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

Carrier separation is 200 kHz, which provides:

       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:

  1. Bm or full rate TCH (TCH/F) - this channel carries information at a gross rate of 22.8 kbit/s.

  1. 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:

  1. Broadcast CHannels (BCH)

  1. Common Control CHannels (CCCH)

  1. 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 

MAPPING OF LOGICAL CHANNELS
TIME SLOTS AND TDMA FRAMES