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0 1 2 3 4 5 6 7

8.25 bit guard band

000 Information F1 Training F1 Information 000


3 bits 57 bits 1 bit 26 bits 1 bit 57 bits 3 bits


148 bit data frame sent in 547us


Figure 3.14 GSM data burst
58 GSM FUNDAMENTALS


Actually each time slot (burst) consists of the GSM burst and the guard band and is
thus 156.25 bits long and takes up 577 µs, with each single bit taking approximately
3.69 µs. Note that the guard band consisting of 8.25 bits is only used as a time inter-
val (approximately 30 µs) between time slots to separate one user transmission from
the next.
Each TDM burst consists of 148 bits. The ¬rst 3 bits and last 3 bits are always zero
and can be used for framing. The F1 bits are referred to as stealing bits and indicate
whether the burst contains user data or control information. While a call is in progress, if
it is decided that a handover to another cell should take place, this handover needs to be
executed very quickly. The fastest way for the network to indicate to the mobile device that
a handover should take place is via its dedicated traf¬c channel (TCH). When the traf¬c
channel is used for this purpose, it is referred to as the fast associated control channel
(FACCH). The stealing bits indicate to the subscriber that this burst is a control message
and not user data. In the uplink, the mobile device will indicate control information to the
network in the same manner. Using the traf¬c channel in such a way exposes the subscriber
to a minimal amount of increased interference while the handover is executed. However,
in most cases, the subscriber will not notice this extra interference. The training ¬eld is
used to synchronize the transmitter and receiver. It is also used to ˜train™ the receiver to
the particular characteristics of the channel being used. The receiver knows exactly what
the transmitter has sent in this ¬eld since this is negotiated beforehand. By using this
knowledge it can check for any distortion between the transmitted and received training
¬eld signal. This information can then be used to ˜equalize™ the information ¬elds that
are received, reducing the possibility of errors.



3.3.3 Control channel multiframe

In addition to the traf¬c channel multiframe, there is also a 51-frame multiframe which
is employed on the control and signalling channels. This multiframe consists of several
logical channels, which incorporate control, timing and signalling. These channels are
highlighted below.


Broadcast channel (BCCH)
The BCCH is a continuous stream of data from the base station containing its identity
and channel status. All mobile stations can monitor the strength of this signal to ensure
that they are still within the cell.


Frequency correction channel (FCCH)
The FCCH is used to ensure that the mobile device adjusts its frequency reference to
match that of the base station so that the mobile device does not drift off frequency,
reducing the voice quality of the call. The base station simply emits a sine wave on this
channel for the duration of a time slot.
3.3 GSM AIR INTERFACE 59


Synchronization channel (SCH)
The synchronization channel is used to frame synchronize the mobile device. The base
station identity code (BSIC) is also broadcast on the SCH so that the mobile device can
tell if the base station it tries to connect to is part of the correct network.

Common control channel (CCCH)
The CCCH is split into three sub-channels; one is used in the uplink and two are used in
the downlink:

• Uplink: the random access channel (RACH), allows a mobile station to request a time
slot on the dedicated control channel, which can then be used, for example, to assign a
traf¬c channel for a voice call. The RACH channel utilizes the slotted-Aloha method.
• Downlink: the paging channel (PCH) is used to alert the mobile station of an incoming
call. The base station will announce the assigned slot on the access grant chan-
nel (AGCH).

Standalone dedicated control channel (SDCCH)
This is used for call setup and location updating of mobiles, as well as for SMS messages
to and from mobile devices which are in idle mode. When an initial request for a con-
nection is made by the mobile device on the RACH, the network responds on the AGCH
by issuing the mobile device an SDCCH channel rather than directing it immediately to a
TCH. By incorporating the SDCCH channel for call setup rather than switching directly
to a TCH, the overall ef¬ciency of the GSM system can be increased. This is possible
since initial call setup does not require the relatively high bandwidth that a TCH supports.
By utilizing the SDCCH, which is a much lower bandwidth channel, the TCH is not tied
up and is available for user calls.

Slow associated control channel (SACCH)
As previously mentioned, the SACCH is a bidirectional channel which is used to transfer
measurement reports to and from the network. The SACCH may also be used for transfer
of SMS data if a traf¬c channel is allocated to the particular subscriber.

Fast associated control channel (FACCH)
The FACCH is an in-band signalling channel which interrupts user data to transfer system
information in both the uplink and downlink. This channel is used when information needs
to be transferred quickly, such as when a handover is required.
Figure 3.15 shows the logical channels de¬ned for GSM. The broadcast, common and
dedicated control channels together are referred to as the signalling channels.
Figure 3.16 shows a control channel, which consists of the FCCH, SCH, BCCH, CCCH,
SDCCH and SACCH channels. This is a single frequency (time slot 0) control channel of
51 frames. However, it actually repeats every 102 frames. Using this method will allow
the other seven time slots on this particular frequency to be used by subscribers for calls.
60 GSM FUNDAMENTALS



Signalling Channels




TCH CCCH SDCCH FACCH BCH SACCH




PCH AGCH BCCH SCH FCCH


(a) Down Link

Signalling Channels




SDCCH SACCH CCCH FACCH TCH



(b) Up Link
RACH



Figure 3.15 GSM logical channels


An alternative method of implementing a control channel is shown in Figure 3.17.
Slot 0 comprises FCCH, SCH, BCCH and CCCH. In this example there would be a
separate channel (time slot 1) for the SDCCH/SACCH. This method would be used where
there is more than one frequency being used in the cell since only six time slots on this
particular frequency will be available for subscriber calls.



