• In a cellular system, a cell is a local area of coverage served by a base station. These base station are connected to an MSC. Since communication may occur over multiple such MSCs, the MSCs provide connectivity between the PSTN and multiple base stations.
• PSTN stands for Public Switched Telephone Network.
• The PSTN forms the global telecommunications grid which connects conventional (landline) telephone switching centers (called central offices or exchanges) with MSCs throughout the world.
• A typical MSC is responsible for connecting around 100 base stations to the PSTN.
• The information transfer in PSTN takes place over landline trunked lines (trunks) comprised of fiber optic cables, microwave links and satellite links.
• The network configuration remains static as it needs to be changed only when the user changes residence and a reprogramming happens at central office.
• The bandwidth for a fixed landline network can be changed by altering the type of material (co-axial cable/fiber-optic).
• It is estimated that there are around 1 billion fixed landline subscribers in whole world )as compared to 600 million mobile subscriber)
• Depending on individual country, the rules for usage of such PSTN systems change. In some countries, certain private parties too are allowed to be part of it.
• In PSTN, each city or a geographical grouping of towns is called a local access and transport area (LATA).
• The various nearby LATAs are connected by a local exchange carrier (LEC).
• Interexchange Carriers (IXC) have a large network of fiber-optic connections connected to LEC throughout a country or continent. Extra fees need to be paid for accessing such connections.
• The figure below is an illustration of a local telephone network called a local exchange.
• As seen in the figure, each CO provide connectivity to
- Customer Premises Equipment (CPE) kept at residences.
- Private Branch Exchanges (PBX) kept at business organizations (e.g. extension lines as in colleges)
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