Channel consept: Cell Allocation (CA) is the subset of the total
frequency band that is accessible for one BTS. It can be seen as the aggregate
transport asset accessible for movement between the BTS and its appended MSs.
One Radio Frequency CHannel (RFCH) of the CA is utilized to convey
synchronization data and the Broadcast Control CHannel (BCCH). This can be any
of the bearers in the cell and it is known as the BCCH transporter or the c0
bearer.
Solid proficiency and quality prerequisites have
brought about a fairly complex method for using the recurrence asset. This
section portrays the fundamental standards of how to utilize this asset from
the physical asset itself to the data transport administration offered by the
BTS.
RADIO FREQUENCY CARRIERS
GSM 900
|
GSM 1800
|
GSM 1900
|
|||
Uplink
|
890 - 915 MHz
|
1710 - 1785 MHz
|
1850 - 1910 MHz
|
||
Downlink
|
935 - 960 MHz
|
1805 - 1880 MHz
|
1930 - 1990 MHz
|
i.
124 pairs of carriers in the GSM 900 band
ii.
374 pairs of carriers in the GSM 1800 band
iii.
299 pairs of carriers in the GSM 1900 band
Utilizing Time Division
Multiple Access (TDMA) each of these bearers is partitioned into eight Time
Slots (TS). One TS on a TDMA edge is known as a physical channel, i.e. on every
duplex pair of bearers there are eight physical channels.
An assortment of data is
transmitted between the BTS and the MS. The data is assembled into diverse
coherent channels. Each intelligent channel is utilized for a particular
reason, for example, paging, ring set and discourse. For instance, discourse is
sent on the sensible channel Traffic CHannel (TCH). The coherent channels are
mapped onto the physical channels.
The data in this section
does exclude channels particular for GPRS (General Packet Radio Service). For
fundamental data on GPRS see section 14 of this documentation.
LOGICAL
CHANNELS
The
logical channels are into two categories. They are traffic channels and
signaling channels.
There are two forms of TCHs:
- Bm or full rate TCH (TCH/F) - this
channel carries information at a gross rate of 22.8 kbit/s.
- Lm or half rate TCH (TCH/H) - this
channel carries information at a gross rate of 11.4 kbit/s.
Signaling channels are subdivided into three
categories:
- Broadcast
CHannels (BCH)
- Common
Control CHannels (CCCH)
- Dedicated
Control CHannels (DCCH)
CHANNEL
COMBINATIONS
Just sure blends of
consistent channels are allowed by GSM proposals. The figure beneath
demonstrates the path in which legitimate channels can be joined on to Basic
Physical Channels (BPC). Numbers showing up in sections after channel
assignments demonstrate sub-channel numbers. Partaking in a BPC inside of a
multiframe structure and regarding it as a different asset shapes a
sub-channel.
(i)
TCH/F
+ FACCH/F + SACCH/TF
(ii)
TCH/H(0,1)
+ FACCH/H(0,1) + SACCH/TH(0,1)
(iii)
TCH/H(0)
+ FACCH/H(0) + SACCH/TH(0) + TCH/H(1)
(iv)
FCCH +
SCH + BCCH + CCCH
(v) FCCH +
SCH + BCCH + CCCH + SDCCH/4(0...3) + SACCH/C4(0...3)
(vi)
BCCH +
CCCH
(vii)
SDCCH/8(0...7)
+ SACCH/C8(0...7)
(viii)
TCH/F
+ FACCH/F + SACCH/M
(ix)
TCH/F
+ SACCH/M
(x)
TCH/FD
+ SACCH/MD
Where CCCH = PCH + AGCH + RACH
SACCH/T means that SACCH
is associated with a TCH while
SACCH/C is
associated with a control channel.
Where the
SMSCB is supported, the CBCH replaces SDCCH sub-channel 2 in cases (v) and
(vii) of the above.
A combined
CCCH/SDCCH allocation (case v) above may only be used when no other CCCH
channel is allocated.
The
difference between channel combinations (ii) and (iii) is that combination (ii)
addresses two different MSs, while combination (iii) addresses one single MS
using both half rate traffic channels, for example, one for speech and the
other for data.
Channel combinations from (viii) through (x) are used for
multislot configurations (e.g. High Speed Circuit Switched Data, described in
more detail in chapter 3), the postfix /M standing for “multislot”, /MD -
“multislot downlink”.
In GSM
system the operator can also use the Adaptive Configuration of the SDCCHs
feature. Time slots can be automatically reconfigured by the BSC to support
channel combinations (i) or (vii), depending on the traffic situation in the
cell. This means that the number of TCH and SDCCH channels will change
according to needs (heavy SMS traffic requiring high SDCCH capacity, multislot
data transmission with big TCH needs, etc.) The reconfiguration is performed per
BPC and takes about two seconds.
In the coming articles i am going to explain about
TIME SLOTS AND TDMA FRAMES