Time can be used because it is a digital communications system. The transmissions can be bursts of data with pauses in between. In these pauses other users may transmit using the same frequency. This is called Time Division Multiple Access (TDMA). For DECT a schema is used of dividing the available time in parts of 10 ms, called frames. In these 10 ms frames 24 'slots' are defined. The first 12 slots are available for the base system to transmit, while the second 12 slots are for the portables to use. Since the frame cycles 100 times per second every slot is available 100 times per second.
This example shows two handsets communicating with one base system. The left handset is using slot 3 to receive digital information from the base system, while it sends in slot 15. The right handset uses slot 11 to receive and 23 to transmit. Note that the receive and transmit slots always are spaced by half a frame, that is 12 slots, and they are on the same frequency. Also note that the handset always receives in the first half of the frame and transmits in the second half, while for the base system this is the other way around. From this it can be derived that handsets never communicate directly with eachother.
This example shows the use of a single frequency. Since there are ten frequencies defined a portable may select any combination of slot(pair) and frequency (called channel) it finds suitable. Therefor the portable has a choice of 10 (freq's) times 12 (slots) is 120 channels. Note that it is the portable which desides which channel to use to communicate with the base system.
Since almost all portables and base system radio's have a single tranciever they are capable transmitting or receiving on one of the ten DECT frequencies per slot. This means that the same slot on the other frequencies cannot be used by that portable or base stations' radio. Depending on the tuning speed of the RF part neighboring slots can also not be used. They are the RF parts' so called 'blind slots'. This devides RF parts is cheaper 6-channel (6-slot would be a better term) and more expensive 12-channel radio's. Since portables usually only use one slotpair at a time, cheaper RP parts can be used, while base systems may have to handle a high traffic load and need 12 channel radio's.
Now that there is a defined set of channels the use of these channels has to be defined. For this purpose several 'packets' are defined. The major difference between these packets is their length and the way they are ended. All packets start with a preamble and a syncfield, 32 bits combined. They are used by the receiver to adjust the timing. Then the data follows. The number of data bits can vary from 64 bits (P00 packet), 384 (P32 packet, typical for speech) to 688 bits (P80 packet, using adjacent slots). The P00 packet contains just enough bits for the Medium Access Control layer to communicate with it's peer on the other end of the radio link. Larger packets have room for 'user-data' e.g digitized speech. Between slots a guard time is observed to handle tuning of the receiver and any timing differences between base systems and portables.
To setup a connection between a portable and a base system the MAC layer first selects a suitable channel and sends an access request in a P00 packet. This is handled by the base system and, if accepted, an acknowledgement is sent back in the appropriate channel (see above). Now the Data Link Control layer can open a data link through this connection.
Handover and quality control go hand in hand. Based on signal quality measurements performed by the Physical Layer the MAC layer continuesly checks for upcoming transmission problems and looks for better channels to either the same or other radio. Once it finds a better channel it sets up a connection there, resulting in two simultanious connections. It then tells the base system to accept user data through the new channel, after which the old connection can be released. All this happens so quickly that the user does not notice this.
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