Voice Channel Structure

Associated with each DTC are two
other channels—the Fast Associated Control Channel (FACCH) and the
Slow Associated Control Channel (SACCH). The FACCH is a signaling
channel used for the transmission of control and supervisory information
between the mobile and the network. For example, if a mobile is to send DTMF tones, then these are indicated on the FACCH. The SACCH is also
used for the transmission of control and supervisory information between
the mobile and the network. Most notably, the SACCH is used by the mobile
to transmit measurement information to the network describing the
mobile’s experience of the RF conditions. This information is used by the
network to determine when and how a handoff should occur.
Figure 3-15 shows the structure of the DTC. It is notable that the figure
does not show the FACCH. This is because the DATA field, which is normally
used to transmit voice, is also used to transmit FACCH information.

In other words, if information is to be sent on the FACCH, then user data is
briefly suspended while the FACCH information is being sent. Figure 3-15
also shows six time slots within the frame structure. In fact, IS-54 enables
two types of mobiles: full-rate and half-rate. A full-rate mobile uses two of
the timeslots in the frame (1 and 4, 2 and 5, or 3 and 6), while a half-rate
mobile uses just a single time slot. A full-rate mobile transmits 260 bits of
speech per time slot (520 bits per frame). Since there are 25 frames per second,
this means that the gross bit rate for speech is 13 Kbps. In practice,
only full-rate handsets are used.

In addition to the user data and SACCH within the DTC, we see a number
of other fields, as follows:
■ Guard Time This field is three symbols (six bits) in duration. It is
used as a buffer between adjacent time slots used by different mobiles
and enables compensation for variations in distance between the
mobile and the base station.
■ Ramp Time This is a three-symbol duration allowing for a ramp up
of the RF power.
■ Sync This is a special synchronization pattern, which is unique for a
given time slot. It is used for correct time alignment.
■ CDVCC This is the Coded Digital Voice Color Code, which is
analogous to the Supervisory Audio Tone used in analog AMPS. It is
used to detect co-channel interference.