In GSM (Global System for Mobile Communications), the total number of channels in a frequency band depends on the specific band and the duplexing scheme used. GSM uses a combination of Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA) to divide the frequency band into multiple channels. The two main frequency bands used in GSM are the 900 MHz band (GSM 900) and the 1800 MHz band (DCS 1800 or GSM 1800). Here’s an overview of the channels in each band:
GSM 900 Band:
1. Frequency Range:
- The GSM 900 band spans the frequency range of approximately 890 MHz to 960 MHz.
2. Downlink Channels (Uplink for Mobile Station):
- The downlink channels (from the base station to the mobile station) are divided into 200 kHz carrier frequencies, and each carrier frequency is divided into eight time slots.
- With eight time slots per carrier, there are a total of 8 time slots×� carrier frequencies8 time slots×N carrier frequencies channels, where �N is the number of carrier frequencies allocated to GSM in that specific frequency band.
3. Uplink Channels (Downlink for Mobile Station):
- The uplink channels (from the mobile station to the base station) follow a similar structure, resulting in the same total number of channels.
GSM 1800 Band (DCS 1800):
1. Frequency Range:
- The GSM 1800 band spans the frequency range of approximately 1710 MHz to 1880 MHz.
2. Downlink Channels (Uplink for Mobile Station):
- Similar to GSM 900, the downlink channels are divided into 200 kHz carrier frequencies with eight time slots each.
3. Uplink Channels (Downlink for Mobile Station):
- The uplink channels follow the same structure as the downlink channels.
Total Channels:
- The total number of channels in a GSM network depends on the available spectrum and how many carrier frequencies are allocated to GSM in that specific frequency band.
In summary, the number of channels in GSM is determined by the frequency band (GSM 900 or GSM 1800) and the allocation of carrier frequencies within that band. The combination of FDMA and TDMA allows GSM to support multiple voice and data channels simultaneously within the allocated spectrum.