Air Interface Channel Structure

Clearly, it does not make sense for these different types of channels to
each be allocated one of the eight timeslots. Firstly, there would simply
not be enough timeslots. Moreover, different data rates apply to the various
types of channels. Instead, a sophisticated framing structure is used
on the air interface to allocate the various channel types to the available
timeslots. The structure includes frames, multiframes, superframes, and
hyperframes.

As mentioned previously, a single frame lasts approximately 4.62 ms and
contains eight timeslots. In standard GSM (as opposed to GPRS), two types
of multiframes are used—a 26 multiframe (containing 26 frames and having
a duration of 120 ms) and a 51 multiframe (containing 51 frames and
having a duration of 235.4 ms). The 26 multiframe is used to carry TCHs
and the associated SACCH and FACCH.The 51 multiframe is used to carry
BCCH, CCCH (including PCH, RACH, and AGCH), and SDCCH (and its
associated SACCH). A superframe lasts 6.12 seconds, corresponding to
51*26 multiframes or 26*51 multiframes. A hyperframe corresponds to
2,048 superframes (a total of 2,715,648 frames, lasting just under 3 hours,
28 minutes, and 54 seconds). When numbering frames over the air interface,
each frame is a numbered modulo of its hyperframe. In other words, a
frame can have a frame number (FN) from 0 to 2,715,467. The reason for
the large hyperframe is to allow for a large value of FN, which is used as
part of the encryption over the air interface.

Certain timeslots on a given RF carrier may be allocated to control
channels, while the remaining timeslots are allocated for traffic channels.
For example, timeslot 0 on the first carrier in a cell is used to carry the
BCCH and CCCH. It may also carry four SDCCH channels. It is also common
to find that timeslot 1 on the first RF carrier in a cell is used to carry
eight SDCCH channels (with the associated SACCHs), with the remaining
timeslots allocated as TCHs. Exactly how much SDCCH capacity is allocated is dependent upon the number of carriers and the amount of
traffic in the cell. Figure 3-8 shows two typical arrangements.

As mentioned, the 26 multiframe is used for the TCH. The structure is
depicted in Figure 3-9, where only one timeslot per frame is shown (only
full-rate TCH is considered in the figure). A given timeslot carries user traffic
(voice) for 24 out of 26 frames. One of the 26 frames is idle and one of the
26 frames carries the SACCH. The FACCH is transmitted by pre-empting
half or all of the user traffic in a TCH.

This overall structure enables a TCH to have a gross bit rate of 22.8
Kbps. Of course, this rate is not allocated completely to user data (such as
speech). Rather, a sophisticated coding and interleaving scheme is applied.
This scheme adds a significant number of bits for error detection and correction,
which reduces the bandwidth available for raw user data. In fact,
for standard GSM full rate (FR) voice coding, the speech is carried at 13
Kbps and for enhanced full rate (EFR), the speech is carried at 12.2 Kbps.
Although it may seem that a great deal of the gross 22.8 Kbps is consumed by coding overhead, it is worth remembering that an RF interface is unreliable
at best, and error-correction overhead is necessary to overcome the
limitations of the medium.

Since the control channels (with the exception of FACCH and SACCH)
are carried on different timeslots from the TCHs, it is possible to have a different
framing structure. In fact, a 51-multiframe structure is used for
transmitting the control channels and this structure applies to any timeslot
that is allocated to control channels.