3.3.4 Frames, multiframes, superframes and hyperframes
Each of the TDMA frames is numbered, and this number is used as an input param-
eter for the encryption process. To ensure that the number is quite large a hyperframe
has been de¬ned. To summarize, a frame consists of 8 TDMA time slots (a total dura-
tion of 4.615 ms), a multiframe consists of a block of 26 of these time slots, primarily
for data (although some control information is actually transferred) and 51 for control
purposes. A superframe consists of 26 — 51 TDMA frames with a duration of approx-
imately 6.12 s. Since 26 is not a factor of 51, these frames ˜slide across™ each other
so that at the end of the 26 — 51 period each of the 26 frames has aligned once with
every one of the 51 control frames. This sliding process is required to allow the mobile
device to monitor the quality of the surrounding cells™ BCCH for handover purposes. The
hyperframe consists of 2048 — 26 — 51 TDMA frames lasting approximately 3 h 28 min
and 54 s.
3.3 GSM AIR INTERFACE 61




IDLE IDLE
SACCH/4 (1) SACCH/4 (3)
SACCH/4 (1) SACCH/4 (3)
SACCH/4 (1) SACCH/4 (3)
SACCH/4 (1) SACCH/4 (3)
SACCH/4 (0) SACCH/4 (2)
SACCH/4 (0) SACCH/4 (2)
SACCH/4 (0) SACCH/4 (2)
SACCH/4 (0) SACCH/4 (2)
SCH SCH
FCCH FCCH
SDCCH/4 (3) SDCCH/4 (3)
SDCCH/4 (3) SDCCH/4 (3)
SDCCH/4 (3) SDCCH/4 (3)
SDCCH/4 (3) SDCCH/4 (3)
SDCCH/4 (2) SDCCH/4 (2)
SDCCH/4 (2) SDCCH/4 (2)
SDCCH/4 (2) SDCCH/4 (2)
SDCCH/4 (2) SDCCH/4 (2)
SCH SCH
FCCH FCCH
SDCCH/4 (1) SDCCH/4 (1)
SDCCH/4 (1) SDCCH/4 (1)
SDCCH/4 (1) SDCCH/4 (1)
SDCCH/4 (1) SDCCH/4 (1)
SDCCH/4 (0) SDCCH/4 (0)
SDCCH/4 (0) SDCCH/4 (0)
SDCCH/4 (0) SDCCH/4 (0)
SDCCH/4 (0) SDCCH/4 (0)
SCH SCH
FCCH FCCH
CCCH CCCH
CCCH CCCH
CCCH CCCH
CCCH CCCH
CCCH CCCH
CCCH CCCH
Figure 3.16 Single time slot for control




CCCH CCCH
CCCH CCCH
SCH SCH
Single Timeslot. Timeslot 0 repeated twice




FCCH FCCH
CCCH CCCH
CCCH CCCH
CCCH CCCH
CCCH CCCH
BCCH BCCH
BCCH BCCH
BCCH BCCH
BCCH BCCH
SCH SCH
FCCH FCCH
62 GSM FUNDAMENTALS




IDLE IDLE IDLE
CCCH IDLE IDLE
CCCH IDLE IDLE
CCCH SACCH/4 (3) SACCH/4 (7)
CCCH SACCH/4 (3) SACCH/4 (7)
CCCH SACCH/4 (3) SACCH/4 (7)
CCCH SACCH/4 (3) SACCH/4 (7)
CCCH SACCH/4 (2) SACCH/4 (6)
CCCH SACCH/4 (2) SACCH/4 (6)
SCH SACCH/4 (2) SACCH/4 (6)
FCCH SACCH/4 (2) SACCH/4 (6)
CCCH SACCH/4 (1) SACCH/4 (5)
CCCH SACCH/4 (1) SACCH/4 (5)
CCCH SACCH/4 (1) SACCH/4 (5)
CCCH SACCH/4 (1) SACCH/4 (5)
CCCH SACCH/4 (0) SACCH/4 (4)
CCCH SACCH/4 (0) SACCH/4 (4)
CCCH SACCH/4 (0) SACCH/4 (4)
CCCH SACCH/4 (0) SACCH/4 (4)
SCH SDCCH/4 (7) SDCCH/4 (7)
FCCH SDCCH/4 (7) SDCCH/4 (7)
CCCH SDCCH/4 (7) SDCCH/4 (7)
CCCH SDCCH/4 (7) SDCCH/4 (7)
CCCH SDCCH/4 (6) SDCCH/4 (6)
CCCH SDCCH/4 (6) SDCCH/4 (6)
CCCH SDCCH/4 (6) SDCCH/4 (6)
CCCH SDCCH/4 (6) SDCCH/4 (6)
CCCH SDCCH/4 (5) SDCCH/4 (5)
CCCH SDCCH/4 (5) SDCCH/4 (5)
SCH SDCCH/4 (5) SDCCH/4 (5)
FCCH SDCCH/4 (5) SDCCH/4 (5)
CCCH SDCCH/4 (4) SDCCH/4 (4)
CCCH SDCCH/4 (4) SDCCH/4 (4)
CCCH SDCCH/4 (4) SDCCH/4 (4)

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