
and this action causes the handset to scan the 40 r.f. channels to find a free one with adequate
signal-to-noise ratio. The handset then signals the base station over the selected channel. The base
station is continually scanning all the 40 r.f. channels, and so it rapidly detects the call from the
handset. Synchronization between handset and base station is established and then the base station
seizes a line to the local telephone exchange or PABX. Dialling tone is then returned to the caller.

CT2 transmits at a power level of about 10 mW and it has a range of about 100 m.
 
 
 
 
 
 

 
 
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