There are substantial differences in the frame structures of the two Figure 34.14 DECT frame structure systems as shown in Figures 34.13 and 34.14. CT2 is a frequency division multiple access, time division duplex (FDMA/TDD) sys- tem arranged as 40 frequencies spaced at 100kHz intervals, with one DECT is a multiple carrier, time division multiple access duplex channel per frequency, using time division to carry the traffic (MC/TDMA) system which uses ten channels at 1.728MHz inter- in both directions. In order to interleave the transmit and receive vals, with 12 duplex channels multiplexed on to each carrier and a bursts and maintain the overall bearer capacity, the transmitted bit transmitted bit rate of 1152kbit/s. Within a DECT base station each rate must be at least twice the overall data rate. For CT2 the time slot may be transmitted on any of the ten carrier frequencies, transmitted bit rate is 72kbit/s. 34/16 PABX and key systems Wired extensions Radio 'N' Extns exchange Mobility control stations 'N' handsets Figure 34.15 A basic cordless PABX arrangement and a single base station may operate with up to 12 handsets 34.8.4 Possible configurations simultaneously. In both systems a transmit or receive burst com- prises a number of ADPCM samples together with control and signalling data. Figure 34.15 shows a simple cordless PABX structure where N The use of time division techniques introduces delay in the speech standard PABX extensions are assigned to N cordless handsets. The paths which may require additional echo control to provide accept- radio exchange links the PABX extensions to the radio base stations able subjective performance, particularly in DECT systems with and also handles handover and location updating functions. The their longer framing delay. PABX provides the normal services. This configuration may be Both CT2 and DECT specifications have adopted a layered ap- added to most existing types of PABX. proach based upon Layers 1 to 3 of the ISO open systems intercon- Figure 34.16 shows a more com