The GSM Air Interface

GSM is a TDMA system, with Frequency Division Duplex (FDD). It uses
Gaussian Minimum Shift Keying (GMSK) as the modulation scheme.
TDMA means that multiple users share a given RF channel on a timesharing
basis. FDD means that different frequencies are used in the downlink
(from network to MS) and uplink (from MS to network) directions.
GSM has been deployed in numerous frequency bands—including the
900-MHz band, the 1800-MHz band, and the 1900-MHz band (in North
America). Table 3-4 shows the frequency allocations for these three bands.
Of course, the amount of spectrum allocated in a given band in a given
country is at the discretion of the appropriate regulatory authorities in that
country. Moreover, even if the entire spectrum in a given band is made
available in a given country, it is likely to be divided among several operators
such that it is extremely rare for a single network operator to have
access to a complete band.

In GSM, a given band is divided into 200-kHz carriers or RF channels in
both the uplink and downlink directions. In addition, a guard band of 200
kHz is located at each end of each frequency band. For example, in standard
GSM 900, the first uplink RF channel is at 890.2 MHz and the last uplink
RF channel is at 914.8MHz, allowing for a total of 124 carriers. Similarly,
DCS 1800 has a maximum of 374 carriers and PCS 1900 has a maximum
of 299 carriers.
As mentioned, in GSM, a given band is divided into a number of RF
channels or carriers, each 200 kHz in both the uplink and downlink. Thus,
if a handset is transmitting on a given 200-kHz carrier in the uplink, then
it is receiving on a corresponding 200-kHz carrier in the downlink. Because
the uplink and downlink are rigidly associated, when one talks about a carrier
or RF channel, both the uplink and downlink are usually implied. A
given cell can have multiple RF carriers—typically one to three in a normally
loaded system, though as many as six carriers might exist in a heavily
loaded cell in an area of very high traffic demand. Note that, when we
talk about a cell in GSM terms, we mean a sector. Thus, a three-sector BTS
implies three cells. This is a somewhat confusing distinction between GSM
and some other technologies.

Each RF carrier is divided into eight timeslots, numbered 0 to 7, and
these are transmitted in a frame structure. Each frame lasts approximately
4.62 ms, such that each time slot lasts approximately 576.9
s. Depending
on the number of RF carriers in a given cell, all eight timeslots on a given
carrier might be used to carry user traffic. In other words, the RF carrier
might be allocated to eight traffic channels (TCHs). There must be, however,
at least one timeslot in a cell allocated for control channel purposes. Thus,
if only one carrier is in a cell, then there is a maximum of seven TCHs, such
that a maximum of seven simultaneous users can be accommodated.