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Airlink van een GSM netwerk
Deze tekst is alleen in het Engels beschikbaar.
Omdat vrijwel alle termen ook engels zijn zou een vertaling een warboel opleveren.
Door: Peter Buijsman
The International Telecommunication Union (ITU),
which manages the international allocation of radio spectrum (among
many other functions), allocated the bands 890-915 MHz for the uplink
(mobile station to base station) and 935-960 MHz for the downlink
(base station to mobile station) for mobile networks in Europe.
Since this range was already being used in the early 1980s by the
analog systems of the day, the CEPT had the foresight to reserve
the top 10 MHz of each band for the GSM network that was still being
developed. Eventually, GSM would be allocated the entire 2x25 MHz
bandwidth.
Multiple access and channel structure
Since radio spectrum is a limited resource shared
by all users, a method must be devised to divide up the bandwidth
among as many users as possible. The method chosen by GSM is a combination
of Time- and Frequency-Division Multiple Access (TDMA/FDMA). The
FDMA part involves the division by frequency of the (maximum) 25
MHz bandwidth into 124 carrier frequencies spaced 200 kHz apart.
One or more carrier frequencies are assigned to each base station.
Each of these carrier frequencies is then divided in time, using
a TDMA scheme. The fundamental unit of time in this TDMA scheme
is called a burst period and it lasts 15/26 ms (or approx. 0.577
ms). Eight burst periods are grouped into a TDMA frame (120/26 ms,
or approx. 4.615 ms), which forms the basic unit for the definition
of logical channels. One physical channel is one burst period per
TDMA frame. Channels are defined by the number and position of their
corresponding burst periods. All these definitions are cyclic, and
the entire pattern repeats approximately every 3 hours.
Channels can be divided into dedicated channels,
which are allocated to a mobile station, and common channels, which
are used by mobile stations in idle mode.
Traffic channels
A traffic channel (TCH) is used to carry speech
and data traffic. Traffic channels are defined using a 26-frame
multiframe, or group of 26 TDMA frames. The length of a 26-frame
multiframe is 120 ms, which is how the length of a burst period
is defined (120 ms divided by 26 frames divided by 8 burst periods
per frame). Out of the 26 frames, 24 are used for traffic, 1 is
used for the Slow Associated Control Channel (SACCH) and 1 is currently
unused. TCHs for the uplink and downlink are separated in time by
3 burst periods, so that the mobile station does not have to transmit
and receive simultaneously, thus simplifying the electronics. In
addition to these full-rate TCHs, there are also half-rate TCHs
defined, although they are not yet implemented. Half-rate TCHs will
effectively double the capacity of a system once half-rate speech
coders are specified (i.e., speech coding at around 7 kbps, instead
of 13 kbps). Eighth-rate TCHs are also specified, and are used for
signalling. In the recommendations, they are called Stand-alone
Dedicated Control Channels (SDCCH).
Control channels
Common channels can be accessed both by idle
mode and dedicated mode mobiles. The common channels are used by
idle mode mobiles to exchange the signalling information required
to change to dedicated mode. Mobiles already in dedicated mode monitor
the surrounding base stations for handover and other information.
The common channels are defined within a 51-frame multiframe, so
that dedicated mobiles using the 26-frame multiframe TCH structure
can still monitor control channels. The common channels include:
Broadcast Control Channel (BCCH)
Continually broadcasts, on the downlink, information including base
station identity, frequency allocations, and frequency-hopping sequences.
Frequency Correction Channel (FCCH) and Synchronisation
Channel (SCH)
Used to synchronise the mobile to the time slot structure of a cell
by defining the boundaries of burst periods, and the time slot numbering.
Every cell in a GSM network broadcasts exactly one FCCH and one
SCH, which are by definition on time slot number 0 (within a TDMA
frame).
Random Access Channel (RACH)
Slotted Aloha channel used by the mobile to request access to the
network.
Paging Channel (PCH)
Used to alert the mobile station of an incoming call.
Access Grant Channel (AGCH)
Used to allocate an SDCCH to a mobile for signalling (in order to
obtain a dedicated channel), following a request on the RACH.
Publicatie uit 2004
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