# A Petri Net Model for Cognitive Radio Internet of Things Networks Exploiting GSM Bands +
## 3 Characterization of the Cognitive Environment
### Characterization of the GSM Primary User  
GSM is an open, digital cellular technology used for transmitting mobile voice and data services. It uses digital technology and Time-Division Multiple Access (TDMA) transmission methods. A circuit switched system is used for phone calls; it divides each 200 kHz channel into eight 25 kHz time-slots. GSM operates in the 900 MHz and 1.8 GHz bands in Europe and the 1.9 GHz and 850 MHz bands in the US. The 850 MHz band is also used for GSM and 3 GSM in Australia, Canada, and many South American countries. To perform frequency reuse and user multiplexing, GSM combines Time-Division 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. Depending on the expected load in a specific area, 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 time slot (or burst period), and it lasts \(\frac{15}{26}\) ms \(\approx\) 0.577 ms. Eight burst periods are grouped into a TDMA frame (\(\frac{120}{26}\) ms \(\approx\) 4.615 ms), which forms the basic unit for the definition of the logical channels. One physical channel is one burst period per TDMA frame. Figure 1 shows the frame structure adopted in GSM900. The organization of the frames in multiframe and superframe levels is out of the scope of our work. When operating in downlink, Time Slot 0 (TS0) is always used to transmit the beacon logical channel (base control channel), which is made up of FCCH (frequency correction channel), SCH (synchronization channel), and BCCH (broadcast control channel). When operating in uplink, TS0 is always used to transmit the logical channel RACH (random access channel). The slots from TS1 to TS7 are usually used to transmit the TCHs (transportation channels) in both downlink and uplink modes; a specific cell configuration can be set in such a way that TS1 is used in downlink to accommodate the SDCCH (stand-alone dedicated control channel) and TS8 in frame 12 is used to accommodate the SACCH (slow associated control channel). Optionally, within an individual cell, TDMA is combined with a slow frequency hopping to address the problems of frequency-selective fading and co-channel interference. In a pre-calculated sequence of different frequencies (algorithm programmed in mobile station), one hop per frame can occur. Our current model of PUs does not take into account frequency hopping. Moreover, GSM implements discontinuous transmission (DTX) in TCH. When DTX is in operation, the transmitter is switched off if no speech is present; this increases system capacity by reducing the co-channel interference, and it also reduces transmitter power consumption (an important consideration for handheld portables). In a typical conversation, each speaker talks for less than 40% of the time, and it has been estimated that DTX could approximately double the capacity of the radio system [38; 39; 40]. The DTX mechanism can be implemented thanks to the presence of a voice activity detector (VAD) block, which is able to analyze the activity status of the speaker through the analysis of the speech signal. The first evolution of the GSM standard provides data services with user bit rates of up to 14.4 kb/s for circuit switching (CS) data and up to 22.8 kb/s for packet switching data through the General Packet Radio Service (GPRS). Both services are based on the original Gaussian minimum shift keying (GMSK) modulation. With the introduction of EDGE, two types of transceivers can be used in a cell: standard GSM transceivers and EDGE transceivers. Each time, the slot used for traffic in the cell can be assigned to support one of the possible services:  
* GSM;
* CSD;
* GPRS;
* Enhanced CSD (ECSD);
* Enhanced GPRS (EGPRS). Standard GSM transceivers only support the first three service types, whereas EDGE transceivers support all five.